preliminary drainage analysis & wetland mitigation …

PRELIMINARY DRAINAGE ANALYSIS & WETLAND MITIGATION PLAN

POLLARD STATION ~ SENIOR HOUSING

Prepared by:

SCO PLANNING & ENGINEERING, INC. 140 Litton Drive, Suite 240

Grass Valley, CA 95945-5079 T (530) 272-5841 / F (530) 272-5880

December, 2010

A. PURPOSE

The purpose of this Preliminary Drainage Analysis and Wetland Mitigation Plan is to:

Analyze pre- and post- development drainage conditions;

Identify goals and objectives for protection of the existing wetland;

Explain qualitatively how the wetland gets its hydrologic support;

Analyze the effect that site development may/may not have on groundwater and surface runoff;

Describe water quality treatment and control of surface runoff from project site;

Outline a wetland protection plan to be implemented during construction and post-project.

B. INTRODUCTION

Project Location/Description

Pollard Station is a proposed residential senior living project to be located on an undeveloped 8.05 acre parcel within the Hilltop Master Plan area in Truckee, CA. The proposed project site is bounded by an undeveloped 24 acre parcel to the south, partially developed residential and commercial parcels to the west and north, and a condominium complex, including paved Pine Cone Road to the east. (APN 19-300-19: 10335 Old Brockway Rd.)

An existing jurisdictional wetland is located on a separate parcel immediately north of the project site. (see Exhibit 1 – Existing Wetland)

Future development of offsite parcels, adjacent to Pollard Station, is anticipated in the Hilltop Master Plan. Further analysis of drainage design and wetland mitigation will be provided for future projects when development plans are available.

Preliminary Drainage Analysis & Wetland Mitigation Plan Pollard Station ~ Senior Housing

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Project Site Topography/Soils

The subject property is ±8 acres in size, generally irregular in shape and is currently undeveloped. Several unimproved roads traverse the site and foundations and septic tank are present at the site from a former house. The site consists of dense to scattered conifer forest with slopes varying from moderate (10% to 15%) to steep (+30%). Site soils generally consist of medium dense to dense silty to clayey sand with gravel, cobbles and boulders. Refer to the geotechnical reports for additional soils information.

C. GOALS AND OBJECTIVES FOR WETLAND PROTECTION

1) Grading disturbance is prohibited within wetland area and buffer zone. (note: wetland property owner may remove previously placed fill if permitted by USACE/RWQCB)

2) Surface runoff to wetland shall not be altered from pre-project levels.

3) Groundwater flow shall not be altered from pre-project levels.

4) Low Impact Development (LID) stormwater management design approach shall be used to control and minimize changes to the local hydrologic cycle or regime.

5) Best Management Practices (BMP’s) for water quality shall be implemented during construction and post-project for removal of potential pollutants from the project site.

6) Project design shall meet setback requirements and all provisions of the Hilltop Master Plan and Design Guidelines per Town of Truckee Resolution No. 2008-03.

D. SITE ASSESSMENT

The following site assessments were conducted for this property and Hilltop master plan. These reports provide qualitative information about the project site, existing wetland and surrounding property. Please contact Bryan McAlister, SCO Planning & Engineering (530) 272-5841 to obtain copies of these reports.

1. Wetland Delineation and Biological Resources Study for Hilltop Master Plan prepared for the Town of Truckee by Eco Synthesis in October 2003;

2. Geotechnical Engineering Report for Hilltop Master Plan prepared for the Town of Truckee by Holdrege & Kull in January 2004;

3. Geotechnical Engineering Report for Hilltop Senior Living Cottages Project prepared for FNC Communities by Holdrege & Kull in December 2009; and

4. Geotechnical Engineering Report for Hilltop Senior Living Lodge Project prepared for FNC Communities by Holdrege & Kull in December 2009.

5. Geotechnical Supplemental Letter, Potential Wetland Impacts for Planned Project Grading for Hilltop Senior Living by Holdrege & Kull dated July 9, 2010.


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Wetland Delineation and Biological Resources Study

This environmental study determined the boundaries of the wetland meadow area, which is located offsite and north of Pollard Station project site (see Exhibit 1 – Existing Wetland). Wetland delineation was determined in conformance with USACE criteria for:

Prevalence of hydrophytic vegetation; Presence of wetland hydrology; and Occurrence of hydric soils.

