National Academies Press: OpenBook

A Watershed Approach to Mitigating Stormwater Impacts (2017)

Chapter: Chapter 7 - Watershed-Based Mitigation Toolbox Case Study

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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Chapter 7 - Watershed-Based Mitigation Toolbox Case Study." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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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 precipi- tation, impairments, etc.). The case study is based on an actual state DOT project: the widening of State Route 410 (SR 410) in Pierce County, WA, performed by the Wash- ington State Department of Transportation. The project impacted a wetland and vegetative buffers. In the following sections, the WBSMT is applied to this project to demonstrate how different mitigation opportunities could be evaluated from a watershed-based perspective. The first three sections detail how the case study information was entered into the WBSMT and discusses the results. The final section includes an evaluation of how the results change if the project was applied to different geographical regions including how altering the stormwater impact mitigation priorities affects the mitigation results. The three alternate geographic regions investigated were Homestead, FL; Manchester, NH; and Ajo, AZ. Assigning Project Parameters The case study is located approximately 2 miles east of Bonney Lake, WA along SR 410 within the Fennel Creek-Puyallup River watershed. The route was being expanded from a two-lane roadway to a four-lane roadway with a center median from 214th Ave E to 234th Ave E; a dis- tance of 1.49 miles. Six perennial wetlands totaling 0.16 acres were permanently impacted and 3.05 acres of vegetative buffer were removed (Washington State DOT 2014a). Washington State DOT constructed 0.23 acres of riparian flood storage and 1.16 acres of riparian enhancement in a 20.2 acre out-of-kind mitigation site adjacent to Fennel Creek to mitigate the impacts. The location of the road widening and mitigation site is shown in Figure 19. Two stormwater dry ponds were constructed, as part of the project, adjacent to the highway for treatment of project area runoff. Project Details The project area was estimated using aerial photos. The main roadway width (including cen- ter median) was determined to be 60 feet. Shoulder widths varied from approximately 6 feet to 20 feet with the majority of the project length tending toward narrower widths. For simplicity, a constant shoulder of 10 feet was assumed for both sides of the main roadway resulting in an Watershed-Based Mitigation Toolbox Case Study To find the WBSMT, go to the TRB website and search for NCHRP Research Report 840.

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. Historical Google Earth imagery was used to estimate the area of pervious ROW that was converted to impervious roadway. Pre-project imagery showed the roadway had an average width of 40 feet (two lanes of 12 feet each with 8 foot shoulders) over the 1.5 mile length of the project, equating to a pre-project highway area of approximately 7.22 acres. Within the WBSMT, approximately 7.06 acres of the total change in roadway area (7.22 acres) was identified as ‘pas- ture replaced by project’ in the Project Description tab. The remaining 0.16 acres was identi- fied as ‘natural land cover replaced by project’ and represents the impacted perennial wetlands. The acreage of the existing highway, also approximately 7.22 acres, was categorized as ‘highway replaced by project’ to make up the total project area of 14.45 acres. The USDA NRCS Web Soil Survey was used to determine the area-weighted hydrologic soil group (HSG) for soils present within the project area. A screenshot of the project soil types is presented in Figure 20. The area-weighted average HSG was determined by assigning numeric values to each HSG: HSG A was assigned a 1, HSG B was assigned a 2, HSG C was assigned a 3, and HSG D was assigned a 4. The respective acreage of each soil type was then multiplied by the HSG. One soil type had a reported HSG of C/D. In this case, the acreage was split between C and D. The average HSG was then determined by summing all of the products and dividing by the total acreage, the result of which was 2.4 or a B HSG. The AADT was 21,000 based on the 2013 Washington State DOT Annual Traffic Report (Washington State DOT 2014b). All of the relevant project areas and information were entered into the Project Description tab of the WBSMT as presented in Figure 21. A variety of stormwater management goals were defined in an overview of the Fennel Creek mitigation site presented to the Bonney Lake City Council in November 2010: (1) reduce TSS and nutrients, (2) regulate and reduce localized flooding, (3) reduce stormwater runoff and velocity, and (4) bank stabilization through revegetation (Washington State DOT 2010). These goals were entered into the WBSMT as presented in Figure 22. Watershed Characteristics In the Watershed Characteristics tab of the WBSMT, the user should not assume the closest rain gauge to the project site is the best option. A search of NOAA cooperative (NOAA COOP) Figure 19. Location of the road widening along SR 410 in Pierce County, Washington. Source: Google Earth Image Location of out-of-kind mitigation site to address impact of the road widening to wetlands and buffers Location of road widening project

