shuswap river project water use plan sugar lake …...december 2007 shuswap river project water use...

December 2007

Shuswap River Project Water Use Plan Sugar Lake Reservoir Shoreline Erosion Study

(Year 1) Reference: SHUMON#2 Shuswap River Water Use Plan Monitoring Program:

Sugar Lake Reservoir Shoreline Erosion Study Period: 2006-2007

Summit Environmental Consultants Ltd.

December 20, 2007 Reference: 251-28.01 Ms. Karen Bray BC Hydro and Power Authority Box 500 Revelstoke, BC V0E 2S0 Dear Ms. Bray:

Re: Sugar Lake Reservoir Erosion Study – Final Report Summit Environmental Consultant Ltd. (Summit) is pleased to provide a final report on the above-noted study. The report summarizes the areas at risk of shoreline erosion under the current normal maximum reservoir level (NMRL) of 601.72 m (geodetic), and provides an assessment of the potential reduction in erosion risk associated with an NMRL of 601.52 m. If you have any questions please call me at 250-545-3672. Yours truly, Summit Environmental Consultants Ltd. Signature on original Signature on original Brian Guy, Ph.D., P.Geo., P.H. Lars Uunila, M.Sc., P.Geo., P.H., CPESC Senior Geoscientist Geoscientist / Hydrologist Attachments Final Report

Summit Environmental Consultants Ltd. FINAL REPORT Project #251-28.01– Sugar Lake Reservoir Erosion ii 20-December-2007

EXECUTIVE SUMMARY

In order to support the specification of the maximum operating level of the Sugar Lake Reservoir, the

Shuswap River Water Use Plan Consultative Committee recommended that shoreline erosion be

studied to identify the potential reduction in erosion that could be achieved by adopting a NMRL of

601.52 (i.e. 20 cm lower than the current level) (BC Hydro 2005). Summit Environmental

Consultants Ltd. (Summit) was retained by BC Hydro to conduct a customized field study of the

reservoir. The study methodology was designed to address concerns over erosion and flood damage,

particularly near private land. The study combined an office review of background information and

aerial photos with the findings of inspections of the reservoir by boat and foot during spring and

summer 2007. During the field inspections, shoreline conditions were noted, photographed and

surveyed. The erosion potential associated with current reservoir operations (NMRL = 601.72 m) and

proposed alternative reservoir operations (NMRL = 601.52 m) was assessed and locations where site-

specific erosion mitigation may be beneficial were identified.

The results of the study indicate that under the current normal maximum reservoir level (NMRL) of

601.72 m, 74% of the shoreline is at low risk of erosion, 20% is at moderate risk of erosion, and 6% is

at high risk of erosion. While the percentages of shoreline with moderate or high erosion potential

are not considered extensive, they are significant, particularly since much of this length of shoreline

classified as high and moderate erosion potential fronts private property or developed areas (such as

campsites on Crown Land).

Assuming a 0.20 m reduction in the normal maximum reservoir level (to 601.52 m), it is estimated

that there will be a decrease of 1,140 m in the length of shoreline with a high potential for erosion

(representing 3% of the total shoreline length), and a decrease of 4,440 m in the length of shoreline

with a moderate potential for erosion (representing 13% of the total shoreline length). While in

overall percentage terms the reductions in erosion potential appear modest, they are significant in

absolute terms, particularly considering that the most noticeable reduction in erosion potential would

occur adjacent to existing private properties. Shoreline now classified as high and moderate erosion

potential is expected to be reduced by 60%, and 62%, respectively, with the 0.20 m reduction in the

NMRL. With the exception of a few locations where the reduced NMRL is unlikely to affect erosion

potential, it is expected that many private property owners would see noticeably reduced rates of

erosion should the NMRL be reduced to 601.52 m.

Summit Environmental Consultants Ltd. FINAL REPORT Project #251-28.01– Sugar Lake Reservoir Erosion iii 20-December-2007

Options for mitigating erosion were identified along 18 shoreline segments. It is recommended that

the costs of implementing site-specific erosion mitigation be estimated, and compared with the costs

associated with reducing the NMRL by 0.20 m. This comparison will assist the Shuswap River

Water Use Plan Consultative Committee to decide on the optimal cause of action to reduce shoreline

erosion.

