willowmoor concept alternatives analysis report

Willowmoor Floodplain Restoration Project

Concept Alternatives Analysis

April 2019

A Service Provider of King County Flood Control District

Prepared for King County by Tetra Tech

1420 Fifth Avenue, Suite 550 Seattle, WA 98101

P 206.728.9655 F 206.728.9670 tetratech.com

Project #100-RCE-T38352 1001

In association with:

1-Alliance Geomatics

Aspect Consulting

Anchor QEA, LLC

JA Brennan and Associates

Northwest Hydraulic Consultants

Stepherson and Associates, Inc.

Willamette Cultural Resources

FUNDING: Funding for this project has been provided by:

Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County i

CONTENTS

EXECUTIVE SUMMARY ....................................................................................................................... 1

1.0 INTRODUCTION ............................................................................................................................. 5

1.1 Overview and Purpose ............................................................................................................ 5

1.2 Flood Control District Directive ................................................................................................ 6

1.3 Project Location and Study Area ............................................................................................. 7

1.4 Public Outreach ....................................................................................................................... 9

1.5 Project Goals and Objectives ................................................................................................... 9

2.0 DESIGN AND EVALUATION CRITERIA ....................................................................................... 11

2.1 Criteria Types ........................................................................................................................ 11

2.2 Design Criteria ....................................................................................................................... 11

2.3 Evaluation Criteria ................................................................................................................. 12

2.3.1 Flood Protection Performance Measure .......................................................................... 12

2.3.2 Habitat Performance Measure ......................................................................................... 13

2.3.3 Sustainability Performance Measure ............................................................................... 14

3.0 PROJECT SETTING ..................................................................................................................... 16

3.1 Lake Sammamish and Tributary Streams .............................................................................. 16

3.2 Sammamish River and Tributary Streams ............................................................................. 16

3.2.1 Sammamish River ........................................................................................................... 16

3.2.2 Transition Zone ............................................................................................................... 17

3.2.3 Tosh Creek ...................................................................................................................... 17

3.2.4 Bear Creek ...................................................................................................................... 17

4.0 EXISTING PROJECT CONDITIONS ............................................................................................. 18

4.1 Hydrology and Hydraulics ...................................................................................................... 18

4.1.1 Lake Sammamish Water Level ........................................................................................ 20

4.1.2 Sammamish River Flow ................................................................................................... 21

4.1.3 Storm Drain Outfall Evaluation ........................................................................................ 22

4.1.4 U.S. Army Corps of Engineers 1964 General Design Memo Evaluation .......................... 23

4.2 Aquatic Habitat ...................................................................................................................... 24

4.2.1 Fish Habitat and Use ....................................................................................................... 24

4.2.2 Wetland and Riparian Conditions .................................................................................... 26

4.3 Geology and Hydrogeology ................................................................................................... 28

4.4 Geomorphology ..................................................................................................................... 29

4.5 Cultural Resources ................................................................................................................ 30

4.6 Recreation ............................................................................................................................. 31

4.7 TZ Maintenance ..................................................................................................................... 32

ii Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County

5.0 CONCEPT DEVELOPMENT ......................................................................................................... 33

5.1 Background ........................................................................................................................... 33

5.2 Transition Zone...................................................................................................................... 33

5.3 TZ Weir ................................................................................................................................. 33

5.4 Side Channel ......................................................................................................................... 34

5.5 Floodplain .............................................................................................................................. 35

5.6 Recreation ............................................................................................................................. 35

5.7 Side Channel Concept Alternatives ....................................................................................... 35

5.8 Cold-Water Supplementation Options .................................................................................... 39

6.0 ALTERNATIVE PERFORMANCE SUMMARY .............................................................................. 44

6.1 General ................................................................................................................................. 44

6.2 Side Channel Concept Alternative 1 Wide Side Channel ....................................................... 44

6.2.1 Flood Protection .............................................................................................................. 44

6.2.2 Habitat ............................................................................................................................. 47

6.2.3 Sustainability ................................................................................................................... 48

6.3 Side Channel Concept Alternative 2 Narrow Side Channel .................................................... 50

6.3.1 Flood Protection .............................................................................................................. 50

6.3.2 Habitat ............................................................................................................................. 54

6.3.3 Sustainability ................................................................................................................... 55

6.4 CWS Option A (“Big Water”) .................................................................................................. 56

6.4.1 Habitat ............................................................................................................................. 56

6.4.2 Sustainability ................................................................................................................... 56

6.5 CWS Option B (“Gravel Interflow”) ......................................................................................... 57

6.5.1 Habitat ............................................................................................................................. 57

6.5.2 Sustainability ................................................................................................................... 57

6.6 CWS Option C (“Groundwater”) ............................................................................................. 58

6.6.1 Habitat ............................................................................................................................. 58

6.6.2 Sustainability ................................................................................................................... 58

7.0 COMBINED ALTERNATIVES AND ESTIMATED PROJECT COSTS .......................................... 59

7.1 Combined Alternatives ........................................................................................................... 59

7.2 Cost Estimates ...................................................................................................................... 59

8.0 PARTNERSHIPS FOR PROJECT IMPLEMENTATION ................................................................ 62

8.1 Grant funding Opportunities for cold-water supplementation.................................................. 62

8.1.1 Salmon Recovery Funding Board .................................................................................... 63

8.1.2 City of Redmond .............................................................................................................. 63

8.1.3 Washington Department of Ecology Floodplains by Design Grants ................................. 63

8.1.4 King County Mitigation Reserves Program ...................................................................... 63

8.1.5 Puget Sound Near-Term Action Agenda.......................................................................... 64

Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County iii

8.1.6 Lake Management Districts ............................................................................................. 64

8.1.7 Other Sources ................................................................................................................. 64

8.2 Funding Partners for OnGoing Maintenance of Cold-Water Supplementation ....................... 65

8.2.1 Grant Opportunities ......................................................................................................... 65

8.2.2 River Management Inter-local Agreement ....................................................................... 66

9.0 PERFORMANCE EVALUATION ................................................................................................... 67

10.0 NEXT STEPS .............................................................................................................................. 68

11.0 REFERENCES ............................................................................................................................ 69

APPENDICES

Appendix A. Hydrologic Modeling Technical Memorandum

Appendix B. Hydraulic Modeling Technical Memorandum

Appendix C. Alternative Analysis Hydraulic Modeling Technical Memorandum

Appendix D. Preliminary Dynamic Weir Analysis Technical Memorandum

Appendix E. Hypolimnetic Cooling of Lake Sammamish Surface Water

Appendix F. Groundwater Analysis

Appendix G. Existing Geomorphic Conditions Technical Memorandum

Appendix H. Sammamish River Conceptual Fish Use Model

Appendix I. Management Technical Paper #1: Beaver Management Tools Literature Review and

Guidance

Appendix J. Planning Level Cost Estimate

Appendix K. Script for Soliciting Input from Grant Agencies Regarding Applicability of Grant Funds for

Ongoing Maintenance of Cold-Water Supplementation

Appendix L. Alternative Evaluation Worksheet

LIST OF TABLES

Table 1. Flood Protection Evaluation Criteria ....................................................................................... 13

Table 2. Habitat Evaluation Criteria...................................................................................................... 14

Table 3. Sustainability Evaluation Criteria ............................................................................................ 15

Table 4. Existing Conditions (2019) Level Exceedance in Lake Sammamish ....................................... 21

Table 5. Existing Conditions (2019) Seasonal Parameters .................................................................. 21

iv Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County

Table 6. Existing Conditions (2019) Peak Lake Level and River Flow Frequency ................................ 21

Table 7. Existing Conditions (2019) Flow Duration Sammamish River Below Bear Creek ................... 22

Table 8. Peak Level at Critical Stormwater Outfalls under Existing Conditions (2019) ......................... 22

Table 9. Fringe Wetlands Inundation Existing Conditions (2019) ......................................................... 28

Table 10. Manually Operated Weir Configuration ................................................................................. 34

Table 11. Side Channel Concept Alternative Summary........................................................................ 36

Table 12. Cold-Water Supplementation Option Summary .................................................................... 39

Table 13. Concept Alternative 1 Wide Side Channel, Level Exceedance in Lake Sammamish ............ 45

Table 14. Concept Alternative 1 Wide Side Channel, Seasonal Parameters ........................................ 45

Table 15. Concept Alternative 1 Wide Side Channel, Sammamish River Flow Frequency

Downstream of Bear Creek .................................................................................................................. 45

Table 16. Concept Alternative 1 Wide Side Channel, Sammamish River Flow Duration

below Bear Creek ................................................................................................................................ 46

Table 17. Concept Alternative 1 Wide Side Channel, Inundated Outfalls ............................................. 47

Table 18. Concept Alternative 1 Wide Side Channel, Lake Fringe Wetlands Inundation ...................... 48

Table 19. Concept Alternative 2 Narrow Side Channel, Level Exceedance in Lake Sammamish ......... 51

Table 20. Concept Alternative 2 Narrow Side Channel, Seasonal Parameters .................................... 51

Table 21. Concept Alternative 2 Narrow Side Channel, Sammamish River Flow Frequency

Downstream of Bear Creek .................................................................................................................. 52

Table 22. Concept Alternative 2 Narrow Side Channel, Sammamish River Flow Duration

below Bear Creek ................................................................................................................................ 52

Table 23. Concept Alternative 2 Narrow Side Channel, Inundated Outfalls .......................................... 54

Table 24. Concept Alternative 2 Narrow Side Channel, Fringe Wetlands Inundation ........................... 55

Table 25. Combined Alternatives ......................................................................................................... 59

Table 26. Planning Level Opinion of Cost Summary ............................................................................ 59

Table 27. Project Cost Allocation for Side Channel Configurations ...................................................... 60

Table 28. Project Cost Allocation for Cold-Water Supplementation Options ......................................... 61

Table 29. Potential Grant Funding Sources for the Willowmoor Floodplain Restoration Project. .......... 62

Table 30. Summary of Responses to Cold-Water Supplementation Grant Inquiries ............................. 65

Table 31. Summary of Agencies That Have Not Yet Responded to Willowmoor Grant Inquiries .......... 66

Table 32. Performance Evaluation Scoring Summary .......................................................................... 67

Table 33. Performance Evaluation Summary for Combined Alternatives ............................................. 67

Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County v

LIST OF FIGURES

Figure 1. The Concept Evaluation as a Component of the Preliminary Design ...................................... 6

Figure 2. Site Features .......................................................................................................................... 7

Figure 3. Willowmoor Floodplain Restoration Project Vicinity ................................................................. 8

Figure 4. Surface Water Features in the Willowmoor Project Vicinity ................................................... 19

Figure 5. Existing Conditions (2019) Average Daily Lake Sammamish Water Levels .......................... 20

Figure 6. Existing Conditions (2019) Daily Lake Level and Sammamish River Flow ............................ 24

Figure 7. Temperature and Fish Use in Sammamish River and Tosh Creek. ....................................... 25

Figure 8. Wetland and Riparian Areas ................................................................................................. 26

Figure 9. Side Channel Concept Alternative 1 – Wide Side Channel ................................................... 37

Figure 10. Side Channel Concept Alternative 2 – Narrow Side Channel .............................................. 38

Figure 11. CWS Option A – Lake Surface Water Heat Exchange (Big Water) ..................................... 40

Figure 12. CWS Option B – Hyporheic Exchange (Gravel Interflow) .................................................... 41

Figure 13. CWS Option C – Deep Groundwater Pumping (Groundwater) ............................................ 43

Figure 14. Concept Alternative 1 Wide Side Channel, Average Daily Lake Sammamish Level ............ 44

Figure 15. Concept Alternative 1 Wide Side Channel, Daily Lake Level and Sammamish River Flow .. 46

Figure 16. Concept Alternative 2 Narrow Side Channel, Average Daily Lake Sammamish Level ......... 51

Figure 17. Concept Alternative 2 Narrow Side Channel, Daily Lake Level

and Sammamish River Flow ................................................................................................................ 53

Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County 1

EXECUTIVE SUMMARY

The Willowmoor Floodplain Restoration Project; Concept Alternatives Analysis is a report describing

the results of an analysis of alternatives for the Willowmoor Floodplain Restoration Project (Project),

located in the Sammamish River Transition Zone (TZ) in Marymoor Park, King County, Washington.

King County, as the local project sponsor for the 1964 U.S. Army Corps of Engineers Sammamish

River Improvement Project, is responsible for maintaining the TZ. Maintenance costs and associated

permitting requirements for this work have increased since the 1960s due to changes in wetland,

shoreline, and endangered species regulations and policies at the federal, state, and local levels. To

address these issues, the King County Flood Control District (District) identified the split-channel

alternative (Alternative 4 described in the Concept Design Summary Report, King County, 2015) as the

selected alternative to proceed to 30 percent design. The split channel alternative best addressed the

need for both additional flood protection and improvement of local habitat for salmonids listed under the

Endangered Species Act and protected by tribal treaty rights. The County, as a service provider to the

District, has undertaken this analysis to evaluate the potential to accomplish the Project goals and

objectives as set forth in District Executive Committee Motion 2016-04.1 (Motion), listed in Table ES-1.

This table summarizes how each of the Motion’s objectives are being addressed and directs the reader

to where each Motion design element is discussed in this report.

The concept design for the selected split channel alternative included a new 3,400-foot side channel,

but little additional detail. This analysis furthers the concept design with more detail, including attention

to both the mainstem weir and side channel control and greater elaboration of the channel alignment

and slope. Two concept alternatives for the split channel were developed: one featuring a wide inset

floodplain side channel; and one featuring a narrow inset floodplain side channel. The wide and narrow

side channel concept alternatives are both paired with a modified, manually operated dynamic weir,

discussed in the Preliminary Dynamic Weir Analysis (Appendix E, King County, 2018). A remotely

operated dynamic weir was considered, but not included by the project team in this alternatives

analysis due to the substantial permitting and operational complexity of the remotely operated weir, as

well as its high risk of impacting Redmond’s stormwater outfall system, just downstream of the weir.

The wide and narrow side channel concept alternatives also include habitat features such as riparian

plantings, variable depth pools, and benching back of the left bank floodplain to improve salmonid

habitat. These features are proposed mitigation for the removal of willow from the existing low flow

channel, a project element intended to increase conveyance capacity and simplify maintenance of the

TZ. In addition to the two split channel concept alternatives that include construction, a third “no-action”

alternative is considered, representing the existing maintained condition.

The two split channel construction concept alternatives are each paired with four cold-water

supplementation options, resulting in a total of nine potential project variations (including no action)

analyzed in this report. The cold-water supplementation options include: 1) no cold-water

supplementation, 2) a “big water” 20 cfs lake withdrawal option, which uses a heat exchanger at the

lake bottom that draws down and cools surface water, 3) a “gravel interflow” option, which forces

surface water in the side channel into manmade deep gravel trenches in the streambed to interact with

and be cooled by groundwater, and 4) a “deep groundwater” option, which pumps cold groundwater up

from 600 feet below the ground surface and into the side channel. The Water Resource Inventory Area

(WRIA) 8 Chinook Salmon Recovery Plan Update (2017) indicates thermal refuge is the top priority for

Sammamish River salmonid habitat restoration; therefore, cold-water supplementation is a prioritized

habitat feature in this analysis for meeting the habitat restoration objective of the Motion. Preliminary

design of these cold-water supplementation features is funded by state salmon recovery grants.

2 Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County

Table ES-1 Design Elements of District Executive Committee Motion 2016-04.1

Design Element from Motion 2016-04.1

How Addressed

1. Develop the split-channel alternative in such a way that balances the objectives of flood control, habitat restoration, fish passage, and sustainability.

Section 5.0 documents the development of eight variations of the split channel alternative that include the following elements to balance the project objectives:

• A widened main channel with willow removed from the central channel area and variable depth pools added to the center low flow channel

• A modified, manually operated dynamic weir to better address climate variability

• A new benched left bank with juvenile salmonid habitat and tall shade trees outside the 2-year floodplain

• A new side channel (one wide and one narrow version)

• A choice of three cold-water supplementation options to address thermal barriers to fish passage and other salmonid health issues

Section 6.0 documents the performance of each of the construction concept alternatives in meeting the stated project objectives relative to the no-action alternative, the existing maintained condition. The alternatives analysis demonstrates that marginal additional flood control could be afforded to lake shore communities through implementation of a construction concept alternative. This benefit is constrained by the need to maintain existing flood protection for downstream communities and the need to protect lake shore wetland resources. Each of the action alternatives provides significant benefit to salmonids and lowers annual operational costs.

2. Include variable depth pools as an enhancement to the split channel alternative.

Section 5.2 documents the proposed addition of variable depth pools in the main channel, and how they will provide adult salmon migratory habitat.

3. Work with the City of Redmond on coordination with city flood control efforts, groundwater issues related to cold-water supplementation, and Bear Creek impacts on Sammamish River flows.

Appendix A and Appendix B document the updated hydrologic and hydraulic modeling used to update the Bear Creek contribution to the flood model and to develop design flows for Redmond flood protection. The City of Redmond provided data and technical support in the development of these models.

Sections 4.1.3, 6.2.1 and 6.3.1 document analysis of flood conditions in Redmond under existing and proposed alternative conditions.

Section 5.8 documents the use of groundwater pumping to achieve temperature reduction targets.

4. Conduct a feasibility analysis of a dynamic weir that includes costs and benefits.

Appendix D is a Preliminary Dynamic Weir Analysis that documents the feasibility of different types of dynamic weirs as project elements. This document concludes that a manually adjustable dynamic weir will provide increased flexibility to manage flood flows in future conditions, while keeping operational costs and risk low. Section 5.3 documents the ability of the manually adjustable dynamic weir (in conjunction with the new side channel) to meet flow targets, even with the increase in Bear Creek flows since the 1964 project construction.

5. Conduct a technical analysis of the split-channel alternative for fish mortality and sustainability.

Sections 6.2.2, 6.2.3, 6.3.2, and 6.3.3 describe technical requirements and features of the narrow and wide side channels as well as the reconfigured mainstem channel that work in tandem to improve fish survival and sustainability. These features include:

• 1:1 pool-to-riffle ratios

• Localized temperature refugia (pools, hyporheic features, Tosh Creek inputs)

• Positive drainage to avoid fish stranding


Each construction concept alternative variation was evaluated for its ability to meet flood protection,

habitat, and sustainability performance criteria relative to the no-action alternative, and a cost estimate

was developed. Table ES-2 describes the nine alternative combinations and provides a summary of

evaluation scores and costs. Evaluation criteria and scoring details can be found in Section 9.0 and

Appendix L. Capital costs were categorized into flood, mitigation, and additional habitat costs. Life cycle

costs were categorized into permitting, maintenance, and mitigation costs. A more detailed elaboration

of project cost assumptions and estimates can be found in Section 7.2, Table 26, Table 27, Table 28

and Appendix J. The information in this alternatives analysis will be presented to the District to inform

its decision on advancing a variation of the split channel alternative forward to 30 percent design.

Table ES-1 (continued) Design Elements of District Executive Committee Motion 2016-04.1

Design Element from Motion 2016-04.1 How Addressed

6. Include a beaver mitigation plan.

Section 6.2.3 discusses beaver activity in the project reach and King County generally. Beavers are present in the TZ, and while they typically do not pose a threat to fish passage, they can create drainage and related flooding concerns. Appendix I is a 2017 County publication that provides technical guidance on managing beavers in a variety of scenarios. Specific beaver mitigation approaches will be selected from this guidance document and written into a more tailored mitigation strategy on selection of a preferred project alternative.

7. Include a maintenance plan for when project is complete.

Sections 6.2 through 6.6 describe the different maintenance activities for each combined side channel alternative and cold-water supplementation option.

