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NCHRP Web-Only Document 333 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects Roger Kilgore Kilgore Consulting and Management Denver, CO Alan Leak Paul Hummel Paul Duda Michael Barzaghi RESPEC Company, LLC Denver, CO Maura Flight Megan Sheahan Christopher Lewis Brent Boehlert IEc Cambridge, MA Blake Meinecke Lee Cook Quest Ecology, Inc. Wimauma, FL Ted Cleveland Lubbock, TX David Sample Virginia Beach, VA Conduct of Research Report for NCHRP Project 25-60 Submitted January 2022 © 2022 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and the graphical logo are trademarks of the National Academy of Sciences. All rights reserved. NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation results in increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research. Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 initiated an objective national highway research program using modern scientific techniquesâthe National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration (FHWA), United States Department of Transportation, under Agreement No. 693JJ31950003. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FTA, GHSA, NHTSA, or TDC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research. They are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; the FHWA; or the program sponsors. The Transportation Research Board does not develop, issue, or publish standards or specifications. The Transportation Research Board manages applied research projects which provide the scientific foundation that may be used by Transportation Research Board sponsors, industry associations, or other organizations as the basis for revised practices, procedures, or specifications. The Transportation Research Board, the National Academies, and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturersâ names appear herein solely because they are considered essential to the object of the report. The information contained in this document was taken directly from the submission of the author(s). This material has not been edited by TRB.
The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, non- governmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. The National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. John L. Anderson is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org. The Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. The mission of the Transportation Research Board is to provide leadership in transportation improvements and innovation through trusted, timely, impartial, and evidence-based information exchange, research, and advice regarding all modes of transportation. The Boardâs varied activities annually engage about 8,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. Learn more about the Transportation Research Board at www.TRB.org.
C O O P E R A T I V E R E S E A R C H P R O G R A M S CRP STAFF FOR NCHRP WEB-ONLY DOCUMENT 333 Christopher J. Hedges, Director, Cooperative Research Programs Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program Sid Mohan, Associate Program Manager, Implementation and Technology Transfer, National Cooperative Highway Research Program Ann M. Hartell, Senior Program Officer Dajaih Bias-Johnson, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Kami Cabral, Editor Jennifer Correro, Assistant Editor NCHRP PROJECT 25-60 PANEL Field of Transportation PlanningâArea of Human and Natural Environment Michael Brad McManus, Georgia Department of Transportation, Atlanta, GA (Chair) Sajjad Ahmad, University of Nevada, Las Vegas, Las Vegas, NV Gillian Davies, BSC Group, Inc., Worcester, MA William B. Fletcher, Oregon Department of Transportation (retired), Portland, OR Garrett Jackson, Washington State Department of Transportation, Olympia, WA Geil E. Rios-Vidal, AECOM, Washington, DC Stephen M. Sisson, Delaware Department of Transportation, Dover, DE Michael A. Stevens, King County (WA), Seattle, WA Nicklas Tiedeken, Minnesota Department of Transportation, St. Paul, MN Joe S. Krolak, FHWA Liaison Melissa Anderson Savage, AASHTO Liaison Christine L. Gerencher, TRB Liaison
iv Table of Contents LIST OF FIGURES .................................................................................................................................................. IX LIST OF TABLES .................................................................................................................................................... XI CHAPTER 1. INTRODUCTION ............................................................................................................................. 1 CHAPTER 2. LITERATURE REVIEW .................................................................................................................. 3 2.1. Landscape Modification Techniques .............................................................................................. 5 2.1.1. Wetland Conservation/Restoration/Creation ................................................................................ 5 2.1.2. Forest Conservation/Restoration/Creation .................................................................................... 6 2.1.3. Stream Stabilization/Restoration/Improvement ............................................................................ 7 2.1.4. Other Techniques ........................................................................................................................... 9 2.2. Stormwater Banking/Trading ...................................................................................................... 10 2.3. BMPs, LID Practices, and GI Practices .......................................................................................... 10 2.4. Physical and Hydrological Modeling ............................................................................................ 11 2.5. Costs and Benefits ....................................................................................................................... 14 CHAPTER 3. DECISION FRAMEWORK FOR THE WATERSHED APPROACH ..................................... 17 3.1. Decision Framework .................................................................................................................... 17 3.2. Mitigation Techniques within the Watershed .............................................................................. 21 3.3. Compatibility of Mitigation Techniques with Existing Regulations ............................................... 23 3.4. Collaboration with External Partners and Stakeholders ............................................................... 