PREDESIGN REPORT
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PREDESIGN REPORT V ALLEY V ILLAGE M ALL
S TORMWATER R ETROFIT P ROJECT W HATCOM C OUNTY , W ASHINGTON
Prepared for
Whatcom County Public Works 322 North Commercial Street, Suite 301
Bellingham, Washington 98225
Prepared by
Herrera Environmental Consultants, Inc.
2200 Sixth Avenue, Suite 1100 Seattle, Washington 98121
Telephone: 206-441-9080
September 25, 2017
C ONTENTS
Engineers’ Stamps ... iii Introduction... 1 Basin Description ... 1 Overall Goal ... 2 Site Description ... 4 Project Description and Design Analysis ... 4 Project Selection and Alternatives Consideration ... 7 Water Quality Benefits ... 8 Phosphorus Removal ... 8 Minimum Requirements – Quantifying the Facility Benefit ... 8 Cost Estimate ... 9 Construction Schedule ... 10 Commitment to Long-Term Operations and Maintenance ... 11 Project Success ... 11 Implementation Recommendations ... 12 References... 13
A TTACHMENTS
Attachment 1 Modeling Report Attachment 2 Cost Estimate
ii
pjj 15-06048-000_predesignrpt_valleyvillagemall.docx
T ABLES
Table 1. Average Annual Phosphorus Loading, Valley Village Mall Stormwater Retrofit
Project. ... 8
F IGURES
Figure 1. Project Area for the Valley Village Phosphorus Reduction Retrofit, Bellingham,
Washington. ... 3 Figure 2. Proposed Bioretention System at Valley Village Mall. ... 5 Figure 3. Typical Bioretention Cross-Section at Valley Village Mall. ... 6
E NGINEERS ’ S TAMPS
This preliminary design report has been prepared under the supervision of the professional engineers registered in Washington State whose seals appear below.
Kristen Kelly Matsumura, PE September 25, 2017
Matthew M. Fontaine, PE September 25, 2017
I NTRODUCTION
In 2008, Whatcom County Public Works (Public Works) completed the Lake Whatcom
Comprehensive Stormwater Plan (Plan), which analyzed threats to water quality and watershed health, and recommended solutions to protect Lake Whatcom. Public Works has subsequently used the Plan to guide water quality treatment project identification and prioritization. However, all capital projects identified in the Plan have either been built or are on the County’s current 6-year capital project plan that is being implemented through the Water Resources
Improvement Program (Program).
To guide future planning efforts under the Program, Public Works updated the capital project section of the Plan in 2017. The Program’s water quality treatment goals are now largely driven by phosphorus reduction targets that stem from the total maximum daily load (TMDL) studies that were performed on Lake Whatcom and the associated Water Quality Improvement Report and Implementation Strategy (Ecology 2016a). Therefore, the selection of potential new capital projects was focused on the goal of reducing phosphorus loading to the lake. Through the Plan update, 29 stormwater capital projects were identified to meet that goal. The 29 projects were prioritized based on cost effectiveness, total phosphorus reduction benefit, and implementation feasibility.
Herrera Environmental Consultants, Inc. (Herrera) has prepared this preliminary design report for one of the priority capital projects, which is at the Valley Village Mall. The project is within the community of Sudden Valley on the south side of the Lake Whatcom watershed. The project would retrofit an existing stormwater ditch with a bioretention system to provide water quality treatment and infiltration of stormwater in a publicly visible location.
B ASIN D ESCRIPTION
Lake Whatcom is a large natural lake in Whatcom County. The northwest end of the lake lies within the city of Bellingham. The lake receives discharge from approximately 22 small streams in its watershed. During parts of the year, water is also diverted from the Middle Fork Nooksack River to Lake Whatcom by a tunnel-and-pipeline system.
Lake Whatcom is an important resource in the region for both humans and aquatic life. For example, Lake Whatcom serves as the drinking water source for approximately 100,000 people in the Bellingham area. The lake is also frequently used for recreation, and the surrounding watershed has become a popular place to live. Lake Whatcom and its tributaries support a native cutthroat trout population.
In 1998, the Washington State Department of Ecology (Ecology) determined that Lake Whatcom had unnaturally low levels of dissolved oxygen and that several tributaries had excess bacteria. A subsequent TMDL study (Ecology 2007) demonstrated the low levels of dissolved oxygen were
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2 Predesign Report—Valley Village Mall Stormwater Retrofit Project
caused by excess phosphorus in stormwater runoff from development around the lake. To address that problem, Ecology developed a Water Quality Improvement Report and
Implementation Strategy for the lake (Ecology 2016a). Ecology’s report and strategy requires the City of Bellingham and Whatcom County to retrofit existing development with the goal of removing 87 percent of the phosphorus that is being generated in excess of what forested lands would generate. In addition, phosphorus generated from new development must not exceed loads that would naturally be generated if the development site were covered by forest or native vegetation. If the phosphorus loading from new development is greater than that, the difference must be mitigated by concurrent reductions of phosphorus from existing development in excess of the 87 percent target.
