Volume Reduction Provided by Eight Residential Disconnected Downspouts in Durham, North Carolina

One major concern of increased development is the proportion of directly connected impervious areas (DCIA) in urbanized watersheds. A cost-efficient opportunity to treat stormwater within existing residential and small-scale commercial developments is to disconnect roof gutter downspouts and direct impervious surface runoff over lawns. Four paired residential downspout disconnection sites in Durham, North Carolina, were studied to quantify volume and peak flow reduction. Hydrologic data were collected from January 22, 2013, to October 8, 2013. For each site, the performance of disconnected downspouts discharging water over existing lawn was compared for three varying factors: slope of lawn, length of run over lawn, and proportion of contributing roof area to receiving lawn area. Data were analyzed from approximately 60 storm events. Performance was evaluated by calculating volume reduction with and without the direct rainfall on the lawn, resulting in cumulative runoff volume reduction ranges of 57–99% and 49–99%, respectively. Findings indicate that this simple and inexpensive stormwater control measure (SCM) might be an important, yet heretofore minimally accounted for, tool to mitigate runoff.