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104 General Approach Supported Types of Mitigation There are four out-of-kind mitigation measures supported in the WBSMT: â¢ Stream improvement techniques: restoration of stream bed and banks, improved floodplain connectivity, channel stabilization. â¢ Upland stabilization: rehabilitation of disturbed areas that contribute to sedimentation of downstream watercourses. â¢ Reducing impervious surface connectivity: placement of buffers to limit the transport of sedi- ment and pollutants through interception of flow. â¢ Wetland restoration/creation: increasing wetland functionality in treating pollutants and/or construction of a wetland to treat pollutants and improve habitat. Based on project particulars, these measures are given a priority score to help the user identify which measure would likely provide the greatest benefit based on watershed processes, eco- system services, and management goals. Mitigation The ranking of the different mitigation measures is controlled by linkages between the water- shed processes, ecosystem services, outstanding loads, and stormwater management goals and identifies the mitigation type(s) that would likely provide the greatest benefit to the watershed. The ranking process is visualized in Figure A-1 and results in mitigation measure priority scores for each mitigation type; a higher score indicates the mitigation measure is better able to provide the desired beneficial uses and stormwater management goals within the land uses found in the watershed. As shown in Figure A-1, four assessments are made by the WBSMT for the determi- nation of the priority score: â¢ Watershed processes based on ecosystem services â¢ Watershed processes based on unmitigated loads â¢ Watershed processes based on stormwater management goals â¢ Opportunities based on surrogate land uses These linkages can be found and adjusted in sheet 10-Advanced-Out of Kind Calcs, and are explained in greater detail here. Out-of-Kind Mitigation Assessment Approach A P P E N D I X A
Out-of-Kind Mitigation Assessment Approach 105 Figure A-1. Mitigation measure linkages and ranking processes. Acronym Key LU â Land Use BU â Beneficial Use ESS â Ecosystem Services Score SLU â Surrogate Land Use
106 A Watershed Approach to Mitigating Stormwater Impacts Watershed Assessment â Ecosystem Services Mitigation The breakdown of land uses (LUs) within the HUC-12 watershed is automatically determined based on the percentages provided by the EnviroAtlas datasets listed in Chapter 6, Table 20, an example of which is provided in Figure A-2. For each land use, a relationship score is given for all five supported ecosystem services: â¢ Biodiversity conservation â¢ Clean and plentiful water â¢ Food, fuels, and materials â¢ Natural hazard mitigation â¢ Recreation, culture, and aesthetics The scores are qualitative and reflect that LUâs interconnectedness to the ecosystem service. A score of 0, 1, or 2 is given based on the definitions provided in Table A-1. The default scores, shown in Figure A-3, can be updated by the user in sheet â10-Advanced-Out of Kind Calcsâ. A weighted land use ecosystem services score, or LU Weighted ESS as shown in Figure A-1, is calculated for each ecosystem service. The LU ESS for a given ecosystem service is the sum product of the percentage of the HUC-12 watershed area each land use makes up (Figure A-2) multiplied by the respective relational score of the land use (Figure A-3), as shown in Equa- tion A-1. The scores are then weighted by dividing the LU ESS for each ecosystem service by the sum of all LU ESSs (Equation A-1). The inverse of the weighted score is then calculated for each ecosystem service. These inverses are then weighted using Equation A-2 and used in determining the user ESS, discussed in the âEcosystem Services Score (ESS)â section below. The normalized inverse is used because land use areas are used to estimate the current value of an ecosystem service; if an ecosystem service is rated high then there is a lower need to enhance that service. â ( )= ÃLU ESS Percent land use Land use relational score (A-1)ecosystem service All land uses Relational Score Description 0 No significant relationship 1 Minor or indirect relationship 2 Significant or direct relationship Table A-1. Description of relational scores used in the linkages. Note: See WBSMT sheet 10-Advanced-Out of Kind Calcs. Figure A-2. Breakdown of HUC-12 LUs in WBSMT.
