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From page 79... ... 79 C H A P T E R 7 Case Study Example: Road Widening in Pierce County, WA This case study shows how the WBSMT can be applied to identify possible mitigation opportunities based on project-specific information (e.g., project area, dominant HSG, stormwater management targets, etc.) and HUC-12 watershed characteristics (e.g., land use distribution, average annual precipitation, impairments, etc.) Read the entire page →
From page 80... ... 80 A Watershed Approach to Mitigating Stormwater Impacts approximate total post-project roadway width of 80 feet. Multiplying the roadway width by the 1.5 mile project length resulted in a project area of approximately 14.5 acres. Read the entire page →
From page 81... ... Figure 20. Web soil survey screenshot. Read the entire page →
From page 82... ... 82 A Watershed Approach to Mitigating Stormwater Impacts precipitation stations determined there are two NOAA COOP stations that closely border the project site: Buckley, WA (COOP ID: 450945) and McMillin, WA (COOP ID: 455224) Read the entire page →
From page 83... ... Watershed-Based Mitigation Toolbox Case Study 83 In-Kind, On-Site Options The Stormwater Management Manual for Western Washington requires stormwater BMPs to be sized to capture and treat 91 percent of the average annual runoff volume (Ecology 2005) Read the entire page →
From page 84... ... 84 A Watershed Approach to Mitigating Stormwater Impacts Out-of-Kind Options The final required input sheet of the WBSMT is sheet 6-Out-of-kind options, which provides a preliminary ranking of out-of-kind mitigation measures based on HUC-12 watershed metrics and user entered information. The sheet consists of two primary sections: (1) Read the entire page →
From page 85... ... Watershed-Based Mitigation Toolbox Case Study 85 Figure 27. Determining the most beneficial out-of-kind BMPs in the WBSMT. Read the entire page →
From page 86... ... 86 A Watershed Approach to Mitigating Stormwater Impacts • Bank instability • Degraded riparian habitat • Lack of a functional wildlife corridor • Limited tree cover • Lack of in-stream woody material • Low frequency of pools Another three of the nine metrics presented were identified as applicable or important to the watershed. Two of these metrics relate to anthropogenic beneficial uses in the watershed while one relates to rare and endangered species. Read the entire page →
From page 87... ... Watershed-Based Mitigation Toolbox Case Study 87 of runoff, 3,500 lbs of TSS, and 6 lbs of TP. As shown in Figure 29, the dry pond results in load reductions similar to a swale for TSS and TP and similar to bioretention for TP. Read the entire page →
From page 88... ... 88 A Watershed Approach to Mitigating Stormwater Impacts to be treated after the implementation of in-kind mitigation measures is presented. Following the summary, the estimated footprints of the four out-of-kind mitigation measures supported in the WBSMT are provided for 100 percent implementation under both self and in lieu cases. Read the entire page →
From page 89... ... Watershed-Based Mitigation Toolbox Case Study 89 The following mitigation options (see Figure 32) were identified using the WBSMT to evaluate the stormwater impacts of the 1.49 mile long SR 410 road widening project near Bonney Lake, WA: • In-Kind On-site: a pair of dry ponds with a combined area of 0.59 acres • Out-of-kind: 0.08 acres of riparian enhancement and the creation of 0.014 acres of wetland restoration/creation measures While the on-site measures were sized to treat 91 percent of the average annual runoff volume from the project area and meet the volume reduction targets, the performance of dry ponds was estimated to be insufficient to meet the TSS and TP load reduction targets. Read the entire page →
From page 90... ... 90 A Watershed Approach to Mitigating Stormwater Impacts At each location, a pre-existing state highway was chosen for a road widening project using the same project characteristics as the SR 410 road widening case study. The watershed characteristics were altered to reflect the new locations and are presented in Table 24. Read the entire page →
From page 91... ... Watershed-Based Mitigation Toolbox Case Study 91 Table 24. Watershed characteristics of case study locations. Read the entire page →
From page 92... ... 92 A Watershed Approach to Mitigating Stormwater Impacts footprint for the reducing impervious surface connectivity mitigation measure is equal to the outstanding equivalent impervious acreages. In all cases, the upland stabilization mitigation measure received the lowest ranking because zero percent of the natural land cover was noted as disturbed area under watershed characteristics. Read the entire page →
From page 93... ... Watershed-Based Mitigation Toolbox Case Study 93 can be completely achieved on-site or not, a decision must be made as to the type and size of on-site BMP to be implemented. If the on-site treatment options have been identified and there are outstanding load reductions to be achieved, potential options for in-kind, off-site BMPs should at least be considered before exploring out-of-kind of options. Read the entire page →

From page 79...

... 79 C H A P T E R 7 Case Study Example: Road Widening in Pierce County, WA This case study shows how the WBSMT can be applied to identify possible mitigation opportunities based on project-specific information (e.g., project area, dominant HSG, stormwater management targets, etc.) and HUC-12 watershed characteristics (e.g., land use distribution, average annual precipitation, impairments, etc.)