A Filter Approach to Turning a City Greener One BMP at a Time

State and Federal Regulations require many cities to develop Long Term Control Plans (LTCPs) to address combined sewer overflows (CSOs). A LTCP characterizes existing sewer system capacity and presents likely improvements required to meet receiving water quality standards. Integrating green infrastructure solutions for targeted areas in a watershed may provide cost effective CSO control opportunities with high public acceptance. The "green filter" approach identifies "green infrastructure" enhancement possibilities such as wetlands and wet ponds, bioretention cells, catch basin retrofits, and retention ponds to help manage and reduce stormwater that enters combined sewer systems. The green filter process utilizes geographic information system (GIS) software and remote sensing data to map topography, vegetation and soils, and open space. At its highest resolution, the green filter analysis can identify widespread micro-scale opportunities such green roofs, rain gardens, and bioretention cells. These opportunities are then evaluated based on catchment area, existing hydrology, existing and future conveyance systems, and overall cost to identify feasible conceptual designs. The final step is a site-by-site cost/benefit comparison between gray and green alternatives to determine locations where green opportunities can cost-effectively reduce combined sewer overflow frequency and volume. Based upon an evaluation of logical targeted locations, LTCPs may incorporate green infrastructure to reduce overflows and enhance the performance of traditional overflow control practices. The green infrastructure enhancements provide added benefits such as water quality treatment, public amenities, and wildlife habitat while helping meet the traditional overflow control goals of reducing frequency and volume of combined sewer overflows.