Monitoring Hydrological Performance of Green Infrastructure Stormwater Controls

The Louisville Metropolitan Sewer District (MSD) has entered into a Consent Decree Agreement with U.S. EPA to reduce the number and volume of combined sewer overflows (CSOs) from the existing combined sewer system (CSS). As an aspect of this agreement MSD is using green infrastructure (GI) stormwater controls to reduce CSOs. The initial phase of the project is implementing GI controls in a small urbanized neighborhood (28 acres) in Louisville, KY, in lieu of a planned concrete retention basin. In December 2011, two permeable interlocking concrete pavement (ICP) controls were constructed. To quantify surface infiltration and subsurface exfiltration performance each control was embedded with time domain reflectometers (TDRs), thermistors, and piezometers. To predict the hydrological performance of the GI controls, a spreadsheet model is developed by utilizing each control's drainage area, hydraulic conductivity values for surrounding soil layers, and dimensions of each GI control. The model calculates the inflow and outflow volumes and predicts the water level inside the control for each rain event. Hydrological performance is defined as a GI control's ability to capture the runoff volume (infiltration capacity) and then pass captured volume into the underlying soil layers (exfiltration performance). The model's predicted water levels are compared with the recorded water levels by the embedded pressure transducers. The developed model predicts the ideal performance of the GI control and compares it to recorded data. This assessment technique works to quantify the progression of clogging at the surface and within the infiltration gallery. This model provides a method for monitoring the hydraulic performance of the GI controls over time.