Section 7.0 and Appendix J include cost estimates for these maintenance activities. Maintenance and mitigation costs are lower for all construction concept alternatives than for the no-action alternative. The proposed alternative channel configurations take advantage of more of the floodplain area and separate habitat and flood conveyance features in a way that requires less frequent and extensive maintenance of salmon habitat features, leading to lower mitigation costs.

8. Pursue grant sources to further evaluate cold-water supplementation.

Sections 8.1 and 8.2 provide a discussion of suitable cold-water funding partnership options, and include details on partnership funding awarded to date ($550,000).

9. Identify funding partners to assume ongoing maintenance costs of cold-water supplementation.

Section 8.2 and Appendix K provide details on partnership options for ongoing maintenance of cold-water supplementation. Several regional salmon recovery grant staff were surveyed to determine whether or not their programs could provide ongoing life cycle maintenance funding for cold-water alternatives. All those surveyed indicated that grant funds have limits ranging from two to five years to closeout grant expenditures. Therefore, grant funds for maintenance are an unlikely option. An innovative approach is suggested here, that a pooled contribution, of less than $2,000 annually, from each of the riverside cities’ District Sub-Regional Opportunity Fund could fund cold-water supplementation maintenance.

10. Work with Parks and Recreation Division of King County Department of Natural Resources and Parks to pursue recreational boater access.

Section 5.6 describes the process and outcome of a recreation access planning public workshop co-hosted by the project team and the King County Parks and Recreation Division. The project team identified potential impacts of project construction and maintenance on existing recreation use and access. Public comment was broad-ranging. The alternatives presented here include design features to preserve and enhance boater access in the project area.

11. Continue existing maintenance during design and permitting phases.

Annual maintenance and stage-discharge monitoring have occurred, and will continue, through project design and permitting. Section 4.7 discusses current maintenance activities and costs.


Table ES-2. Performance Evaluation Summary for Combined Alternatives

a. All scores are relative to the maintained existing condition. The maintained existing condition scored below zero because

ongoing maintenance is degrading the existing riverside tree canopy at the project site. All point scores have been adjusted

upward so that the lowest score is zero in order to normalize the point scale to a logical origin.

Combined Alternative Variation

Side Channel Alternative Cold-water Supplementation Option Cost

Points (zero

origina)

1 Wide Side Channel None $10,506,000 138

1A Wide Side Channel Big Water - Lake surface water heat exchange

$19,245,000 162

1B Wide Side Channel Gravel Interflow - Hyporheic side channel features

$11,463,000 147

1C Wide Side Channel Deep Groundwater - Deep pumped groundwater

$13,662,000 144

2 Narrow Side Channel None $8,482,000 130

2A Narrow Side Channel Big Water - Lake surface water heat exchange

$17,221,000 154

2B Narrow Side Channel Gravel Interflow - Hyporheic side channel features

$9,439,000 139

2C Narrow Side Channel Deep Groundwater - Deep pumped groundwater

$11,638,000 136

3 Maintained Existing Condition

None $3,054,000 0


1.0 INTRODUCTION

1.1 OVERVIEW AND PURPOSE

The Sammamish River is a low-gradient (0.02-percent slope) channel that drops 13 feet over a

distance of 13.5 miles from Lake Sammamish to Lake Washington. Historically, the Sammamish River

was a slow-moving, highly sinuous slough with extensive connections to floodplain wetlands. It has

been highly modified from its historical condition, primarily as a result of three major actions: (1) the

lowering of Lake Washington associated with the construction of the Lake Washington Ship Canal and

Locks in 1917; (2) straightening of the river to facilitate farming, primarily conducted prior to the 1930s;

and (3) construction of the Sammamish River Improvement Project in 1964.

Since 1964, outflows from Lake Sammamish have been controlled by a weir at the lake outlet and

conveyed by a constructed channel known as the Transition Zone (TZ) from the weir to the

Sammamish River, a length of about 1,400 feet. The TZ and the weir (referred to as the TZ weir) were

constructed by the U.S. Army Corps of Engineers (USACE) as part of the Sammamish River

Improvement Project. This project deepened the river channel by about 6 feet and resulted in the

substantial lowering of winter maximum water surface elevations in Lake Sammamish. The primary

objective of the 1964 project was to reduce flooding in the Sammamish River valley downstream of

Lake Sammamish. Since then, King County has sought to balance the hydraulic impact of the weir and

TZ on lake levels and river flows with beneficial habitat function in the TZ.

Maintenance of the TZ has been a challenge in recent years, as the County has had to balance the

environmental function that the TZ provides in terms of habitat and water quality with the hydraulic

function that it provides in influencing lake elevations and river flows. The purpose of the Willowmoor

Floodplain Restoration Project is to provide environmental enhancements that would mitigate future

maintenance impacts on habitat and water quality so that hydraulic function can be maintained. The

King County Flood Control District (District) identified the split-channel alternative (Alternative 4

described in the Concept Design Summary Report, King County, 2015) as the selected alternative to

proceed to 30 percent design. The split channel alternative best addressed the need for both additional

flood protection and improvement of local habitat for salmonids listed under the Endangered Species

Act and protected by tribal treaty rights. The County has undertaken this analysis to evaluate the

potential to accomplish the Project goals and objectives as set forth in District Executive Committee

Motion 2016-04.1 (see Section 1.2),

This concept alternatives analysis report is part of a body of work that will inform the Willowmoor

Floodplain Restoration Preliminary Design (Figure 1). The findings presented in this report will be used

to select a preferred weir and side channel configuration and whether and what type of cold-water

supplementation will be included in the preliminary design. All alternatives will be evaluated relative to

the existing maintained condition, which is the current configuration of the weir and transition zone with

annual maintenance activities that include trimming willow in the navigation channel, mowing the high

flow channel on both river banks, and removing all trimmings and clippings out of the flood conveyance

channel.


Figure 1. The Concept Evaluation as a Component of the Preliminary Design

1.2 FLOOD CONTROL DISTRICT DIRECTIVE

The Flood Control District’s Executive Committee Motion 2016-04.1 (Motion) directs this project to

balance objectives related to flood control, habitat restoration, fish passage and sustainability,

recreational access, and ongoing maintenance. The following list from the Motion identifies 11 specific

design elements for both flood and habitat benefits:

The King County Flood Control Zone District will proceed to thirty percent design for the

Willowmoor Floodplain Restoration Project, which will include, but is not limited to, the following

design elements, technical studies and plans:

1. Develop the split channel alternative in such a way that balances the objectives of flood control, habitat restoration, fish passage, and sustainability;

2. Include variable depth pools as an enhancement to the split channel alternative; 3. Work with the City of Redmond on coordination with city flood control efforts, groundwater

issues related to cold-water supplementation, and Bear Creek impacts on Sammamish River flows;

4. Conduct a feasibility analysis of a dynamic weir that includes costs and benefits; 5. Conduct a technical analysis of the split channel alternative for fish mortality and

sustainability; 6. Include a beaver mitigation plan; 7. Include a maintenance plan for when the project is complete; 8. Pursue grant sources to further evaluate cold-water supplementation; 9. Identify funding partners to assume ongoing maintenance costs of cold-water

supplementation; 10. Work with the Parks and Recreation Division of the King County Department of Natural

Resources and Parks to pursue recreational boater access; and 11. Continue existing maintenance during design and permitting phases.


1.3 PROJECT LOCATION AND STUDY AREA

The project is located in Marymoor Park, intersecting two jurisdictions: the City of Redmond and

unincorporated King County. The project extends from West Lake Sammamish Parkway on the west to

the top of the right bank of the Sammamish River on the east (see Figure 2 and Figure 3). The project

area encompasses approximately 60 acres and includes about 3,300 linear feet of the river and both

right and left river banks. The project area generally follows the alignment of the Sammamish River

starting about 2,000 feet downstream of the outlet from Lake Sammamish and extending approximately

3,300 feet downstream. The Sammamish Rowing Association (SRA) boathouse and an out-of-

commission County wastewater pond are located at the upstream end of the project site. The project

area boundary at the southwest corner is configured to exclude a single private residence.

Figure 2. Site Features


Figure 3. Willowmoor Floodplain Restoration Project Vicinity


The ground surface in the project area is relatively flat, with slopes ranging from 0 to 8 percent,

according to a site topographic survey. Steeper slopes are located on the southwest boundary of the

project area associated with West Lake Sammamish Parkway East and the transition to residential

development on the hillslopes outside the project area. The ground elevation ranges from about 6 to

16 feet NGVD29 (National Geodetic Vertical Datum of 1929) (10 to 20 feet NAVD88 (North American

Vertical Datum of 1988)).

The project study area includes affected area outside the project footprint. It extends from the outlet of

Lake Sammamish to the end of the TZ and also includes associated King County-owned lands to the

southwest of the river (Figure 2). This encompasses approximately 4,500 feet of channel from Lake

Sammamish to the downstream end of the TZ and approximately 40 acres southwest of the channel.

The study area also includes the Sammamish River downstream to the confluence with Bear Creek,

approximately 2,200 additional feet. Potential project impacts along the entire Sammamish River and in

Lake Sammamish and tributary stream confluences are also important.

1.4 PUBLIC OUTREACH

A 22-member Stakeholder Advisory Committee has supported development of Project objectives and

design features for the concept design phase. The Committee was active, meeting nine times for three

hours each between 2013 and 2015. Many Committee members participated in a public hearing on the

project conceptual alternatives on June 6, 2016.

The project team reconvened the Stakeholder Advisory Committee at Marymoor Park on October 4,

2018, to present the decision to proceed to 30 percent design with the split-channel alternative. The

team reacquainted the committee with the project and gave technical presentations on a hydrologic

model update and preliminary weir analysis.

A public meeting was held immediately following the October Stakeholder Advisory Committee meeting

to solicit input on recreation aspects of the project, particularly how the project may temporarily impact

existing access and use during construction. The meeting started with a presentation on park

management considerations, site history, existing recreation use, and site issues and opportunities.

After the presentation, the public was invited to participate in group discussions about recreation

opportunities for the left bank, right bank, and the Sammamish River. The project team shared that,

while existing access may be preserved or enhanced as needed related to project construction impacts,

any new recreation amenities would require discussion with King County Parks and partnership funding

for construction and ongoing maintenance.

A second meeting of the Stakeholder Advisory Committee was held at Marymoor Park on February 26,

2019. This meeting included discussion of an updated hydraulic model and the constraints the project

location created for further enhancing flood protection features. A draft alternatives evaluation matrix

was shared at the meeting. During the meeting, Committee members requested a third meeting to

discuss this alternatives analysis, prior to moving a preferred alternative to 30 percent design. The

District’s Executive Director concurred, and a third meeting will be scheduled.

1.5 PROJECT GOALS AND OBJECTIVES

The District identified the split-channel alternative (Alternative 4 described in the Concept Design

Summary Report, King County, 2015) as the selected alternative to proceed to 30 percent design. The


selected split channel alternative was chosen by the District based on an alternatives analysis and

feedback from the public, municipal and regulatory agencies, and tribal agencies. The split channel

alternative best addressed the need for both flood protection and improvement of local habitat for

salmonids listed under the Endangered Species Act and protected by tribal treaty rights. Alternatives

development and analysis work presented here builds on feasibility work that has been ongoing since

2013. The County has undertaken this analysis to evaluate the potential to accomplish the body of work

set forth in the Motion presented in Section 1.2.

Cold-water supplementation is an optional project element intended to counteract lethal and sub-lethal

water temperatures in the project reach that currently far exceed State water quality standards that

have been developed to support fish survival and productivity (WAC 173-201A). There is strong interest

from the salmon recovery community in incorporating cold-water supplementation as part of the

Willowmoor project as an experimental feature. Design of this element is currently funded by salmon

recovery partners and may be implemented as an integrated part of the Project, or as a standalone

project by salmon recovery partners at a later date.


2.0 DESIGN AND EVALUATION CRITERIA

2.1 CRITERIA TYPES

The following design and evaluation criteria for the Project address the District Motion, regulatory

requirements, existing contractual park use agreements between the County and community

organizations, and public safety. A draft version of these criteria was presented at the February 26,

2019 Stakeholder Advisory Committee meeting as noted above. The criteria have been refined for this

analysis to improve clarity of the basis for scoring each item.

Design criteria are distinguished from evaluation criteria in that design criteria have a minimum

threshold that every alternative must achieve and they are not scored. Evaluation criteria, in contrast,

are scored relative to performance against the existing maintained condition. Design criteria set the

minimum requirements. Evaluation criteria on the other hand, reflect the range of improvement that is

possible among the optimized alternatives. The alternative with the best performance relative to the

existing condition for a given evaluation criteria gets the highest score.

A nuance of the design criteria is a sub-category called design guidance, and these items apply to

recreation issues. Considering existing recreation access and use is part of the SEPA regulatory

process and therefore a design requirement. However, there are no explicit regulatory thresholds for

recreation access, so these criteria are set forth to define the minimum recreation standard that all

alternatives should strive to achieve for this Project. Recreation design at this conceptual stage will be

addressed with a concept plan showing preservation or improvement of existing recreation access

features common to all alternatives.

2.2 DESIGN CRITERIA

Flood Protection Design Criteria

o Do not increase downstream peak flood flows.

o Maintain minimum lake level above 25.4 feet NGVD29 ) (29.0 feet NAVD88), per the 1962

USACE General Design Memo.

o Maintain lake level below elevation 29.0 feet NGVD29 (32.6 feet NAVD88) for flows less

than 1,500 cubic feet per second (cfs) in the Sammamish River below Bear Creek, per 1962

USACE General Design Memo.

• Habitat Design Criteria

o Maintain or improve upstream and downstream fish passage through the project area

(Sammamish River and Tosh Creek).

o Maximize canopy cover in riparian and wetland plant communities.

o Avoid or minimize effects in lake fringe wetlands and native plant communities. Maintain

frequency and duration of lake levels that sustain lake fringe wetlands and affect nearshore

habitats.

• Recreation Design Guidance

o Preserve public boat access at the Sammamish Rowing Association dock for the association

and public use.


o Preserve access and safety and reduce sedimentation to the river from dog access points

upstream of the TZ in the off-leash dog area.

o To the extent feasible, preserve passive recreation amenities for observing the river and

wildlife.

o Consider improvements to passive recreation access opportunities on the left bank

floodplain in collaboration with the King County Wastewater Treatment Division, Lake Hills

Trunk Connector project and the City of Redmond West Lake Sammamish Parkway road

widening project, both of which are considering extending the bike lane through the upland

portion of the project area.

o Maintain navigation for small boats through the TZ at current or improved seasonally

affected levels.

2.3 EVALUATION CRITERIA

An evaluation process was established to compare the range of improvements that project alternatives

could achieve for three broad Project performance measures:

• Flood Protection

• Habitat (includes fish passage)

• Sustainability

For each performance measure evaluation, criteria were established that allowed a quantitative

assessment of each alternative. For each of the criteria, a rating scale was developed that allowed a

point score to be assigned based on how well an alternative meets the criteria.

2.3.1 Flood Protection Performance Measure

The flood protection performance measure evaluates the ability of the alternative to meet flood

protection objectives for Lake Sammamish and limit downstream impacts on City of Redmond

stormwater infrastructure. Three lake levels are evaluated (27.0, 28.0 and 29.0 feet NGVD29) to

determine the reduction in lakeshore inundation compared to existing conditions. Also evaluated were

seasonal minimum and maximum lake levels. Downstream impacts on City of Redmond stormwater

infrastructure were evaluated by comparing peak flood flow magnitude, as well as the duration of high

flows for each alternative as they relate to the existing condition. Flood protection evaluation criteria are

listed in Table 1.


Table 1. Flood Protection Evaluation Criteria

Evaluation Criterion Number & Description

District Design

Elementa

Concept Design

Evaluation Criteriab Regulatory

Nexus

FP-1—Maintain downstream Sammamish River levels at or below current 100-year flood levels.

1, 3, 4, 7, 11 HH / 3 Yes

Evaluation Criteria: Determined by hydraulic model output for alternatives at or below the 2010 (current) Federal Emergency Management Agency 100-year water surface profile.

FP-2—Minimize impacts on tributary drainage systems along downstream river corridor.

1, 3, 4, 7, 11 HH / 6 Yes

Evaluation Criteria: Maintain or reduce frequency and duration of increases in river level relative to existing levels greater than existing conditions 2-year return interval when flooding is reported to start to occur in the Sammamish River downstream of Bear Creek.

FP-3—Reduce average winter lake level. 1, 3, 4, 7, 11 HH / 7 No

Evaluation Criteria: From November 1 through March 1, reduce the long-term average lake level from current level, estimated to be 26.7 feet NGVD29 (30.3 feet NAVD88).

FP-4—Reduce frequency and duration of winter and spring lake levels relative to existing conditions.

1, 3, 4, 7, 11 HH / 8 No

Evaluation Criteria: Reduce average number of days per year lake levels exceed 27.0 feet NGVD29 (30.6 feet NAVD88).


1, 3, 4, 7, 11 HH / 9 No



1, 3, 4, 7, 11 HH / 8 No


a. See Section 1.2 for a listing of design elements identified in ECM 2016-04.1.

b. See Concept Design Summary Report, Project Goals and Objective, (King County, 2015).

2.3.2 Habitat Performance Measure

The habitat performance measure evaluates the extent to which the alternative restores or improves

habitat within the project area and downstream of the project area. Habitat function improvement is

evaluated for aquatic, wetland and riparian habitats. The evaluation for aquatic habitat considers

instream flows, improving channel complexity, and development of holding and rearing habitat for fish.

Temperature criteria evaluate the ability of the cold-water supplementation alternatives to improve

water quality to mitigate potential thermal barriers to fish passage and to reduce metabolic stress on

migrating adult salmonids. The evaluation for wetland and riparian habitat considers enhancing or

reestablishing riverine wetlands and improving hydrologic connectivity with the river. Habitat evaluation

criteria are listed in Table 2.


Table 2. Habitat Evaluation Criteria


District Design

Elementa

Concept Design


Nexus

HAB-1—Maintain discharge at or above minimum existing summer flows for fish use.

1, 4, 5, 7, 11 HH / 4 Yes

Evaluation Criteria: From August 1 to October 1, maintain flows (as measured at Marymoor Bridge) at or above 25.7 cfs, the computed current conditions average minimum flow from 1948 to 2018.

HAB-2—Provide thermal refuge within project reach (between the Lake Sammamish outlet and the Bear Creek confluence) to improve habitat conditions for rearing salmonids and minimize thermal barriers for migrating adult salmon.

1, 2, 5, 8, 9 HAB / 1 Yes

Evaluation Criteria: Reduce temperature within project reach during critical migration period.

HAB-3—Reduce incidence of incipient lethal water temperature (21°C) for adult salmon between August 15 and September 31 within project reach (between the Lake Sammamish outlet and the Bear Creek confluence).

1, 5, 8, 9 HAB / 2 Yes

Evaluation Criteria: Reduce average index of thermal stress in project reach during critical migration period.

HAB-4—Reduce temperature beyond project reach (downstream of the Little Bear Creek confluence) to minimize thermal barriers and pre-spawn mortality for migrating adult salmon.

1, 5, 8, 9 HAB / 2 Yes

Evaluation Criteria: Reduce average index of thermal stress downstream of project reach during critical migration period.

HAB-5—Reduce thermal heat loading to the river in the project reach.

1, 5, 8, 9 HAB / 4 Yes

Evaluation Criteria: Increase surface areas of river shaded within project reach.