Figure 20. Web soil survey screenshot. Note: Includes a breakdown of the project’s soil types and their respective HSG ratings Source: NRCS 2015 Figure 21. Input of case study description into the WBSMT. Figure 22. SR 410 road widening stormwater management goals and calculated mitigation priorities in the WBSMT.

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). The average annual precipitation at each site is 48 inches and 42 inches, respectively. The closest match in the list of the dropdown menu in the WBSMT, shown in Figure 23, is the Olympia airport rain gauge, with an average annual precipitation of 46 inches. This value (46 inches) falls between the two NOAA COOP station values and was used in the analysis. Volume and Load Reduction Targets For the purposes of the case study, the volume and TSS load reduction targets were set at the default values and the phosphorus and nitrogen reduction targets were set to 50 percent and 0 percent, respectively. These values were selected to address the stormwater runoff, sediment, and nutrient enrichment concerns for the watershed and were input into the tool as shown here and in Figure 24: • Volume Reduction = 20 percent • TSS Load Reduction = 80 percent • Total Phosphorus (TP) Load Reduction = 50 percent • Total Nitrogen (TN) Load Reduction = 0 percent Figure 23. WBSMT watershed characterization dropdown menus. Figure 24. Setting load reductions and estimating average annual BMP performance in the WBSMT.

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). Assuming the detention basin was sized according to this standard, a runoff capture volume of 91 percent was entered as the design basis in the toolbox. The volume loss was assumed to be 20 percent of the average annual runoff volume. This value as well as the percent capture vol- ume could have been estimated by using an average infiltration rate estimate, design assump- tions, and a continuous simulation model such as the Western Washington Hydrology Model (Ecology 2015) or applicable spreadsheet model such as the NCHRP Report 792 BMP Evalu- ation Tools. The input for the stormwater design basis is located on sheet 4-In-Kind On-Site Options of the WBSMT presented in Figure 25. The area of the on-site constructed stormwater ponds was estimated in Google Earth and the BMP footprint to effective drainage area ratio was adjusted on sheet 4 until the calculated BMP footprint area (last input shown in Figure 25) matched the estimated area of 0.592 acres. In-Kind, Off-Site Options No in-kind off-site mitigation options were specified for the SR 410 road widening project. This was reflected in the WBSMT by recording a drainage area of zero into the first box of sheet 5-In-Kind Off-Site Options as shown in Figure 26. If an option was specified, the sum of the areas of the five different land types presented (highway, urban area, natural land cover, cropland, and pasture) would need to match the total drainage area. The off-site load reduction adjustment factor alters the effectiveness of the proposed off-site BMP by dividing the calculated load reduction by the number specified as a “conservative and reasonable assurance” for treating an area other than the project impact area. This factor can be used as a mitigation ratio (along with the in lieu multiplier) to account for uncertainty in performance and potential differences in loading when mitigating in an off-site location. Any factor above 1 reduces the calculated effectiveness of off-site mitigation measures. Figure 25. Example of entered in-kind on-site mitigation options in the WBSMT.