Summit Environmental Consultants Ltd. FINAL REPORT Project #251-28.01– Sugar Lake Reservoir Erosion iv 20-December-2007

TABLE OF CONTENTS

LETTER OF TRANSMITTAL ................................................................................................. i

EXECUTIVE SUMMARY....................................................................................................... ii

TABLE OF CONTENTS......................................................................................................... iv

LIST OF TABLES.................................................................................................................... v

LIST OF FIGURES .................................................................................................................. v

1.0 INTRODUCTION ....................................................................................................... 1 1.1 Project Background.................................................................................................. 1 1.2 Project Objectives .................................................................................................... 2 1.3 Overview of Shoreline Processes............................................................................. 2

2.0 METHODS .................................................................................................................. 4 2.1 Pre-Field Tasks ........................................................................................................ 4 2.2 Field Tasks ............................................................................................................... 4 2.3 Post-Field Tasks....................................................................................................... 6 2.4 Estimating Erosion Potential ................................................................................... 7 2.5 Reliability of Estimates............................................................................................ 7

3.0 SUGAR LAKE RESERVOIR ..................................................................................... 9 3.1 Location ................................................................................................................... 9 3.2 Reservoir Description .............................................................................................. 9 3.3 Reservoir Operations ............................................................................................. 10

4.0 PREVIOUS RESERVOIR SHORELINE EROSION STUDIES.............................. 12

5.0 SHORELINE EROSION POTENTIAL .................................................................... 13 5.1 Summary of Shoreline Observations ..................................................................... 13 5.2 Representative Cross-sections ............................................................................... 13 5.3 Summary ................................................................................................................ 14

6.0 MITIGATION OF SHORELINE EROSION............................................................ 16

7.0 CONCLUSIONS........................................................................................................ 17

8.0 REFERENCES .......................................................................................................... 19

Appendix A List of properties surrounding the Sugar Lake reservoir

Map 1 Erosion potential of Sugar lake shoreline for NMRL = 601.52 m and

NMRL = 601.72 m

Attachment 1 (CD) Table 5.1 in Microsoft Excel and Microsoft Access format

Digital photographs

Summit Environmental Consultants Ltd. FINAL REPORT Project #251-28.01– Sugar Lake Reservoir Erosion v 20-December-2007

LIST OF TABLES

(Tables are presented at the end of the report)

Table 2.1 Guidelines used in determining the erosion potential of a shoreline segment

Table 5.1 Percentage of shoreline with high, moderate or low erosion potential

LIST OF FIGURES

(Figures are presented at the end of the report)

Figure 1.1 Location of Sugar Lake

Figure 1.2 Aerial view northwards of the west shore of the Sugar Lake Reservoir.

Figure 3.1 Historic Sugar Lake Reservoir water levels

Figure 5.1 Cross-section and photograph of shoreline at 5,700 m




Summit Environmental Consultants Ltd. FINAL REPORT Project #251-28.01– Sugar Lake Reservoir Erosion 1 20-December-2007

1.0 INTRODUCTION

1.1 PROJECT BACKGROUND

Sugar Lake, located 70 km east of Vernon, BC (Figures 1.1 and 1.2) has been used as a

storage reservoir for 78 years. In 1929, a 5.1 m high overflow structure [the Sugar Lake

(Peers) Dam] was constructed at the outlet of the lake on the site of Brenda Falls. In 1942,

low level outlets were added, and the dam was raised to 13 m (Shuswap River Water Use

Plan Consultative Committee 2002). Currently, the Sugar Lake Reservoir normally operates

within a water level range of 7.02 m (from 594.70 m to 601.72 m) (Shuswap River Water

Use Plan Consultative Committee 2002).

There are increasing concerns that the shoreline of Sugar Lake, particularly in developed

areas (i.e. on private land), is susceptible to erosion and flood damage when the reservoir is

at or near full pool (601.72 m); however, according to property owners around the reservoir,

erosion and flood damage is reduced when the reservoir is maintained at 601.52 m (i.e. when

one less stop-log is in place at the Sugar Lake Dam) (Shuswap River Water Use Plan

Consultative Committee 2002). While there have been some recent studies of shoreline

erosion at the Sugar Lake Reservoir (BC Hydro 1993a, Richmond 2004), there remains some

uncertainty of the extent of erosion under the current NMRL (601.72 m) relative to lower

levels. Accordingly, the Shuswap River Water Use Plan Consultative Committee

recommended that shoreline erosion be studied to reduce the uncertainty, and identify the

potential reduction in erosion that could be achieved by adopting a NMRL of 601.52 (i.e. 20

cm lower than the current level) (BC Hydro 2005). This information would be used to

support any revisions to the operating procedures of the Sugar Lake Dam in future.

