Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Conduct of Research Report (2022)

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1 Chapter 1. Introduction This report describes work completed for the National Cooperative Highway Research Program (NCHRP) Project 25-60, “Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects.” The overall goal of the project was to: “Develop guidance for State DOTs for developing and implementing watershed- based strategies and techniques for mitigating hydrologic impacts of transportation projects.” The project focused on meeting needs and constraints of state departments of transportation (DOTs) facing increasingly stringent requirements for stormwater quantity and quality while looking for opportunities to collaborate with regulatory agencies and other potential external partners/stakeholders for solutions where the whole is greater than the sum of its parts. This project is focused on out-of-kind landscape modifications for mitigation. The intended result of the research on completion of the project is guidance on how to: • Identify opportunities in a watershed to conserve, restore, enhance, or create landscape features for hydrologic mitigation. • Plan, site, and design features for hydrologic mitigation. • Quantify the hydrologic outcomes from selected features. • Develop strategies for implementing a watershed-based approach to hydrologic mitigation. Engineers, scientists, and planners use traditional, well-accepted techniques – including a wide variety of best management practices (BMPs) – to achieve stormwater quantity and quality mitigation of transportation impacts in-kind. In-kind mitigation addresses the negative effect of a transportation project with a counterbalancing positive effect measured by the same metric. For example, if the impact is an increase in the loading of nutrients measured in lbs/yr, the mitigation decreases the loading of nutrients in lbs/yr through some mechanism. Similarly, if the impact is an increase in the peak runoff of the 10-yr event measured in ft3/s, the mitigation decreases the peak runoff of the 10-yr event in ft3/s. Engineers, scientists, and planners typically apply these techniques at a project site but may apply them off-site. An example of an off-site in-kind BMP is a regional stormwater management pond that mitigates runoff from many parts of a watershed. In-kind stormwater mitigation techniques may provide ecosystem services that are in addition to the primary hydrologic benefits of the mitigation. The benefits associated with ecosystem services, known as co-benefits, either are not associated with the in-kind mitigation or they are not considered. This research does not explicitly consider traditional in-kind on-site mitigation techniques though they may be part of a watershed approach to mitigation. However, the watershed approach offers a strategy for State DOTs applicable in many situations and particularly when on-site mitigation may not be feasible. Out-of-kind mitigation addresses the negative impact of a project by compensating for that impact using the same metric applied to quantify the impact while also providing co-benefits by improving a different, but related, watershed function. In some cases, the most attractive purpose of out-of-kind mitigation could be one or more of the co-benefits rather than reducing the

2 hydrologic impact of the project. Out-of-kind mitigation is typically located off-site but can also be on-site. Many out-of-kind mitigation techniques can be characterized as landscape modifications as they change the characteristics of the landscape that makes up the watershed. It is common for these techniques to be newer and less well accepted (i.e., less well documented regarding their impacts and effectiveness) than traditional techniques. This research project focused on out-of-kind landscape modifications for mitigation. Information on the rationale for using a watershed-level approach is important background and contextual information that will be provided in the guidebook produced by this project and in this project research report. This context provides important incentives and justification for State DOTs to test and apply the concepts and approaches developed in this research. Following this introductory chapter, Chapter 2 summarizes the project literature review which is found in Appendix A. Chapter 3 describes the overall decision framework for State DOTs to work within a watershed approach. The decision framework describes steps and decision points for a State DOT and highlights the role of potential external partners/stakeholders in the process. The next two chapters provide guidance and tools needed for successful application of the watershed approach using the decision framework. Chapter 4 addresses hydrologic guidance and tools including selection of watersheds, choosing hydrologic metrics, and representing mitigation techniques in watershed models. Chapter 4 also introduces both screening and detailed tools for assessing the hydrologic impacts of transportation projects as well as the effectiveness of out-of- kind mitigation techniques in reducing those impacts. Chapter 5 provides complementary screening and detailed tools and guidance for evaluating the co-benefits of out-of-kind mitigation techniques. Chapter 6 provides an overview of the opportunities for stormwater banking and the relationship between this concept and the well-accepted practice of wetland mitigation banking. The chapter also provides an outline of a stormwater banking program modeled after wetland banking. Chapter 7 demonstrates application of the screening analyses and detailed analyses. Screening analyses in Washington, Massachusetts, and Colorado illustrate the application of the hydrologic screening tool and the co-benefits screening. The chapter provides a detailed case study of the Colorado site and compares the results of the screening and detailed analyses. Finally, Chapter 8 provides a summary and recommendations followed by the report references, a list of abbreviations, and a glossary of terms. Four appendices provide detailed information on selected topics discussed in the main report. Appendix A provides an annotated bibliography that supplements the literature review summarized in Chapter 2. Appendix B (Watershed Model Summaries), Appendix C (Existing Screening Tools), and Appendix D (Mitigation Ratio Tables) support the selection and development of the screening and detailed hydrologic tools and guidance described in Chapter 4. This project also produced a Guidebook for State DOTs and external partners/stakeholders on the watershed approach covered in this project report. This guidebook provides the essence of the process and specific guidance for step-by-step application of the approach.