Out-of-Kind Mitigation Assessment Approach 107 â ( )=LUWeighted ESS LU ESS LU ESS (A-2)ecosystem service ecosystem service ecosystem serviceAll ecosystem services Beneficial Uses The next step in the prioritization of out-of-kind mitigation options is the identification of the type and health of the beneficial uses (BUs) found within the watershed. This is accomplished by filling out the beneficial use applicability table shown in Figure A-4. This table, an editable version of which is found at the top of sheet 6-Out of Kind Options, identifies the nine BUs that are supported in the WBSMT. The user must provide a score of 0, 1, or 2 for each use; scores are defined in Table A-2. After the applicability scores are adjusted, relational scores must be defined between the BUs and ecosystem services using the scoring rubric previously described in Table A-1. The default relationship table is shown in Figure A-5. Once the applicability and relational tables have been adjusted for the project, the BU Weighted ESS is determined using the same procedure as the LU Weighted ESS. First, a BU ESS is calculated for each ecosystem service by summing the product of the applicability score and the relational score for each BU (Equation A-3). BU ESS BU Applicability Score BU Relational Score (A-3)ecosystem service All beneficial uses â= Ã Figure A-3. Relationships between land use and ecosystem services. Figure A-4. Beneficial use applicability table.
108 A Watershed Approach to Mitigating Stormwater Impacts Using the values provided in Figure A-4 and Figure A-5, the BU ESS for the biodiversity conservation ecosystem service as computed using Equation A-3 would be 15, as shown below. BU ESS 2 2 0 0 1 0 0 0 1 1 0 0 2 2 1 2 2 2 15BC ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )= Ã + Ã + Ã + Ã + Ã + Ã + Ã + Ã + Ã ï£® ï£°ï£¯ ï£¹ ï£»ï£º = The BU Weighted ESS for each ecosystem service is then determined using the same pro- cedure as the LU Weighted ESS but substituting BU ESS values for the LU ESS values in Equation A-2. Ecosystem Services Score The ESS is the weighted sum of the LU Weighted ESS and BU Weighted ESS for each ecosys- tem service. The user ESS is used to calculate the first of three watershed processes assessment scores, notated as ESS Weighted Processes in Figure A-1. For each watershed process, an ESS Processes score is calculated using the user ESS scores and relational scores between the pro- cess and the various ecosystem services. A total of 15 watershed processes are included in the WBSMT; these processes are summarized in Table A-3. The default relational scores between the watershed processes and the ecosystem services are shown in Figure A-6 and follow the same convention as the land use and beneficial use relation- ship tables. Each ESS Processes score is the sum product of the user ESS scores and the watershed process relational scores for all ecosystem service types (Equation A-4). The ESS Weighted Pro- cesses score is then determined in the same manner as the LU Weighted ESS and BU Weighted Applicability Score Description 0 Beneficial use is not applicable or important 1 Beneficial use is applicable or important 2 Beneficial use is applicable and impaired Table A-2. Description of beneficial use applicability scores. Figure A-5. Relationships between BUs and ecosystem services.
Out-of-Kind Mitigation Assessment Approach 109 Watershed Processes Increases interception Increases infiltration Enhances evapotranspiration Enhances pollutant uptake and retention by upland vegetation Builds soil quality Reduces upland erosion Reduces overland flow Increases surface/subsurface storage Creates channel and floodplain storage Increases filtration Increases sedimentation Increases aeration Reduces in-stream erosion and sediment transport Increases shading Enhances pollutant uptake and retention by riparian vegetation Table A-3. List of watershed processes considered. Figure A-6. Relationships between watershed processes and ecosystem services. ESS with the exception that the scores are summed across all of the watershed processes rather than the ecosystem services, as shown in Equation A-5. ESS Processes User ESS Process Relational Score (A-4)wp All ecosystem services â= Ã â ( )=ESSWeighted Processes ESS Processes ESS Processes (A-5)wp wp wpAllWP Where wp is watershed process.
110 A Watershed Approach to Mitigating Stormwater Impacts Watershed Assessment â Unmitigated Loads The second assessment made by the WBSMT is the watershed processes assessment based on unmitigated loads. This assessment considers the outstanding loads (volume, TSS, TN, and TP) that were not mitigated with in-kind on-site and off-site options. Due to the variation of load magnitudes and the resultant bias introduced when weighting loads, the outstanding loads are converted from total loads (cu.ft or lbs) into an equivalent urban acreage as shown in Figure A-7. Similar to the ecosystem services rankings, a relational table between watershed processes and stormwater load categories is used to determine a Loads Processes score for each water- shed process. Default relationships between the watershed processes and stormwater load categories are given in Figure A-8. The Loads Process score is calculated by summing the prod- ucts of each load categoryâs outstanding equivalent urban acreage by their relational score for Figure A-7. Description of unmitigated project loads in sheet 10-Advanced-Out of Kind Calcs. Figure A-8. Relationships between watershed processes and stormwater load types.