In order to determine the potential of shoreline erosion at the Sugar Lake Reservoir, BC

Hydro retained Summit Environmental Consultants Ltd. (Summit) to conduct a customized

field study of the reservoir, with specific focus on the shoreline adjacent to developed land.

This report summarizes the work, which was conducted over the spring and summer of 2007.


1.2 PROJECT OBJECTIVES

The main objective of the study is to determine the long-term extent of shoreline erosion and

flooding that will likely occur at the Sugar Lake Reservoir from operating the reservoir at a

maximum elevation of 601.72 m versus 601.52 m. In other words, the study is addressing

the following management questions:

1. Does operating the reservoir at full pool (601.72 m) cause extensive shoreline

erosion?

2. Would operating the reservoir below full pool (i.e. 601.52 m) significantly reduce

shoreline erosion over the long-term?

In addition, the study investigated options to mitigate the potential for erosion at specific

locations and identified where further study is warranted. In order to answer the

management questions, an assessment supported by field measurements and photographs was

conducted.

1.3 OVERVIEW OF SHORELINE PROCESSES

It is not uncommon for erosion to occur along shorelines of hydroelectric reservoirs due to

changes in the natural stability of the shorelines associated with reservoir management. The

degree of shoreline erosion depends in part on the range of reservoir levels; the number and

rate of filling and draining cycles; reservoir orientation, prevailing winds (speed and

direction), waves, and currents; shoreline configuration (both in planform and profile); and

the materials in which the shoreline has formed.

Wave action resulting from large infrequent storms, particularly when a reservoir is near its

normal maximum reservoir level (NMRL) or “full pool”, is generally the major cause of

shoreline erosion (BC Hydro 1993a; Richmond 2004), and under non-storm conditions, wave

action can also shape the shoreline, but typically on a smaller scale. Wave erosion, often

combined with long-shore transport, is referred to as “beaching”. Waves erode shorelines

and deposit the sediment as a beach slope, or beach. Over time, beach profiles generally

become stable; thereby limiting shoreline erosion, but where long-shore currents are


common, beach profiles can remain unstable due to the continual removal of sediment

forming the beach slope. Under extreme situations, beaching has led to landslides; a result of

water waves undercutting and steepening the toe of large slopes (BC Hydro 1993b).

Changes to the groundwater elevation, brought on through reservoir implementation, can also

result in shoreline instability, and possibly landslides (BC Hydro 1993b). This can occur

when flooding at the toe of a large shoreline slope extends above the original piezometric

level, resulting in changes to the piezometric characteristics, and/or reductions in shearing

stress (BC Hydro 1993b). Since the majority of natural shoreline slopes are not

homogeneous, changes to the natural patterns in groundwater can initiate slides along

defects, such as joints, bedding planes, foliations, or shears (BC Hydro 1993b). Also, due to

continual water level fluctuations, shoreline slopes (soils and/or bedrock) are constantly

undergoing wetting and drying cycles which can weaken their makeup and potentially result

in shoreline slope failures.

Finally, in some reservoirs ice has lead to shoreline instabilities, but its affect may not be

pronounced in reservoirs that remain below full pool over the winter, since its effects are

usually obscured once the shoreline is inundated in spring.


2.0 METHODS

The following section outlines the methods used in this study, organized by pre-field, field,

and post-field tasks.

2.1 PRE-FIELD TASKS

At the beginning of the study, background information on the Sugar Lake Reservoir was

compiled and reviewed. This included:

• 1:25,000 scale aerial photos commissioned by BC Hydro dated September 21, 2001;

• 1:20,000 scale topographic maps;

• Reports prepared on behalf of BC Hydro (Arc Environmental Ltd. 2001; BC Hydro

1993a, 1993b, 1993c, 2005; Shuswap River Water Use Plan Consultative

Committee 2002; Richmond 2004);

• Water Survey of Canada hydrometric records for Sugar Lake (Station No.

08LC041); and

• Cadastral maps showing property ownership around Sugar Lake (see Appendix A).

To provide an effective means for data collection and presentation, Summit prepared an

orthophoto base map of Sugar Lake using 2001 aerial photos provided by BC Hydro. The

base map developed by Summit includes cadastral information as well as basic topographic

information and UTM coordinates. To avoid confusion, we have used a system to reference

locations along the perimeter of the reservoir. Distance is noted along the perimeter of the

reservoir (in metres) moving clockwise from the west side of the Sugar Lake Forest Service

Road bridge (Map 1).