HAB-6—Provide side-channel structure and complexity. 1, 2, 5 HAB / 7 No

Evaluation Criteria: Install large wood and pools to create habitat diversity in side-channel

HAB-7—Provide suitable juvenile rearing habitat 1, 2, 5, 6 HAB / 7 No

Evaluation Criteria: Provide shallow water for both rearing and refuge for juvenile salmonids

HAB-8—Enhance wetland connectivity. 1, 5 HAB / 12 No

Evaluation Criteria: Reconnect floodplain wetlands to the river

HAB-9—Enhance riparian and wetland quality 1, 2 HAB / 6, 8, 9, 11 No

Evaluation Criteria: Improve riparian wetland plant community condition, cover and complexity of habitat



2.3.3 Sustainability Performance Measure

This performance measure covers evaluation criteria related to long-term sustainability of the project

and the ability to continue to function as designed. As such, these criteria are primarily related to

construction and operation and maintenance activities:

• Operational complexity

• Impacts on fish and wildlife during construction

• Impacts on fish and wildlife during operation and maintenance activities

• Impacts on cultural resources during construction


• Operational resilience to potential short and long-term climate changes

• Neighborhood impacts

• Permitting risk

Operational complexity refers to the effort needed (passive and active) to control the TZ weir operation.

Impacts on fish and wildlife relate primarily to disturbance of sensitive areas during construction and

maintenance. The potential to disturb culturally sensitive areas during construction also was evaluated.

Project phasing (the number of construction seasons) and the volume of construction traffic are

evaluated as neighborhood impacts. Sustainability evaluation criteria are listed in Table 3.

Table 3. Sustainability Evaluation Criteria


District Design

Elementa

Concept Design


Nexus

SUS-1—Minimize long-term operational complexity and costs for weir, side channel and cold-water supplementation facility.

1, 4, 5, 6, 7, 11 C-O&M / 1, 2 No

Evaluation Criteria: Priority should be given to a passive outlet flow control or minimal necessary adjustment. Also consider reconfigured TZ channel’s ability to accommodate mature riparian vegetation and large wood, while still meeting flow conveyance criteria.

SUS-2—Minimize construction impacts on ESA-listed species, other native species and high-value habitat.

1, 11 C-O&M / 3 Yes

Evaluation Criteria: To the extent reasonable and feasible, design should avoid impacting existing high-value wetlands, native vegetation, and other beneficial habitat.

SUS-3—Minimize future operation and maintenance impacts to provide long-term protection to ESA-listed species and other flora and fauna.

1, 6, 7, 11 C-O&M / 4 Yes

Evaluation Criteria: To the extent feasible, practices defined in the operation and maintenance manual should avoid impacting ESA listed species, other fish, birds and wildlife, wetlands, native vegetation, and other beneficial habitat.

SUS-4—Minimize construction impacts on cultural resources. 1 New Yes

Evaluation Criteria: To the extent reasonable and feasible, design should avoid impacting sensitive cultural-resource areas.

SUS-5—Design for resilience to potential short- and long-term changes in conditions (climate, development, vegetation, beaver activity).

1, 3, 4, 6, 7 C-O&M / 5, 6 No

Evaluation Criteria: Flow control structures designed to be easily modified for hydrologic changes.

SUS-6—Comparison of neighborhood traffic impacts. 11 New No

Evaluation Criteria: Number of truck trips for hauling sediment and other material during construction.

SUS-7—Permitting risk. 1, 4, 5, 6, 7, 8, 9, 11

New Yes

Evaluation Criteria: Likelihood of mitigation requirement for future maintenance.




3.0 PROJECT SETTING

3.1 LAKE SAMMAMISH AND TRIBUTARY STREAMS

The project area is in the Sammamish River watershed. The Sammamish River system includes Lake

Sammamish, Bear Creek, Issaquah Creek and other tributary streams that have the potential to affect

(or be affected by) conditions in the project area.

Lake Sammamish is about 7 miles long and about 1.5 miles wide, with a surface area of about

4,900 acres, oriented along a north-south axis. The maximum depth of the lake is about 105 feet and

the average lake level is 26.4 feet NGVD29 (30.0 feet NAVD88). The average lake level fluctuates

between elevation 25.6 and 27.1 feet NGVD29 (29.2 and 30.7 feet NAVD88). Lake levels are controlled

at a minimum level by a concrete weir located at the upstream end of the TZ.

The lake is surrounded by urban areas, including the City of Sammamish on the east shore, the City of

Issaquah on the south shore, the City of Bellevue on the west shore and the City of Redmond on the

north shore. Marymoor Park is at the north end of the lake and Lake Sammamish State Park is at the

south end.

The Lake Sammamish watershed has an area of about 97 square miles. The largest contributor of

inflow to the lake is Issaquah Creek, which enters from the south through Lake Sammamish State Park.

Numerous smaller streams contribute inflow to the lake, including George Davis Creek, Ebright Creek,

Pine Creek and Laughing Jacobs Creek on the east side, and Tibbets Creek, Schneider Creek, Lewis

Creek, Vasa Creek and Idylwood Creek on the west side.

3.2 SAMMAMISH RIVER AND TRIBUTARY STREAMS

Lake Sammamish outlets to the Sammamish River at Marymoor Park on the north end of the lake. The

Sammamish River flows 13.5 miles north and west through the Cities of Redmond, Woodinville, Bothell,

and Kenmore, where it discharges into Lake Washington.

3.2.1 Sammamish River

The Sammamish River is a low-gradient (0.02 percent slope) channel that drops 13 feet over a distance

of 13.5 miles from Lake Sammamish to Lake Washington. Historically, the Sammamish River was a

slow-moving, highly sinuous slough with dense forest vegetation and extensive connections to

floodplain wetlands. It has been highly modified from its historical condition, primarily as a result of

three major actions: (1) the lowering of Lake Washington associated with the construction of the Lake

Washington Ship Canal and Locks in 1917; (2) straightening of the river to facilitate farming, primarily

prior to the 1930s; and (3) construction of the Sammamish River Improvement Project in 1964, which

deepened the river channel by approximately 6 feet and incidentally resulted in the substantial lowering

of winter maximum water level in Lake Sammamish. The primary objective of the 1964 project was to

reduce flooding impacts on agricultural land uses in the Sammamish River valley downstream of Lake

Sammamish.


3.2.2 Transition Zone

The two primary features of the 1964 Project are the TZ and the TZ weir. The TZ weir, located in the

river channel within the project area, was designed to maintain minimum Lake Sammamish water

levels. It acts as the primary control on lake level and discharge during low to moderate flows. In 1998,

the USACE replaced the original grouted riprap weir with a concrete weir that included a low-flow notch

to improve water depths for fish passage during low flows. The TZ, which occupies much of the river

channel within the project area, is a constructed trapezoidal channel with a 55-foot-wide, low-flow

channel in the center and a 200-foot wide high-flow channel. The low-flow channel has the capacity to

convey about 60 cfs. In order to maintain effective flood flow conveyance, the TZ was lined with angular

rock and was grass-lined in its high-flow channel benches. The TZ serves as a transition between the

original river level at the Lake Sammamish outlet and the deepened river channel downstream. The TZ

is the highest-gradient section of the entire river, dropping about 6.8 feet over approximately 1,400 feet

in length (0.5 percent slope). Though this part of the Sammamish River is steeper, backwater extends

into the TZ from the much flatter river channel downstream for all but the lowest flows.

3.2.3 Tosh Creek

Tosh Creek enters Lake Sammamish just upstream of the TZ weir. It drains an urban area of

115 acres. Increased stormwater flows following urbanization have caused substantial erosion of areas

of Tosh Creek and its tributaries, so it transports a relatively high quantity of suspended and bed

material sediment. Tosh Creek is the only source of sediment to the Sammamish River between the

lake outlet and Bear Creek. In 2013, the City of Redmond replaced the culvert that conveys Tosh Creek

under West Lake Sammamish Parkway, removed a sediment basin located upstream of the culvert,

and restored approximately 700 feet of the creek channel downstream to a more sinuous channel,

including placement of large wood and establishment of a riparian zone. The lower 300 feet of the

creek before it enters Lake Sammamish was not included in the work noted above, and remains

confined to a linear ditch along a former property boundary.

3.2.4 Bear Creek

Bear Creek enters the Sammamish River approximately 0.5 miles downstream of the project site,

immediately downstream of SR-520. The Bear Creek watershed (including Evans Creek) is about

49 square miles. Bear Creek is a significant contributor of flow to the Sammamish River, with

discharges that sometimes exceed flows in the Sammamish River. Additionally, analyses and

observations have revealed Sammamish River backwater conditions during peak flows from Bear

Creek that extend up to the TZ weir and reduce the capacity of the weir (see Appendix A).


4.0 EXISTING PROJECT CONDITIONS

4.1 HYDROLOGY AND HYDRAULICS

The Concept Design Summary Report (King County, 2015) provided a characterization of the current

hydrologic and hydraulic setting in the Lake Sammamish/Sammamish River system. This analysis

utilized a continuous hydrologic and hydraulic modeling approach to capture the interplay of various

factors and the range of climate conditions, both seasonally and from year to year. Existing conditions

in the project area form the baseline condition against which to compare the performance of

alternatives. Figure 4 shows the surface water features in the vicinity of the project and identifies

hydraulic analysis points to compare alternative performance to baseline conditions.

The hydrologic and hydraulic models of the Lake Sammamish/Sammamish River system developed in

2015 during the conceptual design phase were recalibrated in 2019 as part of the preliminary design

phase to capture updates to land use in the watershed and to provide a higher level of calibration

needed for design. The models from the 2019 update were used to evaluate design alternatives and

will be used to support design of the selected alternative.

The 2019 updated hydrologic model was used to develop a continuous 69-year record of inflow to Lake

Sammamish and the Sammamish River for the water years 1949 to 2017. The hydrologic inflow record

was applied to the updated hydraulic model to determine long-term water level in Lake Sammamish

and level and flow in the Sammamish River. The 2019 hydraulic model for the Sammamish River

extended from Lake Sammamish downstream to Lake Washington but the detailed study area was

limited to about 4.3 miles downstream of the lake outlet—to NE 116th Street (City of Redmond limits).

The 2019 hydrologic analysis is documented in Appendix A and the 2019 hydraulic analysis is

documented in Appendix B.

Summary stream flow and lake level statistics from the 2019 hydrologic and hydraulic analysis are

documented in Appendix C and summarized in the sections below. These analyses define the baseline

performance (existing conditions) that will be used to evaluate the ability of concept design alternatives

to meet the project guidance and criteria described in Section 2.0.

The results shown are based on a synthetic flow record developed using the updated 2019 hydrologic

model (see Appendix A). Using a synthetic inflow record from a calibrated model provides a more

complete picture of the seasonal and inter-annual flow condition than would be obtained from a limited

number and duration of observed values. The flow time-series is based on current land cover, which

generates higher runoff rates due to increased development than has occurred in the watershed since

start of the analysis period.

The 2019 hydraulic modeling of existing conditions (See Appendix B and Appendix C) assumed that

current vegetation maintenance practices in the TZ, which started in 2011, would apply for the entire

simulation period. Historical vegetation and sediment management activities in the TZ were not

considered because current practices are expected to continue into the future. This assumption

provides a comparable baseline for evaluating the ability of the alternatives to meet flood protection

objectives. The existing conditions analysis is not able to, nor is it intended to, replicate historical lake

level and river flow characteristics prior to 2011 when current vegetation management practices in the

TZ began.


Figure 4. Surface Water Features in the Willowmoor Project Vicinity


4.1.1 Lake Sammamish Water Level

Water level in Lake Sammamish fluctuates about 1.5 feet on average over the course of a year. With

passive control provided by the TZ weir, the lake level follows the hydrologic cycle—higher lake levels

occur in the wetter winter period and lower levels in the drier summer period. Average daily lake level is

relatively constant during winter, fluctuating between 26.8 and 27.1 feet NGVD29 (30.4 and 30.7 feet

NAVD88). Average lake level in summer ranges from 25.7 to 25.8 feet NGVD29 (29.3 to 29.4 feet

NAVD88). Figure 5 shows the minimum, average, and maximum level for Lake Sammamish for the

69-year simulation period.

Figure 5. Existing Conditions (2019) Average Daily Lake Sammamish Water Levels

Table 4 shows that, on average, lake level is above 27.0 feet NGVD29 (30.6 feet NAVD88) about 57

days a year and above elevation 29.0 feet NGVD29 (32.6 feet NAVD88) slightly more than 1 day a

year. The average winter base water level in Lake Sammamish is about 26.9 feet NGVD29 (30.4 feet

NAVD88). The minimum summer water level is 25.4 feet NGVD29 ((28.9 feet NAVD88); see Table 5).


Table 4. Existing Conditions (2019) Level Exceedance in Lake Sammamish

Lake Level Number of Days Exceededa

feet NGVD29 feet NAVD88 Total for Simulation Period Average Annually

27.0 30.6 3,898 57

28.0 31.6 444 6

29.0 32.6 75 1

a. Number of days per year exceeding a given lake level shown for existing conditions here differs from what is shown in the Concept Design Summary Report (King County, 2015). This is primarily driven by use of a single rating curve to represent conveyance through the TZ in the current analysis, indicative of recent (2011-2017) TZ vegetation management practices. Previous analyses incorporated rating curve information representing previous TZ maintenance practices.

Table 5. Existing Conditions (2019) Seasonal Parameters

Parameter Value

Lake Sammamish Minimum Summer Level (July 1 – September 15)

25.4 feet NGVD29 (29.0 feet NAVD88)

Sammamish River Average Minimum Summer Flow (July 1 – September 15)

25.7 cfs

Lake Sammamish Average Winter Level (November 1 – March 1)


4.1.2 Sammamish River Flow

Fluctuating water levels in Lake Sammamish provide flood storage during large events to moderate

peak flows in the Sammamish River. This is evident by the relatively narrow range of estimated annual

peak flows computed for the 2- through 100-year return periods. Table 6 and Table 7 show computed

Sammamish River flood-frequency and flow duration statistics, respectively, using long-term output

from the 2019 model. The modeled peak flow downstream of Bear Creek is 1,250 cfs for the 2-year

event and 2,040 cfs for the 100-year event. The estimated 10-year peak flow of 1,660 cfs is higher than

the peak flow estimates of 1,500 cfs used by the USACE in its 1964 General Design Memo. Higher

peak-flow estimates are a result of the 1998 USACE project that increased the capacity of the lake

outlet channel (King County, 2013) and development in the Bear Creek and East Lake Sammamish

watersheds, which likely increased runoff from the urbanized areas. Less significant contributors are

the use of more refined methods in the 2019 update and the more rigorous analysis of flood-frequency

at this location provided by the 69-year record.

Table 6. Existing Conditions (2019) Peak Lake Level and River Flow Frequency

Return Period (years)

Exceedance Probability

(%)

Lake Sammamish Level (feet NGVD29, (feet NAVD88))

Peak Flow, Sammamish River at Lake Sammamish Outlet

(cfs)

Peak Flow, Sammamish River Downstream of Bear

Creek (cfs)

1 99 26.9, (30.5) 415 675

1.25 80 27.6, (31.2) 650 1,010

2 50 28.2, (31.8) 820 1,250

10 10 29.5, (33.1) 1,130 1,660

50 2 30.6, (34.2) 1,350 1,940

100 1 31.0, (34.6) 1,430 2,040


Table 7. Existing Conditions (2019) Flow Duration Sammamish River Below Bear Creek

Flow (cfs) Number of Days Exceeded over Simulation Period Time Exceeded (%)

1,500 53 0.2

1,000 726 2.9

500 4,322 17.2

100 16,026 63.6

4.1.3 Storm Drain Outfall Evaluation

City of Redmond staff reported that overbank flooding occurs in the City downstream of Bear Creek

starting at about the 2-year peak flow (1,250 cfs). There are also 63 storm drain outfalls in the City that

discharge to the Sammamish River downstream of the Bear Creek. To quantify downstream impacts

from the project site, the depth of inundation during peak flood events was evaluated for five critical City

outfalls identified by City staff at the following locations: NE 95th Street, NE 90th Street (east bank), NE

85th Street (includes Westpark), and Redmond Way (see Figure 4).

Existing conditions peak river level frequency analyses were computed and compared to survey invert

elevations at each of the critical outfall locations to understand the magnitude of outfall inundation

under existing conditions for a given Sammamish River peak level frequency. Table 8 shows the peak

river level at each outfall location. Note that for more frequent events (2-year and smaller), peak river

levels reported in this table are slightly higher than those associated with peak flow-frequency, due to

the variation in channel roughness from seasonal elodea growth in the Sammamish River.

Table 8. Peak Level at Critical Stormwater Outfalls under Existing Conditions (2019)

Sammamish River Peak Level (feet NGVD29, (feet NAVD88))

Critical Outfall Location River Mile 1-year 1.25-year 2-year 10-year

NE 95th Street 10.92 22.0, (25.6) 24.5, (28.1) 25.7, (29.3) 27.3, (30.9)

NE 90th Street along East Bank 11.18 22.3, (25.9) 24.8, (28.4) 26.0, (29.6) 27.6, (31.2)

NE 85th Street, Westpark 11.42 22.6, (26.2) 25.0, (28.6) 26.3, (29.9) 27.8, (31.4)

Redmond Way 11.76 22.9, (26.5) 25.4, (29.0) 26.7, (30.3) 28.3, (31.9)

The findings from the existing conditions outfall analysis are summarized below:

• NE 95th Street—The Stormwater Management Model (SWMM) developed by NHC (2006) for

the 95th Street storm drain system was used to evaluate the effect of the peak river level

increase in this system. The SWMM analysis of the 95th Street storm drain system assumed a

2-year runoff event coincident with the 1-year peak level in the Sammamish River. This analysis

showed that peak level increase would project riverward a quarter-mile to 151st Avenue NE and

be contained within the storm drain system. The 2006 NHC analysis indicated flooding would

occur in the business park adjacent to the river.

• NE 90th Street along the east bank—No detailed storm drain routing information was available

for this system. Impacts were analyzed by comparing peak river level to the ground elevation

adjacent to this outfall. Existing-conditions peak river level during the 1-year event was


compared to the ground elevation adjacent to this outfall (41.0 feet NGVD29, 44.60 feet

NAVD88) and found to be about 18.9 feet below ground.

• Westpark Outfall and NE 85th Street Outfall—No detailed storm drain routing information was

available for these systems. Impacts were analyzed by comparing peak river level to the ground

elevation adjacent to this outfall. Existing-conditions peak river level during the 1-year event was

compared to the ground elevation adjacent to these outfalls (32.1 and 30.4 feet NGVD29, 35.7

and 34.0 feet NAVD88) and found to be about 9.5 to 7.8 feet below ground.

• Redmond Way Outfall—As-built plans (14-0479) provided by the City of Redmond show two

outfalls at this location. One outfall is a 54-inch-diameter pipe with a control weir at the upstream

end. The weir sill elevation at the outfall is 23.5 feet NGVD29 (27.1 feet NAVD88), about 0.5

feet higher than the existing-conditions 1-year peak level. The second outfall is a 30-inch-

diameter pipe connected to a series of water quality treatment vaults. The as-built drawings

show the vaults to be above elevation 26.4 feet NGVD29 (30 feet NAVD88), which is about 3.5

feet higher than the existing-conditions 1-year peak level.

Peak-level frequency for existing conditions was computed at these locations to determine Sammamish

River level at the outfalls. Table 8 shows the peak level at each outfall location. Note that for more

frequent events, peak level reported in this table is slightly higher than the level associated with peak

flow-frequency, due to the variation in channel roughness from seasonal elodea growth in the

Sammamish River.

