Stormwater conveyance networks contribute to stream degradation and instability either through direct discharges of sediment or by eroding gullies downslope of pipe outlets. To limit erosion downslope of pipe outlets, current regulations require the 10-yr, 24-hr peak velocity discharged to a vegetated area to be less than the downslope soils’ permissible velocity. Otherwise, the conveyance system must be redesigned. The effectiveness of this design standard has not been evaluated. North Carolina State University assessed 60 pipe outlets in the Piedmont and Mountain physiographic regions of North Carolina to identify which watershed and downslope characteristics influence the severity of erosion caused by pipe outlets. The effectiveness of the current standard was also assessed. Six assessed sites in Raleigh were additionally monitored for hydrologic, hydraulic, and water quality impacts. These data were also used to evaluate the effectiveness of a swale with (1) rip-rap, (2) well-established vegetation, (3), check dams with mowed turf grass, and (4) check dams with well-established vegetation to limit downslope erosion. The results indicate the current standard does not limit downslope erosion, and designers should consider limiting the 1-yr, 24-hr peak velocity to the permissible velocity. Designers should also consider the downslope soil and vegetative conditions to reduce the potential for gully erosion. Designers can use the predictive tools developed from the data to help determine the optimal discharge point for pipe outlets. Preliminary results suggest turf grass swales with check dams may mitigate the potential for erosion downslope of pipe outlets. However, this design should be validated with field-scale studies before standards for pipe outlets are revised.
