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3 C H A P T E R 1 Project Overview State DOTs are increasingly required to meet more complex federal and state regulations to manage the quantity and quality of stormwater runoff and to protect water resources. The Clean Water Actâs National Pollutant Discharge Elimination System (NPDES) program, in particular, requires DOTs to spend significant amounts of financial resources on stormwater management. Under the NPDES program, DOTs are considered municipal separate storm sewer system (MS4) owners and operators. New highway development and redevelopment projects that disturb greater than or equal to one acre are required to develop, implement, and enforce a program to ensure that controls are in place to prevent or minimize water qual- ity impacts. Implementing stormwater best management practices (BMPs) to manage stormwater run- off quality and quantity can be quite challenging for roadway drainage designers. DOTs face unique limitations and design challenges due to the linear nature of the road network and limited space within the right-of-way (ROW) for stormwater management. This is particu- larly true in urban and ultra-urban areas that are built out and have limited land available to purchase additional ROW for stormwater management or in areas with unique physical or political geographic features (such as soil, topography, or political boundaries that restrict mitigation options). Even when these challenges do not exist, there is a growing need to think beyond the ROW and within the watershed to determine the best and most cost-effective option to mitigate the impact of particular stormwater stressors or pollutants to ensure that it provides the greatest water quality and ecosystem benefit while preserving the stewardship of public funds. Address- ing stormwater issues in a project-by-project fashion can be very inefficient and much less effective in actually addressing important watershed water quality and ecosystem issues. In constrained sites, this can result in very expensive and often unsustainable treatment alterna- tives. The goal of the watershed-based approach to mitigate stormwater impacts is to allow DOTs to identify environmentally superior alternatives for compliance within existing and emerging regulations. This NCHRP publication provides guidance on watershed-based stormwater mitigation strate- gies and techniques that can be used to evaluate on- and off-site mitigation options that are applicable to DOTs across the country. In addition to the report, readers are encouraged to download the associated Watershed-Based Stormwater Mitigation Toolbox (WBSMT). This is an Excel-based planning level tool that utilizes nationally available geographic information systems (GIS) data to assist DOTs in identifying and prioritizing potential stormwater runoff mitigation opportunities. Introduction
4 A Watershed Approach to Mitigating Stormwater Impacts Institutional and Cost Implications DOTs spend an estimated eight to 10 percent of their total transportation construction and repair project budgets on environmental compliance annually (Larson 2001). The breakdown of expenditures is difficult to fully ascertain because DOTs do not generally separate out costs between what is attributable to project elements considered essential to basic transportation needs versus costs that are a part of compliance with environmental regulations (ICF Inter- national et al. 2008 and Macek 2006). An evaluation of 44 transportation projects in four different years by the Washington State DOT found that the percent of construction cost related to environmental mitigation ranged from 13.5 percent to 18.1 percent (Washington State DOT 2013). Based on these numbers, environmental costs related to stormwater, wetlands, endangered species, and other regulations can clearly affect the overall cost of transportation projects. Adherence with regulations under Section 402 of the Clean Water Act (CWA) requires DOTs to develop, implement, and enforce a stormwater management program designed to reduce the discharge of pollutants from their MS4s via the NPDES permitting process. State DOTs are anticipating that NPDES compliance costs will increase as total maximum daily loads (TMDLs), waste load allocations (WLAs), and other similar regulatory requirements to treat existing impervious area are incorporated into NPDES stormwater permits. Costs also result when DOT projects impact a wetland under Sections 401 and 404 of the CWA. Other envi- ronmental regulations that have the ability to affect DOT project costs include but are not limited to: ⢠The Endangered Species Act; ⢠The Fish and Wildlife Coordination Act; ⢠The National Environmental Policy Act; ⢠The Rivers and Harbors Act; ⢠The Coastal Zone Management Act; ⢠State-specific or local regulations related to wetlands, water quality, watershed protection, shoreline management, habitat, or other environmental concerns; and ⢠State-specific regulations related to construction and post-construction activities. Of the environmental regulations impacting DOT projects, those related to stormwater man- agement and NPDES permit implementation are thought to have the greatest impact on overall project costs. Failure to integrate stormwater requirements early in the project development process can cause project delays, leading to additional costs. In the Washington State DOT study referenced earlier, the percent of construction cost related to stormwater management alone ranged from 4.4 percent to 10.7 percent for each sample year, or between 31.8 percent and 67.2 percent of the total costs associated with environmental mitigation (including temporary stormwater management measures) (Washington State DOT 2013). Addressing stormwater issues in a project-by-project fashion can be very inefficient and much less effective in addressing important watershed water quality and habitat issues. In constrained sites, this can result in very expensive and often unsustainable treatment alternatives. This report is intended to provide guidance to DOTs on potential approaches, data, and tools that can be used to help make informed planning decisions when evaluating alternative runoff management strategies using a watershed-based approach. Due to the complexity and operations of the many state DOTs, this report does not provide a detailed methodology of how each individual DOT might incorporate a watershed approach. It provides general guidance for the process and topics for discussion with the planning and regulatory community.
Introduction 5 Report Organization This report is divided into eight chapters. Chapters 1, 2, and 3 provide a foundation for understanding stormwater runoff impacts on a watershed scale, identifying and collecting the data necessary for watershed-based information assessments, and outlining the methodology for identifying watershed baseline conditions and comparing mitigation options. The WBSMT utilizes a process to identify mitigation opportunities as introduced in Chapter 4. These include identifying connections between project-level stormwater runoff impacts, water- shed processes, and possible mitigation opportunities to achieve regulatory objectives. Where available, the toolbox provides DOTs the flexibility to incorporate more localized data and plan- ning information from existing watershed, development, or master planning efforts (extant planning info). In Chapter 5, information is provided on how and where to find extant planning information and provides guidelines for determining the adequacy of such information for DOT or regula- tory needs when evaluating and siting water quality improvement options. Chapter 6 describes how the toolbox may be utilized to evaluate off-site mitigation opportunities. In Chapter 7, the WBSMT is applied to a case study to demonstrate its use for identifying mitigation opportuni- ties. The report concludes with Chapter 8. About the WBSMT The WBSMT is a Microsoft Excel-based program to facilitate the char- acterization of the project watershed and the identification of mitigation options at the planning level. The toolbox relies on nationally available data- sets to limit the data collection burden while providing DOTs the flexibility to incorporate more localized watershed, development, or extant planning information when available. This report and the WBSMT are intended for use by DOT personnel at the project level. Results derived from the toolbox and background information provided in the report can be used by DOTs to identify and select mitigation options based on performance estimates. It should be noted that this information is valuable for planning-level analysis, but is not intended to replace the need for field verifica- tion of results. To find the WBSMT, go to the TRB website and search for NCHRP Research Report 840.
