research location Retrofitting with Bioretention and a Bioswale to Treat Bridge Deck Stormwater Runoff

(Bio)swale

Created at: 04 Jan 2020

Description

Stormwater runoff from roadways is a source of surface water pollution in North Carolina. The North Carolina Department of Transportation (NCDOT) is required to implement stormwater control measures (SCMs) in the linear environment. NCDOT has specific interest in runoff from bridge decks, which is often discharged directly to a stream below. The research presented herein focuses on retrofit stormwater SCMs for bridge deck runoff management. Two bioretention basins and a swale were constructed in the easement of a bridge deck on I-540 at Mango Creek in Knightdale, North Carolina. One bioretention basin was sized to capture (without overtopping) runoff from the 0.7 inch event. The second basin was undersized by one-half (as compared to the large cell) and captured runoff from the 0.4 inch event. Undersized bioretention basins might often be used in retrofit situations when space is limited; therefore, it is important to understand how an undersized bioretention basin performs with respect to hydrology and water quality. Both bioretention basins employed 20 in (51 cm) of fill media, and had an internal water storage layer (IWS) of 2 ft (0.6 m) including the gravel drainage layer. The swale was designed to convey the ten-year storm event without overtopping. Runoff was piped from the northbound and southbound lanes to the bioretention basins and swale, respectively. Data collection began in October 2009 and continued through April 2010. Weirs and stage recorders were used to monitor inflow to and outflow from each SCM. Flow-proportional, composite water quality samples were obtained at the inlet and outlet of each SCM. Monitored water quality parameters included TKN, NO2,3-N, NH4-N, TN, TP, TSS, Cu, Zn, and Pb. TN was calculated by summing TKN and NO2,3-N. For small storms [those with less than 1 in (25 mm) rainfall depth], flow volume reductions for the large and small bioretention basins were 69% and 47%, respectively. This shows the hydrologic importance of properly sizing bioretention basins when space is available. However, some benefit is also associated with undersized systems, if space is too limited to allow for “full sizing.” There was a 23% volume reduction benefit associated with the swale.


Samenvatting (Dutch description)

research location Retrofitting with Bioretention and a Bioswale to Treat Bridge Deck Stormwater Runoff

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climatescan

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