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) the weighting of watershed metrics (Steps 2 and 3 in Figure 27) and (2) the ranking of out-of-kind mitigation measures based on the weightings of priority and opportunity scores (Step 4 in Figure 28). The user may alter the priority and opportunity scores to better reflect what opportunities are known and feasible in the project area. The final score (or ranking) in Step 5 of Figure 28 will reflect these alterations. While weighting factors may be based on the desirability of improving an ecosystem service, the ultimate selection of an out-of-kind mitigation measure is expected to be driven by DOT requirements, regulatory agency acceptance, and watershed needs and opportunities. Out of the nine watershed metrics presented, three were given a ranking of 2 to indicate that the beneficial use was applicable or deemed impaired with respect to aquatic and terrestrial wildlife. The rankings were based on the Fennel Creek mitigation site overview (Washington State DOT 2010) which listed the following problems within the Fennel Creek watershed: • Stormwater impacts – Flooding – Water quality – Temperature Figure 26. Inputting in-kind off-site mitigation parameters in the WBSMT.

Watershed-Based Mitigation Toolbox Case Study 85 Figure 27. Determining the most beneficial out-of-kind BMPs in the WBSMT. Figure 28. Ranking of out-of-kind BMP options. Note: The final score, or rank, presented in step 5 is the normalized product of the user defined priority and opportunity scores.

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. The City of Bonney Lake is creating a multi-use trail system within the vicinity of Fennel Creek, a portion of which runs along the out-of-kind mitigation site, warranting the ranking of 1 for both the recreation and aesthetics beneficial uses. Although there are no rare and endangered species in the reach of Fennel Creek where the mitigation site is located, there may be a federal species of concern: the West Slope cutthroat trout (City of Bonney Lake 2007). The remaining three metrics (drinking water supply, fish consumption, and industrial uses) were ranked with a 0 indicating the beneficial use was not applicable or deemed a priority for this watershed. The reach of Fennel Creek which will be impacted by the mitigation site is not a supply for municipal or industrial uses and does not support an active fishery. In terms of ecosystem services, the default priority scores were accepted. These could have been modified if a more detailed assessment of ecosystem services values were developed. The default scores reflect the estimated dependence that ecosystem services have on the distribution of land uses within the watershed and the beneficial uses of the receiving water. The next step in evaluating out-of-kind options is to assess mitigation measure priorities. The default priorities are set based on the relationship between mitigation measures and water- shed processes that have been weighted by ecosystem services priorities, outstanding loads to be mitigated, and stormwater management goals (see sheet 10-Advanced out-of-kind calcs for details). The default opportunity scores are based on an assessment of the land use areas that are most amenable to the various mitigation measures. These land use areas are used as surrogate measures of potential out-of-kind mitigation opportunities. Opportunity scores consider the surrogate land use areas within the HUC-12 watershed relative to other HUC-12 watersheds contained within its parent HUC-4 watershed (see Chapter 6 for details). The default priority and opportunity scores (shown in Figure 28) resulted in wetland restoration/ creation and stream improvement receiving the highest final scores. Analysis of Mitigation Options The Analysis tab of the toolbox provides the opportunity to review results and change levels of implementation between on-site, off-site, and out-of-kind mitigation. The load reduction targets are summarized first followed by a summary of the estimated loads removed by on-site, in-kind BMPs. The load reduction targets and results for the use of an on-site, in-kind dry pond treating the effective area are presented in Figure 29. If the site had space constraints, the level of implementation could be reduced until the BMP footprint could fit into the available space. Any unachieved targets could then be addressed by off-site, in-kind BMPs or out-of-kind mitigation measures. In this example, the on-site area available was adequate to meet the entire dry detention pond footprint. Therefore, 100 percent was entered into the Effective % of Area Draining to On-site, In-Kind BMP field. The annual runoff volume from the road widening is just over 1,600,000 cubic feet per year, transporting an annual average of approximately 4,400 lbs TSS, 12 lbs TP, and 103 lbs TN. Target load reductions of 20 percent, 80 percent, 50 percent, and 0 percent for volume, TSS, TP, and TN, respectively, amount to overall load reduction targets of approximately 320,600 cubic feet

