Traditional stormwater management practices are implemented at the local level. This is for good reason, as controlling the quantity and quality of runoff from land development is most effectively managed by applying site-specific techniques close to the source. Low-Impact Development (LID) and stormwater management Best Management Practices (BMPs) embrace this concept and are appropriately designed for the individual parcel or lot level. However, regardless of approach, these management methods may still convey large volumes of stormwater to community drainage systems that are interconnected and only have so much capacity. Major rainfall events in particular will exceed this capacity and create flood hazards both at the site and downstream with stormwater overflows, backflows, and increased velocities with potentially destructive impacts to the built and natural environment.
Watersheds are larger areas of land where all water flows across or through and drains into a common stream, river, lake, or ocean. They include all the natural and structural channels designed to convey stormwater, the floodplains which store and transport floodwaters, and all other lands up to the highest elevation. No matter how they are delineated, all watersheds meet this definition and thus are critical for understanding and taking a holistic approach to stormwater and floodplain management. All communities are affected by development that takes place upstream in their watershed, and similarly will have an impact on downstream communities through their own development activity. Consequently, communities should plan on a watershed-wide scale.
- One of the first steps to implementing such an approach is the completion of a watershed master plan that assesses the potential impact of development on existing and future conditions – including impervious surfaces, drainage and stormwater flows, natural systems, and structures throughout the watershed. Plans should also assess the potential impacts to the community from larger rainfall events that exceed the performance standards (design storm) used for existing stormwater facilities.
- Once these impacts are known, a more comprehensive program can be created to prevent or minimize adverse impacts including new or revised regulations for development, enhanced macro-scale techniques for stormwater or floodplain management, or capital projects for flood control and mitigation. Such a program will help the community identify opportunities to address problems before and as they arise.
In taking the watershed view, communities are better positioned to consider additional tools or strategies for risk reduction and avoid making development decisions that lead to increased flood hazard vulnerability. Watershed-based planning can also help facilitate regional or multi-jurisdictional coordination on stormwater and flood-related issues that traverse political boundaries. These enhanced efforts can lead to more integrated, coordinated, and systematic solutions across the watershed versus dealing with stormwater solely by locality. For example, by working with neighboring jurisdiction, communities may be able to consider the development and use of regional retention or detention measures. Pursuing these and other types of larger-scale projects may be used to leverage more non-traditional sources of funding for plan implementation. For these reasons, watershed-scale planning is often best executed at the regional scale, with coordination and creation of a regional planning entity with appropriate authority.