4.1.4 U.S. Army Corps of Engineers 1964 General Design Memo Evaluation

The design intent for the TZ weir was for lake water level not to exceed 29.0 feet NGVD29

(32.6 NAVD88) when flow in the Sammamish River downstream of Bear Creek is less than 1,500 cfs.

However, as shown in Figure 6, the 29.0-foot lake level was exceeded on about 10 days over the

69-year simulation period. However, it may not be possible to meet this target because the updated

hydrologic analyses performed for this project showed that the peak flow rate has increased to 1,660

cfs (see Table 6) from the 10-year peak flow of 1,500 cfs identified prior to the construction of the

USACE project (see section 4.1.2).


Figure 6. Existing Conditions (2019) Daily Lake Level and Sammamish River Flow

4.2 AQUATIC HABITAT

4.2.1 Fish Habitat and Use

Aquatic habitat in the Sammamish River in the project area consists of 22 percent riffles, 3 percent

pools, and 75 percent glide habitat. The TZ comprises all of the riffle habitat, which has shallow

turbulent flow but does not function like a natural riffle because of the constructed channel with angular

rock. From the TZ downstream to the Bear Creek confluence, the river is predominantly glide habitat

with approximately three pools (R2 1999; King County 2013). Currently, the low-flow channel (riffle) in

the TZ is bordered on both sides by a narrow (approximately 30-foot-wide canopy) willow (Salix sp.)

and shrub zone that provides 30 to 40 percent canopy cover over the low-flow channel. The river banks

and high-flow channel area have been maintained in a mowed condition since 2013, so only

herbaceous vegetation is present.

Six species of salmonids are known to be present in the Sammamish River watershed: Chinook, coho,

pink, and sockeye salmon/kokanee, and steelhead/rainbow and cutthroat trout (King County 2014). The

presence of bull trout has not been confirmed. Other fish species known or likely to occur in the

Sammamish River watershed include native species such as Pacific lamprey, river lamprey, Western


brook lamprey, mountain whitefish, longfin smelt, northern pike minnow, peamouth chub, three-spine

stickleback, large-scale sucker, redside shiner, longnose dace, speckled dace, Olympic mudminnow,

and several species of sculpin, and non-native species such as yellow perch, smallmouth bass,

largemouth bass, brown bullhead, bluegill, pumpkinseed sunfish, green sunfish, tench, black crappie,

grass carp and common carp (Wydoski & Whitney, 2003; USACE and King County, 2002).

Within the study area, adult and juvenile salmonids and lamprey pass through on their upstream and

downstream migrations. Of particular importance to this project is the migration of adult Chinook and

sockeye salmon through the study area during late summer and early fall (August through October, see

Figure 7). Figure 7 shows the monthly mean, mean maximum, and mean minimum Sammamish River

water temperatures (at the weir, Gage 51M, and near the railroad bridge downstream of Bear Creek,

Gage 51L). Even with cooler Bear Creek water flows, temperatures downstream of Bear Creek are still

high, but reduced by approximately 2ºC (3.6ºF) compared to temperatures near the weir.

The current Washington Department of Ecology (Ecology) water temperature standard is a maximum

7-day moving average temperature of 17.5ºC (63.5ºF) for salmonid spawning, rearing and migration

(September 16 to June 14) and 16ºC (60.8ºF) for core salmonid summer habitat (June 15 to September

15; Ecology 2012). The 16ºC standard applies to the project area during summer. Figure 7 shows the

Sammamish River water temperatures along with the general timing of salmon presence in the river, and

the Ecology temperature standards. Water temperatures exceed Ecology water temperature standards

during July and August—even the minimum (nighttime) temperatures. The mean monthly maximum

temperatures exceed Ecology standards from April through October.

Figure 7. Temperature and Fish Use in Sammamish River and Tosh Creek.

Sockeye

Chinook

Coho


Because surface flows from Lake Sammamish are the source of water to the upper Sammamish River,

seasonally high water temperatures can occur. High water temperatures in the Sammamish River,

typically from July through September, are potentially a significant limiting factor for salmonids in the

river. High water temperatures have occurred during observations of Chinook, sockeye, and coho

salmon prespawn mortality (R2 2015; Cardno 2017; Fresh, et al. 1999).

Water temperatures as high as 27ºC (80ºF) have been measured in the project area during late July

(King County gage data, see Figure 7). Water temperatures are regularly above 20ºC (68ºF) from May

through October and can exceed 25ºC (77ºF) daily during July and August. As the discharge from the

lake that enters the Sammamish River is from the surface, the river temperatures are naturally seasonally

high.

Fish habitat and use are documented in Appendix H.

4.2.2 Wetland and Riparian Conditions

Vegetation in the study area includes native and non-native trees, shrub, grass, and herbaceous

species associated with upland, wetland, and riparian habitats. Wetland and riparian conditions are

shown on Figure 8. The majority of the study area was previously farmland that has remained

undeveloped and is now part of Marymoor Park, with wetland and riparian habitat associated with the

Sammamish River and Tosh Creek and their floodplains. Five wetlands have been delineated within

and adjacent to the study area, totaling approximately 17 acres of the study area.

Figure 8. Wetland and Riparian Areas


Wetland habitats include depressional, riverine, and lake-fringe wetland systems. Approximately

4.6 acres of riverine wetlands occupy the high-flow channel within the TZ and a depressional wetland,

approximately 9 acres in size, is located in the northwest area of the study area. A large lake-fringe

wetland is present along the outlet of Lake Sammamish upstream of the weir, but only about 3 acres of

this wetland is present within the study area.

Historically, much of the study area may have been inundated by Lake Sammamish or been primarily

lake-fringe wetland. Now, much of the floodplain is disconnected from the river and the depressional

wetland is isolated, with primary water sources from upland stormwater and a high groundwater table

from upslope sources.

Vegetation communities mapped in the study area include cottonwood deciduous forest, red alder and

big leaf maple deciduous forest, Douglas fir forest, willow wetland, Douglas spirea wetland, non-native

Scot’s broom and Armenian blackberry dominated shrublands, non-native grassland and wetlands, and

emergent sedge wetland. Non-native species are widely distributed throughout the study area and

dominate the shrub and herbaceous layers in most vegetation communities.

The riparian and floodplain wetland communities at the project site are generally in poor condition, with

numerous non-native species, few large trees, limited cover over the river and wetlands, and a lack of

large wood. Floodplain wetlands were rated as Category 3 and 4 in 2014 (King County, 2014). The

floodplain wetlands do provide flow attenuation and pollutant removal. The study area is part of the

large and diverse Marymoor Park and supports a diverse assemblage of native and non-native wildlife

species throughout the year, including resident and migratory birds, large and small mammals,

amphibians, and reptiles (King County, 2014). Over 200 bird species have been documented as

occurring in Marymoor Park (Friends of Marymoor Park, 2018).

Lake Sammamish lake fringe wetlands at the project site are in substantially better condition than the

floodplain wetlands at the project site. Lake fringe wetlands within the site were rated as Category 2 in

2014 and are primarily dominated by native plant species and would be accessible to juvenile

salmonids for rearing. They may have more than one source of hydrologic input, including from upslope

drainage, but are primarily sustained by surface water inundation or saturation of the soils based on

lake levels. The lake fringe wetlands to the east of the river within Marymoor Park have not been

evaluated for this report. They are assumed to be a contiguous area of up to 100 acres of Category 1

and Category 2 wetlands. The extent of inundation from the lake can be affected by frequent wave

action above a static lake level. Anaerobic conditions that create hydric soil characteristics and affect

the plant community (hydrophytic vegetation) are typically understood to occur when the soil is

saturated at a frequency and duration during the growing season (when soil microbes are active) that is

long enough to deplete soil oxygen. In turn, the depletion of oxygen in the soil dictates the type of

vegetation that can establish in the areas ordinarily inundated by the lake.

The lake fringe wetland design criteria was developed to evaluate the potential for Willowmoor project

alternatives to affect wetlands fringing Lake Sammamish as the alternatives may change the frequency

and duration of lake level to reduce flooding of lakeshore residents. The only potential effect on wetland

hydrology from the alternatives would be on the average annual number of days the lake is at specific

static lake level of interest: 27 feet NGVD29 (30.6 feet NAVD88), 28 feet NGVD29 (31.6 feet NAVD88)

and 29 feet NGVD29 (32.6 feet NAVD88).

Lake fringe wetlands as defined in the state of Washington “are located on the water side of the

Ordinary High Water Mark (OHWM) of the lake” (Hruby, 2014) and thus by definition have their primary

hydrologic source from the lake. Thus, lake fringe wetlands are sustained and predominantly affected


by lake levels that are maintained for a sufficient duration during the growing season. However,

because the growing season is highly variable from location to location and from year to year, the

National Research Council (NRC 2005) concluded that a reasonable working definition is that wetlands

require 14 continuous days of inundation or saturation (within 1 foot of the ground surface) in 5 out of

10 years to be sustained. This is the working definition used for wetland delineations accepted by

regulatory agencies in Washington (USACE 2010). Unique wetland plant communities may also occur

at elevations that are inundated or saturated for a “long duration” during the growing season, defined as

30 continuous days in 5 out of 10 years.

The criteria developed for analyzing potential effects on lake level was thus based on analyzing

potential change in the duration (number of days) at:

• The static lake level that occurs continuously for 30 or more days in 5 out of 10 years

• The static lake level that occurs continuously for 14 or more days in 5 out of 10 years

In addition, because lake fringe wetlands occur waterward of the OHWM, the criteria also includes an

analysis of the potential change in the duration (number of days) at the 2-year lake level, which can be

indicative of the location of OHWM. Due to other environmental factors, including wave action, the field-

delineated OHWM could be different, but as the project would only have effects on the static lake level,

the analysis is only directed at the static lake level. Federal and state permitting requires recent field

wetland and OHWM delineations, which will be determined during the final design phase of the project.

Table 9 summarizes the fringe wetland inundation lake levels.

Table 9. Fringe Wetlands Inundation Existing Conditions (2019)

Parameter

Existing Conditions Lake Level

(feet NGVD29 (feet NAVD88))

Lake level that occurs continuously for 30 or more days in 5 out of 10 years 26.6 (30.2)

Lake level that occurs continuously for 14 or more days in 5 out of 10 years 26.8 (30.4)

2-year lake level, which can be indicative of the location of OHWM. 28.2 (31.8)

4.3 GEOLOGY AND HYDROGEOLOGY

Surficial geology in the project area is mapped as alluvium derived from the Sammamish River,

consisting primarily of organic-rich fine sand, silt, and clay and extending to a depth of 70 to 100 feet

below the ground surface. Fill materials from dredging and straightening of the Sammamish River, and

to a much lesser extent Tosh Creek, are also present at or near the surface. Primarily silty sand, silt,

and clay deposits underlie the alluvium and are interpreted to be glacial and interglacial deposits of pre-

Fraser glaciation age.

Drilling for an exploratory deep groundwater well installed at the site encountered water at a depth of

about 18 feet in the silty, gravelly sand. The silty sands extending to a depth of 73 feet produced limited

water while drilling through this unit, likely due to a higher content of fine-grained silt. Little or no water

was produced in interbedded fine sand units and clay layers until encountering sands and gravels at a

depth of about 570 feet. Groundwater in the deeper sand and gravel deposit was under flowing artesian

conditions, with water levels rising to about 17 feet above ground surface. A 6-inch-diameter well

screen was installed and developed within this deep aquifer, and aquifer testing was conducted to

evaluate deep groundwater as a cold-water supplementation alternative. The potential yield for a single

production well is a pumping rate of 1.3 cfs for 60 days. More wells could be installed to achieve a


greater yield; however, drawdown interference among pumping wells would diminish the yield from any

individual well such that three wells could produce 2.55 cfs.

Groundwater quality appears to be within water quality criteria for protection of freshwater systems.

Temperature was measured to be 14.4ºC (57.9ºF) and pH was measured at 7.5. The exception was

dissolved oxygen, which was very low at 0.1 mg/L. The groundwater investigation is documented in

Appendix E.

4.4 GEOMORPHOLOGY

The geomorphology of the project site is heavily influenced by its history of glaciation. Both the lake and

river occupy a trough scoured by subglacial meltwater during the most recent period of lowland

glaciation in southern Puget Sound (Booth and Hallet, 1993; Booth, 1994). Various erosion and

deposition processes, since the glaciers retreated, have resulted in filling the trough and creating the

current valley. This valley had an extremely low gradient. Early historical records indicate that the river

was highly sinuous and the valley bottom was persistently wet. The channel was bounded by extensive

wetlands (seasonally flooded by Lake Washington and the Sammamish River) that extended from

valley wall to valley wall and included more deeply flooded channels (Tetra Tech, 2002). Following

European settlement of the region, three major changes were imposed on the Sammamish Valley:

• Lowering of Lake Washington

• Channelization of the Sammamish River

• Construction of the TZ to control the Lake Sammamish outlet.

These modifications to the system are described in Section 3.0.

Existing sediment supply is an important consideration in developing the design of project

modifications, particularly the side channel. To be sustainable, the ability of the side channel to

transport sediment needs to be in relative balance with the sediment supply, indicating dynamic

equilibrium and avoiding significant erosion or deposition.

Tosh Creek is the primary potential source of sediment to the side channel. The creek has built a delta

where it enters the Sammamish River approximately 700 feet upstream of the weir, which is mostly

submerged and fills the over-excavated channel above the weir. An estimated 925 cubic yards of sandy

sediment has accumulated in the delta. It is assumed that this accumulation began at the time of the

USACE project (1964). Therefore, the estimated rate of sediment delivery to the river from Tosh Creek

is about 17 cubic yards per year.

Tosh Creek transports some course sediment (gravel) past West Lake Sammamish Parkway but not

down to the delta at the Sammamish River. This material accumulates in a portion of the creek channel

upstream and may be transported into the proposed side channel, depending on the confluence

location and channel elevation relative to Tosh Creek's profile. Based on material removed from a

sediment basin, 12 to 40 cubic yards of coarse sediment was delivered annually to Tosh Creek at West

Lake Sammamish Parkway from 2004 to 2012. This sediment basin was removed as part of the West

Lake Sammamish Parkway Culvert Replacement Project in 2013, so sediment is now transported

downstream. This sediment is much coarser than that accumulating in the delta and is primarily gravel.

A technical memorandum on existing geomorphic conditions in the project area (Appendix G) provides

details on the historical setting as well as important considerations for developing the project design.


4.5 CULTURAL RESOURCES

A desktop review of available cultural resources information was performed to synthesize findings from

previous studies of the project area. Data from those studies were used to model subsurface layers in

the project area, identifying layers with relatively high archaeological sensitivity.

The archaeological record of the project vicinity spans the past ~12,500 years, from the end of the last

ice age to the early historic period of Redmond and Willowmoor Farm. The area’s subsurface geology

preserves evidence of environmental shifts that have occurred over that period. Straightening of the

Sammamish River and in-filling of the old channel in the early 1960s substantially disturbed some

portions of the project area, while further burying other parts with fill. Areas with fill and more recently

deposited natural sediments may protect more deeply buried archaeological deposits.

Two previously inventoried archaeological sites are in or near the project area, and several isolated

artifacts were found. The two known archaeological sites in the project area should be avoided by

ground disturbing activity. The isolated artifacts found in the project area were found in disturbed fill

deposits that moved the artifacts from the places where they were originally deposited. The artifacts

were not formally recorded when initially identified, therefore they do not have site numbers or inventory

forms.

Previous archaeological investigations in the project vicinity indicate artifacts found in three depositional

settings: fill, elevated areas underlain by glacial sediments, and buried surfaces under diatom-rich

sediments and peat. The estimated extent and depth of these depositional settings is as follows:

• Fill was broadcast throughout the project area as the Sammamish River was channelized. Fill

deposits are relatively thick where the old channel was filled, and relatively thin in locations that

were natural high points before the 1960s. Artifacts have been found in near-surface fill deposits

that have been displaced and no longer retain their original archaeological context.

• Well-drained landforms that once rose above the surrounding floodplain before being buried by

subsequent natural or artificial processes can feature surfaces that are quite old. Because these

landforms are advantageous places for humans to live and acquire resources, they carry high

potential for archaeological sites. However, the nature of alluvial deposits and landforms on the

site is poorly understood without further field data collection.

• Buried surfaces under diatom-rich sediments and peat are identified based on the known extent

of buried peat deposits within the project area. In several places, the peat is relatively deep

below the present-day ground surface—the same as the peat overlying the Bear Creek

archaeological site deposits. The nearby Bear Creek archaeological site yielded stone tools

from a thin, formerly stable surface buried under (i.e., older than) peat that began forming

10,000 years ago. The peat was covered by sediments rich in the fossilized remains of tiny,

aquatic organisms called diatoms (such sediments are known as diatomaceous earth).

Aerial photographs indicate that several agricultural buildings were located in the project vicinity in the

early 20th century, and some of their likely remnants were identified during a previous survey. The

vicinity of those buildings has a higher sensitivity for structural remains and other associated features

and artifact concentrations.

A subsurface geoarchaeological investigation is planned for the summer of 2019 to identify the potential

for encountering sensitive artifacts during construction. This investigation will be performed prior to

initiating final design so potential impacts can be mitigated during design.


4.6 RECREATION

The project area is fully contained within Marymoor Park; a 600-acre regional park, and has a variety of

active programming including climbing, entertainment, a velodrome, and sports fields. The 40-acre off-

leash dog area draws visitors from all over the region. The southern part of the park near the lake is the

most similar to the side channel portion of the project site. A loop trail takes users along the

Sammamish River through the off-leash area, out toward the lake, and then back toward the center of

the park through a forested area. The Sammamish River runs along the west edge of the park and is

enjoyed by off-leash area users and boaters in small, non-motorized crafts and the occasional stand-up

paddleboard.

Within the project area, the left bank and the right bank of the Sammamish River have very different

recreational opportunities. The left bank, which constitutes the majority of the project area, is where the

new side-channel and habitat improvements will be constructed. Project design on this side of the river

will seek to limit human disturbance of wildlife and habitat following construction of the project. Current

use includes occasional dog walkers and birding. Parks and SRA staff report illegal encampments in

the summer as well.

The off-leash area at Marymoor Park, along the right bank of the Sammamish River, comprises fields,

woods, five formal water access beaches, paths, elevated boardwalks, and bridges. The river bank

sees frequent use as dogs access the river. Improvements along the right bank may include interpretive

signage to explain the flood works, habitat features, and maintenance activities and measures to

reduce sedimentation from the eroding water access beaches. A hand carry boat launch downstream of

the off-leash area may be desired, but would require an organizational sponsor for ongoing

maintenance.

Access to the west side of Marymoor Park along West Lake Sammamish Parkway NE is limited. Most

of Marymoor Park is east of the Sammamish River. At this time, the only route to cross the Sammamish

River is at the northwest corner of the park on either the Marymoor Way NE vehicular bridge or the

Sammamish River Trail bridge. Along most of West Lake Sammamish Parkway NE there is no

infrastructure for pedestrians or cyclists, and a popular bike path abruptly ends at NE 51st Street. The

City of Redmond’s 2017 Parks, Arts, Recreation, Culture, and Conservation Plan identifies a future

regional trail connection along West Lake Sammamish Parkway NE continuing the multiuse trail from

its current terminus at NE 51st Street south to Idylwood Park.

