Stormwater Symposium

Oral

393825 - Quantifying Volume Reduction in Grass Buffers and Grass Swales

Monday, June 4
10:30 AM - 12:00 PM
Location: Greenway AB
Co-Authors: T. Andrew Earles, Denver, CO – Wrightewater Engineering Company; Jim Wulliman, Lakewood, CO – Muller Engineering Company

The latest Colorado Phase II General Permit for Municipal Separate Storm Sewer Systems (MS4s) included a new design standard for stormwater runoff reduction. Development of criteria for implementation of this design standard was a collaborative effort involving multiple agencies, literature review, analysis of several field tests performed in Colorado, as well as detailed SWMM modeling.

Developing guidance for the MS4 permit “runoff reduction standard” involved modeling and field-based data collection to understand how receiving pervious areas (RPAs) on a site can help to reduce the rate and volume of runoff. This is a simple concept rooted in disconnecting impervious area; however, the analysis considered some of the hidden complexities in this practice related to ratios of unconnected impervious area (UIA) to RPA, length to width ratios, infiltration rates of RPAs, and slope characteristics needed to fully infiltrate/reduce runoff for a given precipitation depth and to perform as an effective stormwater control measure without the need for excessive maintenance. The modeling effort was conducted using SWMM and included sensitivity analysis of various parameters in an effort to develop and fine-tune a simplified method for quantifying volume reduction at the individual lot scale. Modeling results were compared to several field tests in the Denver Metropolitan area. These previously conducted field tests had been performed on existing grass buffer and grass swale installations constructed in areas of different land use and hydrologic soil group.

Criteria derived from this work will be summarized in a Fact Sheet and accompanied by an Excel-based workbook tool.


Holly Piza, P.E.

Standards Development Manager, Operations and Development
Urban Drainage and Flood Control District

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