Combined Sewer Overflow Planning and Downspout Disconnect Program

The City of Wilmington has 4,300 acres along the Delaware River at the confluence of Brandywine Creek and the Christina River. Almost all of the City’s 72,000 residents and a large commercial business population are served by a combined sewer system. Greeley and Hansen is currently serving as program manager for the update, refinement and implementation of the City’s Long Term Control Plan (LTCP) to address combined sewer overflows (CSO).

Greeley and Hansen established a sustainable approach to CSO reduction utilizing green infrastructure and source controls. The includes enhancements to the current stormwater utility structure for improved source control credits, investigating a stormwater management ordinance to reduce runoff from new and redevelopment, and examining a variety of pilot programs that will deploy various green infrastructure elements along city streets or in public areas such as rain gardens, planters, bioswales, and tree pits. Based on the performance of the green infrastructure projects, the standard for future City water and sewer replacement projects will be examined for long term incorporation of green infrastructure elements.

The project also includes successfully addressing a sensitive CSO through a downspout disconnect project using green infrastructure including design and construction management of residential rain gardens, infiltrating basins, and green roofs. A comprehensive pre‐ and post‐construction modeling and monitoring program used rainfall and sewer flow data along with SWMM and Mike Urban framework to show overflow reduction was achieved. Through the use of XP‐SWMM, existing collection system performance was modeled. Design verification and capture efficiencies were performed with new CSO controls. New technologies were investigated and incorporated in the final revised plan helping to reduce the overall cost of the City’s program while achieving a higher level of capture. Greeley and Hansen has assisted the city in reducing the cost of the CSO program while keeping the support of the regulators.

Project Highlights: 

• Green infrastructure planning and design 
• Rain gardens, infiltration planters, tree pits, bioswales, porous pavements 
• Downspout disconnect program 
• Green roof design 
• Hydraulic modeling for CSO planning