24 CHAPTER 4. HYDROLOGIC GUIDANCE ......................................................................................................... 26 4.1. Defining the Watershed and Assessment Points .......................................................................... 26 4.1.1. Relative Watershed Definition ..................................................................................................... 26 4.1.2. Fixed Watershed Definitions ........................................................................................................ 30 4.1.3. Tools for Defining a Watershed .................................................................................................... 32 4.1.4. Recommendations for Defining a Watershed and APs ................................................................ 33 4.2. Hydrologic Impacts, Objectives, and Metrics ............................................................................... 34 4.2.1. Possible Hydrologic Metrics ......................................................................................................... 34 4.2.2. State DOT Hydraulic Design Standards ......................................................................................... 37 4.2.3. Available Software/Tools for Calculating Hydrologic Metrics ...................................................... 37 4.2.4. Recommendations for Hydrologic Metrics ................................................................................... 39
v 4.3. Simulation of Mitigation Techniques in Hydrologic Modeling ...................................................... 39 4.3.1. Wetland Restoration/Creation ..................................................................................................... 40 4.3.2. Forest Restoration/Creation ......................................................................................................... 40 4.3.3. Stream Stabilization/Restoration/Improvement .......................................................................... 40 4.3.4. Uplands Restoration ..................................................................................................................... 41 4.3.5. Agricultural Practices Modification or Land Conversion .............................................................. 41 4.4. Screening Evaluation of the Hydrologic Impacts and Effectiveness of Mitigation Techniques ....... 41 4.4.1. Watershed Model Run Matrix ...................................................................................................... 42 4.4.2. Detailed Modeling Results for Screening Tool Development ....................................................... 43 4.4.2.1. General Methodology ........................................................................................................ 43 4.4.2.2. Watershed Model Locations .............................................................................................. 43 4.4.2.3. Transportation Project Impacts and Mitigation Technique Effectiveness ......................... 47 4.4.2.4. Hydrologic Metrics ............................................................................................................. 48 4.4.2.5. Modeling Limitations ......................................................................................................... 49 4.4.2.5.1. Mitigation from Pervious Land Covers ........................................................................ 50 4.4.2.5.2. Watershed Size and Project Impact Area ................................................................... 50 4.4.3. Hydrologic Screening Tool ............................................................................................................ 52 4.4.3.1. Determination of Mitigation Ratios ................................................................................... 53 4.4.3.2. Land Cover Conversion Mitigation Techniques ................................................................. 55 4.4.3.3. Stream Restoration Mitigation Techniques ....................................................................... 56 4.5. Detailed Evaluation of the Hydrologic Impacts and Mitigation .................................................... 57 4.5.1. Adaptation of Existing Watershed Models ................................................................................... 57 4.5.1.1. Upatoi Creek Example ........................................................................................................ 57 4.5.1.2. Cherry Creek Example ........................................................................................................ 62 4.5.2. Development of New Site-Specific Watershed Models ............................................................... 65 CHAPTER 5. IDENTIFICATION AND EVALUATION OF CO-BENEFITS ................................................ 66 5.1. Linkage of Co-Benefits and Mitigation Techniques....................................................................... 67 5.2. Screening Assessment of Co-benefits and Costs of Mitigation Techniques ................................... 71 5.2.1. Establish the Project Context for Co-benefits and Cost Evaluation (Step 8B1) ............................ 72 5.2.2. Identification of Co-benefits through Established Causal Chains (Step 8B2) ............................... 73 5.2.2.1. Wetland Restoration and Creation .................................................................................... 74 5.2.2.2. Forest Restoration and Creation ........................................................................................ 80 5.2.2.3. Stream Stabilization, Restoration, and Improvement ....................................................... 87 5.2.2.4. Uplands Restoration .......................................................................................................... 91 5.2.2.5. Agricultural Practices Modification and Land Conversion ................................................. 96 5.2.2.6. Tracking Ecosystem Services Co-Benefits ........................................................................ 101 5.2.3. Assess the Relative Magnitude of Co-Benefits (Step 8B3) ......................................................... 104 5.2.4. Screening Evaluation of Costs (Step 8B4) ................................................................................... 109 5.2.5. Compare Costs and Benefits across Alternatives to Rank Mitigation Options Based on Project Objectives (Step 8B5) ....................................................................................... 110 5.2.6. Synthesis of Co-Benefits Screening Process and Expected Level of Effort ................................. 111