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. The low TP load reductions are due to the default effluent quality of the dry pond (based on the International Stormwater BMP Database) being greater than the estimated influent quality of the runoff; as a result, the load reduction is only realized because of the 20 percent reduction in runoff volume. This is also the situation for the swale and bioretention BMPs. While all of the on-site, in-kind BMP options provide equal volume reductions (based on entered volume reduction goals), the calculated pollutant load reductions are based on a combination of volume reduction and change in effluent concentration by each BMP. Default effluent concentrations are provided and can be modified in sheet 9-Advanced. With no off-site, in-kind mitigation measures selected, the remaining load reductions to be met must be addressed with out-of-kind mitigation measures. The WBSMT allows the user to choose the implementation rate of the four measures, as shown in Figure 30. The user can choose between self-implementation and in lieu implementation. In lieu implementation applies only to areas with an established program and may require an additional multiplier, depending on the local jurisdictional requirements. The program should define the in lieu multiplier, but for the purposes of the case study the factor was set at the default of 1.1 (e.g., a 10 percent multi- plier). In Figure 30, a summary of outstanding load reductions and equivalent impervious area Figure 29. Results of WBSMT on-site in-kind load reductions evaluation. Note: One hundred percent of the stormwater runoff volume is routed to dry ponds.

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. The approximate footprints required to address the equivalent impervious surface area yet to be treated to meet the targets are primarily based on the unit impervious area loading rate for the watershed, the performance of in-kind mitigation strategies selected, and the user footprint ratios entered in sheet 6-Out-of-kind options. The actual footprints for individual mitigation measures are finally computed based on user specification of the degree of implementation (e.g., reasonable amount of stream reach or area in which mitigation measures may be applied) of each measure until the progress toward meeting outstanding equivalent impervious area to be treated is 100 percent. In this example, 75 percent is allocated toward stream improvement techniques and 25 percent toward wetland restoration/creation. Report Summary The top of the Report tab of the WBSMT provides a summary of the information provided in sheets 1-3 on the project, watershed, and load reduction targets as shown in Figure 31. According to the 2013 Fennel Creek 2 mitigation site monitoring report (Washington State DOT 2014a), the mitigation area consisted of 0.23 acres of flood storage creation and 1.16 acres of riparian enhancement; flood storage creation was implemented as wetland creation and the riparian enhancement as a stream improvement technique. The percent implementation was determined as the ratio of areas for each distinct mitigation type. Figure 30. Out-of-kind mitigation approaches implementation section.

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. Stream improve- ments and wetland restoration or creation were identified as the most applicable out-of-kind mitigation measures for this watershed to achieve the outstanding load reductions and meet project and watershed priorities. Outstanding load reductions are those required to offset storm- water impacts beyond those addressed with the on-site BMPs. While the out-of-kind measures were deemed sufficient to completely offset stormwater impacts caused by the project, they were not necessarily sufficient to mitigate for the loss of wetlands and riparian buffers that were also a part of the initial project. In the case study, the project had impacted 0.16 acres of wetland and 3.05 acres of buffer. While 0.01 acres of wetland and 0.08 acres of riparian enhancement were estimated by the WBSMT to be adequate to mitigate for the outstanding stormwater impacts from the project, regulatory agencies deemed 0.23 acres of wetland and 1.16 acres of riparian enhancement were necessary to mitigate for the loss of habitat associated with the change in land use. Effects of Altering Project Locations Three locations were chosen to illustrate the effect project location has on the calculated mitigation measures in the WBSMT. Each location is representative of a distinct climatological region in the United States: • Homestead, FL • Manchester, NH • Ajo, AZ Figure 31. Summary information provided in sheet 8-Report. Note: Summary information includes information on the project location, watershed characteristics, and load reduction targets.