Following a review of the background materials, a field plan was developed, which is

described below.

2.2 FIELD TASKS

Two field reconnaissance trips were conducted during the study:


• Trip #1 took place on May 29, 2007; and

• Trip #2 took place on July 12, 2007.

The first trip was conducted by Lars Uunila, P.Geo. (geoscientist / hydrologist) and Brian

deJong, C.E.T. (surveyor) during relatively low water levels (water levels ranged in elevation

between 598.042 and 598.057 m). Under optimal (i.e. clear and calm) weather conditions,

the crew slowly circumnavigated the perimeter of the reservoir1 by boat and stopped at

several locations for detailed ground inspections and level surveys. Given that the main

erosion concerns are on private land on the west side of the lake, much of the focus of the

observations was placed in that area.

During the reconnaissance, the following was conducted:

• The physical characteristics of the shoreline were noted;

• Representative photographs were taken, with locations recorded by GPS and marked

on the base map;

• Level surveys2 were conducted at selected sites to characterize the cross-sectional

profile of the shoreline;

• The potential for shoreline erosion under NMRL of 601.72 and 601.52 m for each

section of shoreline was assessed and mapped. (Three classes of erosion potential

were used, which are described in Section 2.4);

• Notes were taken of land-uses affecting shoreline stability; and

• Options for erosion of flood damage mitigation were noted.

The second field trip, conducted by Lars Uunila, P.Geo. and Drew Lejbak (geoscientist) on

July 12, 2007, was timed to observe water levels near full pool. Water levels during this trip

ranged from 601.560 m to 601.570 m (i.e. only 0.040 m to 0.050 m above the proposed

1 Unsafe shallow conditions prevented access to the extreme north end of the reservoir and the flooded inlet near the mouth of Sitkum Creek. Observations of these areas were made using binoculars. 2 Automatic level surveys were conducted in favor of differential GPS or other survey methods. The level surveys were referenced to the local reservoir level (as reported by BC Hydro) at the precise time of the survey; the estimated accuracy was +/-0.01 m, which is sufficient for the purpose of this study.


alternative NMRL of 601.52 m). Both the reservoir level and weather conditions were

optimal and facilitated direct observation of the reservoir under conditions similar to the

proposed alternative NMRL. During the second trip, nearly the entire perimeter of the

reservoir and the islands were observed by boat and several sites were inspected on the

ground. The second field reconnaissance provided a means to confirm and refine earlier

observations based on witnessing water levels near the proposed alternative NMRL. This

further increased confidence in the assessment findings.

2.3 POST-FIELD TASKS

Following the field trips, all data was summarized and the report was prepared. Key

elements of the report include the following:

• Map 1, which highlights

o the current erosion potential using a system of red, orange and green colours

to represent shoreline segments with high, moderate and low erosion

potential, respectively (discussed in Section 2.4);

o the locations of the detailed cross-sections; and

o the locations of all photographs.

• A field reconnaissance summary outlines the shoreline conditions and erosion

potential under both NMRLs of 601.72 and 601.52 m. This summary table is

provided in Table 5.1 and digitally on the attached CD as a Microsoft Excel

spreadsheet and a Microsoft Access database;

• Cross-sections and photographs of the shoreline at the selected locations are

provided in Figures 5.1 through 5.4; and

• All digital photographs are provided on the attached CD (Attachment 1).

Photographs are labelled using the convention “AA-BB”, where AA is the photo

location (shown on Map 1) and BB is the frame. The first frame at all locations

shows the location of the photograph as recorded in the field and is labelled “AA-0”.


2.4 ESTIMATING EROSION POTENTIAL

Erosion potential within this report is defined as the proximity of the shoreline to its final

equilibrium state. The assessment of erosion potential was based on the guidelines outlined

in Table 2.1 (a total of eight factors were utilized to assess the erosion potential), with the

erosion potential for each shoreline segment rated as “high”, “moderate”, or “low” through

professional judgement. The assessment generated limited information upon which to base

assessments of actual erosion rates. Therefore in this report we classify the shoreline in

terms of erosion potential, but do not identify actual erosion rates. Reference is made to

erosion rates only where we comment on the likely changes to erosion rates associated with

changes in erosion potential.

