Gravel Wetland Design and Performance for Stormwater Management

A gravel wetland, designed and constructed, by the University of New Hampshire Stormwater Center is demonstrated to outperform other technologies in stormwater management. Gravel wetlands have sparse published data in stormwater management and minimal design guidelines exist for them. They have been occasionally used as lined systems in lieu of septic systems. The subject gravel wetland was monitored side-by-side with a range of other stormwater technologies, and was routinely the top performer for year-round water quality improvements and storm-volume reduction. This system was designed as a series of horizontal flow-through treatment cells, preceded by a sedimentation basin (forebay). The technology is designed to retain and filter the entire Water Quality Volume (associated with the first one inch of rainfall runoff from an impervious surface): 10% in the forebay, and 45% in each of the respective treatment cells. For the small, frequent storms, each treatment cell filters 100% of the WQV. Gravel wetlands are designed as flow through treatment, where the stormwater passes through a gravel substrate that is a microbe rich environment. Side slopes should be 3:1 or flatter for maintenance, as standing water of significant depth is not expected other than large rainfall events. The wetlands should be continuously saturated below a depth of 10 cm from the ground surface in order to promote near-anaerobic water quality treatment conditions and to support wetland vegetation. To force this near-surface ground water condition, the system outlet pipe has an invert 10 cm below the wetland ground surface. In the event of a high rainfall event, the water quality volume is stored above the wetlands, and drains into a perforated riser on one end of the wetland, and into the substrate. Seasonal performance evaluations indicate that the gravel wetland routinely achieves close to 99% removal efficiency for total suspended solids, total petroleum hydrocarbons-diesel, and zinc, and > 80% for nitrate. Additionally, the subsurface nature of the system results in minimal seasonal stormwater treatment variations for such a northern climate.