Using SWMM and pre-construction flow monitoring to forecast the hydrologic impacts of residential street permeable pavers

Scott Dierks

ABSTRACT

As part of a project to re-design and re-construct Easy Street in Ann Arbor, Michigan the hydrologic and water quality benefits of proposed Low Impact Development (LID) best management practices (BMPs) have been assessed with flow and water quality field monitoring and SWMM modeling (EPA SWMM v.5.0.006b) of wet weather events. In a novel approach for the City of Ann Arbor, part of the existing street asphalt surface is being replaced with porous pavers.

This evaluation has two phases: the first phase, described in this paper, is a field and modeling evaluation of existing conditions and a preliminary model evaluation of proposed conditions. Phase 2 will include field monitoring following installation of the porous pavers and calibration of the SWMM model to post-construction conditions.

For Phase 1, flow and water quality measurements were taken over four months to support the SWMM modeling and pollutant load calculations of existing conditions. A SWMM model of proposed conditions was also created to forecast the changes due to the proposed improvements. The improvements include two ribbons of porous pavers between 3.0 – 3.4 feet wide on either side of the street along with additional vegetated swale area.

The SWMM model was calibrated to two flow meters, one at the outlet for Buhr Park and one at the outlet of Easy Street for three storm events that occurred between November 2005 and May 2006 (Buhr Park drains to the middle of the modeled storm sewer in Easy Street). Except for one event at the downstream station, the ratio of predicted to measured peak flows were within +25% for all events for all locations. Except for two events at the same location, all predicted and measured event volumes were within +20%.

Frequent street flooding during monitoring proved a challenge to capture in the model. In addition, attempts to use SWMM’s groundwater routine to simulate exfiltration into the paver underdrains had to be abandoned in favor of an “exfiltration pump” workaround. Comparison of the calibrated model to the proposed conditions model, showed that for small events (<0.5 inches) street flooding could virtually be eliminated, peak flows could be reduced by 20-50%, while volumes could be reduced between 40-50%. Peak flow and total flow volume reductions for design events, 2-year (2.5 inches), 10-year (3.5 inches) and 100-year (4.9 inches), ranged between 7-13% and 0-20%, respectively.


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