As a result of the aerial photo review and field inspections, the shoreline of the Sugar Lake

Reservoir was divided into 59 segments (Map 1 and Table 5.1). Each segment represents a

length of the shoreline with roughly homogeneous exposure to prevailing winds, shoreline

configuration (in planform and profile), and materials in which it has formed. Based on these

characteristics along with information obtained from level surveys, the potential for erosion

of each shoreline segment was first assessed assuming the current NMRL of 601.72 m with a

design wave height up to 0.75 m (depending on exposure and fetch). Then it was assessed

assuming an alternative NMRL of 601.52 m with a design wave height up to 0.75m

(depending on exposure and fetch).

2.5 RELIABILITY OF ESTIMATES

Two main sources of uncertainty are associated with the assignment of an “erosion potential”

class to a shoreline segment:

1) Determining erosion potential at each shoreline segment; and

2) Identifying the exact location of each segment and transferring that information to a

map.

To minimize the uncertainty associated with the first source, we:


• Developed and utilized a straightforward, three class erosion potential system, based

on eight key factors considered responsible for shoreline erosion at Sugar Lake, as

defined in Table 2.1;

• Used an experienced professional to conduct all observations;

• Conducted two field reconnaissance trips at separate water levels to ensure we were

able to observe the entirety of the shoreline;

• Took photographs of almost the entirety of the shoreline to provide a reference during

follow-up office analysis; and

• Selective surveys and ground inspections were conducted to confirm close-up

shoreline configuration and materials.

To minimize the uncertainty associated with the second source of error, we:

• Used differential GPS and recent orthophotos to identify field locations. It is

estimated that the error on the map line work (i.e. breaks between erosion potential

classes) is ±10 m.

Other uncertainties may still exist, including the fact that some portions of the shoreline were

obscured by vegetation making assigning erosion potential difficult, but these other sources

of error are considered minor and are not expected to significantly influence the results.

The results can be considered reasonable, given the methods, assumptions, limitations, and

budget associated with the study.


3.0 SUGAR LAKE RESERVOIR

3.1 LOCATION

The Sugar Lake Reservoir is oriented roughly north-south in the Monashee Mountains

(Figure 1.1). It is accessed from Vernon, BC via Highway 6 and the Sugar Lake Road north

of Cherryville, BC. Much of the west side of the reservoir is accessed by the Sugar Lake

Forest Service Road (FSR). While most of the shoreline is Crown Land or is owned by BC

Hydro (Appendix A), it is along this road where private development has been largely

concentrated; however, development has also occurred along the southeast shore of the lake.

The Kokanee Lodge, Fraser Lodge Campground, several Forest Service campgrounds, and

many cabins, trailers, and seasonally used vacation properties are currently located close to

the shore of the Sugar Lake Reservoir (Appendix A)3.

3.2 RESERVOIR DESCRIPTION

Sugar Lake is approximately 11 km long by 1 to 4 km wide and covers an area of 21 km2

(BC Hydro 1993a). The length of shoreline is approximately 32 km (excluding islands). The

mean and maximum depths are 35 m and 83 m, respectively (Ministry of Environment 2007).

Reservoir bathymetry varies considerably around the lake, with large relatively shallow

zones lined with wetlands at the north and south ends of the reservoir and near the mouth of

Sitkum Creek (on the east side of the reservoir) – these were terrestrial areas prior to the

construction of the Sugar Lake Dam.

Heavily forested steep bedrock slopes mantled by glacial deposits and colluvium surround

much of the reservoir. The shoreline is mostly formed by the process of beaching whereby

unconsolidated surficial materials, including till, colluvium, glaciofluvial, and fluvial

sediments are eroded. The latter two types of sediments are found on relatively low gradient

fans near the lower end of steep gradient creeks. Beaches located at the margins of such fans

3 All property information was supplied by BC Hydro (BC Hydro 1993c). Summit did not verify the accuracy or completeness of the information in Appendix A.


range in sediment texture from sandy to bouldery. Bedrock forms the shoreline at many

locations and accounts for about 40% of the total shoreline of the reservoir (Richmond 2004).