Page 12 of this study, states:

The meadow (wetland) appears to be supported largely by groundwater, which daylights near the toe of the slope. Therefore, it is hydrologically largely “autonomous.” However, a portion of the wetland is probably also supported in part by runoff from the nearly flat topography on the north side of the wetland; provision of at least a narrow upland buffer zone would help mitigate the potential loss of watershed.

Mitigation measures as described in the biological study and in the Town approved Hilltop Master Plan include interpretative signage; removal of previously placed fill (by owner of wetland property); avoidance of any impact from grading, surface runoff or snowmelt; setback and buffer zones; and requirement for physical barrier such as split rail fencing or plantings.

Geotechnical Engineering Reports

The Geotechnical Engineering Reports explained the results of various soil test pits dug at various locations throughout the site to determine if the depth and soil types are adequate to support the proposed development. The results reflected that where development/ buildings and parking areas are proposed, adequate soil depth and stability were determined. Test pits varied in depth from 5 feet to 18 feet. (see Test Pit Logs in referenced reports). Ground water seepage was not encountered in any of the exploratory test pits.

The effect of site development and recommended mitigation is summarized in the geotechnical supplemental letter (included herein as an Attachment), as stated:

At the time of our subsurface exploration, groundwater was not encountered within the planned excavation depths for the project. However, seasonal near-surface groundwater seepage will likely be intercepted by the planned earthwork grading…

The re-infiltration of intersected groundwater and surface water runoff into the near-surface soil is important to the protection of the wetlands. Our geotechnical engineering report recommends Low Impact Development (LID) methods for storm water management. LID is a storm water management and land development strategy that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic natural hydrologic functions.


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E. DRAINAGE ANALYSIS

Existing Drainage Condition (Pre-Development)

In the existing drainage condition, site runoff consists of sheet flow from natural slope areas and partially developed residential areas to the south. There are no defined drainage courses within the watershed boundary and runoff currently consists of sheet flow to the north through undeveloped forested slope areas.

The overall watershed area consists of two distinct drainage areas. The westerly drainage shed area (Area A-2) is sloped toward Brockway Road which has existing roadway drainage facilities that convey storm water runoff to Truckee River. The easterly drainage shed area (Area A-1) slopes toward wet meadow areas that are located directly south of the Pollard Station property. This wet meadow is a part of a series of meadows along the south side of Brockway Road from the project site to the Truckee-Tahoe Airport. The meadow areas do not directly discharge to any water bodies, however subsurface water trends in a north-east direction toward Martis Creek which is tributary to Martis Creek Reservoir and the Truckee River. A portion of the wet meadow is jurisdictional wetland as shown in Exhibit 1 – Existing Wetland.

(See Attachment: Pre-Development Hydrology Map)

Proposed Drainage Condition (Post-Development)

In the proposed drainage condition, surface drainage from impervious roadway surfaces and buildings will be collected and conveyed toward Low Impact Development (LID) water quality treatment facilities as described below. LID stormwater management approach being implemented with this project is to manage rainfall at or near the source of runoff using uniformly distributed decentralized micro-scale controls. Water quality treatment facilities infiltrate, filter, store, evaporate, and detain runoff close to its source through the use of roof line infiltration systems, buffer strips, bio-retention swales and retention basins.

Storm water runoff from the development area will ultimately be conveyed to pre-existing drainage facilities on Brockway Road and/or the existing wet meadow. Through the use of onsite retention facilities and recommended implementation of LID drainage approach outlined herein, there should be no increase or diversion of ground or surface water flow to the existing wetland meadow in the easterly drainage shed.

All drainage facilities are designed to accommodate the required 10-year and 100-year storm events with no net increase in runoff beyond pre-project conditions in accordance with Town of Truckee requirements.

(See Attachment: Post-Development Hydrology Map)


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F. WATER QUALITY TREATMENT

Storm drainage from impervious areas (roads, walks, roofs) will be collected and routed through water quality treatment facilities for removal of potential pollutants. This consists of a Multiple Treatment System which includes two or more of the following BMPs in series prior to discharge of flow to existing drainage facilities.