There are several trails that tie into the off-leash dog area parking lot from areas north and east of the

park, providing easy access to the right bank of the project area. North of the park entrance there are

sidewalks on both sides of West Lake Sammamish Parkway NE that lead to Redmond’s downtown

area. The intersection of West Lake Sammamish Parkway NE and NE Marymoor Way is an entry point

to a major regional trail system. The Sammamish River Trail runs west to connect to the Burke-Gilman

Trail, and the Marymoor Connector Trail runs east to tie into the East Lake Sammamish Trail. Together,

this trail system connects Issaquah to Seattle around the top of Lake Washington.

An unpaved multi-use trail links Bridle Trails State Park to Marymoor Park running west along NE 60th

Street and through Westside Park before connecting to the bike trail along West Lake Sammamish

Parkway NE.

The Sammamish River is also a segment of the Lakes to Locks water trail (a water trail system for

small non-motorized craft) that connects Lake Sammamish and Lake Washington to Puget Sound. The


two launch sites within Redmond that are upstream from the project area are the SRA dock and

Idylwood Park. The primary existing boating access is from private property on Lake Sammamish and

the SRA dock, which has a hardened boat launch and a wooden float in the river just upstream of the

weir. The rowing club uses this launch to access Lake Sammamish. An existing crushed rock parking

lot is located off West Lake Sammamish Parkway NE for access to the rowing club building and launch

via a gated access drive.

Wildlife is plentiful in the southern portion of Marymoor Park between the off-leash area and Lake

Sammamish. Many people enjoy bird-watching and glimpsing other fauna as they explore the paths.

Interest has been expressed in planning for a viewing platform on the right bank of the river upstream of

the off-leash area.

4.7 TZ MAINTENANCE

Intensive maintenance of the TZ has been occurring annually since 2011. These efforts include brush

clearing from the banks of the TZ and sediment removal from the channel. Maintenance is performed

according to conditions specified in a Hydraulic Project Approval permit from the Washington

Department of Fish and Wildlife.

Since 2011, King County has expended $440,000 on vegetation and sediment removal and $100,000

for a pre-spawn mortality study as required mitigation for impacts associated with maintenance. Total

expenditures since 2011 were used to develop a 50-year life cycle cost of $3,054,000, assuming the

current level of maintenance activity. Life-cycle costs are documented in Appendix J.


5.0 CONCEPT DEVELOPMENT

5.1 BACKGROUND

The King County Flood Control District identified the split-channel alternative (Alternative 4 in the

Concept Design Summary Report) as the selected alternative to proceed to 30 percent design. This

alternative was refined during preliminary design to include the following features:

• TZ modifications, including a low-flow channel, floodplain benches, pools and backwater

alcoves.

• Widening the existing TZ within its current alignment to convey floodwater.

• Construction of a flow-through side channel in the left bank floodplain to provide additional flood

conveyance in winter and improved aquatic habitat for fish. Flow into the channel would be

influenced by modifications to the TZ weir. Tosh Creek would connect to this side channel.

• Enhancement of the riparian zone and wetlands in the left bank floodplain.

• Manually adjustable weirs.

• Cold-water supplementation to improve water quality.

• Recreation enhancements to provide passive recreation elements on the left bank, recreational

use in the TZ, and improvements at river access points in the dog park.

An operations and maintenance plan to include fish passage, invasive species management, and

beaver management activities will be developed during the 30 percent design.

5.2 TRANSITION ZONE

The transition zone would be modified to include a low-flow channel meandering through the TZ, with

three pools of varying depth to provide refuge for migrating salmonids. A floodplain bench would be

constructed along the left bank. The bench elevation would be at about the 2-year recurrence interval

peak flood water surface elevation. Tall trees would be planted on both banks landward of the bench to

shade the TZ. These features are proposed mitigation for the removal of willow from the existing low

flow channel, a project element intended to increase conveyance capacity and simplify maintenance of

the TZ.

5.3 TZ WEIR

The existing TZ weir would be modified to a manually adjusted dynamic weir configuration, to provide

for seasonal adjustment (twice a year) of the fixed weir structure elevation to more precisely control

lake levels during late winter and spring (February 15 through June 30). The existing TZ weir would be

retrofitted as a multi-chamber flash-board structure with concrete, aluminum, or wooden stop logs

between slotted piers. The piers would be attached to the upstream face of the existing TZ weir on an

approximate 10-foot spacing.

Seasonal adjustment of the TZ weir is required because the preliminary weir analysis showed that a

static weir was unable to meet flood protection objectives in Lake Sammamish and also maintain the

function of lake-fringe wetlands. A seasonal adjustment allows higher outflow from the lake during the


wetter late-fall/early winter period to reduce lake levels when levels are the highest and reduces outflow

during the late winter and spring period to maintain levels to preserve function of lake-fringe wetlands.

Development of the manually adjusted dynamic weir is described within the Preliminary Dynamic Weir

Analysis Technical Memorandum (Appendix D). The preliminary weir analysis defined the weir

configuration as described in Table 10.

Table 10. Manually Operated Weir Configuration

Manually Operated Weir

Component Existing

Condition Flood Protection Lake-Fringe Wetland

Preservation

Notch Invert Elevation (feet NGVD29, (feet NAVD88)) 23.48 (27.05) 24.24 (27.81) 24.76 (28.33)

Notch Width (feet) 4.0 4.0 4.0

Notch Depth (feet) 2.2 1.45 1.20

Weir Sill Overflow Elevation (feet NGVD29, (feet NAVD88))

25.68 (29.25) 25.68 (29.25) 25.96 (29.53)

Adjustment Period None July 1 – Feb. 14 Feb. 15 – June 30

The manually operated weir would maintain the simple operational concept of the existing weir but

allow for the weir height to be adjusted as needed to meet flow targets. The addition of the stop log

structure would be a relatively minor retrofit to the existing weir. Additional maintenance expense would

be associated with seasonal adjustment of the stop logs. Intermediate posts holding the stop logs may

collect debris and may require more frequent cleaning. Also, regular inspection of the post connections

would be required to ensure the integrity of the connections.

A remotely operated dynamic weir was considered, but not included by the project team in this

alternatives analysis due to the substantial permitting and operational complexity of the remotely

operated weir, as well as its high risk of impacting Redmond’s stormwater outfall system just

downstream of the weir.

5.4 SIDE CHANNEL

The design of the proposed side channel intends to provide a number of aquatic habitat enhancements:

a fish-passable channel year-round, incorporating high-quality and complex fish habitat, large wood,

high-quality forested riparian habitat, and cold-water inputs from Tosh Creek and hyporheic features in

the channel. Key elements of the concept development included maximizing channel length through a

sinuous alignment that also avoids existing large trees and known cultural resource locations. Channel

sinuosity will allow for the immediate creation and long-term formation of riffles and pools associated

with meander bends and large wood in the side channel. The excavated side channel would include the

placement of streambed gravel to provide suitable habitat for preferred salmonid prey species and

stability under proposed velocities.

Site constraints did not allow for appreciable variation in the alignment of the side channel, so a single

optimized alignment was selected for use in both alternatives. Topography, the presence of known

archeological resources, and the need to minimize impacts on the existing habitat all constrained the

selection of the alignment of the new side channel. The channel profile needed to be elevated to

facilitate the connection to the relatively high profile of Tosh Creek. Channel slope was also configured

to accommodate the sediment supplied by Tosh Creek.


The side channel was assumed to have a compound trapezoidal section with a smaller, inset channel

to convey low flows with adequate depth and velocity for migrating fish during low flow periods and a

wider floodplain channel to convey high flows. A 2:1 side-slope was assumed for the low-flow channel

and 3:1 side-slope assumed for the floodplain channel. The final alignment has a total length of 3,200

feet. Side channel development is described in Appendix C.

5.5 FLOODPLAIN

The existing left bank floodplain area was highly modified by the 1964 Sammamish River Improvement

Project that filled in an old channel meander and side-cast excavated material from deepening and

widening the river to construct the TZ. The area of the left bank floodplain that is mapped within the

100-year year floodplain (King County 2018) extends into some of the areas mapped as wetlands,

though many of the 17 acres of wetlands on the site are disconnected from surface water of the river.

Increasing river connectivity to floodplain wetlands will provide critical, missing flood refugia for juvenile

salmonids, benefitting their growth and survival. The two alternatives described in Section 5.7 provide

two scales of floodplain connectivity: one that includes excavation along the higher left bank terrace to

promote flow into the floodplain; and one that primarily relies on connectivity between the side channel

and future floodplain wetlands.

5.6 RECREATION

A recreation concept plan is currently under development. This plan will include passive elements on

the left bank such as trails or viewing areas. On the right bank, recreation elements will include

constructed recreation facilities, such as dog access points and a possible kayak or canoe portage in

the TZ.

5.7 SIDE CHANNEL CONCEPT ALTERNATIVES

Two side-channel concept alternatives were developed to meet the project objectives and design

criteria (see Section 2.0). The two concept alternatives are designated “Wide” and “Narrow”, based on

the width of the floodplain that would be associated with the side channel. The concept alternatives are

described in Table 11 and shown on Figure 9 and Figure 10. The current TZ channel and maintenance

condition was evaluated as a third concept alternative, representing the baseline condition.


Table 11. Side Channel Concept Alternative Summary

Alt. 1 Wide Side

Channel

Alt. 2 Narrow

Side Channel

Transition Zone:

• Modify weir with manually adjusted stop-logs.

• Excavate 3 large pools for adult salmon holding and juvenile rearing.

• Excavate floodplain bench along the left bank at approximately the 2-year flow elevation.

• Plant tall trees along the left and right banks of the TZ.

● ●

Side Channel:

• Excavate 3,220-foot-long meandering side channel that flows off the Sammamish River

• Connect channel approximately 1,200 feet upstream of the existing TZ weir

• Construct with 0.03 percent slope in the 1,725-foot upper section (upstream of Tosh Creek confluence) and a 0.3 percent slope in the 1,495-foot lower section (downstream of Tosh Creek confluence).

• Connect channel at the downstream end of the TZ.

• Excavate new meandering 610-foot-long channel for Tosh Creek that joins the side channel approximately 1,800 feet downstream of the inlet.

● ●

Compound Side Channel Cross Section:

• Upstream of Tosh Creek: 100-foot wide floodplain with 2.5-foot-deep, 15-foot-wide, inset channel (25-foot-wide top width).

• Downstream of Tosh Creek: 40-foot wide floodplain channel with 2.0-foot-deep, 6-foot-wide inset channel (10-foot-wide top width).

• Floodplain width based on a 4:1 entrenchment ratio as a typical upper limit for natural

floodplains in lowland environments.

●

• Upstream of Tosh Creek: 50-foot-wide floodplain channel with 2.5-foot-deep, 25-foot-

wide inset channel (25-foot-wide top width).

• Downstream of Tosh Creek: 20-foot-wide floodplain channel with 2.0-foot-deep, 10-foot-wide inset channel.

• Floodplain width based on a 2:1 entrenchment ratio as a typical lower limit for natural

floodplains in lowland environments.

●

Floodplain Modifications

• Manage invasive species throughout the floodplain and wetlands.

• Plant native trees, shrubs, and emergent wetland species in the floodplain and wetlands.

● ●

• Excavate to promote floodplain connections above a 2-year flow from Lake Sammamish above the weir into floodplain wetlands and the side channel. ●


Figure 9. Side Channel Concept Alternative 1 – Wide Side Channel


Figure 10. Side Channel Concept Alternative 2 – Narrow Side Channel


5.8 COLD-WATER SUPPLEMENTATION OPTIONS

Three cold-water supplementation (CWS) options have been identified as part of the concept

development. Table 12 summarizes features of the three options.

Table 12. Cold-Water Supplementation Option Summary

CWS Option A.

Big Water

CWS Option B.

Gravel Interflow

CWS Option C. Ground-

water

Lake Surface Water Heat Exchange System

• Heat exchange through deep lake manifold.

• Shallow intake with fish screen.

• 16,000-foot pipe length manifold on bottom of lake.

• 1,800-foot-long underground transmission line to pump facility.

• 200-foot-long underground transmission line from pumping facility to side channel.

●

Hyporheic Exchange

• Over-excavate side channel about 18 inches and line with porous streambed gravels to promote hyporheic exchange of surface water with cooled groundwater.

• Install extended gravel bars and engineered sediment wedges with clay dams to promote hyporheic flow and upwelling in the side channel.

●

Pumped Groundwater:

• Install three wells equally spaced on 1,000-foot center parallel to the Sammamish River.

• Construct well house with 25HP pump and controls for each well.

• Install 3,000 feet of 8” pipeline to connect wells to a discharge point at the upstream end of the proposed side channel.

●

CWS Option A—the “big water” option—would pump surface water from Lake Sammamish into a heat

exchange piping system located on the bed of Lake Sammamish where temperatures are cold. The

cooled water would then be pumped during an estimated 60-day period from August 1 to September 30

to the side channel and river to discharge 20 cfs of water at 18ºC (64ºF) or cooler. This pumped water

would be replacement rather than supplemental flow into the river. Pumping would cause a small drop

in lake level, which would slightly lower outflow rates from the lake during the pumping period.

Appendix E describes the analysis of this option. Figure 11 shows the key features of the option.

CWS Option B—the “gravel interflow” option—would line the channel with porous streambed gravels to

promote hyporheic exchange of surface water with groundwater. To promote hyporheic exchange,

gravel bar deposits would be placed at the inside of meander bends and engineered sediment wedges

would be installed in the side channel downstream of the Tosh Creek confluence with the side channel.

Figure 12 shows the key features of the option.


Figure 11. CWS Option A – Lake Surface Water Heat Exchange (Big Water)


Figure 12. CWS Option B – Hyporheic Exchange (Gravel Interflow)


CWS Option C—the “groundwater” option—would pump groundwater from the deep artesian aquifer to

provide cold water to the side channel during an estimated 60-day period from August 1 to September

30. A test well was drilled in 2018 near the southwest corner of the study area to a depth of 635 feet.

The water appears to be of good quality except for low dissolved oxygen and suitable for cold-water

supplementation with a temperature of 14.4ºC (57.9ºF). An aquifer test to determine the potential

sustainable flow rate estimated that one well could produce 1.3 to 1.75 cfs and three wells could

produce 2.55 to 3 cfs. The three-well option is included in this analysis to meet the original intent of

having a mid-range volume option to evaluate. This option is presented in Appendix F. Figure 13 shows

the key features of the option. Information provided by the City of Redmond supported development of

the estimated well flow rates.


Figure 13. CWS Option C – Deep Groundwater Pumping (Groundwater)


6.0 ALTERNATIVE PERFORMANCE SUMMARY

6.1 GENERAL

This section documents the performance of the side-channel concept alternatives and CWS options to

meet design requirements and the performance evaluation criteria outlined in Section 2.0. The

performance is compared to the performance of the existing baseline condition (Side Channel Concept

Alternative 3), documented in Section 4.0.

6.2 SIDE CHANNEL CONCEPT ALTERNATIVE 1 WIDE SIDE CHANNEL

6.2.1 Flood Protection

Lake Sammamish Level

For Concept Alternative 1 Wide Side Channel, the seasonal operation of the TZ weir results in a

0.2-foot reduction in average daily lake level during the higher-level months of November through

February but increases average lake level by about 0.1 feet from March through July. Figure 14 shows

a comparison of the average daily Lake Sammamish level for existing conditions and Concept

Alternative 1 for the 69-year simulation period.

Figure 14. Concept Alternative 1 Wide Side Channel, Average Daily Lake Sammamish Level


Table 13 shows that Concept Alternative 1 is able to reduce lake level exceedance above 27.0 feet

NGVD29 (30.6 feet NAVD88) by about 6 days per year on average but exceedance above 29.0 feet

NGVD29 (32.6 feet NAVD88) by less than one day per year on average. The average minimum

summer lake level is slightly higher, and the average winter base level is slightly lower with this

alternative (see

Table 14).

Table 13. Concept Alternative 1 Wide Side Channel, Level Exceedance in Lake Sammamish

Lake Level (feet) Average Number of Days Exceeded Annually

NGVD29 NAVD88 Existing Conditions (2019) Concept Alternative 1 Difference

27.0 30.6 57 50 -7

28.0 31.6 6 5 -1

29.0 32.6 1 1 0a a. The computed difference is around a 10 percent reduction (0.1 feet); however, this is reported as zero because the sensitivity of this

analysis is limited so only whole day increments are reported.

Table 14. Concept Alternative 1 Wide Side Channel, Seasonal Parameters

Parameter Existing Conditions (2019) Concept Alternative 1 Difference

Lake Sammamish Min. Summer Level (July 1 – September 15)



0.1

Sammamish River Avg. Min Summer Flow (July 1 – September 15)

25.7 cfs 25.0 cfs -0.7 cfs

Lake Sammamish Avg. Base Winter Level (November 1 – March 1)



-0.1 ft

Sammamish River Flow

Peak flow rates with Concept Alternative 1 in the Sammamish River downstream of Bear Creek are

slightly higher than existing conditions for the 2-year or lower return period. For events greater than the

2-year, peak flow is slightly lower (see Table 15). Similarly, the duration of flows increases at lower flow

level, but there is no significant difference in flow duration above 100 cfs (see Table 16). However, the

average annual minimum summer flow is slightly lower for this alternative (see

Table 14). Figure 15 shows that the USACE design intent for the TZ is not met with this alternative, but

the days of exceedance are reduced from 10 to 6.

Table 15. Concept Alternative 1 Wide Side Channel, Sammamish River Flow Frequency Downstream

of Bear Creek

Return Period Exceedance Sammamish River Peak Flow Downstream of Bear Creek (cfs)

(years) Probability (%) Existing Conditions (2019) Concept Alternative 1 Difference

1 99 675 715 40

1.25 80 1,010 1,030 20

2 50 1,250 1,250 0

10 10 1,660 1,650 -10

50 2 1,940 1,920 -20

100 1 2,040 2,030 -10


Table 16. Concept Alternative 1 Wide Side Channel, Sammamish River Flow Duration below Bear

Creek

Percent of Time Flow Exceeded (%)

Flow (cfs) Existing Conditions (2019) Concept Alternative 1 Difference

1,500 0.2 0.2 0

1,000 2.9 2.9 0

500 17.2 17.1 -0.1

100 63.6 64.5 0.9

Figure 15. Concept Alternative 1 Wide Side Channel, Daily Lake Level and Sammamish River Flow

Storm Drain Outfall Evaluation

Flow frequency reported in Table 15 shows that peak flows increase for return periods less than the

2-year rate. Because river level would also increase with flow, a more rigorous assessment of flooding

potential in the storm drain systems was performed. Under Alternative 1, the depth of inundation at

storm drain outfalls in the City of Redmond during peak flood events would increase 0.1 feet from

existing conditions for the 1-year event (see Table 17). Peak river level would be slightly lower relative

to existing conditions for events greater in magnitude than the 1-year event. A similar drop would be

expected for larger events, based on change in peak flood frequency reported in Table 15.


Table 17. Concept Alternative 1 Wide Side Channel, Inundated Outfalls

Peak River Level (feet NGVD29 (feet NAVD88)); Difference from Existinga (feet)

Critical Outfall Location 1-year 1.25-year 2-year 10-year

NE 95th Street 22.2 (25.7); 0.0 24.5 (28.0); -0.0 25.6 (29.2); -0.1 27.2 (30.8); -0.1

NE 90th Street along East bank

22.4 (26.0); 0.1 24.7 (28.3); -0.0 25.9 (29.5); -0.1 27.5 (31.1); -0.1

NE 85th Street, Westpark

22.7 (26.2); 0.1 25.0 (28.6); -0.0 26.2 (29.8); -0.1 27.8 (31.4); -0.1

Redmond Way 23.1 (26.6); 0.1 25.4 (29.0); -0.0 26.6 (30.2); -0.1 28.2 (31.8); -0.1

a. See Table 8 for existing conditions peak level. Positive value indicates peak level is higher for the alternative than for existing conditions.

