Green Infrastructure and the Watershed Studies

Green Infrastructure (GI) is smaller infrastructure that filters and absorbs stormwater where it falls. GI uses plant or soil systems, permeable pavement or other permeable surfaces or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters. Madison is committed to installing GI. However, as part of the watershed studies GI has been reviewed and it is generally not considered to be a significant tool for flood management. GI does many things well but conveying or reducing the peak flows in a 1% chance storm event are not one of its core strengths, even when implemented on a large scale. You may have read of cities on the east and west coasts using GI, however that is generally to address the problem of combined (storm and sanitary share a pipe) sewer overflows to the waters of the state (rivers, lakes, stream…). The City of Madison has separate storm and sanitary sewers, so GI is not needed for that application.

As part of the watershed studies a Green Infrastructure Effectiveness Analysis was conducted for one of the watersheds in the City. The purpose of the analysis was to specifically understand how much green infrastructure would be needed to meet flood reduction targets. The numbers generated by that analysis indicate that GI is not a cost effective measure to manage the 1% chance or 100 year event flooding. As noted above however, GI has other environmental, economic and social benefits. Again this analysis only reviewed GI’s impacts on flood control.

Runoff is stormwater that does not soak (infiltrate) into the ground. Stormwater that lands on hard or impervious surfaces such as roofs, driveways, streets all becomes runoff. This runoff ends up in our stormwater system if it doesn’t flow over a pervious surface and soak into the ground. The runoff from hard surfaces that flows directly into our stormwater system without infiltrating into a pervious surface is considered “directly connected impervious area.” For example, a roof downspout drains onto a driveway that then goes to the street is considered directly connected impervious area.  The study found that all of the directly connected impervious area in the watershed needs to be treated with green infrastructure to meet the flood targets.

The estimated cost to construct all that green infrastructure is twice as much as the estimated cost for constructing traditional stormwater management. For purposes of the watershed studies, traditional stormwater management include items such as installing larger pipes or creating bigger ponds. Additionally, GI is extremely maintenance intensive and more prone to failure in places where it snows—sand and salt applied to streets and parking lots can clog pervious pavements, seal soils that are intended to infiltrate, and freeze thaw cycles can cause failure of pervious pavement.

The study concludes that even though green infrastructure could be used to meet the flood study targets, it is cost-prohibitive to only use green infrastructure for flood reduction.

The City will be using green infrastructure where feasible.  But, it will not be the only part of the solution.

Green infrastructure also continues to be implemented citywide through other City initiatives.

Water Quality and the Watershed Studies

Water quality, or pollution reduction, was not included in the watershed studies. This is because these particular watershed studies focus on flooding. The analysis for flooding is typically for large storm events. The analysis for water quality is typically for small storm events which results in the “first flush” effect of concentrated pollutants entering the stormwater system with the initial part of the rainfall. The water quality analysis also typically uses different computer modeling software.

The City does have a separate program and model for water quality, as described in its Wisconsin Pollution Discharge Elimination System (WPDES) Municipal Separate Storm Sewer (MS4) Permit. Many of the proposed watershed study solutions do include water quality components as part of the design.

Ecological Impacts of Solutions

During the design phase of each solution, engineers work with landscape architects, and vegetation specialists to incorporate native vegetation that will aid the stormwater infrastructure to serve its purpose.

Deep roots of native vegetation help hold soil in place and help water infiltrate. Additionally, native vegetation is essential food for insects which are the base for our local food web.

Also, during the design phase, engineers work with Wisconsin Department of Natural Resources (WDNR) staff to comply with all natural resources rules and permitting.

Ecological Restoration in Stormwater Ponds and Greenways

The City of Madison is actively working to restore its stormwater ponds and greenways. See the Ecological Restoration In Stormwater Ponds and Greenways Story Map for more information.