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 characteris- tics were altered to reflect the new locations and are presented in Table 24. Note the differences in rainfall, soils, and land use distribution. Load Reduction Results Load reduction results for each study location are presented in Table 25 for a 90 percent impervious drainage area of 14.45 acres. Included is an estimation of the equivalent impervi- ous area to be addressed through out-of-kind mitigation to reach the load reduction targets. Locations with higher average annual precipitation, impervious/pervious area ratios, and poorly- drained soils experience greater loading; the Washington and New Hampshire locations have very similar loadings for these reasons. The equivalent impervious areas to be treated by out-of-kind mitigation are all relatively similar because the value is based on the equivalent load produced from the average impervious area of the watershed. Ajo, AZ has the largest equivalent outstanding impervious area footprint due to the higher percentage of natural land cover in the watershed producing a lower average load per impervious area than the other watersheds. Out-of-Kind Mitigation Rankings The out-of-kind mitigation measures supported in the WBSMT are shown in Table 26 with the calculated acreages necessary to achieve the same watershed benefits as treating the load- ings from the outstanding equivalent impervious acreages presented in Table 25. The estimated Figure 32. Summary of mitigation measures and load reductions.

Watershed-Based Mitigation Toolbox Case Study 91 Table 24. Watershed characteristics of case study locations. Watershed Characteristics & Variables Units Bonney Lake, WA Homestead, FL Manchester, NH Ajo, AZ HUC 12 NA 171100140501 030902061605 10700060802 150702020208 Watershed area sq.mi 27 51 42 23 Average annual precipitation (HUC) in. 43 54 43 8 Average annual precipitation (Rain gauge) in. 46 60 39 8 Watershed area average HSG NA B C B A Percent urban area % 50.9 52.4 42.4 13.1 Percent natural land cover % 37.0 19.8 53.8 86.9 Percent cropland % 1.6 27.5 0.3 0.0 Percent pasture % 10.4 0.0 2.1 0.0 Percent potentially restorable wetlands % 3.0 26.0 0.0 0.0 Percent natural land cover in buffer % 39.5 38.8 56.4 89.3 Percent of natural land cover - disturbed % 0.0 0.0 0.0 0.0 Table 25. Comparison of load reduction results between case study locations. Bonney Lake, WA Homestead, FL Volume (ft3) TSS (lbs) TP (lbs) TN (lbs) Volume (ft3) TSS (lbs) TP (lbs) TN (lbs) Project Pre-BMP loads 1,603,011 4,403 12.0 103.1 2,290,366 6,291 17.2 147.3 Target load reductions 320,602 3,523 6.0 0.0 458,073 5,033 8.6 0.0 On-site load reductions 320,602 2,351 2.4 20.6 458,073 3034 3.4 29.5 Off-site load reductions 0 0 0.0 0.0 0 0 0.0 0.0 Outstanding load reductions 0 1,171 3.6 0.0 0 1,999 3.4 0.0 Outstanding equivalent impervious acreage for out-of- kind mitigation 1.1 1.3 Manchester, NH Ajo, AZ Volume (ft3) TSS (lbs) TP (lbs) TN (lbs) Volume (ft3) TSS (lbs) TP (lbs) TN (lbs) Project Pre-BMP loads 1,590,536 4,369 11.9 102.3 314,422 864 2.4 20.2 Target load reductions 318,107 3,495 4.8 0.0 62,884 691 0.94 0.0 On-site load reductions 318,107 2,107 2.4 20.5 62,884 417 0.47 4.0 Off-site load reductions 0 0 0.0 0.0 0 0 0.0 0.0 Outstanding load reductions 0 1,388 2.4 0.0 0 274 0.47 0.0 Outstanding equivalent impervious acreage for out-of- kind mitigation 1.3 1.4 Note: Same targets and design bases were used.

