Can Traditionally-Sized Green Infrastructure be Used for Flood Control?

The City of Madison, Wisconsin wanted to understand the effectiveness of green infrastructure to help meet the City’s flood mitigation goals. Because the City is also evaluating traditional flood control solutions (increase pipe capacity, detention basins, etc.), the City is referring to green infrastructure as “volume control infrastructure”. For the purposes of this analysis, volume control infrastructure will be infrastructure meant to infiltrate stormwater. City Engineering conducted a pilot planning level analysis for the Pheasant Branch Watershed. The purpose of the analysis was to understand the data available and how it could efficiently be used to evaluate the effectiveness of volume control infrastructure. Engineering staff conducted an iterative analysis with check‐ins and discussion. The resulting analysis is described in this document. Pheasant Branch Watershed is located on the far west side of Madison and comprises approximately 3,300 acres of urban and agricultural land use. The hydrologic and hydraulic model for the watershed was built in XP‐SWMM. The hydrology and stormwater infrastructure are modeled in 1D while the large conveyance channels and overland flow is modeled in 2D. The watershed is broken up into 451 subcatchments. The hydrology for the watershed utilizes SWMM hydrology and Horton Infiltration. Each subcatchment is broken into three components:
- Directly Connected Impervious Area (DCIA),
- Non‐Directly Connected Impervious area, and ###- Pervious Area. In each subcatchment, the NDCIA is routed to the pervious area through subcatchment routing.