Unfortunately there are no climate stations at Sugar Lake that can provide wind speed and

direction data useful to estimate wave heights. Due to the strong topographic effects, the

available data from other, rather distant, climate stations such as Vernon or Revelstoke are

not likely to be representative of conditions at Sugar Lake. Therefore, rather than modelling

wave heights during a design storm based on non-representative data, we have assumed a

maximum wave height of up to 0.75 m may occur when the reservoir is at NMRL. This

assumption is consistent with Richmond (2004) and with our field observations and level

surveys. Furthermore, we have assumed the largest summer storms typically have winds

from the east or southeast direction, which are based on anecdotal accounts reported by

Richmond (2004).

3.3 RESERVOIR OPERATIONS

The Sugar Lake Reservoir is used to provide storage for hydroelectric generation 29 km

downstream at Wilsey Dam on Shuswap River. In order to maintain the reservoir within the

normal operating range (594.70 m to 601.72 m), stoplogs and low level gates are utilized at

the Sugar Lake Dam. Normally, the reservoir level is lowest in mid-April (Figure 3.1) prior

to spring freshet. With the onset of spring freshet, the reservoir begins filling, and usually by

the end of May the reservoir is rising rapidly. During freshet, no stoplogs are in place and

the four gates are left open. The volume of water passing the dam during freshet is

significant since on average inflows to the reservoir are 8-12 times the capacity of the

reservoir (Shuswap River Water Use Plan Consultative Committee 2002). Following the

peak of freshet (typically in early June), six (6) stoplogs are installed (in accordance to dam

safety requirements) bringing the top of the dam to 601.72 m (Shuswap River Water Use

Plan Consultative Committee 2002), which results in more water being stored in the

reservoir. Once full pool is reached it is maintained until late fall. Water is then released

from the reservoir over the fall and winter for power generation and to maintain fisheries

(conservation) flows.


Figure 3.1 presents the historical range in average daily Sugar Lake water levels as recorded

by the Water Survey of Canada (station 08LC041) between 1971 and 2006. It shows that

maximum reservoir levels typically occur in early August, however, water levels at or above

the NMRL of 601.72 m have occurred as early as June 2 and as late as December 14.

Occasionally, heavy precipitation or snowmelt, possibly accompanied by strong winds and

wave action, can cause a flood surcharge and raise water levels above the NMRL. The

highest recorded daily water level of 602.7 m occurred on July 13, 1997 (Figure 3.1).


4.0 PREVIOUS RESERVOIR SHORELINE EROSION STUDIES

Two of the documents reviewed in this study make specific reference to shoreline erosion at

the Sugar Lake Reservoir. This includes BC Hydro (1993a) and Richmond (2004). Both

studies are summarized below.

BC Hydro (1993a)

This was an overview study that examined the degree and significance of physical impacts

upon shorelines of several BC Hydro reservoirs. Sugar Lake was designated a Class A

reservoir, where adverse reservoir impacts upon existing shoreline developments or

properties with development potential were identified and where further studies were

justified. The report noted ongoing “beaching” and erosion were occurring and that a

number of properties on low shorelines could be adversely impacted by high reservoir levels

and wind generated waves.

Richmond (2004)

This is a draft report of an assessment conducted to define “impact-line”4 setbacks for private

land development. The report concludes that much of the shoreline that is either bedrock or

wetlands will not experience significant erosion from reservoir operations. Low gradient

beaches along the margins of fans have largely stabilized and have a moderate potential for

erosion. Areas with a high potential for ongoing erosion are largely limited to the relatively

steep east facing shorelines formed in silty surficial materials.

4 Impact-lines are boundaries based on technical criteria which demarcate areas influenced by reservoir-related processes (Richmond 2004).


5.0 SHORELINE EROSION POTENTIAL

5.1 SUMMARY OF SHORELINE OBSERVATIONS

A summary of existing shoreline conditions, organized by shoreline segment, is provided in

Table 5.1 and is shown on Map 1. Table 5.1 identifies the following:

• the location and length of each shoreline segment identified,

• the legal description of properties bordering the shoreline segment,

• the erosion potential under a NMRL of 601.72 m and 601.52 m with an assumed

wave height of up to 0.75 m,

• brief characteristics of the shoreline segment,

• other notes;

• location(s) from which the shoreline segment was photographed; and

• recommended mitigation options for the shoreline segment or portion of the segment.

5.2 REPRESENTATIVE CROSS-SECTIONS

Four (4) cross-sections on the west side of the reservoir were surveyed to document the

common shoreline conditions present. These cross-sections are described below.