Out-of-Kind Mitigation Assessment Approach 111 each watershed process as shown in Equation A-6. A weighted score is then calculated using Equation A-7. â= ÃLoads Process OEA Load Type Relational Score (A-6)wp wp All Load Types where OEA is outstanding equivalent acreage. Loads Weighted Process Loads Process Loads Process (A-7)wp wp wpAll wpâ ( )= Watershed Assessment â Stormwater Management Goals The third assessment is the watershed process assessment based on stormwater management goals. This assessment considers the answers to the five yes/no stormwater goal questions in sheet 1-Project Details. Depending on the answer to each question, a stormwater management goal table in sheet 10-Advanced-Out of Kind Calcs will be populated with a user value of 1 or 0 for Yes or No answers, respectively. For example, in Figure A-9, the user answered Yes to the first four questions and No to the last question. The relationship between each management goal category and the 15 watershed processes must be defined in order to compute the Goals Processes score. The default relationships are shown in Figure A-10 and can be altered/updated in sheet 10-Advanced-Out of Kind Calcs. An overall relational score, the Goals Processes score, is calculated for each watershed process by cross-multiplying each stormwater management goal user value with the goalâs relational score (Equation A-8). These scores are then weighted using Equation A-9. Goals Process User Value MGRelational Score (A-8)wp wp All MG â= Ã where MG is stormwater management goal. Figure A-9. Stormwater management goal table in WBSMT. Note: See WBSMT sheet 10 â Advanced Out-of-Kind Calcs.
112 A Watershed Approach to Mitigating Stormwater Impacts â ( )=Goals Weighted Process Goals Process Goals Process (A-9)wp wp wpAll wp Watershed Mitigation Option Priorities Once the weighted watershed assessment scores (ESS Weighted Processes, Loads Weighted Processes, and Goals Weighted Processes) have been calculated for each watershed process, the Mitigation Options Priority score is calculated. The priority score is determined using the three weighted watershed assessment scores, relational scores between the watershed processes and the four out-of-kind mitigation options, and a weighting factor. The weight- ing factor is used to increase/decrease the relative importance of the three priority factors and their associated assessment scores: (1) ecosystem services, (2) outstanding loads, and (3) stormwater management goals. By default, each priority factor is given a weight of 1. Figure A-11 provides the default relational scores between the watershed processes and the out-of- kind mitigation options in the WBSMT. Under each watershed process, an intermediate score is calculated for the four out-of-kind mitigation options using Equation A-10. The intermediate scores for all of the watershed pro- cesses are then summed under each mitigation option (Equation A-11), determining the TOTAL score shown in Figure A-12. The Mitigation Options Priority score (represented in Figure A-12 as the Normalized Mitigation Options Score) is the TOTAL score for each mitigation option divided by the sum total of the TOTAL scores for each option (Equation A-12). The Mitigation Options Priority score is one of two scores, the other being the Mitigation Options Opportunity Figure A-10. Relationship between watershed processes and stormwater management goals.
Out-of-Kind Mitigation Assessment Approach 113 Figure A-11. Relationship between watershed processes and out-of-kind mitigation options. score discussed in the next section, used to calculate the final out-of-kind mitigation options score found in Step 5 of sheet 6-Out-of-Kind Options. ( ) ( ) ( ) = Ã Ã + Ã + Ã ï£® ï£° ï£¯ï£¯ï£¯ï£¯ ï£¹ ï£» ï£ºï£ºï£ºï£º Option Score MORelational Score WF ESSWeighted Process WF Loads Weighted Process WF Goals Weighted Process (A-10)wp wp ES wp Loads wp Goals wp â=TOTAL Option Score (A-11)MO wp AllWP â ( )=Mitigation Options Priority TOTAL TOTAL (A-12)MO MO MOAll MO Where MO is mitigation option and WF is weighting factor. Opportunity Assessment Influential Watershed Characteristics The out-of-kind mitigation opportunity assessment ranks the ability to implement the differ- ent mitigation options based on analyzing the occurrence of surrogate LUs for each out-of-kind mitigation option in the HUC-12 watershed. The surrogate LUs are defined in Figure A-13 and
114 A Watershed Approach to Mitigating Stormwater Impacts Figure A-12. Out-of-kind mitigation options priority scoring. Figure A-13. Surrogate land use descriptions for the out-of-kind mitigation options.
