Urban Green Stormwater Infrastructure: How Much Is Infiltrating?

There is an emerging trend in urban stormwater management as more and more major U.S. cities are considering green stormwater infrastructure (GSI) to reduce stormwater impacts to their separate and combined sewers. Philadelphia is implementing the most ambitious GSI program in the U.S. to meet its combined sewer overflow control obligations. A key feature of GSI is the capture and infiltration of stormwater before it reaches the sewer system. Because of the scale of the Philadelphia program, Philadelphia Water has implemented a sophisticated monitoring program to answer a number of basic questions about the functioning of urban GSI. The program includes monitoring of individual GSI practices to develop storm by storm water balances, and to calculate inflow, infiltration/evapotranspiration and slow release back to the sewer. It also includes hundreds of infiltration tests and an examination of the infiltration test results compared to actual infiltration rates for completed projects.

Groundwater monitoring wells have been installed adjacent to GSI to answer the basic question of the impacts of urban infiltration on the water table and potential flooding of nearby basements, particularly in right-of-way systems (rain gardens, infiltration tree trenches, etc.). Monitoring well data are coupled with continuous monitoring data collected within the GSI to evaluate the magnitude and timing of water table increases due to different storm events and the recovery period when the water table recedes to baseline depths (pre-storm conditions). Monitoring wells are also being routinely monitored at a regional scale to evaluate how the regional water table is responding to large scale implementation of GSI.

Philadelphia Water is using monitoring data to develop design guidelines, assess infiltration impacts, and to show regulatory agencies how GSI can be an effective means of reducing combined sewer overflows.