Cross-section at 5,700 m

This cross-section characterizes a considerable percentage of the shoreline surrounding the

reservoir. It consists of a relatively stable gentle to moderately sloping sand and fine gravel

beach beneath a low wave-cut bank comprised of colluvium with a considerable proportion

of fine-grained sediment (Figure 5.1). Mature vegetation is dense upslope of the edge of the

bank. A dense root mat as well as abundant large woody debris along the face of the bank

limit wave action and tend to promote bank stability. This bank segment has a low erosion

potential under current reservoir operations. It will also have a low erosion potential under

the proposed alternative NMRL.



This cross-section represents a shoreline segment with a high erosion potential under the

current reservoir operations and under the proposed alternative NMRL It consists of a

relatively steep (86%) 5 m high bank of silty sandy glaciofluvial material and colluvium that

is actively being undercut and ravelling (Figure 5.2). Below the steep bank, an apron of

sand, gravel and cobbles has formed a moderately steep (47%) slope. Given the height,

steepness, and sediment texture of this segment of shoreline, unless mitigation is carried out,

erosion will continue for the foreseeable future.


The cross-section at 8,700 m characterizes the low gradient beaches that have formed along

the margins of several alluvial fans. The cross-section has a gently sloping (8%) beach

consisting of sand and gravel and occasional cobbles (Figure 5.3). Near the NMRL a small

wave-cut step has formed, above which grass and scattered trees grow. At such locations,

the shoreline has generally stabilized and the potential for erosion under current and

proposed alternative reservoir operations is low.


The cross-section at 10,400 m is similar in material type to that at 8,450 m, but its bank and

beach slopes are smaller and woody debris stranded along the toe of the bank provides some

protection from wave action (Figure 5.4). The reduced slope of the bank has an effect of

moderating the erosion potential of this segment. Overall, the erosion potential of this

segment is moderate under both current reservoir operations and under the alternative

NMRL.

5.3 SUMMARY

Depending on the shoreline materials and the profile, the erosion potential at any given

shoreline segment will be either reduced or remain the same with a reduction in the NMRL

from 601.72 m to 601.52 m. At locations where the water level assuming an NMRL of

601.52 m drops below a significant break-in slope or encounters a less erosive material (than


it currently does), the erosion potential will decrease (usually by one class). If not, the

erosion potential under the proposed alternative NMRL will remain unchanged.

To provide a semi-quantitative measure of the change in the overall erosion potential that

would occur by adopting the alternative NMRL of 601.52, the total length of shoreline for

each erosion potential was summed for the current and alternative NMRL. The values shown

below represent the length of shoreline under each erosion potential classification.

Length of shoreline (m) Normal Maximum Reservoir Level Low erosion

potential Moderate

erosion potential

High erosion potential

Total

601.72 m (current) 25,543 7,190 1,910 34,643 601.52 m (alternative) 31,123 2,750 770 34,643 Difference +5,580 -4,440 -1,140

Notes: 1) Total shoreline length is 34,600 m

The results above indicate that with the 0.20 m reduction in the normal maximum reservoir

level, it is estimated that there will be 1,140 m decrease in the length of shoreline with a high

potential for erosion, and a 4,440 m decrease in the length of shoreline with a moderate

potential for erosion. Overall, this represents a 3% and 13% reduction in total shoreline

length classified as high and moderate erosion potential, respectively.

While in overall percentage terms these reductions in erosion potential seem modest, they are

significant in absolute terms, particularly considering that the most noticeable reduction in

erosion potential would occur adjacent to existing private properties on the west side of the

reservoir. Approximately 60% of the shoreline now classified as high erosion potential is

expected to become moderate erosion potential, and 62% of shoreline presently classified as

moderate erosion potential is expected to be classified as low erosion potential. With the

exception of a few locations (e.g. near 8,450 m), it is expected many private property owners

would see reduced rates of erosion should the NMRL be reduced to 601.52 m.


6.0 MITIGATION OF SHORELINE EROSION

Although a reduction in the NMRL from 601.72 m to 601.52 m is expected to significantly

reduce erosion potential along the Sugar Lake Reservoir shoreline, it is possible to mitigate

erosion also on a site-specific basis, with or without changing the NMRL. During the field

reconnaissance we identified 18 locations where mitigation is recommended to reduce the

flood damage and erosion risk. These are noted in Table 5.1 and are summarized in general

terms below. The recommended mitigation or prescriptions are at a conceptual level only.