vi 5.3. Detailed Analysis of Co-Benefits and Costs of Mitigation Techniques ........................................ 116 5.3.1. Need for Detailed Analysis ......................................................................................................... 116 5.3.2. Ecological Modeling .................................................................................................................... 117 5.3.3. Economic Valuation Methods .................................................................................................... 120 CHAPTER 6. STORMWATER BANKING OPPORTUNITIES .................................................................... 124 6.1. Wetland Banking as a Potential Model for Stormwater Banking ................................................ 124 6.2. Compatibility of Stormwater Banking with Existing Regulations ................................................ 126 6.3. A Framework for Stormwater Mitigation Banking ..................................................................... 127 6.3.1. Objectives ................................................................................................................................... 128 6.3.2. Site Selection .............................................................................................................................. 128 6.3.3. Site Protection Instrument ......................................................................................................... 129 6.3.4. Baseline Information .................................................................................................................. 129 6.3.5. Determination of Credits ............................................................................................................ 130 6.3.6. Mitigation Work Plan .................................................................................................................. 131 6.3.7. Maintenance Plan ....................................................................................................................... 131 6.3.8. Performance Standards .............................................................................................................. 131 6.3.9. Monitoring Requirements .......................................................................................................... 132 6.3.10. Long-Term Management Plan .................................................................................................. 132 6.3.11. Adaptive Management Plan ..................................................................................................... 133 6.3.12. Financial Assurances ................................................................................................................. 133 6.4. Summary ................................................................................................................................... 133 CHAPTER 7. CASE STUDIES ............................................................................................................................ 135 7.1. Introduction .............................................................................................................................. 135 7.2. Screening Analyses .................................................................................................................... 141 7.2.1. Jenkins Creek Case Study ............................................................................................................ 141 7.2.1.1. Hydrologic Screening ....................................................................................................... 142 7.2.1.2. Co-Benefits Screening ...................................................................................................... 149 7.2.1.2.1. Establish the Project Context (Step 8B1) .................................................................. 150 7.2.1.2.2. Identify Co-benefits through Established Causal Chains (Step 8B2) ......................... 151 7.2.1.2.3. Assess Relative Magnitude of Co-Benefits (Step 8B3) .............................................. 151 7.2.2. West Branch Housatonic (2030) Case Study .............................................................................. 159 7.2.2.1. Hydrologic Screening ....................................................................................................... 159 7.2.2.2. Co-Benefits Screening ...................................................................................................... 164 7.2.2.2.1. Establish the Project Context (Step 8B1) .................................................................. 164 7.2.2.2.2. Identify Co-Benefits through Established Causal Chains (Step 8B2) ......................... 167 7.2.2.2.3. Assess Relative Magnitude of Co-Benefits (Step 8B3) .............................................. 167 7.2.3. Piney Creek Case Study............................................................................................................... 179 7.2.3.1. Hydrologic Screening ....................................................................................................... 179 7.2.3.1.1. Stream Restoration Mitigation Scenario ................................................................... 180 7.2.3.1.2. Uplands Restoration Scenario ................................................................................... 180 7.2.3.2. Co-Benefits Screening ...................................................................................................... 183
vii 7.2.3.2.1. Establish the Project Context (Step 8B1) .................................................................. 183 7.2.3.2.2. Identify Co-Benefits through Established Causal Chains (Step 8B2) ......................... 185 7.2.3.2.3. Assess Relative Magnitude of Co-Benefits (Step 8B3) .............................................. 185 7.2.4. Screening Evaluation of Costs..................................................................................................... 191 7.2.4.1. Screening Evaluation of Costs (Step 8B4) ........................................................................ 191 7.2.4.2. Compare Costs and Benefits Across Alternatives to Rank Mitigation Options Based on Project Objectives (Step 8B5) ................................................................................... 191 7.3. Detailed Piney Creek Case Study ............................................................................................... 192 7.3.1. Hydrologic Analysis and Results ................................................................................................. 