Hydrologic and water quality performance of regenerative stormwater conveyance installed to stabilize an eroded outfall

Conveying concentrated urban runoff to outfall locations frequently leads to stream bank erosion, channel incision, and the formation of headcuts. Installed in an incised eroded channel, regenerative stormwater conveyance (RSC) is a grade-control and treatment system that uses a series of pools and cascades with an underlying sand bed. As a recently developed practice, RSC performance evaluations are needed to improve the design and guide regulatory accreditation. An eroded channel with a 10% longitudinal slope was stabilized with a five-cell system and monitored for a 14-month period. A small capture volume (6-mm design event) and groundwater intrusion resulted in minimal hydrologic benefit. Hydrologic mitigation was attained only for storms less than 12.7 mm. The RSC converted a substantial 50% of inflow to media flow at the second cell; however, the media flow entirely reemerged as surface flow in the saturated downstream pools. Small but statistically significant reductions in event mean concentrations occurred: 17% total suspended sediment (TSS), 17% total phosphorus (TP), and 3% total nitrogen (TN). Water quality improvements occurred primarily between the upstream and second cell, indicating limited processing by the wetter downstream pools. Comparisons between inter-event outflow grab samples and storm event outflow concentrations show nitrogen export during inter-event periods (47% increase in TN concentration). Due to elevated inter-event concentrations, overall mass loading of nitrogen increased. Hydrologic and water quality benefits were garnered only in the first three cells where groundwater intrusion was minimal. Future implementations of RSC are encouraged to account for seasonal high water table elevations that interact with the RSC media and adequately size pool and sand layer capture volumes.