STORMWATER MANAGEMENT IN A SOUTHERN APPALACHIAN FEN

Riverbend is a natural wetland community in Western North Carolina (USA) that supports a variety of rare flora, including the federally-endangered mountain sweet pitcher plant (Sarracenia jonesii) and federally-threatened swamp pink (Helonias bullata). It is located at the base of a slope and—as its name implies—on an alluvial terrace deposited within a horseshoe-shaped bend in the river downgradient.

Hydrologic monitoring, major-ion geochemistry, and thermal imagery indicate Riverbend is a “flow-through” wetland, with groundwater originating from the adjacent hillslope flowing through and below the wetland before it discharges to the river below. Upgradient residential development and agricultural activity have impacted the hydrologic regime, generating large stormwater pulses that carry nutrients and invasive species into the fen. Water levels were monitored to determine the magnitude and frequency of flood events, and game cameras were employed to delineate the areal extent of overland flow.

This presentation focuses on a hydrologic restoration project that was recently implemented to reduce the flow of stormwater, sediment, nutrients, and exotic invasive seeds into the wetland. A sequence of bioretention cells was constructed on the slope immediate adjacent to the wetland for runoff storage and infiltration. The total storage volume was limited by a high water table (0.7-1.7m below ground surface), and low infiltration rates (Ksat=2.0E-5 cm/s) were expected to limit stormwater retention after back-to-back or particularly large storm events. In the first six months after construction, the first retention cell overtopped eight times, resulting from 24-hour rain events ranging from 1.56 to 3.88in (4.0 to 9.9cm). Meanwhile, there were no storm surges in the most sensitive areas of the wetland (i.e. highest concentration of Sarracenia), and infiltration beneath the retention basins helped maintain consistent water levels throughout the fen during drier time intervals.