The outfall inundation analysis summarized in Table 17 shows that the potential for increased flooding

at City of Redmond outfalls downstream of the project site is limited to the 1-year peak event only.

However, this increase is minimal and does not affect the flood condition at the critical outfalls. Potential

flood impacts for individual outfall locations are summarized as follows:

• NE 95th Street— At two of the locations with flooding identified in the 2006 NHC analysis, the

alternatives analysis showed no increase in level. At the two other locations, the increase was

limited to 0.01 feet.

• NE 90th Street along the east bank—For the 1-year event, the peak level was found to be

about 18.8 feet below the ground elevation adjacent to the outfall (41.0 feet NGVD29, 44.60 feet

NAVD88).

• Westpark Outfall and NE 85th Street Outfall—For the 1-year event, the peak level was found

to be about 9.4 and 7.7 feet below the ground elevation adjacent to these outfalls (32.1 and 30.4

feet NGVD29, 35.7 and 34.0 feet NAVD88).

• Redmond Way Outfall—The 1-year peak level for this alternative is about 0.4 feet lower than

the weir sill elevation (23.5 feet NGVD29, 27.1 feet NAVD88) and about 3.4 feet below the

bottom of the water quality treatment vaults (26.4 feet NGVD29, 30 feet NAVD88).

6.2.2 Habitat

Fish Use

Concept Alternative 1 (Wide Side Channel) would improve holding and suitable migratory habitat for

adult salmon by creating three large pools in the TZ (~5,000 cubic feet each), a side channel with an

estimated 1:1 riffle-to-pool ratio, minimum depths of 12 inches through the modified weir, and minimum

depths of 12 inches in the side channel. Concept Alternative 1 also would provide shallow water habitat

for winter and spring juvenile fish rearing and refuge by excavating a floodplain bench along the left

bank of the TZ (~2.5 acres), promoting floodplain connectivity and creating a larger inset floodplain

along the side channel (~7 acres).

Concept Alternative 1 would include installation of the recommended key piece large wood loading in

the side channel or about 110 key pieces (75th percentile per Fox & Bolton, 2007) and three large

wood structures associated with the pools in the TZ. The willows along the existing TZ low-flow channel


would be removed, and willows would be installed on the floodplain bench to provide cover for the

pools. Tall native trees would provide shading to the mainstem and side channel.

Routing Tosh Creek into the side channel and anticipated minor cooling associated with relatively cool

water from the creek and gravel bars in the side channel would potentially cool the side channel to

~22ºC without additional cold-water supplementation and provide localized temperature refugia (pools

and Tosh Creek confluence) for fish. The side channel is intended to provide high quality holding and

migratory habitat for adult and provide juvenile rearing habitat.

Wetland and Riparian

Concept Alternative 1 would provide 5.5 acres of floodplain bench willow habitat, 19 acres of enhanced

riparian/floodplain forest, 13.5 acres of enhanced emergent and shrub wetland, 1 acre of tall tree

planting, and 33 acres of invasive species management. Overall, the wetland and riparian

enhancement would likely result in approximately 40-percent shading over the Sammamish River (over

the long-term when trees mature) and greater than 60-percent shading of the side channel and

wetlands (Ecology, 2011).

Table 18 summarizes the analysis of the potential for Concept Alternative 1 to affect lake level and

fringe wetland inundation frequency. This table shows that the seasonal operation of the TZ weir is able

to maintain the existing level of lake-fringe wetland inundation.

Table 18. Concept Alternative 1 Wide Side Channel, Lake Fringe Wetlands Inundation

Existing Conditions (2019) Lake Level

(feet NGVD29, (feet NAVD88))

Concept Alternative 1 Lake Level


Difference (feet)

Lake level that occurs continuously for 30 or more days in 5 out of 10 years

26.6, (30.2) 26.6, (30.2) 0.0


26.8, (30.4) 26.8, (30.4) 0.0

2-year lake level, which can be representative of the OHWM elevation.

28.1, (31.7) 28.1, (31.7) -0.1

6.2.3 Sustainability

Concept Alternative 1 (Wide Side Channel) addresses the key objectives of the project and includes

features to promote the long-term function and sustainability of the site for fish and wildlife. Specific

features included in the concept design include sizing the weir notch and stop logs to ensure fish

passability (minimum 1 foot depth), sizing the side channel to ensure fish passability (minimum 1 foot

depth during low flows), creating deep pools for adult salmon holding habitat in the TZ and side

channel, creating localized temperature refuge in the side channel (pools and Tosh Creek confluence),

and creating multiple return flow paths from the floodplain to avoid fish stranding. During further phases

of design, additional details will be developed to also specifically address management of invasive

species to ensure success of the riparian and floodplain plantings, monitoring and management of

beaver use to ensure the native plantings survive and mature, maintenance of fish passage and flow

passage, and avoidance of negative impacts on the channel and adjacent infrastructure.


Concerns have been raised about the potential effects of beavers on the sustainability of the side

channel and whether fish passage and flow capacity down the side channel could be maintained.

Beavers are a natural component of Pacific Northwest ecosystems, and numerous studies have shown

that beaver presence and activities, including the creation of beaver dams, can improve overall water

quality, raise the groundwater table, retain organic matter in stream systems for food web support, and

increase the quantity and quality of riparian and instream habitats (Johnston and Naiman 1987, Naiman

et al. 1988; Balodis 1994 [references cited in Parish 2016]). Cover provided by beaver activities has

been shown to have beneficial impacts on salmon occupancy, more than other habitat features (Parish

2016). In general, salmonid presence, growth, and productivity increases in streams with beavers

present (Bustard and Narver 1975). Salmonids have co-existed with beaver for many thousands of

years and while there has been much speculation that beaver dams can impede fish migration, a recent

survey of literature (Kemp et al. 2012) indicated that most field data show little effect on fish migration.

Beavers can significantly modify local drainage and create flood impacts. Beavers are active in the

transition zone area and from time to time have created partial channel blocking dams in the TZ. The

Washington Department of Fish and Wildlife has in the past offered reasonable regulatory

accommodation to remove beaver dams where they interfere with flood works. Should more pressing

beaver management needs arise, the County has recently developed a series of technical papers

based on best available science that describe tools and related regulations for a wide variety of beaver

management needs (see Appendix J, King County, 2017).

Operational Complexity

The seasonal operation of the TZ weir would require additional annual maintenance activities

associated with adjustments to the TZ weir overflow sill. It is expected that this would be a relatively

short-duration activity requiring a boat with a small lifting device for placement of stop logs.

Environmental Impacts of Maintenance Activity

Concept Alternative 1 would require ongoing maintenance activities including the following:

• Initial period of 3 to 5 years of invasive species management (mowing, spraying, pulling of

individual plants)

• Invasive species management once every 10 years after initial 3- to 5-year period.

• Reestablishment of plantings in year 3 after construction.

• Periodic mowing of the TZ (estimated every 3 to 4 years)

• Occasional trimming and debris along the side channel if obstructions to fish passage are

observed (estimated every 10 years).

The ongoing maintenance activities would require obtaining permits (estimated every 5 years) and may

require mitigation to compensate for vegetation and debris removal. Permitting maintenance requires

demonstrating to regulators that maintenance actions will result in no net loss of salmon habitat.

Maintenance of existing TZ willow has been shown to diminish the canopy over time. Maintenance

mitigation costs for the no-action alternative are anticipated to escalate over time with further decline of

the willow canopy. For both side channel concept alternatives, vegetation maintenance is assumed to

be less impactful to salmonid habitat. Tree species for the mainstem will be selected and placed

outside of the zone where frequent mowing is required. The side channel will require infrequent,

periodic maintenance, which should be exempt from mitigation requirements as it is a habitat

restoration feature. The habitat enhancement measures included in this alternative may partially


compensate for future maintenance. A current working assumption is that ongoing maintenance of cold-

water supplementation Option A (Big Water), could provide ongoing mitigation credit for vegetation

management activities associated with either concept alternative. It is assumed that a larger volume of

cold-water supplementation provides greater temperature reduction, including cooling the river.

Construction Impacts

Generally, construction impacts for Concept Alternative 1 Wide Side Channel would be more

substantial than those of Concept Alternative 2 Narrow Side Channel due to the larger area and greater

volume of excavation. Construction-related impacts would be as follows:

• Construction would occur over two seasons to facilitate water management during construction.

• Construction activities would generate about 5,000 truck trips to remove about 85,000 cubic

yards of excavated material from the site and to deliver new construction material. This activity

would have direct impacts on traffic on West Lake Sammamish Parkway. Haul routes have not

been established, but it is likely that other streets would also be affected by construction traffic.

• The excavation of the side channel and the new Tosh Creek channel would convert

approximately 3.9 acres of existing wetland to side channel or creek channel. Additionally, it

would convert approximately 8.6 acres of existing river or wetland buffer to side channel or inset

floodplain (would remain as buffer).

Cultural Resources

Channel excavation would range from approximately 8 to 12 feet deep. Shallow excavations that do not

extend below fill (e.g., plantings) are very unlikely to impact intact archaeological deposits, although

occasional Native American artifacts have been found within the fill. Any project excavations that would

extend below the fill (e.g., side-channel excavation and large-wood anchoring) may disturb naturally

deposited sediments, with potential to contain intact archaeological material. Deeper ground

disturbance confined to the alignment of the filled former river channel may be less likely to impact

significant archaeological resources along the alignment of the filled former river channel, but

archaeological deposits may be present below the bottom of that former channel.

6.3 SIDE CHANNEL CONCEPT ALTERNATIVE 2 NARROW SIDE CHANNEL

6.3.1 Flood Protection

Lake Sammamish Level

For Concept Alternative 2 Narrow Side Channel, seasonal operation of the TZ weir results in a 0.2-foot

reduction in average daily lake level during the higher-level months of November through February but

increases average lake level by about 0.1 feet from March through July. Figure 16 compares the

average daily Lake Sammamish level for existing conditions and Concept Alternative 2. Table 19 shows

that Concept Alternative 2 is able to reduce lake level exceedance above 27.0 feet NGVD29 (30.6 feet

NAVD88) by about 6 days per year on average but exceedance above 29.0 feet NGVD29 (32.6 feet

NAVD88) by less than one day per year on average. The average minimum summer lake level is

slightly higher, and the average winter base level is slightly lower with this alternative (see Table 20).


Figure 16. Concept Alternative 2 Narrow Side Channel, Average Daily Lake Sammamish Level

Table 19. Concept Alternative 2 Narrow Side Channel, Level Exceedance in Lake Sammamish

Lake Level (feet) Average Number of Days Exceeded Annually

NAVD88 NGVD29 Existing Conditions (2019) Concept Alternative 2 Difference

30.56 27.0 57 51 -6

31.56 28.0 6 6 0

32.56 29.0 1 1 0a a. The computed difference is around a 10 percent reduction (0.1 feet); however, this is reported as zero because the sensitivity of this

analysis is limited, so only whole day increments are reported.

Table 20. Concept Alternative 2 Narrow Side Channel, Seasonal Parameters

Parameter Existing Conditions (2019) Concept Alternative 2 Difference

Lake Sammamish Min. Summer Level (July 1 – September 15)



0.1

Sammamish River Avg. Min Summer Flow (July 1 – September 15)

25.7 cfs 25.0 cfs -0.7 cfs

Lake Sammamish Avg. Base Winter Level (November 1 – March 1)



-0.1 ft


Sammamish River Flow

Peak flow rates with Concept Alternative 2 Narrow Side Channel in the Sammamish River downstream

of Bear Creek are slightly higher than existing conditions for the 2-year or lower return period. For

events greater than the 2-year, peak flow is slightly lower (see Table 21). Similarly, the duration of flows

increases at lower flow level, but there is no significant difference in flow duration above 100 cfs (see

Table 22). However, the average annual minimum summer flow is slightly lower for this alternative (see

Table 20). Figure 17 shows that the USACE design intent for the TZ is not met with this alternative, but

the days of exceedance are reduced from 10 to 7.

Table 21. Concept Alternative 2 Narrow Side Channel, Sammamish River Flow Frequency Downstream

of Bear Creek

Return Period Exceedance Sammamish River Peak Flow Downstream of Bear Creek (cfs)

(years) Probability (%) Existing Conditions (2019) Concept Alternative 2 Difference

1 99 675 715 40

1.25 80 1,010 1,030 20

2 50 1,250 1,250 0

10 10 1,660 1,650 -10

50 2 1,940 1,920 -20

100 1 2,040 2,030 -10

Table 22. Concept Alternative 2 Narrow Side Channel, Sammamish River Flow Duration below Bear

Creek

Percent of Time Flow Exceeded

Flow (cfs) Existing Conditions (2019) Concept Alternative 2 Difference

1,500 0.2 0.2 0

1,000 2.9 2.9 0

500 17.2 17.1 -0.1

100 63.6 64.5 0.9


Figure 17. Concept Alternative 2 Narrow Side Channel, Daily Lake Level and Sammamish River Flow

Storm Drain Outfall Evaluation

Flow frequency reported in Table 21 shows that peak flows increase for return periods less than the

2-year rate. Because river level would also increase with flow, a more rigorous assessment of flooding

potential in the storm drain systems was performed. The depth of inundation at storm drain outfalls in

the City of Redmond during peak flood events would increase 0.1 feet from existing conditions for the

1-year event (see Table 23). Peak level would be slightly lower for events greater in magnitude than the

1-year. A similar drop would be expected for larger events, based on change in peak flood frequency

reported in Table 21. The change in tailwater for Concept Alternative 2 is identical to Concept

Alternative 1 so see Section 6.2.1 for a discussion of impacts.

The outfall inundation analysis summarized in Table 23 shows that the potential for increased flooding

at City of Redmond outfalls downstream of the project site is limited to the 1-year peak event only.

However, this increase is minimal and does not affect the flood condition at the critical outfalls.


Table 23. Concept Alternative 2 Narrow Side Channel, Inundated Outfalls

Peak River Level (feet NGVD29 (feet NAVD88)); Difference from Existinga (feet)

Critical Outfall Location 1-year 1.25-year 2-year 10-year

NE 95th Street 22.2 (25.7); 0.1 24.5 (28.0); -0.0 25.6 (29.2); -0.1 27.2 (30.8); -0.1

NE 90th Street along East bank

22.4 (26.0); 0.1 24.8 (28.3); -0.1 25.9 (29.5); -0.1 27.5 (31.1); -0.1

NE 85th Street, Westpark

22.7 (26.3); 0.1 24.0 (28.6); -0.0 26.21 (29.8); -0.1 27.8 (31.4); -0.2

Redmond Way 23.1 (26.6); 0.0 25.4 (29.0); -0.0 26.6 (30.2); -0.1 28.2 (31.8); -0.2

a. See Table 8 for existing conditions peak level. Positive value indicates peak level is higher for the alternative than for existing conditions.

6.3.2 Habitat

Fish Use

The narrow side channel would maximize holding and suitable migratory habitat for adult salmon by

creating 3 large pools in the TZ (~5,000 cubic feet each), a side channel with an estimated 1:1 riffle to

pool ratio, minimum depths through the modified weir of 12 inches, and minimum depths in the side

channel of 12 inches. Concept Alternative 2 would provide a lesser area than Concept Alternative 1 of

shallow water habitat for winter and spring juvenile fish rearing and refuge by excavating a floodplain

bench along the left bank of the TZ (~2.5 acres) and creating a larger inset floodplain along the side

channel (~4 acres).

Concept Alternative 2 Narrow Side Channel would also include installation of 75 percent of the

recommended key piece large wood loading in the side channel or about 110 key pieces (per Fox &

Bolton, 2007) and 3 large wood structures associated with the pools in the TZ. The willows along the

existing TZ low-flow channel would be removed, and willows would be installed on the floodplain bench

to provide cover for the pools.

Routing Tosh Creek into the side channel would cool the side channel to ~22ºC without additional cold-

water inputs.

Wetland and Riparian

Concept Alternative 2 Narrow Side Channel would provide 3 acres of floodplain bench willow habitat,

21 acres of enhanced riparian/floodplain forest, 14 acres of native planting to enhance existing

wetlands, 1 acre of tall tree planting for shading along the banks of the TZ, and 33 acres of invasive

species management. Overall, the wetland and riparian enhancement would likely result in

approximately 60 percent shading over the Sammamish River (over the long-term when trees mature,

Ecology, 2011) and greater than 60 percent shading of the side channel and wetlands.

Table 24 summarizes the analysis of the potential for Concept Alternative 2 to affect lake level and

fringe wetland inundation frequency. This table shows that the seasonal operation of the TZ weir is able

to maintain the existing level of lake-fringe wetland inundation.


Table 24. Concept Alternative 2 Narrow Side Channel, Fringe Wetlands Inundation

Existing Conditions (2019) Lake Level

(feet NGVD29 (feet NAVD88)

Concept Alternative 2 Lake Level


Difference (feet)


26.6, (30.2) 26.6, (30.2) 0.0


26.8, (30.4) 26.8, (30.4) 0.0

2-year lake level, which can be indicative of the location of OHWM.

28.2, (31.8) 28.1, (31.7) -0.1


Concept Alternative 2 addresses the key objectives of the project and includes features to promote the

long-term function and sustainability of the site for fish and wildlife. Specific features included in the

concept design include sizing the weir notch and stop logs to ensure fish passability (minimum 1 foot

depth), sizing the side channel to ensure fish passability (minimum 1 foot depth during low flows),

creating deep pools for adult salmon holding habitat in the TZ and side channel, creating localized

temperature refugia in the side channel (hyporheic features, pools and Tosh Creek confluence), and

creating positive drainage downstream to avoid fish stranding. During further phases of design,

additional details will be developed to specifically address management of invasive species to ensure

success of the riparian and floodplain plantings, monitoring and management of beaver use to ensure

the native plantings survive and mature, maintenance of fish passage, and avoidance of negative

impacts on the channel and adjacent infrastructure.


The seasonal operation of the TZ weir would require additional maintenance activities associated with

adjustment of the weir overflow. It is expected that this would be a relatively short-duration activity

requiring a boat with a small lifting device to place the stop logs in place.

Environmental Impacts of Maintenance Activity

Concept Alternative 2 Narrow Side Channel would require ongoing maintenance activities including:

• Initial period of 3 to 5 years of invasive species management (mowing, spraying, pulling of

individual plants).

• Invasive species management once every 10 years after initial 3-5-year period.

• Reestablishment of plantings in year 3 after construction.

• Periodic mowing of the TZ (estimated every 3 years)

• Occasional trimming along the side channel if obstructions to fish passage are observed

(estimated every 5 years on average).

The ongoing maintenance activities would require obtaining permits (estimated every 5 years) and may

require mitigation to compensate for vegetation and debris removal. The habitat enhancement

measures included in this alternative may partially compensate for future maintenance, but without the

addition of effective cold-water supplementation, periodic requirement for mitigation is likely. It is


assumed that a larger volume of cold-water supplementation provides greater temperature reduction,

including cooling the river.


Construction-related impacts for Concept Alternative 2 Narrow Side Channel would be as follows:

• Construction would occur over two seasons to facilitate water management during construction.

• Construction activities would generate about 3,800 truck trips to remove about 54,000 cubic

yards of excavated material from the site and also to deliver new construction material. This

activity would have direct impacts on traffic on West Lake Sammamish Parkway. Haul routes

have not been established but it is likely that other streets would also be affected by

construction traffic.

