Enhancing Stormwater Management Benefits of Rainwater Harvesting via Innovative Technologies

Rainwater harvesting (RWH) systems provide the dual, but often opposing, benefits of acting as alternate water supply sources and providing detention/retention of roof runoff that would otherwise become stormwater runoff. A major challenge that exists when using a RWH system to simultaneously accomplish stormwater and water-conservation benefits in nonarid regions is that these systems are often underutilized, thus remaining full a large portion of the time. For a system to mitigate stormwater runoff, however, there must be sufficient room available in the tank to store a runoff event. Two novel approaches were implemented to improve the ability of RWH systems to serve as both water-conservation practices and stormwater-management practices: an active release technology and a passive release technology. Two locations in Craven County, North Carolina, had RWH systems installed to capture roof runoff and store it for nonpotable uses. One system was equipped with the passive release mechanism, which divided the storage tank into a detention storage volume that was slowly drained between storm events, and a retention storage volume, which was retained for harvesting. The other system included an active release mechanism that automatically released harvested water based on real-time forecasted precipitation and current RWH system conditions. Despite minimal usage, both systems provided substantial stormwater mitigation. The passive release system averaged 82 and 90% volume and peak flow reductions, respectively, while the active release system reduced volumes and peak flows by an average of 91 and 93%, respectively. Both of these mechanisms exhibit great promise in revolutionizing rainwater harvesting system use to meet both water-conservation and stormwater-management goals.