Field work required to develop more detailed prescriptions with associated estimated costs is

beyond the present scope of work. However, such work should be conducted in future to

establish the aggregate costs associated with mitigating erosion on a site-specific basis. This

cost can then be compared with the costs associated with lowering the NMRL to 601.52 m.

Both passive and active techniques of shoreline protection were identified in Table 5.1 as

options. They include:

• Discouraging recreation site users from disturbing unstable shoreline segments, either

through fencing, signage or a combination of both;

• Selectively armouring sections of bank with riprap;

• Strategically placing large woody debris along the bank to reduce wave action; and

• Establishing and promoting the growth of shoreline vegetation with deep effective

root structures.

There are several erosion control products (e.g. turf reinforcement mats and cabled-concrete

mats) that could replace riprap and prove more suitable at some locations. These options

should be investigated during the development of detailed prescriptions. To optimize

resources, these detailed prescriptions should focus on the shoreline segments with moderate

and high erosion potential (Map 1).


7.0 CONCLUSIONS

A customized field study of shoreline erosion at the Sugar Lake Reservoir has been

completed. The study methodology was designed to address concerns identified by property

owners and the Shuswap River Water Use Plan Consultative Committee. The study

combined an office review of background information and aerial photos with the findings of

inspections of the reservoir by boat and foot during spring and summer 2007. During the

field inspections, shoreline conditions were noted, photographed and surveyed. The erosion

potential associated with current reservoir operations (NMRL = 601.72 m) and proposed

alternative reservoir operations (NMRL = 601.52 m) was assessed and locations where site-

specific erosion mitigation may be beneficial were identified.

Based on the above-noted study of the shoreline of the Sugar Lake Reservoir, the following

conclusions are drawn:

• Under the current normal maximum reservoir level (NMRL) of 601.72 m,

o 74% of the shoreline is at low risk of erosion,

o 20% is at moderate risk of erosion, and

o 6% is at high risk of erosion.

• While the above-noted percentages of shoreline with moderate or high erosion

potential are not considered extensive, they are significant, particularly considering

much of this length of shoreline fronts private property or developed areas.

• Assuming a 0.20 m reduction in the normal maximum reservoir level (to 601.52 m), it

is anticipated that:

o the length of shoreline with a high erosion potential will decrease by 1,140 m.

This represents 3% of the total shoreline length, and

o the length of shoreline with a moderate erosion potential will decrease by

4,440 m. This represents 13% of the total shoreline length.


• While in overall percentage terms the reductions in erosion potential appear modest,

they are significant in absolute terms, particularly considering that the most

noticeable reduction in erosion potential would occur adjacent to existing private

properties.

• With a change in NMRL from 601.72 m to 601.52 m, approximately 60% of the

shoreline now classified as high erosion potential is expected to become moderate

erosion potential, and 62% of shoreline now classified as moderate erosion potential

is expected to be classified as low erosion potential.

• With the exception of a few locations (e.g. near 8,450 m), it is expected many private

property owners would see noticeably reduced rates of erosion should the NMRL be

reduced to 601.52 m.


8.0 REFERENCES

Arc Environmental Ltd. 2001. Shuswap River Fish/Aquatic Information Review. Prepared for BC Hydro, February 2, 2001.

BC Hydro. 1993a. Reservoir Shoreline Investigations, Reservoir Shoreline Impact Overview

Study. Prepared by BC Hydro Hydroelectric Engineering Division. Report No. H2644. March 1993.

BC Hydro. 1993b. Geotechnical Guidelines for Determining Slope Stability and

Groundwater Impacts for Land Use Purposes. Prepared by BC Hydro Hydroelectric Engineering Division. Report No. H2293. March 1993.

BC Hydro. 1993c. Sugar Lake Reservoir Property Inventory, Property Ownership. Supplied

by K. Bray, Revelstoke Environmental & Social Issues, November 15, 2006. BC Hydro. 2005. Shuswap River Water Use Plan (Shuswap Falls & Sugar lake Project),

Revised for Acceptance by the Comptroller of Water Rights. August 18, 2005. Ministry of Environment. 2007. Habitat Wizard. Online database of bathymetric maps.

http://www.env.gov.bc.ca/habwiz/. Richmond, G. 2004. Sugar lake Shoreline Impact Lines – Draft. Prepared for BC Hydro,

September 14, 2004. Shuswap River Water Use Plan Consultative Committee. 2002. Consultative Committee

Report: Shuswap River Water Use Plan, A Product of BC Hydro. December 2002.

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