BMP #

TC-10 Infiltration trenches will be installed at roof downspouts for both retention of storm water runoff and for capturing pollutants prior to discharge. Runoff is stored in the void space between the stones and infiltrates into surrounding soil.

TC-50 Water Quality Inlets are used to collect runoff from pavement areas. Each Storm Drain Inlet includes a 1’ sump. The sump, located below the storm drain outlet, captures sand and sediment and includes weep holes for infiltration.

TC-30 Earthen Swales and Rock Lined Swales are utilized to collect and slowly convey runoff to downstream discharge points. They are designed to treat runoff through filtering and trapping sediment with angular rock lining and/or vegetation in the channel, filtering through a subsoil matrix and infiltration into the underlying soils.

TC-31 Vegetated Buffer Strips are used in lieu of curb/gutter or asphalt dike and accept storm water sheet flow from adjacent pavement. This consists of a 2’ wide gravel shoulder and an adjacent vegetated area that provides filtration of pollutants and provides infiltration to underlying soils.

TC-11 Retention basins provide the final stage of water quality measure by allowing settling of suspended solids and additional filtration through vegetation. This facility stores runoff until it gradually infiltrates through the amended topsoil and underlying soil and rock. The attenuation of peak flows reduces runoff volume and rate of discharge and recharges natural groundwater.

During construction, additional BMP’s including temporary erosion control facilities shall be implemented to control pollutants that have a potential to affect the quality of storm water discharges from the construction site. Implementation of BMP's for Construction Activities will be in accordance with California State Water Resources Control Board (SWRCB) requirements.


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G. WETLAND PROTECTION PLAN

Exhibit 2 shows the proposed plan for wetland protection. This includes measures to be implemented during construction and permanent protective measures to be implemented and maintained upon project completion. The following is a list of proposed protective measures:

Prior to Grading - Stake property lines on south side and west side of wetland; Place orange fence to delineate limit of work and no disturbance area.

Place BMP’s for erosion control, sediment control, tracking control, wind erosion control and waste management/materials control in accordance with the project SWPPP. Place erosion control fence along north property line.

Place Split Rail Fence along south and west side of wetland. Fence shall be placed at the property line and shall include native plantings with soil amendments at various locations (+/-20’ spacing) along the fence line.

Install Interpretive Signage at a minimum of four locations along property line. Signage shall explain the function of the wetlands and their water quality benefit.

Construct linear Bio-Retention Swale adjacent to the split rail fence at north property line. Bio-Retention Swale shall be relatively flat (gradient of 1% or less), and shall convey runoff from the onsite water quality treatment facilities.

Construct Onsite Water Quality Treatment Facilities including Water Quality Inlets, Buffer Strips, Earthen / Rock Lined Swales, Retention Basins and Roof Line Infiltration Trenches.

Prepare a BMP Maintenance Plan to be implemented by the property owner and/or homeowners association upon completion of the project. BMP Maintenance Plan shall provide a descriptive overview of the functionality and required maintenance of the LID water quality features and drainage systems. This document shall be prepared in compliance with NPDES / Lahontan requirements and Town of Truckee Stormwater Management Plan.

Future development of adjacent parcels may necessitate changes to fence alignment, however no portion of the development shall encroach into the wetland buffer zone as described in the Hilltop Master Plan.

Snow Storage Areas shall be located outside of the identified wetland protection area. Runoff from snow storage area shall be routed through water quality treatment facilities.

As a part of the future development of the parcel which includes the jurisdictional wetland, the existing fill materials previously placed in the wetland meadow shall be removed. The wetland shall be restored pursuant to the Corps of Engineers and/or Lahontan Regional Water Quality Control Board requirements or guidelines.

Exhibit 1

EXISTING WETLAND

Exhibit 2

WETLAND PROTECTION PLAN

Attachment 1

HYDROLOGY MAPS

PRE-DEVELOPMENT, and POST-DEVELOPMENT

Attachment 2

HYDROLOGY CALCULATIONS

Attachment 3

GEOTECHNICAL SUPPLEMENTAL LETTER

POTENTIAL WETLAND IMPACTS FOR PLANNED PROJECT GRADING

HOLDREGE & KULL, July 9, 2010

preliminary drainage analysis & wetland mitigation …

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