Out-of-Kind Mitigation Assessment Approach 115 are available EnviroAtlas datasets discussed in Table 20 found in Chapter 6. The percentages are based on the watershed characteristics defined in sheet 2-Watershed Characteristics. In the figure, a natural land disturbance threshold is also shown. The threshold is used to calculate land use percentile thresholds for the disturbed natural land cover surrogate land use, which are in turn used to assign a weight for the opportunity to implement upland stabilization in the HUC-12 watershed. The percentile thresholds are discussed in further detail for all of the surrogate LUs in the next section. Mitigation Options and Influential Watershed Characteristic Thresholds The relative importance (weight) of the surrogate LUs are determined by assessing their occurrence in the project and adjacent watersheds. The greater the importance, or occurrence, of the surrogate land use, the greater the weight, or opportunity, that surrogate land use and its equivalent mitigation option is given in the WBSMT, with one exception. The weight for the stream improvement technique mitigation option, represented by the natural land cover within a buffer area extending 30 meters to each side of a stream, increases as the occurrence of the sur- rogate land use decreases, as shown in Figure A-14. This is due to the inability to implement the stream improvement techniques mitigation option if the land within the buffer zone is existing natural cover. The percentile thresholds (10 percent, 50 percent, and 90 percent) are calculated from the sur- rogate land use distributions of all of the HUC-12 watersheds that fall within the HUC-4 project watershed. The thresholds for the disturbed natural land cover (upland stabilization mitigation option) are calculated using the natural land cover land use distributions and multiplying the resultant percentiles by the disturbance threshold percentage given in Figure A-13. This calcula- tion is necessary because disturbed natural land cover is not a separate EnviroAtlas dataset that could be leveraged for the analysis. Project weights are given to each mitigation option dependent on where the projectâs HUC-12 surrogate land use distributions fall in comparison to the per- centile thresholds. These weights constitute the mitigation measureâs opportunity score, which is used in the final score calculations. In Figure A-14, if the distribution falls between the 10th and 50th percentiles, the mitigation measure is given the weight shown between those two thresholds (e.g., 0.4 for potential wetland restoration/creation). A distribution greater than the 90th per- centile threshold is given the far-right weight whereas a distribution less than the 10th percentile threshold would be given a weight of 0.8, 0.2, 0.2, or 0.2 for the stream improvement techniques, upland stabilization, reducing impervious surface connectivity, and wetland restoration/creation mitigation options, respectively. Figure A-14. Percent thresholds of occurrence and weighting of surrogate LUs.
116 A Watershed Approach to Mitigating Stormwater Impacts Watershed Opportunities Characterization The opportunity score for the different out-of-kind mitigation measures is the weight given to that measure based on the relative occurrence of the surrogate land use in the project HUC-12 watershed. A final score is calculated for each mitigation option using Equation A-13. The final score is then weighted using Equation A-14. This is the final score presented in sheet 6-Out of Kind Options. = ÃFinal Score Mitigation Options Priority Opportunity Score (A-13)MO MO MO â ( )=Final Weighted Score Final Score Final Score (A-14)MO MO MOAll MO
Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACIâNA Airports Council InternationalâNorth America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FAST Fixing Americaâs Surface Transportation Act (2015) FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TDC Transit Development Corporation TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S.DOT United States Department of Transportation
TRA N SPO RTATIO N RESEA RCH BO A RD 500 Fifth Street, N W W ashington, D C 20001 A D D RESS SERV ICE REQ U ESTED N O N -PR O FIT O R G . U .S. PO STA G E PA ID C O LU M B IA , M D PER M IT N O . 88 A W atershed A pproach to M itigating Storm w ater Im pacts N CH RP Research Report 840 TRB ISBN 978-0-309-44615-0 9 7 8 0 3 0 9 4 4 6 1 5 0 9 0 0 0 0