192 7.3.1.1. Stream Restoration Mitigation Scenario ......................................................................... 192 7.3.1.2. Uplands Restoration Mitigation Scenario ........................................................................ 193 7.3.1.3. Model Analysis Results .................................................................................................... 193 7.3.2. Co-benefits Analysis and Results ................................................................................................ 194 7.3.2.1. Stream Restoration .......................................................................................................... 196 7.3.2.1.1. Project Costs ............................................................................................................. 196 7.3.2.1.2. Increased or Improved Recreation Opportunities .................................................... 196 7.3.2.1.3. Climate Stabilization ................................................................................................. 198 7.3.2.1.4. Climate Resiliency ..................................................................................................... 199 7.3.2.1.5. Non-Use and Cultural Values .................................................................................... 199 7.3.2.1.6. Commercial Fishing Benefits ..................................................................................... 200 7.3.2.2. Uplands Restoration ........................................................................................................ 200 7.3.2.2.1. Project Costs ............................................................................................................. 200 7.3.2.2.2. Improved Landscape Aesthetics ............................................................................... 201 7.3.2.2.3. Increased or Improved Recreation Opportunities .................................................... 201 7.3.2.2.4. Climate Stabilization ................................................................................................. 203 7.3.2.2.5. Climate Resiliency ..................................................................................................... 205 7.3.2.2.6. Non-Use and Cultural Values .................................................................................... 205 7.3.2.2.7. Increased Property Values ........................................................................................ 206 7.3.2.3. Summary .......................................................................................................................... 207 7.4. Summary ................................................................................................................................... 211 7.4.1. Screening Analysis Results .......................................................................................................... 211 7.4.1.1. Jenkins Creek ................................................................................................................... 212 7.4.1.2. West Branch Housatonic (2030) ...................................................................................... 212 7.4.1.3. Piney Creek ...................................................................................................................... 212 7.4.2. Detailed Analysis Results and Comparison with Screening ........................................................ 213 7.4.3. General Observations ................................................................................................................. 213
viii CHAPTER 8. SUMMARY ................................................................................................................................... 215 REFERENCES ....................................................................................................................................................... 218 ABBREVIATIONS ............................................................................................................................................... 229 GLOSSARY ............................................................................................................................................................ 231 APPENDIX A. ANNOTATED BIBLIOGRAPHY ............................................................................................ 233 APPENDIX B. WATERSHED MODEL SUMMARIES ................................................................................... 268 B.1. Hydrologic Simulation Program FORTRAN (HSPF) ............................................................................. 268 B.2. Storm Water Management Model (SWMM) ..................................................................................... 272 B.3. Soil and Water Assessment Tool (SWAT) ........................................................................................... 274 B.4. System for Urban Stormwater Treatment and Analysis IntegratioN (SUSTAIN) ............................... 275 B.5. Watershed Management Optimization Support Tool (WMOST) ...................................................... 276 APPENDIX C. EXISTING SCREENING TOOLS ............................................................................................. 279 C.1. National Stormwater Calculator ........................................................................................................ 279 C.2. Western Washington Hydrology Model (WWHM) ............................................................................ 282 C.3. Model My Watershed ........................................................................................................................ 285 C.4. Watershed Management Optimization Support Tool (WMOST) ...................................................... 286 C.5. Hydrologic and Water Quality System (HAWQS) ............................................................................... 287 C.6. US Army Corps of Engineers HEC-HMS .............................................................................................. 289 C.7. Adaptations Required ........................................................................................................................ 290 APPENDIX D. MITIGATION RATIO TABLES .............................................................................................. 292 NCHRP Web-Only Document 333: Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects is associated with NCHRP Research Report 1011: Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide. Readers can read or purchase NCHRP Report 1011 on the National Academies Press website (www.nap.edu).
