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Effective Impervious Area Lake Minnetonka.

Fitch, Sarah, Ryan Navis, Sam Trebesch, Stephen Borden, and Zeinab Takbiri

Prepared by students in CE 5511: Urban Hydrology and Land Development (Instructor John Gulliver, Civil Engineering) as part of the 2012-2013 Resilient Communities Project partnership with the City of Minnetonka.

Effective Impervious Area (EIA) is the percentage of area in a given watershed that contributes to outflow volume within a given pipeshed during a storm event. EIA is of concern for urban hydrologists as the volume of water that flows into the underground pipeshed compared to the volume of rainfall over a given watershed can affect many aspects of stormwater management. If too much water flows into the pipeshed for a given event flooding can occur. Also, storm water which is received from an impervious area has a much greater possibility of containing high levels of pollutants, sediment, and other contaminants of interest. This water often discharges downstream without receiving treatment before entering the receiving water body. Precipitation which falls upon pervious land has a greater chance of infiltration, and therefore has a lower possibility of causing harm to nearby bodies of water.

Effective impervious area is also needed in a variety of engineering applications. Specifically, the EIA is needed as an input when modeling infiltration rate, outflow volume, and a variety of other watershed aspects in computer programs such as the Storm Water Management Model (SWMM).

The EIA was found for two areas of Minnetonka for which we had flow monitoring data; one residential and one commercial. The results for EIA were then applied to larger watersheds within Minnetonka to give an estimate of the EIA for areas which did not have flow monitoring data.

For this work two different methods to determine EIA were used; the GIS method and the method of Boyd et.al. The method of Boyd et.al took into account flow monitoring data and runoff volumes and was used as a more accurate approach to finding EIA as it accounted for only the sections of the watershed that contributed runoff directly into the pipeshed. The GIS method was a more simple approach and did not require any field data but only accounted for the total impervious areas of the watershed, rather than the connected impervious areas. As a result, the percent difference between the method of Boyd et. al and the GIS method was used as a calibration when determining the EIA using what was obtained from the GIS method for the larger area of the Minnetonka watershed.

Publication date: 
Minneapolis: Resilient Communities Project..
This project was supported by the Resilient Communities Project (RCP), a program at the University of Minnesota that convenes the wide-ranging expertise of U of M faculty and students to address strategic local projects that advance community resilience and sustainability. RCP is supported by the Center for Urban and Regional Affairs (CURA) and the Institute on the Environment.
17 pp
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