• Excavation of the side channel and the new Tosh Creek channel would convert approximately

2.2 acres of existing wetland to side channel or creek channel. Additionally, it would convert

approximately 4.9 acres of existing river or wetland buffer to side channel or inset floodplain

(would remain as buffer).

Cultural Resources

Potential impacts on archeological resource areas for Concept Alternative 2 Narrow Side Channel

would be similar to those for Concept Alternative 1 but possibly lower due to the smaller footprint of the

side channel.

6.4 CWS OPTION A (“BIG WATER”)

6.4.1 Habitat

CWS Option A would likely reduce water temperature in the side channel and Sammamish River to less

than 21ºC and would eliminate the incidence of incipient lethal water within the project reach. However,

the Index of Thermal Stress (ITS, cumulative hours per year with temperatures above threshold) above

17.5ºC would only be reduced by about 30 percent. Temperature is the only habitat-related measure

affected by this option.

The 20-cfs flow contribution from this option would be in place of existing river flow and therefore would

not increase base flow during the summer (see Section 5.8).



Seasonal operation of the surface lake water heat-exchange facility would require additional annual

maintenance to initiate operation of the pump and fish screening. This would be a relatively short-

duration activity requiring a visit to the facilities. Access to the pump facility would be from the land, but

a boat would be needed to access the fish screen in Lake Sammamish. More intensive maintenance

activity would occur every 5 years to clean the intake assemble, maintain the pump facility, and conduct


underwater inspection of the heat exchange manifold. The pumps would likely need to be replaced

every 20 years.

Providing cold-water supplementation that improves salmon survival and productivity would additionally

enhance the project area habitat quality and provide additional compensation for future maintenance,

thus reducing the potential need for ongoing mitigation.


CWS Option A would generate about 300 truck trips to remove about 600 cubic yards of excavated

material from the site and to deliver new construction material. This activity would have direct impacts

on traffic on West Lake Sammamish Parkway. Haul routes have not been established, but it is likely

that other streets would also be affected by construction traffic.

Cultural Resources

Pipeline excavation would range from 6 to 10 feet deep. Any project excavations that would extend

below the fill may disturb naturally deposited sediments, with potential to contain intact archaeological

material.

6.5 CWS OPTION B (“GRAVEL INTERFLOW”)

6.5.1 Habitat

CWS Option B would likely be able to reduce temperature to 21ºC or below only in the side channel

downstream of Tosh Creek. It would have only a minor benefit to cool the constructed channel

immediately downstream of the side channel outlet. This option would not measurably reduce

temperature downstream in the Sammamish River.



There would be cost associated with CWS Option B to perform effectiveness monitoring of groundwater

level after construction.

Providing cold-water supplementation that improves salmon survival and productivity would additionally

enhance the project area habitat quality and provide additional compensation for future maintenance,

thus reducing the potential need for ongoing mitigation.


Construction of CWS Option B would generate about 400 truck trips to remove about 3,500 cubic yards

of excavated material from the site and to deliver new construction material such as clean gravels. This

activity would have direct impacts on traffic on West Lake Sammamish Parkway. Haul routes have not

been established, but it is likely that other streets would also be affected by construction traffic.


Cultural Resources

Potential impacts on archeological resource areas for CWS Option B would be similar to those of

Concept Alternative 1 but possibly higher due to the deeper excavation needed to install the hyporheic

gravel layer.

6.6 CWS OPTION C (“GROUNDWATER”)

6.6.1 Habitat

CWS Option C would likely be able to reduce temperature to 21ºC or below only in the side channel

downstream of Tosh Creek. It would have only a minor benefit to cool the constructed channel

immediately downstream of the side channel outlet. This option would not measurably reduce

temperature downstream in the Sammamish River.

The flow contribution from this option would supplement existing river flow and therefore would increase

base flow by 2 to 3 cfs during the summer. Groundwater would require oxygenation prior to discharge

to the side channel surface water.



Seasonal operation of the pumped groundwater facility would require additional annual maintenance to

initiate operation of the pump. This would be a relatively short-duration activity requiring a visit to the

facilities. Access to the well houses and pump facilities would be from the land. More intensive

maintenance activity would occur every 5 years to maintain the pump facilities and the discharge

outfall. The pumps would likely need to be replaced every 20 years.


CWS Option C construction would generate about 300 truck trips to remove about 3,300 cubic yards of

excavated material from the site and to deliver new construction material. This activity would have

direct impacts on traffic on West Lake Sammamish Parkway. Haul routes have not been established,

but it is likely that other streets would also be affected by construction traffic.

Cultural Resources

Pipeline excavation would range from 6 to 10 feet deep. Any project excavations that would extend

below the fill may disturb naturally deposited sediments with potential to contain intact archaeological

material. The pipeline route will traverse a known archaeological hot-spot, so the likelihood of impacts

is high.


7.0 COMBINED ALTERNATIVES AND ESTIMATED PROJECT COSTS

7.1 COMBINED ALTERNATIVES

The side-channel concept alternatives were paired with cold-water supplementation options to develop

the combined alternatives listed in Table 25. Costs were estimated and the overall performance was

evaluated for each combined alternative.

Table 25. Combined Alternatives

Combined Alternative Side Channel Concept Alternative Coldwater Supplementation Option

1 Wide Side Channel None

1A Wide Side Channel Big Water

1B Wide Side Channel Gravel Interflow

1C Wide Side Channel Groundwater

2 Narrow Side Channel None

2A Narrow Side Channel Big Water

2B Narrow Side Channel Gravel Interflow

2C Narrow Side Channel Groundwater

3 Existing maintained condition None

7.2 COST ESTIMATES

The capital project costs and life cycle costs for the alternatives are shown in Table 26. No property

acquisition will be needed for any of the alternatives, so capital project costs are for construction only.

The planning level opinion of probable construction cost includes all phases of construction, including

clearing, erosion control, mass grading, installation of project features and site restoration. Additionally,

all construction costs include a 30 percent contingency and are expressed in year 2022 dollars,

assuming 3 percent escalation per year. Construction management and engineering services during

construction are not included in these costs.

Table 26. Planning Level Opinion of Cost Summary

Concept Alternative 1 Wide Side

Channel Concept Alternative 2 Narrow Side

Channel

No

CWS

CWS A. Big

Water

CWS B. Gravel

Interflow

CWS C. Ground-

water No

CWS

CWS A. Big

Water

CWS B. Gravel

Interflow

CWS C. Ground-

water

Concept Alt. 3

(Baseline)

Combined Alternative 1 1A 1B 1C 2 2A 2B 2C 3

2022 Construction Cost ($M)

TZ-Floodplain $9.493 $9.493 $9.493 $9.493 $7.460 $7.460 $7.460 $7.460

CWS $8.751 $1.180 $3.031 $8.751 $1.180 $3.031

Subtotal Cost $9.493 $18.244 $10.672 $12.524 $7.460 $14.389 $7.058 $9.135

50-Year Life-cycle Cost ($M)

TZ-Floodplain $1.013 $0.604 $0.604 $0.604 $1.022 $0.613 $0.613 $0.613

CWS $0.397 $0.187 $0.534 $0.397 $0.187 $0.534

Subtotal Cost $1.013 $1.001 $0.791 $1.138 $1.022 $1.010 $0.800 $1.147 $1.790

Project Cost ($M) $10.506 $19.245 $11.463 $13.662 $8.482 $15.399 $9.439 $11.638 $3.054


The life cycle costs represent the present worth value of the maintenance, assuming a 50-year design

life. This cost includes facility and structure maintenance, vegetation removal, equipment replacement,

and permitting cost. The present worth value discount rate is assumed to be 2.88 percent based on the

federal rate for 2020.

The capital project cost and life cycle costs are documented in Appendix J.

Capital project costs were categorized into costs associated with flood protection, habitat mitigation for

impacts caused by flood protection facilities, and habitat enhancements above what is needed to

mitigate for project impacts. Generally, flood protection costs are associated with the TZ channel and

weir modifications and side channel construction. Plantings within the footprint of the TZ and side

channel, pools, large wood and invasive species control are considered habitat mitigation for project

impacts. Additional habitat mitigation is associated with floodplain plantings outside the riparian buffer,

planting in wetlands not directly impacted by the project, and floodplain reconnection elements. All cold-

water supplementation techniques are considered additional mitigation.

Life-cycle costs were categorized into permitting cost, maintenance cost, and mitigation cost for

maintenance impacts. Permitting and mitigation costs are assumed to be lower for flood protection

projects paired with CWS because cooling the side channel and river are assumed to provide broader

benefits to salmonids beyond the project area and more effectively provide advanced mitigation. This

assumption will need to be confirmed with agencies during permitting for the project.

Table 27 and Table 28 show the allocation of these cost categories for the side channel concept

alternatives and the cold-water supplementation options.

Table 27. Project Cost Allocation for Side Channel Configurations

Concept Alternative 1 Wide

Side Channel Concept Alternative 2 Narrow Side Channel

Alternative 3 (Baseline)

Capital Project Cost $9,493,000 $7,460,000 $0

Acquisition $0 $0 $0

Capital - Flood $5,791,000 $3,954,000 $0

Capital - Habitat (Mitigation) $1,898,000 $1,865,000 $0

Capital - Additional Habitat $1,804,000 $1,641,000 $0

Life-cycle Costs (w/o, w/ CWS)a $1,013,000 $604,000 $1,022,000 $613,000 $3,054,000

Permitting $203,000 $95,000 $215,000 $95,000 $214,000

Maintenance $557,000 $509,000 $552,000 $518,000 $672,000

Mitigation $253,300 $0 $266,000 $0 $2,317,000

Total Cost $10,506,000 $10,098,000 $8,482,000 $8,073,000 $3,054,000

a. Permitting and maintenance mitigation costs lower when paired with a CWS option.


Table 28. Project Cost Allocation for Cold-Water Supplementation Options

Option A

(Big Water) Option B

(Gravel Interflow) Option C

(Groundwater)

Capital Project Cost $8,751,000 $1,179,000 $3,031,000

Acquisition $0 $0 $0

Capital - Flood $0 $0 $0

Capital - Habitat (Mitigation) $0 $0 $0

Capital - Additional Habitat $8,751,000 $1,179,000 $3,031,000

Life-cycle Costs $397,000 $187,000 $534,000

Permitting $48,000 $0 $134,000

Maintenance $349,000 $187,000 $401,000

Mitigation $0 $0 $0

Total Cost $9,148,000 $1,366,000 $3,565,000


8.0 PARTNERSHIPS FOR PROJECT IMPLEMENTATION

8.1 GRANT FUNDING OPPORTUNITIES FOR COLD-WATER SUPPLEMENTATION

District Motion 2016-04.1 directs King County’s Water and Land Resources Division to pursue grant

sources to further evaluate cold-water supplementation (District Motion Item 8) and to identify funding

partners to assume ongoing maintenance costs of cold-water supplementation (District Motion Item 9).

A list of potential grant sources has been developed for the project, as shown on Table 29. Most of

these grant sources could apply to the project as a whole or to cold-water supplementation specifically.

The one exception is the Ecology’s Centennial Grant, which can be used only for water quality projects.

Table 29. Potential Grant Funding Sources for the Willowmoor Floodplain Restoration Project.

Grant Agency Grant Name Award Range

King County Flood Control District Cooperative Watershed Management Grant

• $1.67 M (WRIA 8 Total)

King County Flood Control District Subregional Opportunity Fund allocation to jurisdictional partners

• $490,000/year King County

• $600,000/year Bellevue

• $210,000/year Redmond

King County Mitigation Reserves Program • No cap

Washington Department of Ecology Centennial Grant • $250 K - $5 M per project

Washington Department of Ecology Floodplains by Design • $300 K - $10 M per project

Washington Recreation and Conservation Office

Salmon Recovery Funding Board

• $18 M (State Total)

• $390 K (WRIA 8 Total)

• $200 K (per project design)

• No cap (per project construction)


Puget Sound Acquisition and Restoration Fund


• $1.5 M (WRIA 8 Total)

• $200 K (per project design)

• No cap (per project construction)


Puget Sound Acquisition and Restoration Fund (Large Cap)

• $12 M cap per project

• $1 M minimum request


Aquatic Lands Enhancement Account

• $5.3 M (State Total)

• $1 M cap per project


Washington Wildlife and Recreation Program


• No cap (Riparian)

• $500K cap (Local park)

U.S. Environmental Protection Agency (through local integrating organization)

National Estuary Program • No Limit, funding is highly variable

U.S. National Oceanic and Atmospheric Administration

Coastal Resilience Grants • $75 K - $1 M

• $2 M cap


The project team has procured $550,000 in grant and partnership funding for the Willowmoor project to

date, amounting to 17 percent of preliminary design costs. This includes two $200,000 grants from the

Washington Recreation and Conservation Office Salmon Recovery Funding Board.

8.1.1 Salmon Recovery Funding Board

The Salmon Recovery Funding Board grants specify the use of funding for habitat features with an

emphasis on cold-water supplementation. The next funding cycle is for 2021-2023. Given this timing,

the Salmon Recovery Funding Board is not likely to be a source of additional design funding, but it

could be among the best venues to request construction funding.

8.1.2 City of Redmond

The City of Redmond contributed $150,000 of its District Sub-Regional Opportunity Fund (Opportunity

Fund) allocation for the Willowmoor Preliminary Design with the intention of supporting a multi-benefit

project design. The City may be amenable to an additional disbursement of District Opportunity Fund

dollars toward the project, although there has been no discussion with the City regarding this.

8.1.3 Washington Department of Ecology Floodplains by Design Grants

The project has been denied funding by the Ecology Floodplains by Design grant program during three

consecutive funding cycles. The first two requests—for the 2015 and 2017 funding cycles—were

declined on the basis of the letter of interest. At the time of these grant cycles, Ecology stated that

because a preferred alternative had not been selected, the grant review committee did not have the

information needed to rank the project. The 2019 project funding request documented the selection of

the split-channel alternative via the District Motion, and the program requested a full proposal. After

submittal of the proposal, funding was declined because the project did not meet Ecology’s minimum

threshold score for the flood hazard evaluation category.

In a phone debrief on December 10, 2019, Ecology staff further explained that despite the number of

lakeshore property owners affected by rising lake levels, the program funding criteria prioritize life

safety risk first and then regional economic impacts. Lakeshore erosion and private recreation

amenities do not rank as high as life safety risks, so the proposal was not competitive for this grant

source. Floodplains by Design is not a likely source of future design or implementation project funding

based on the existing screening criteria.

8.1.4 King County Mitigation Reserves Program

Sound Transit and the King County Mitigation Reserves Program contacted the project team in fall of

2018 with an interest in funding wetlands enhancement and creation within the Willowmoor project

footprint. Sound Transit was seeking mitigation sites for stream and wetland impacts within the

proposed Sound Transit 3 railway alignment in the Redmond area. Project staff worked with the King

County Mitigation Reserves Program to estimate the potential for the project to serve as a mitigation

site.

A proposal for an in-lieu fee partnership was raised at the Mitigation Reserves Program Interagency

Review Team meeting in fall of 2018. The team rejected the proposal on the basis that the project is


already funded by the District and in the WRIA 8 Chinook Salmon Recovery Plan. The explanation

provided to the project team was that converting an already planned salmon recovery project intended

to improve habitat into a mitigation project would effectively “lower the bar” for regional salmon recovery

efforts. The Mitigation Reserves Program is therefore not a likely source of future design or construction

funding.

8.1.5 Puget Sound Near-Term Action Agenda

The Puget Sound Partnership has placed Willowmoor as a top-tier project on the Puget Sound Near-

Term Action Agenda. This list is used by the U.S. Environmental Protection Agency and other salmon

recovery partners to identify significant projects that impact endangered species, such as Chinook

salmon in the case of Willowmoor. There may be a nexus to design or construction funding through this

listing, but there is not an application period for Puget Sound Partnership funding at this time. The

project team will periodically check for updates on funding availability with the South Central Caucus

Area Action Group, the local integrating organization that prioritizes project funding for the Puget Sound

Partnership.

8.1.6 Lake Management Districts

Lake management districts may be formed by a legislative County authority or by petition of 10 or more

shoreline landowners (RCW 36.61.030). In 2015, the Lake Geneva Property Owners Association was

awarded a grant from the District’s Flood Reduction Grant Program to develop a lake management plan

to address lake outlet management, nutrient loading, aquatic weeds, and fish and wildlife habitat for

Lake Geneva in Federal Way, Washington (Award 4.14.14). A lake management district may be an

appropriate funding mechanism for aspects of lake management that are of interest to Sammamish

lakeshore land owners. As an example, the remotely operated dynamic weir was not included in the

split-channel alternative because it is projected to have substantial operational requirements and risks

relative to a manually operated dynamic weir. The manual weir meets USACE project performance

standards so it is a cost-effective choice for the District. If there is interest from the community in

funding a more complex weir operation, a lake management district could be an appropriate venue.

Lakeshore municipalities could support a lake management district by pooling a portion of their

allocation of the District Opportunity Fund or another mutually agreeable mechanism. The City of

Sammamish has experience with this process, managing the budget for the Beaver Lake Management

District, and, on the advice of the citizen-led board, is authorized to contract work for District projects.

8.1.7 Other Sources

Additional grant funding sources that may have a nexus to funding Willowmoor cold-water

supplementation design or construction are listed in Table 29. They have not been pursued at this time

because the grant applications are prescriptive in a way that does not indicate water quality

improvement via methods other than tertiary treatment or riparian vegetation planting would rank well.

Staff can pursue these other grant sources at the direction of the District.


8.2 FUNDING PARTNERS FOR ONGOING MAINTENANCE OF COLD-WATER SUPPLEMENTATION

8.2.1 Grant Opportunities

No sources of grant funding have yet been identified for ongoing maintenance of the Willowmoor cold-

water supplementation options. King County reached out to potential sources in April 2019 to explore

the feasibility of procuring grants specifically for the cold-water supplementation aspect of design,

construction and long-term maintenance. A script with four main questions related to the applicability of

the grant funding source and relevance to ongoing maintenance was used in phone conversations with

grant program staff (see Appendix K for text of script). Table 30 summarizes responses to the

questions asked regarding eligibility for funding and the types of funding each program provides.

Table 30. Summary of Responses to Cold-Water Supplementation Grant Inquiries

Grant Agency Grant Name Cold-Water

Supplementation Eligible?

Design (D), Capital (C), or Maintenance (M) Funding?

Awards Long-Term

Maintenance Funding?

Knows Other

Maintenance Funding?

King County Flood Control District

Cooperative Watershed

Management Grant

Yes, but policy is to not award grants if project is in the District’s capital budget

D, C, Short-Term M

No No

King County Mitigation Reserves Program

Yes, but must be registered with program as

mitigation site; program staff have indicated this is

unlikely to happen

D, C, Long-Term M

Yes

Washington Department of Ecology

Floodplains by Design

Yes, but must document severity of flooding

problems

D, C, Short-Term M

No No

U.S. Environmental Protection Agency (through local integrating organization)

National Estuary Program

Yes, Willowmoor is on the top tier of Action Agenda

list

D, C, Short-Term M

National Fish and Wildlife Foundation

Coastal Resilience Grants

Yes D, C Only No No

U.S. National Oceanic and Atmospheric Administration

Coastal & Marine Habitat Restoration

Grants

Yes D, C Only No No

Based on these conversations, as well as past experience the project team has had with the granting

entities, the most likely funding partners are salmon habitat grants with the Washington Recreation and

Conservation Office and Coastal Resilience Grants from the National Fish and Wildlife Foundation.