ix List of Figures Figure 3.1. Decision framework. ................................................................................................................. 17 Figure 4.1. Watershed determined by project location (from USGS StreamStats). ................................... 28 Figure 4.2. Watershed determined by the junction immediately downstream from project (from USGS StreamStats). .................................................................................................................................. 28 Figure 4.3. Example near a confluence with a larger waterway (from USGS StreamStats). ...................... 29 Figure 4.4. Example near a confluence with a larger waterway moved to just upstream of the next junction (from USGS StreamStats). ................................................................................................ 29 Figure 4.5. Example near a confluence with a larger waterway moved to just downstream of the next junction (from USGS StreamStats). ................................................................................................ 30 Figure 4.6. Hydrologic Unit Code hierarchy (source: USGS). ...................................................................... 31 Figure 4.7. StreamStats coverage (source: USGS). ..................................................................................... 32 Figure 4.8. Peak discharge return period used for design (FHWA 2012).................................................... 38 Figure 4.9. Geographic location of watershed models. .............................................................................. 44 Figure 4.10. Piney Creek (Cherry Creek), Colorado. ................................................................................... 44 Figure 4.11. Upper Pine Knot Creek (Upatoi Creek), GA. ............................................................................ 45 Figure 4.12. Bluff Creek (Lower Minnesota River), MN. ............................................................................. 46 Figure 4.13. Average annual runoff. ........................................................................................................... 51 Figure 4.14. Average annual precipitation. ................................................................................................. 51 Figure 4.15. Stream classification by sinuosity (source: TxDOT 2019). ...................................................... 56 Figure 4.16. HSPF integration with EPA BASINS. ........................................................................................ 58 Figure 4.17. Hypothetical Georgia transportation project example using HSPF. ....................................... 58 Figure 4.18. WinHSPF user interface. ......................................................................................................... 59 Figure 4.19. BASINS flow duration plot. ...................................................................................................... 60 Figure 4.20. HSPF streamflow mitigation results for the higher flow rates. .............................................. 61 Figure 4.21. HSPF short term hydrograph results. ...................................................................................... 61 Figure 4.22. HSPF-BASINS Cherry Creek Colorado model interface. .......................................................... 62 Figure 4.23. Hypothetical Cherry Creek Colorado highway example using HSPF. ...................................... 63 Figure 4.24. Cherry Creek flow-duration curves. ........................................................................................ 64 Figure 4.25. Close-up of the higher flows from the Cherry Creek flow-duration curves............................ 64 Figure 4.26. HSPF time-series plot. ............................................................................................................. 65 Figure 5.1. Examples of mitigation techniques, environmental changes, and ecosystem services. .......... 68 Figure 5.2. Factors to consider in establishing the mitigation context. ...................................................... 73 Figure 5.3. Generic causal chain for wetland restoration and creation mitigation. ................................... 75 Figure 5.4. Generic causal chain for forest restoration and creation mitigation. ...................................... 82 Figure 5.5. Generic causal chain for stream restoration and improvement mitigation. ............................ 88 Figure 5.6. Generic causal chain for uplands restoration mitigation. ......................................................... 92 Figure 5.7. Example causal chain diagram for agricultural conservation mitigation with the goal of enhancing water quality (Source: This diagram is Figure 1 from Wainger et al. (2017)). ............. 96
x Figure 5.8. Example causal chain diagram for nitrogen management practices (Source: NESP (2020)). ... 97 Figure 5.9. Example causal chain diagram for phosphorous management practices (Source: NESP (2020)). .................................................................................................... 98 Figure 7.1. Modeling and case study locations. ........................................................................................ 136 Figure 7.2. Piney Creek (CO) watershed location. .................................................................................... 139 Figure 7.3. Jenkins Creek (WA) watershed location. ................................................................................ 139 Figure 7.4. West Branch of the Housatonic River (MA) watershed location. ........................................... 140 Figure 7.5. Hypothetical transportation project in the Jenkins Creek watershed. ................................... 141 Figure 7.6. Land cover distribution at the Upper AP in the Jenkins Creek watershed from EPA BASINS. ....................................................................................................... 144 Figure 7.7. Land cover in the Jenkins Creek watershed. ........................................................................... 