STATUS:  The research was published as NCHRP Report 840, A Watershed Approach to Mitigating Stormwater Impacts.

BACKGROUND

State departments of transportation (DOTs) and natural resources and regulatory agencies have begun to consider a watershed approach for complying with a variety of state and federal environmental requirements that apply to transportation project construction, and system operations and maintenance. The current legislative and regulatory climate has placed a spotlight on watershed protection, stormwater pollutant reduction, and hydrologic impairment. State DOTs are exploring a variety of approaches and regulatory mechanisms to achieve compliance with water quality permits that are based on watershed-based Total Maximum Daily Load limitations and receiving water quality standards. State DOTs are sometimes unable to meet stormwater management requirements on site or in close proximity to the impacted area; therefore, additional approaches to accomplishing mitigation requirements are needed. These approaches could include off-site mitigation and non-traditional techniques and strategies that would allow greater environmental benefits to be achieved. Research is needed to provide a comparative decision-making framework within existing regulations that is based on watershed needs.

OBJECTIVES

The objectives of this research were to produce (1) a practical watershed-based decision-making framework that will enable DOTs to identify and implement cost-effective and environmentally beneficial water quality solutions for stormwater impacts and (2) a toolbox of feasible water quality solutions for stormwater impacts.

The framework is designed to:

• be consistent with the watershed approach developed by the U.S. Environmental Protection Agency (available here),
• accommodate varying degrees of extant watershed planning,
• be applicable in a variety of watershed types,
• include methods to demonstrate the net environmental benefit of potential water quality solutions,
• address implementation and feasibility issues including but not limited to information needs and sources, and
• include a retroactive demonstration of its applicability in at least one or more real-world examples. 

The toolbox addresses conventional best management practices (BMPs) and off-site water quality solutions (e.g., outside of the project area or right-of-way), and at a minimum should consider:

• conditions under which conventional onsite BMPs are infeasible or inadequate,
• mitigation options that augment conventional BMPs,
• solutions that mitigate water quality impacts from existing impervious surfaces, and
• solutions that mitigate water quality impacts from new impervious surfaces.