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Relating Stormwater Design to Stream Health: A Case Study in Chapel Hill, NC

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  2010 Pittsburgh, Pennsylvania, June 20 - June 23, 2010  1008880.(doi:10.13031/2013.29747)
Authors:   Erica D Tillinghast, Dr. William Hunt, Dr. Gregory Jennings
Keywords:   Best Management Practice, geomorphology, stream erosion, Chapel Hill NC, hydraulics, hydrology, wetland, stormwater management, urbanization, shear stress, PCSWMM, EPA SWMM 5.0

Stormwater control measures (SCMs) commonly use peak flow attenuation to return an urbanized, developed watershed back to its pre-developed peak flow conditions. The discharge design standards for the SCMs normally target low-frequency storms (usually 2- and/or 10-year events), but ignore the smaller, more frequent events. Recent emphasis has been placed on these smaller storms as well as the impact of SCMs on the geomorphic stability of streams. The more common sub-bankfull flows produced by SCMs are subjecting streams to longer and more frequent periods of erosion, increasing the instability of the stream. A stream experiencing extreme bank erosion in Bolin Creek Watershed, Chapel Hill, NC was modeled using continuous simulations in the Stormwater Management Model (SWMM) to determine a design discharge rate or volume control standard for a stormwater wetland that would reduce the frequency and duration of erosional events the stream was currently experiencing. The post-development erosion potential of the critical portion of the flow frequency curves was compared to the pre-developed or natural state of the watershed. Stormwater Wetlands with a variety of orifices within the outlet structure were sized for peak attenuation as well as erosion protection to determine the optimal discharge design standard for Bolin Creek. This study will hopefully influence other SCM design standards to protect stream stability.

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