144 Figure 7.8. Concentration of high intensity land cover in the Jenkins Creek watershed. ........................ 145 Figure 7.9. Aerial imagery of developed land in Jenkins Creek watershed. ............................................. 146 Figure 7.10. Area of wetlands in the Jenkins Creek watershed. ............................................................... 147 Figure 7.11. âDeveloped Open Spaceâ in the Jenkins Creek watershed. ................................................... 147 Figure 7.12. Aerial imagery of grassland in the Jenkins Creek watershed. ............................................... 148 Figure 7.13. Developed land near wetlands in the Jenkins Creek watershed. ......................................... 149 Figure 7.14. Hypothetical transportation project in the West Branch Housatonic watershed ................ 160 Figure 7.15. Land cover distribution at the Upper AP in the West Branch Housatonic watershed from EPA BASINS ........................................................................................................ 162 Figure 7.16. Area of agricultural land in the West Branch Housatonic watershed. ................................. 162 Figure 7.17. Land cover distribution at the downstream AP in the West Branch Housatonic watershed from EPA BASINS. ....................................................................................................... 163 Figure 7.18. Land cover distribution in the West Branch Housatonic watershed from EPA BASINS. ...... 163 Figure 7.19. Concentration of developed land near Pittsfield, MA. ......................................................... 164 Figure 7.20. The hypothetical project and Piney Creek Watershed near Denver, CO. ............................. 179 Figure 7.21. The upper subbasins of the Piney Creek Watershed. ........................................................... 181 Figure 7.22. Developed area in the Piney Creek Watershed. ................................................................... 182 Figure 7.23. Aerial imagery of a developed area in the Piney Creek Watershed. .................................... 182 Figure 7.24. Potential stream restoration area in the Piney Creek Watershed. ...................................... 193 Figure C.1. National Stormwater Calculator â location specification. ...................................................... 280 Figure C.2. National Stormwater Calculator â watershed definition. ....................................................... 281 Figure C.3. National Stormwater Calculator â LID cover. ......................................................................... 281 Figure C.4. National Stormwater Calculator â summary of results. ......................................................... 282 Figure C.5. WWHM â site selection. ......................................................................................................... 283 Figure C.6. WWHM â predevelopment subbasin creation. ...................................................................... 284 Figure C.7. WWHM â outputs. .................................................................................................................. 284 Figure C.8. Model My Watershed â HUC selection. .................................................................................. 285 Figure C.9. Model My Watershed â results. ............................................................................................. 286 Figure C.10. WMOST interface. ................................................................................................................. 287
xi Figure C.11. HAWQS â HUC selection. ...................................................................................................... 288 Figure C.12. HAWQS â flow duration plot. ............................................................................................... 289 Figure C.13. HEC-HMS user interface. ....................................................................................................... 290 List of Tables Table 2.1. Classification of modification techniques from the literature review. ........................................ 5 Table 2.2. Hydrologic modeling software from the literature review. ....................................................... 12 Table 4.1. Attributes of streamflow for ecosystems (Richter et al. 1996). ................................................. 35 Table 4.2. Piney Creek, Colorado, hypothetical baseline watershed summary. ........................................ 45 Table 4.3. Upper Pine Knot Creek, Georgia, hypothetical baseline watershed summary. ......................... 46 Table 4.4. Bluff Creek, Minnesota, hypothetical baseline watershed summary. ....................................... 47 Table 4.5. Example model results for percent change in the 100-year peak flow using forest mitigation of impervious areas downstream of a 2 percent transportation project site. ............. 53 Table 4.6. Example mitigation ratios for the 100-year peak flow for forest restoration of impervious areas located downstream of the project site. ........................................................... 54 Table 5.1. Descriptions of categories of environmental changes resulting from mitigation techniques. .. 69 Table 5.2. Description of categories of co-benefits resulting from environmental changes. .................... 70 Table 5.3. Site-specific factors for wetland restoration and creation mitigation. ...................................... 76 Table 5.4. Site-specific factors for forest restoration and creation mitigation. ......................................... 83 Table 5.5. Site-specific factors for stream stabilization, restoration, and improvement mitigation.......... 90 Table 5.6. Site-specific factors for uplands restoration mitigation. ............................................................ 93 Table 5.7. Site-specific factors for agricultural practice modification mitigation. ...................................... 99 Table 5.8. Identifying relevant co-benefits by mitigation technique. ....................................................... 102 Table 5.9. Summary of potential benefit-relevant indicators by co-benefit and mitigation technique. . 