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From page 64... ... 64 Determining Off-Site Mitigation Options in the WBSMT The WBSMT utilizes several key datasets, user inputs, and computational approaches to link project impacts and mitigation targets in order to provide potential off-site mitigation alternatives. This chapter outlines the methods for linking project impacts to off-site mitigation options, along with a clearly defined procedure for ranking and prioritizing mitigation opportunities. Read the entire page →
From page 65... ... Linking Project Impacts to Mitigation Alternatives in the WBSMT 65 estimated footprint of in-kind on-site BMPs and, with the acreage, is used to calculate on-site loadings pre- and post-BMP. Figure 7 shows the project characterization table in the WBSMT. Read the entire page →
From page 66... ... 66 A Watershed Approach to Mitigating Stormwater Impacts project and whose average annual precipitation most closely matches that of the project location. If the HUC-12 ID is not known, a lookup link is provided to help identify and select the HUC-12 ID for the project location (see Figure 8) Read the entire page →
From page 67... ... Linking Project Impacts to Mitigation Alternatives in the WBSMT 67 NRCS Web Soil Survey. The soil data explorer application allows the user to define the area of interest and generate a soil report of the soil's estimated hydrologic properties. Read the entire page →
From page 68... ... 68 A Watershed Approach to Mitigating Stormwater Impacts is the percent of the erosional sediment mass that is predicted to be delivered to the receiving water as suspended sediment. This is also known as the sediment delivery ratio and depends on numerous factors, such as basin size, slope, soil type, particle size distribution, vegetative cover, gully and channel density, sediment storage areas, and rainfall depth and intensity. Read the entire page →
From page 69... ... Linking Project Impacts to Mitigation Alternatives in the WBSMT 69 assessments be completed to the extent possible to arrive at locally-derived delivery ratios for a watershed. The default ratio in the WBSMT is 10 percent. Read the entire page →
From page 70... ... 70 A Watershed Approach to Mitigating Stormwater Impacts is assumed to be routed to an on-site BMP (defined in sheet 1-Project Details) Read the entire page →
From page 71... ... Linking Project Impacts to Mitigation Alternatives in the WBSMT 71 • Bypassed volume: runoff which is not captured by the BMP • Reduced volume: runoff which is captured by the BMP and locally infiltrated • Treated volume: runoff which is captured by the BMP, treated, and released The bypassed, reduced, and treated volumes are calculated using Equations 10, 11, and 12, respectively. These calculations also use the percent capture and percent volume reduction design bases defined by the user. Read the entire page →
From page 72... ... 72 A Watershed Approach to Mitigating Stormwater Impacts highway land use, dominant HSG, and the breakdown of the total drainage area into individual land use acreages and average imperviousness values as shown in Figure 13. Supported BMP Types and Performance Assumptions The BMP types, performance characteristics, and design basis for in-kind off-site mitigation options are identical to those for in-kind on-site mitigation, but the influent to off-site BMPs differs. Read the entire page →
From page 73... ... Linking Project Impacts to Mitigation Alternatives in the WBSMT 73 project area as highway, the in-kind off-site mitigation approach calculates pre-BMP runoff volumes for each specified land use (i.e., highway, urban area, natural land cover, cropland, and pasture) based on Equation 7 and the provided land use acreage and percent imperviousness. Read the entire page →
From page 74... ... Figure 14. Ranking of watershed beneficial uses and ecosystem services. Read the entire page →
From page 75... ... Linking Project Impacts to Mitigation Alternatives in the WBSMT 75 priority score and user opportunity score (Equation 17) divided by the sum of all of the mitigation measure's final scores (Equation 18) Read the entire page →
From page 76... ... 76 A Watershed Approach to Mitigating Stormwater Impacts Figure 16. Project loading overview and in-kind load reduction reporting from sheet 7-Analysis. Read the entire page →
From page 77... ... Linking Project Impacts to Mitigation Alternatives in the WBSMT 77 Figure 17. Out-of-kind mitigation overview from sheet 7-Analysis. Read the entire page →
From page 78... ... 78 A Watershed Approach to Mitigating Stormwater Impacts Reporting The report sheet in the WBSMT summarizes all of the project conditions and load reduction targets, and quantifies the total load reduction (TLR) percentage attributed to each mitigation type (on-site in-kind, off-site in-kind, and out-of-kind in the form of outstanding reductions) Read the entire page →

From page 64...

... 64 Determining Off-Site Mitigation Options in the WBSMT The WBSMT utilizes several key datasets, user inputs, and computational approaches to link project impacts and mitigation targets in order to provide potential off-site mitigation alternatives. This chapter outlines the methods for linking project impacts to off-site mitigation options, along with a clearly defined procedure for ranking and prioritizing mitigation opportunities.