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Pages 36-57

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From page 36...
... 36 Background The development of efficient mitigation strategies requires an understanding of the impacts of stormwater management and required mitigation potential of on-site and off-site treatment measures, along with the primary watershed processes that these measures are protecting. This includes the establishment of linkages between stormwater impacts and effective mitigation options that account for key watershed processes and their effect on water quality.
From page 37...
... Framework for Evaluating Off-Site Mitigation Options 37 beyond the existing project boundary and ROW and within the watershed. The USEPA supports pollutant trading for sediment and nutrients, but does not yet support trading for metals or other toxics (USEPA 2009)
From page 38...
... 38 A Watershed Approach to Mitigating Stormwater Impacts versus being directly measured using in-stream instrumentation. This is presented in the following equation: Ω = ρ (6)
From page 39...
... Framework for Evaluating Off-Site Mitigation Options 39 • Evapotranspiration – The process of water movement from the ground through a plant and its evaporation from aerial parts, such as from leaves, stems, and flowers. • Uptake and retention of pollutants by vegetation – The action of trees, shrubs, and plant roots to accumulate and retain pollutants via their natural metabolic processes.
From page 40...
... 40 A Watershed Approach to Mitigating Stormwater Impacts The relationship between the impact of upland and in-stream/near stream physical processes identified in Table 13 and each of the four water quality stressors of concern (increased TSS, eutrophication, increased volume, and increased stream power) are identified and discussed here.
From page 41...
... Framework for Evaluating Off-Site Mitigation Options 41 energy but may also increase base flow during drier periods which may increase stream power/ energy. Increased base flow is likely to represent more natural, undeveloped conditions.
From page 42...
... 42 A Watershed Approach to Mitigating Stormwater Impacts • Eutrophication: Indirect Negative Effect (-1) – Overland flow can result in stormwater runoff coming into contact with nutrient rich soils that could lead to a higher eutrophication potential.
From page 43...
... Framework for Evaluating Off-Site Mitigation Options 43 • Volume: Negligible Effect (0) – Sedimentation within receiving waters does not significantly reduce in-stream volumes.
From page 44...
... 44 A Watershed Approach to Mitigating Stormwater Impacts appropriate mix of on-site and off-site practices. On-site options are located on a project site or within the ROW adjacent to the project and directly associated with the project area.
From page 45...
... Framework for Evaluating Off-Site Mitigation Options 45 Table 14 summarizes in-kind BMPs commonly used by state DOTs. Configuration and sizing will vary by state.
From page 46...
... 46 A Watershed Approach to Mitigating Stormwater Impacts Many of the out-of-kind mitigation approaches listed above may already be required by a DOT's NPDES permit (e.g., reducing illicit discharges, public education, etc.) and therefore may not qualify for mitigation credits.
From page 47...
... Table 15. Out-of-kind BMP mitigation tools and associated pollutant reductions.
From page 48...
... EDUCATION Nutrient management Reducing the excess amount of fertilizer applied in public places and agricultural areas can reduce nitrogen loading to water bodies. Chesapeake Stormwater Network 2014 Branosky et al.
From page 49...
... Framework for Evaluating Off-Site Mitigation Options 49 (and in-kind) mitigation measures for these stormwater impacts as shown in Figure 4 would be selected based on site characteristics.
From page 50...
... 50 A Watershed Approach to Mitigating Stormwater Impacts Equivalency Using Ecosystem Services Ecosystem services can be evaluated through a combination of quantitative and qualitative approaches to demonstrate how the impacts to watershed functions at one location in a watershed can be offset by providing improvements to the watershed functions in another part of the watershed. One approach is to characterize how society uses and values the services provided by the watershed and then identify improvements that can enhance those services.
From page 51...
... Framework for Evaluating Off-Site Mitigation Options 51 the mechanisms through which the mitigation options contribute to the improvement of environmental conditions. Once these physical processes for stormwater impacts, mitigation options, and ecosystem services have been identified and agreed upon with the regulatory agencies and other stakeholders, they can be used to assign weighted links between stormwater impacts, mitigation options, and ecosystem services (see The SEQ Ecosystem Services Framework 2014)
From page 52...
... 52 A Watershed Approach to Mitigating Stormwater Impacts Identifying Candidate Off-Site Mitigation Options A project planner/designer has the flexibility to select a mix of on-site and/or off-site practices that best or most cost-efficiently achieve the stormwater management goals. The mitigation opportunities identified in Figure 4 designate off-site mitigation options as in-kind (using typical DOT stormwater management BMPs)
From page 53...
... Framework for Evaluating Off-Site Mitigation Options 53 • Step 2: Identify off-site BMP locations and BMP type – One or more candidate off-site locations and stormwater BMP types are selected based on evaluation of physical watershed characteristics, stormwater improvement needs, or existing watershed plans. • Step 3: Summarize watershed land use characteristics – Land use characteristics for the tributary drainage area are then determined for the proposed off-site BMP using available spatial datasets.
From page 54...
... 54 A Watershed Approach to Mitigating Stormwater Impacts evaluating the ecosystem services that can be linked to stormwater impacts of a DOT project, five groups of ecosystem services were identified for consideration: (1) biodiversity conservation; (2)
From page 55...
... Framework for Evaluating Off-Site Mitigation Options 55 Table 18. Linkages between mitigation options and ecosystem services based on upland physical processes.
From page 56...
... 56 A Watershed Approach to Mitigating Stormwater Impacts Table 19. Linkages between mitigation options and ecosystem services based on in-stream/near stream physical processes.
From page 57...
... Framework for Evaluating Off-Site Mitigation Options 57 information that can incorporate data from local and targeted watershed stakeholders and public agencies. Information on how to collect this data to customize and refine the scale or local interest within the tool is discussed in Chapter 5.

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