These types of grants typically provide funds for design, construction, and short-term monitoring and

maintenance. Maintenance is typically limited to the life of the construction grant and not more than five

years. The Mitigation Reserves Program does award longer-term maintenance funding for selected

projects, however Willowmoor was not eligible for this funding when last reviewed in fall 2018. The

programs and agencies listed in Table 31 were also contacted, but have not yet replied to this inquiry.


Table 31. Summary of Agencies That Have Not Yet Responded to Willowmoor Grant Inquiries

Grant Agency Grant Name

Washington Department of Ecology Centennial Grant/319 Grant


• Salmon Recovery Funding Board

• Puget Sound Acquisition and Restoration Fund

• Aquatic Lands Enhancement Account

• Washington Wildlife and Recreation Program

National Fish and Wildlife Foundation Coastal Resilience Grants

8.2.2 River Management Inter-local Agreement

The Sammamish River winds through several local jurisdictions, including Bellevue, Redmond,

Woodinville, Bothell, Kenmore, and unincorporated King County. Cold-water supplementation would

benefit salmonids as they travel through all of these jurisdictions along the waterway. Each of the

jurisdictions has a stated goal of restoring and protecting salmonid habitat, and there may be interest in

developing an interlocal agreement to fund cooperative river management plans such as the

Sammamish River Corridor Action Plan (USACE, 2002) or the Sammamish River Integrated Aquatic

Vegetation Management Plan (King County, 2013). The Aquatic Vegetation Management Plan

identifies a need for municipal coordination to address aquatic weeds and the Corridor Action Plan

suggests researching and implementing a large-scale cold-water supplementation project, such as the

lake water heat exchange option described in this report.

An interlocal agreement could be developed that addresses these needs and develops a funding

mechanism to implement maintenance activities such as these that may be best addressed in a holistic,

cooperative manner. As an example, municipalities are entitled to pool their annual District Opportunity

Fund allocations toward common goals. For the most expensive cold-water supplementation (Option A

(Big Water), the 20-cfs heat exchange option), the six entities listed above could each contribute less

than $2,000 per year of their fund allocation to pay for operation and maintenance costs. A larger

pooled allocation could support control of Brazilian elodea and other noxious weeds along the river

shorelines.


9.0 PERFORMANCE EVALUATION

Each alternative was evaluated using the performance measures, metrics and evaluation criteria

described in Section 2.0. For each performance measure, the scores for all evaluation criteria were

normalized for each performance measure and then summed to determine an overall score. A

complete description of the performance measures, metrics, and point assignment for scoring the

alternatives is provided in Appendix J.

The project consultant team performed a preliminary evaluation of each alternative using the

performance measures. The evaluation was then presented to the larger team of King County and

consultant staff during a workshop. The team considered the categories, point assignments, and

qualitative assessment of alternatives. The final evaluation was adjusted based on input received from

the larger team and comments from the Stakeholder Advisory Committee and from the District.

Table 32 and Table 33 summarize the scoring.

Table 32. Performance Evaluation Scoring Summary

Alternative Cost

Flood Protection

Score Habitat Score

Sustainability Score

Total Score Normalized to

100 per Category

Concept Alternative 1 when paired with CWS

$10,506,000 $10,097,000

5 11 0 103

Concept Alternative 2 when paired with CWS

$8,482,000 $8,073,000

5 9 1 95

Concept Alternative 3 (Baseline) $3,054,000 0 -5 -2 -35

CWS Option A (Big Water) $9,148,000 0 5 -1 24

CWS Option B (Gravel Interflow) $1,366,000 0 1 1 9

CWS Option C (Groundwater) $3,565,000 0 3 -3 6

Table 33. Performance Evaluation Summary for Combined Alternatives

Combined Alternative Combined Alternative Name Cost Points

Points

(Zero Origina)

1 Wide Side Channel $10,506,000 103 138

1A Wide Side Channel with "Big Water" Lake Heat Exchanger

$19,245,000 127 162

1B Wide Side Channel with "Gravel Interflow" Hyporheic Side Channel Features

$11,463,000 112 147

1C Wide Side Channel with "Groundwater" Deep Pumped Groundwater

$13,662,000 109 144

2 Narrow Side Channel $8,482,000 95 130

2A Narrow Side Channel with "Big Water" Lake Heat Exchanger

$17,221,000 119 154

2B Narrow Side Channel with "Gravel Interflow" Hyporheic Side Channel Features

$9,439,000 104 139

2C Narrow Side Channel with "Groundwater" Deep Pumped Groundwater

$11,638,000 101 136

3 Maintained Existing Condition $3,054,000 -35 0

a. Point scores adjusted by lowest score to normalize point scale to start at zero.


10.0 NEXT STEPS

The information developed in this alternatives analysis will be presented to the District to inform its

decision on advancing a concept alternative variation of the split channel alternative forward to

30 percent design. If authorized, the project team will develop a 30 percent preliminary design based on

the selected alternative. The findings from the alternatives analysis will be presented at a third Strategic

Advisory Committee meeting.


11.0 REFERENCES

Balodis, M. 1994. Beaver population of Latvia: history, development and management. Proceedings of

the Latvian Academy of Sciences B 7/8: 122-127.

Booth, D. B. 1994. Glaciofluvial infilling and scour of the Puget Lowland, Washington, during ice-sheet

glaciation. Geology, 22(8), 695–698.

Booth, D. B., and Hallet, B. 1993. Channel networks carved by subglacial water: Observations and

reconstruction in the eastern Puget Lowland of Washington. Geological Society of America Bulletin,

105(5), 671–683. doi:10.1130/0016-7606(1993)105<0671:CNCBSW>2.3.CO;2.

Bustard D. and D. Narver. 1975. Aspects of the winter ecology of juvenile Coho salmon (Oncorhynchus

kisutch) and steelhead trout (Salmo gairdneri). Journal of the Fisheries Research Board of Canada 32:

667-680.

Cardno, 2017. 2016 Upper Sammamish River Pre-Spawn Mortality Data Report. Prepared for King

County Department of Natural Resources and Parks, October 31, 2017.

Fox, M. & S. Bolton. 2007. A Regional and Geomorphic Reference for Quantities and Volumes of

Instream Wood in Unmanaged Forested Basins of Washington State, North American Journal of

Fisheries Management, 27:1, 342-359

Friends of Marymoor Park. 2018. Species Lists for Marymoor Park. Available at:

http://marymoor.org/wildlife.htm

Hruby, T.R. 2014. Washington State Wetland Rating System for Western Washington, 2014 update.

Washington Department of Ecology Publication 14-06-029.

Johnston, C. and R. Naiman. 1987. Boundary dynamics at the aquatic-terrestrial interface: The

influence of beaver and geomorphology. Landscape Ecology 1: 47-57

Kemp, P. S., T. A. Worthington, T. E. Langford, A. R. Tree, and M. J. Gaywood. 2012. Qualitative and

quantitative effects of reintroduced beavers on stream fish. Fish and Fisheries 13:158-181.

King County. 2013. Willowmoor Restoration Design Hydrology, Phase 1 – Hydrologic Characterization.

Prepared by NHC for King County River and Floodplain Management Section.

King County. 2014. Willowmoor Existing Habitat, Fish and Wildlife Report. Prepared by Tetra Tech, Inc.

King County. 2015. Willowmoor Floodplain Restoration Project. Concept Design Summary Report.

Prepared by Tetra Tech for King County Department of Natural Resources and Parks.

King County, 2018. King County iMap Interactive Mapping Tool. Accessed June 25, 2018. Available at

https://www.kingcounty.gov/services/gis/Maps/imap.aspx.

National Research Council (NRC). 1995. Wetlands characteristics and boundaries. Washington, DC:

National Academy Press.

http://marymoor.org/wildlife.htm

https://www.kingcounty.gov/services/gis/Maps/imap.aspx


National Technical Committee for Hydric Soils. 2000. Technical standards for hydric soils. Technical

note 11. https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/use/hydric/?cid=nrcs142p2_053973

Northwest Hydraulic Consultants (NHC). 2006. DRAFT - Stormwater Master Plan Modeling – Model

Documentation and Application in Willows Creek and 95th Street Pilot Watersheds. Memorandum

prepared for City of Redmond.

Parish, M.M. 2016. Beaver bank lodge use, distribution, and influence on salmonid rearing habitats in

the Smith River, California. Master’s thesis, Humboldt State University.

R2 Resource Consultants, 2015. Salmonid Pre-Spawn Mortality Study, Sammamish River. Prepared

for: King County Department of Natural Resources and Parks. December 2015.

R2 Resource Consultants. 1999. Habitat survey Sammamish River, King County, Washington: 1999

data report. Prepared for U.S. Army Corps of Engineers, Seattle District, Washington, Redmond,

Washington.

U.S. Army Corps of Engineers (USACE) and King County. 2002. Sammamish River Corridor Action

Plan. Final Report. Prepared by Tetra Tech, Inc., Seattle WA.

U.S. Army Corps of Engineers (USACE). 2010. Regional supplement to the Corps of Engineers

Wetland Delineation Manual: Western Mountains, Valleys, and Coast Region (J. S. Wakeley, R. W.

Lichvar, and C. V. Noble, ed.). ERDC/EL TR-10-3. Vicksburg, MS: U.S. Army Engineer Research and

Development Center, Environmental Laboratory.

Washington Department of Ecology (Ecology). 2011. Green River Temperature Total Maximum Daily

Load, Water Quality Improvement Report. Publication No. 11-10-046. June, 2011.

Wydoski, R.S. and R.R. Whitney. 2003. Inland Fishes of Washington. Second Edition, Revised and

Expanded. American Fisheries Society with University of Washington Press, Seattle, WA.

https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/use/hydric/?cid=nrcs142p2_053973

APPENDIX A. HYDROLOGIC MODELING TECHNICAL MEMORANDUM

APPENDIX B. HYDRAULIC MODELING TECHNICAL MEMORANDUM

APPENDIX C. ALTERNATIVE ANALYSIS HYDRAULIC MODELING TECHNICAL MEMORANDUM

APPENDIX D. PRELIMINARY DYNAMIC WEIR ANALYSIS TECHNICAL MEMORANDUM

Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County F-1

APPENDIX E. HYPOLIMNETIC COOLING OF LAKE SAMMAMISH SURFACE WATER

This concept evaluated the withdrawal of 10 or 20 cfs of surface water from Lake Sammamish and

pumping it through a pipe system at the bottom of the lake to cool the water to 18ºC (65ºF) via heat

exchange. The water would then be piped to discharge immediately downstream of the Transition Zone

weir. The water temperature at 20 meters (66 feet) in the lake averages less than 10ºC (50ºF) during the

July to September time period (Figure 1). This concept avoids potential water quality and fish concerns

regarding withdrawing water from the hypolimnion by using surface water that is the current natural

source of discharge over the weir and uses the heat exchange possibilities of the cold-water at the bottom

of the lake. The cooled water discharge would replace the passive outflow from the Lake during a 60-day

critical time period (generally July 15 to September 15). However, the 10 cfs alternative did not sufficiently

cool the discharge to justify the smaller flow rate, and only the 20 cfs alternative was carried forward.

Figure 1. Monthly maximum and mean water temperatures at Lake Sammamish Hydro Data Site 612 at

1, 10, 15, and 20 meter depths

In order to conduct the heat exchange where the lake is sufficiently deep (20 meters or greater depth),

the surface withdrawal would be located about 14,000 feet into the lake. It would draw surface through a

vertical cylinder fish screen to an intake pipe above heat exchange piping on the lake bottom. The heat

exchange system divides and slows the flow in 6 parallel pipe loops to maximize contact time with the

colder lake bottom water. Flows are then be merged back to a single pipe and drawn to a small upland

pump station on the south side of the Sammamish River, opposite Marymoor Park.

Figure 2. Floating Surface Fish Screen Intake Figure 3. Heat Exchange Manifold System

Primary features of this alternative include:

• Floating surface intake and 36-inch HDPE pipeline to heat exchange piping on Lake Sammamish

bottom, see Figure 2.

• Heat exchange manifold with six 24-inch aluminum pipe loops, 2,000 linear feet (LF) in each, for

a 700 foot long manifold piping system, see Figure 3.

• 14,000 LF 36-inch HDPE pipeline submerged on Lake Sammamish bottom, with 2,000 LF 36-

inch pipeline on Sammamish River bottom, from heat exchange piping to pump station.

• Upland pump station proposed near Sammamish Rowing Association river access for existing

power and access purposes.

• Submerged 2,000 LF 30-inch pipeline on Sammamish River bottom from pump station to

discharge at existing weir.

• Discharge to side channel and river and cool overall river temperatures. The 20 cfs would be

approximately half the discharge, since current mean monthly weir discharge during July, August

and September is typically 50 cfs.

The calculation of the potential cooling provided by a heat exchanger is based on fundamental heat

transfer equations (ASHRAE 2009) for three types of heat exchange that will occur in the pipe: 1) forced-

convection through the pipe; 2) thermal conduction at the pipe wall per unit length; and 3) natural-

convection as the heat is dissipated away from the pipe where the lake serves as a heat sink. The forced

convection component is a function of the Nusselt number derived from correlations based on empirical

data that are a function of the thermal conductivity of water and the diameter of the pipe for fully developed

turbulent flow. The thermal conduction through the pipe wall is a function of the thermal conductivity of

the pipe material (aluminum) and the pipe wall thickness. Aluminum is a good conductor of heat, whereas

HDPE is a very low conductor. Natural convection of heat from the pipe to the lake, acting as a heat sink,

is similarly derived from the Nusselt number.

Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County F-3

Assuming a lake surface temperature of 24ºC (75ºF) flowing through the heat exchanger piping array on

the bottom of the lake, the 20 cfs cooled output water is estimated to be 18ºC (65ºF) to cool the 24ºC

(75ºF) river temperature downstream of the weir. A 10 cfs output water was estimated to cooled to 17ºC

(62ºF), not a significantly different temperature than the 20 cfs calculations and half the flow rate.

Earlier evaluations did not include the effect on temperature of the water inside the pipe as it rises

through the warmer lake waters towards discharge. The 20 cfs, with its greater mass was estimated to

have greater effectiveness in reducing water temperature experienced by fish downstream of the

discharge so the 10 cfs alternative was dropped from further consideration.

The intake fish screen surface area is large enough to meet WDFW approach velocity maximums to both

protect the fish and significantly reduce entrance losses at the intake. The system head losses and heat

exchange performance were then calculated interactively, iterating pipe system diameters as required.

Adding looped heat exchange piping increased the length of heat exchange pipe for more effectiveness,

while keeping the manifold short enough to stay in the colder lake bottom water. Using a manifold heat

exchange system also reduced velocity derived line losses and fitting losses to reduce pump energy

requirements.

The pipelines and concrete anchors would be assembled on land, floated into the lake as it is assembled,

positioned via barge or work boats, and then submerged to the lake bottom; see Figure 4. The only land

elements are the pump station and the discharge. Consideration should be given to limited wetland

disturbance, looking at pipe trench excavation approaching and downstream of the pump station to

compare with submerged pipe and anchors. Pump performance and temperature gain from warmer lake

waters will need to be evaluated during design.

Figure 4. Example of Positioning Floating Pipeline and Anchors

Potential concerns or risks of this alternative include the potential effects to navigation from having a 36

or 30-inch diameter pipeline running along the bed of the lake outlet channel upstream of the weir. The

bathymetry of the existing Sammamish River is assumed to be in the range of 6-10 feet, so a submerged

pipe is considered feasible and also reduces wetland disturbance of an upland trench alignment. Another

constraint to this alternative is the fish screen at the intake that would need a cleaning mechanism. The

cylinder screens are designed to rotate over brushes. Considerations include running power from shore

to a 1 Hp motor, using the pipe velocity to rotate a propeller driven rotation (head loss unknown), or

manually cleaning the screens seasonally.

The planning level cost estimate for the 20 cfs alternative is $8.3 million. HDPE pipe costs have risen

substantially since 2014 concept development. Operation and maintenance (life-cycle) requirements

would include electricity for pumping and annual start-up/maintenance/shut-down labor and equipment

costs for the pump. An estimated net present value of operation and maintenance is $340,000 over

50 years.

REFERENCES

American Society of Heating, Refrigerating, and Air-Conditioning Engineers. 2009. Fundamentals. 880

pages. On the web at: https://www.ashrae.org/resources--publications/handbook/description-of-the-

2013-ashrae-handbook--fundamentals

Burkholder, B. 2007. Influence of hyporheic flow and geomorphology on temperature of a large, gravel-

bed river, Clackamas River, Oregon, USA. Master’s thesis, Oregon State University.

King County. 2014. Willowmoor Cold-Water Supplementation Concepts. Prepared by Tetra Tech, Inc.

Seattle, WA. Final June 2014.

R2 Resource Consultants. 2010. Assessment of summer temperatures and feasibility and design of

improved adult Chinook salmon thermal refuge habitat in the Sammamish River. Prepared for the

Muckleshoot Indian Tribe, Auburn, WA.

https://www.ashrae.org/resources--publications/handbook/description-of-the-2013-ashrae-handbook--fundamentals

https://www.ashrae.org/resources--publications/handbook/description-of-the-2013-ashrae-handbook--fundamentals

APPENDIX F. GROUNDWATER ANALYSIS

APPENDIX G. EXISTING GEOMORPHIC CONDITIONS TECHNICAL MEMORANDUM

APPENDIX H. SAMMAMISH RIVER CONCEPTUAL FISH USE MODEL

APPENDIX I. MANAGEMENT TECHNICAL PAPER #1: BEAVER MANAGEMENT TOOLS LITERATURE REVIEW AND GUIDANCE

APPENDIX J. PLANNING LEVEL COST ESTIMATE

Willowmoor Floodplain Restoration Project; Concept Alternatives Evaluation – King County L-1

APPENDIX K. SCRIPT FOR SOLICITING INPUT FROM GRANT AGENCIES REGARDING APPLICABILITY OF GRANT FUNDS FOR ONGOING MAINTENANCE OF COLD-WATER SUPPLEMENTATION

The Willowmoor Floodplain Restoration project will reconfigure the Sammamish River at Marymoor

Park and reconnect the adjacent floodplain with a new side channel. The project will balance flood

protection, habitat restoration, fish passage, recreational access and ongoing maintenance. Cold-water

supplementation is a proposed element of the project as it is identified in the WRIA 8 Salmon Recovery

Plan Update (2017) as a top priority in salmon recovery for Issaquah Fall Run Chinook. The cold-water

supplementation portion of the project is being considered at a broad range of scales from individual

pools with enhanced hyporheic connection, all the way up to a 20 cfs solution that involves running lake

surface water through a cooling unit at the bottom of the lake.

The costs of developing cold-water supplementation and maintaining it are highly variable at these

different scales and King County is conducting an analysis of grants and other partnership funding

opportunities to determine which funding sources may be appropriate for capital and ongoing

maintenance costs. I am calling to get a sense of compatibility of the cold-water supplementation

elements of our project with your grant program goals.

1. Firstly, would this type of restoration project, specifically one that features cold-water

supplementation, be eligible for your funding?

2. Does your program provide design, capital, or maintenance funding?

3. Has your program ever awarded a lump sum for maintenance funding that a project sponsor

could place in a dedicated fund? The assumption is the cost estimate basis for the fund would

be the net present value of a 50-year life cycle operations and maintenance cost. As an

example, for the larger scale cold-water supplementation alternatives, annual maintenance is

estimated at $500,000 for the 50-year life cycle, or $10,000 per year on average.

4. Do you know of other grant programs in or out of your organization where this type of project

funding may be available?

APPENDIX L. ALTERNATIVE EVALUATION WORKSHEET

APPENDIX M.

willowmoor concept alternatives analysis report

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