106 Table 5.10. Process, data sources, and length of time by co-benefits screening-analysis step. .............. 112 Table 5.11. Examples of ecosystem service benefit modeling tools potentially available for use in quantifying co-benefits. ............................................................................................... 119 Table 5.12. Ecosystem service co-benefit valuation methods and data requirements. ........................... 122 Table 7.1. Modeling site summary. ........................................................................................................... 136 Table 7.2. Case study site summary. ......................................................................................................... 137 Table 7.3. Case study site land cover summary. ....................................................................................... 137 Table 7.4. Mitigation alternatives evaluated in the case studies. ............................................................ 138 Table 7.5. Mitigation ratios for consideration in the Jenkins Creek case study (area of mitigation per unit area of new impervious highway impact). ................................................... 142 Table 7.6. Upstream mitigation amounts to compensate for 7.2 acres of impervious impact for the Jenkins Creek case study. ..................................................................................... 143 Table 7.7. Downstream mitigation amounts to compensate for 7.2 acres of impervious impact for the Jenkins Creek case study. ..................................................................................... 149
xii Table 7.8. Potential partners for Jenkins Creek mitigation alternatives................................................... 151 Table 7.9. Screening assessment of potential co-benefits for forest restoration at Jenkins Creek. ........ 152 Table 7.10. Screening assessment of potential co-benefits for wetland restoration at Jenkins Creek. ... 155 Table 7.11. Comparison of mitigation alternatives for Jenkins Creek. ..................................................... 158 Table 7.12. Mitigation ratios for consideration in the West Branch Housatonic case study (area of mitigation per unit area of new impervious highway impact). ...................................... 160 Table 7.13. Mitigation amounts to compensate for 18.5 acres of impervious impact for the West Branch Housatonic (20230) case study. ............................................................................. 161 Table 7.14. Potential partners for West Branch Housatonic mitigation alternatives. .............................. 167 Table 7.15. Screening assessment of potential co-benefits for wetland restoration at the West Branch Housatonic watershed. .......................................................................................... 168 Table 7.16. Screening assessment of potential co-benefits for forest restoration at West Branch Housatonic. ............................................................................................................. 171 Table 7.17. Screening assessment of potential co-benefits for agricultural practices modification at West Branch Housatonic. ................................................................................... 174 Table 7.18. Comparison of mitigation alternatives for West Branch Housatonic. ................................... 177 Table 7.19. Potential partners for Piney Creek mitigation alternatives. .................................................. 184 Table 7.20. Screening assessment of potential co-benefits for stream restoration at Piney Creek. ....... 186 Table 7.21. Screening assessment of potential co-benefits for uplands restoration at Piney Creek. ...... 188 Table 7.22. Comparison of mitigation alternatives for Piney Creek. ........................................................ 190 Table 7.23. Metrics at the Piney Creek Upper AP (Subbasins 272 and 274). ........................................... 194 Table 7.24. Metrics at the Piney Creek Downstream AP (Outlet of Reach 278). ..................................... 194 Table 7.25. Recreation use value per person per day, Western United States (2020 dollars). ................ 202 Table 7.26: Carbon density and sequestration rates by tree species (metric tons per acre). .................. 204 Table 7.27: Social cost of carbon (2020 dollars per metric ton). .............................................................. 205 Table 7.28. Property value increases using benefit transfer. ................................................................... 208 Table 7.29: Comparison of stream and uplands restoration mitigation alternatives for Piney Creek. .... 209 Table C.1. Summary of existing screening level tools. .............................................................................. 279 Table D.1. Mitigation ratio for AP = downstream, mitigation location = downstream, mitigation site = impervious (area of mitigation per unit area of new impervious highway impact). ......... 293 Table D.2. Mitigation ratio for AP = project, mitigation location = upstream, mitigation site = impervious (area of mitigation per unit area of new impervious highway impact). ................... 293 Table D.3. Mitigation ratio for AP = downstream, mitigation location = upstream, mitigation site = impervious (area of mitigation per unit area of new impervious highway impact). ......... 293 Table D.4. Mitigation ratio for AP = downstream, mitigation location = downstream, mitigation site = pervious (area of mitigation per unit area of new impervious highway impact). ............. 294 Table D.5. Mitigation ratio for AP = project, mitigation location = upstream, mitigation site = pervious (area of mitigation per unit area of new impervious highway impact). ....................... 294 Table D.6. Mitigation ratio for AP = downstream, mitigation location = upstream, mitigation site = pervious (area of mitigation per unit area of new impervious highway impact). ............. 294 Table D.7. Stream restoration mitigation (feet per acre of new impervious highway impact). ............... 295