The Sudden Valley community was developed in the 1970s as a recreation/resort village. It has since become one of the most highly developed areas in the Lake Whatcom watershed and is now occupied primarily by permanent residents. The Sudden Valley community, with leadership from the Sudden Valley Community Association (SVCA), has made improving water quality conditions in Lake Whatcom a priority. Sudden Valley is covered by Whatcom County’s current National Pollution Discharge Elimination System (NPDES) Phase II Municipal Stormwater Permit and is subject to other regulatory protections that apply to the Lake Whatcom watershed
pursuant to Chapter 20.51 of the Whatcom County Code (WCC). As part of WCC 20.51.420(1)(h), the SVCA has a preapproved phosphorus mitigation plan, which requires all new development to meet phosphorus mitigation requirements of WCC 20.51 by using preapproved phosphorus treatment systems. The SVCA has also entered into several joint agreements, restrictive
covenants, and property exchanges with Whatcom County and the Lake Whatcom Water and Sewer District in order to reduce the density of development within the community.
The proposed project provides a unique opportunity for Whatcom County and the SVCA to partner with a common goal of reducing phosphorus and other pollutant loading to Lake Whatcom by retrofitting an existing stormwater ditch with a bioretention system in a high use area (Valley Village Mall) of the Sudden Valley community. The project would discharge to an existing Whatcom County stormwater conveyance system that drains to Austin Creek, which then flows approximately 4,800 feet (0.9 mile) to Lake Whatcom (Figure 1).
O VERALL G OAL
The overall goal of the Valley Village Mall stormwater retrofit project is to reduce phosphorus loading to Lake Whatcom. That goal is consistent with the Water Quality Improvement Report and Implementation Strategy (Ecology 2016a) for Lake Whatcom. Additional goals include public education and involvement, and reduction of flow rates to Austin Creek.
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Figure 1.
Project Area for the Valley Village Phosphorus Reduction Retrofit, Bellingham, Washington.
Project discharges to Austin Creek
Project Area
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4 Predesign Report—Valley Village Mall Stormwater Retrofit Project
S ITE D ESCRIPTION
The Valley Village Mall includes a gas station and three commercial buildings. A portion of the driveway and large planter strip drain to an open ditch located between the parking lot and Lake Whatcom Boulevard. Additionally, Lake Whatcom Boulevard is crowned and the west portion of the roadway drains to the ditch.
Lake Whatcom Boulevard experiences moderate traffic volumes, and the parking lot experiences frequent turnover. Lake Whatcom Boulevard had a weekday average daily traffic (ADT) count of 5,000 that remained constant between 2004 and 2007. The ADT was measured 0.3 mile north of the proposed project location at Sudden Valley Gate 3 (WCOG 2012).
Land use in the area draining to the open ditch is primarily commercial. The drainage area includes approximately 0.13 acre of impervious surface and 0.51 acre of pervious area. The ditch currently drains untreated stormwater through a culvert and downstream ditches to Austin Creek approximately 4,800 feet (0.9 mile) upstream of Lake Whatcom. The drainage ditch is sloping, but the grade is uneven, creating standing water at times. Oil sheens were observed on the ditch water during field visits. Some runoff from the commercial development drains to a rain garden, which was observed to be adequately infiltrating runoff during a 2016 rain event.
P ROJECT D ESCRIPTION AND D ESIGN A NALYSIS
The proposed project will retrofit an existing drainage ditch on the west side of Lake Whatcom Boulevard at the Valley Village Mall with a bioretention treatment facility using high
performance media to remove phosphorus and provide enhanced treatment of stormwater runoff from the roadway and landscaped areas (Figure 2). In the preliminary design, a
1,800-square-foot bioretention facility is sized to infiltrate 91 percent of the long-term runoff volume, using a long-term design infiltration rate of 0.25 inch per hour. The proposed
bioretention system is 80 feet long, with a 10-foot bottom width. The facility will include a 70-foot cell that infiltrates to native soil and a downstream 10-foot cell with an underdrain that filters water that doesn’t infiltrate. It contains a minimum of 18 inches of high performance bioretention soil media and uses a maximum of 6 inches of ponding. The preliminary design provides a minimum of 6 inches of freeboard. Figure 3 shows a typical cross-section for a bioretention facility that infiltrates to native soil (i.e., lacking an underdrain). The long-term design infiltration rate was set at 0.25 inch per hour based on experience with other projects in the basin and because the existing bioretention system (rain garden) nearby appears to be performing well with no overflow structure, indicating it infiltrates even large storm events. The proposed facility will also meet the low impact development (LID) flow duration standard of the 2012 Stormwater Management Manual for Western Washington, as Amended in December 2014 (2014 Ecology Manual; Ecology 2014). The modeling report is included in Appendix A.
Runoff from large storm events that do not infiltrate will overflow and discharge to the existing culvert at the northern edge of the project area. The culvert directs flow north to a downstream open ditch on the west side of Lake Whatcom Boulevard that drains to Austin Creek.
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