Stormwater Retrofits BMP Expert Panel Report

Description

Over the last two decades, the Chesapeake Bay states have pioneered new techniques for finding, designing and delivering retrofits to remove pollutants, improve stream health and maintain natural hydrology in developed watersheds. Several important regulatory drivers are likely to increase the amount of future stormwater retrofit implementation across the Chesapeake Bay watershed. Some communities need to install retrofits to meet pollutant reduction targets under recently issued municipal stormwater permits or meet local TMDLs. In addition, each of the seven Bay states are considering greater use of urban stormwater retrofits as part of an overall strategy to meet nutrient and sediment load reduction targets for existing urban development under the Chesapeake Bay TMDL. 

Stormwater retrofits are a diverse group of projects that provide nutrient and sediment reduction on existing development that is currently untreated by any BMP or is inadequately treated by an existing BMP. The Panel classified retrofits into two broad project categories -- new retrofit facilities and retrofits of existing BMPs. These two categories encompass a broad range of potential local retrofit options and applications including new constructed wetlands, green streets or rain gardens, as well as conversion, enhancements or restoration of older BMPs to boost their performance. 

Given the diversity of possible retrofit applications, the Panel decided that assigning a single universal removal rate was not practical or scientifically defensible. Instead, the Panel elected to develop a protocol whereby the removal rate for each individual retrofit project is determined based on the amount of runoff it treats and the degree of runoff reduction it provides. The Panel conducted an extensive review of recent BMP performance research and developed a series of retrofit removal adjustor curves to define sediment, nitrogen and phosphorus removal rates. The Panel then developed specific calculation methods tailored for different retrofit categories. Recommendations from the Expert Panel are outlined in this report.