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. Reducing impervious surface connectivity received the highest final score for New Hampshire and Arizona because the watersheds in those locations had no potentially restorable wetlands. In Washington and Florida, the wetland restoration/creation measure received higher scores than the stream improvements measure because wetlands have a greater capacity to treat pollutants while providing substantial eco- system benefits. Discussion While the case studies presented above illustrate hypothetical, but realistic applications of the WBSMT, there are many additional steps and issues that a DOT will need to consider before pursuing out-of-kind mitigation. The first and foremost is whether on-site treatment could be sized and designed to achieve 100 percent of the stormwater management targets. The on-site detention basin could have potentially been enlarged to treat a larger fraction of the annual runoff volume or a different type of BMP could have been selected altogether, such as a wet pond, which could achieve higher removals of TSS and phosphorus. Typically, some mini- mum level of on-site treatment would be required. Therefore, regardless of whether the targets Table 26. Ranking of out-of-kind mitigation measures and estimated footprints per mitigation measure using same targets and design bases. Bonney Lake, WA Mitigation Measure Implementation Final Score Rank Estimated Footprint (ac) Stream improvement techniques 0.37 H 0.11 Upland stabilization 0.00 L 0.19 Reducing impervious surface connectivity 0.23 M 1.10 Wetland restoration/creation 0.40 H 0.06 Homestead, FL Mitigation Measure Implementation Rank Estimated Footprint (ac) Stream improvement techniques 0.43 H 0.13 Upland stabilization 0.00 L 0.32 Reducing impervious surface connectivity 0.26 M 1.33 Wetland restoration/creation 0.31 H 0.07 Manchester, NH Mitigation Measure Implementation Final Score Final Score Rank Estimated Footprint (ac) Stream improvement techniques 0.45 H 0.13 Upland stabilization 0.00 L 0.22 Reducing impervious surface connectivity 0.55 H 1.33 Wetland restoration/creation 0.00 L 0.07 Ajo, AZ Mitigation Measure Implementation Final Score Rank Estimated Footprint (ac) Stream improvement techniques 0.45 H 0.14 Upland stabilization 0.00 L 0.04 Reducing impervious surface connectivity 0.55 H 1.37 Wetland restoration/creation 0.00 L 0.07

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 reduc- tions to be achieved, potential options for in-kind, off-site BMPs should at least be considered before exploring out-of-kind of options. In-kind mitigation options are much easier to dem- onstrate equivalency, but obtaining regulatory approvals may still be a significant challenge. If a water quality crediting policy or program has been established for the state or watershed, or if language within the NPDES permit explicitly allows for water quality trading or watershed- based approaches, or acceptance of these approaches has already been established, then there is greater likelihood of obtaining the necessary approvals for off-site mitigation. Locating mitiga- tion areas closer to the project impacts may also increase acceptance by regulatory decision- makers. Also, coordinating mitigation efforts with local agencies and nonprofit organizations will often be more favorably perceived by both the public and regulators and thereby increasing the likelihood of success. After in-kind options have been explored and the benefits reasonably quantified, either through the use of the WBSMT or other computational approaches, out-of-kind mitigation options should be considered. The WBSMT will provide ranked out-of-kind mitigation measures based on mitigation priorities and estimated watershed needs and opportunities. The WBSMT does not rank out-of-kind options based on ease of implementation, cost, or regulatory accep- tance, so these items must also be considered in the decision process. For example, implementing mitigation measures within any waterbody considered a Waters of the United States requires many jurisdictional reviews by various local, state, and federal agencies, which may impact project schedules and costs. In general, the fewer parties involved, the simpler the mitigation measure will be to implement.

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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 840: A Watershed Approach to Mitigating Stormwater Impacts provides a practical decision-making framework that will enable state departments of transportation (DOTs) to identify and implement offsite cost-effective and environmentally beneficial water quality solutions for stormwater impacts when onsite treatment and/or mitigation is not possible within the right-of-way.

The report is accompanied by the Watershed-Based Stormwater Mitigation Toolbox, a Microsoft Excel-based program to facilitate the characterization of the project watershed and the identification of mitigation options at the planning level.

Disclaimer - This tool is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences, Engineering, and Medicine or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

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