Bank erosion of legacy sediment at the transition from vertical to lateral stream incision

Streambank erosion is a primary source of suspended sediments in many waterways of the US Atlantic Piedmont. This problem is exacerbated where banks are comprised of fine sediment produced by the intensive land use practices of early European settlers. A stream in this region, Richland Creek incises into banks comprised of three stratigraphic layers associated with historic land use: pre-European settlement, early European agriculture and development, and water-powered milldam operation. This study aims to identify the bank processes along a reach of Richland Creek that is eroding towards its pre-disturbance elevation. The volume of material that has eroded along this stream since the milldam breached was calculated by differencing a reconstructed surface of the pond bed and an aerial lidar digital terrain model (DTM). Immediately downstream from the study reach, the channel is floored by bedrock and immediately upstream the rate of channel erosion approximately doubled along the longitudinal profile of Richland Creek, which indicate that the study reach spans the transition from a channel dominated by vertical incision in the upstream direction to horizontal widening in the downstream direction. The combined hydrometeorological conditions and dominant processes causing reach-scale cut bank erosion were investigated with analyses of stream stage, precipitation, and streambank volumetric and surfaces change that was measured during nine terrestrial lidar surveys in 2010-2012. The spatial variability of erosion during a simulated precipitation event was examined in a field-based experiment. Erosion was greatest where mill pond sediment columns detached along vertical desiccation and horizontal seepage cracks. This sediment accumulated on the bank toe throughout the study and was a source of readily-entrained fine sediment contrary to the upper reaches where depositional accommodation space is more limited. Findings suggest that hotspots of sediment excavation progress upstream, indicating that restoration efforts should focus upon stabilizing banks at these locations. Copyright (c) 2015 John Wiley & Sons, Ltd.