Does Removing Large Wood From Streams Reduce Large‐Magnitude Flood Risk? A Case Study From North Carolina, USA

Abstract

ABSTRACTLarge wood (LW) removal from streams is one of the most common river channel management activities and is often undertaken with little analysis of the impacts. A primary objective of LW removal is often flood risk reduction, but reports of actual flood reduction benefits have varied widely and were often anecdotal in nature. The objective of this study was to quantify the flood reduction impacts of removing LW from the stream channel. Uncalibrated two‐dimensional hydraulic models were used to simulate the impact of various levels of wood accumulation at three study sites and one‐dimensional modeling was used to conduct a sensitivity analysis of channel slope, floodplain width, and channel blockage across a range of flood‐frequency return periods. The existing accumulation resulted in very little change in upstream water surface elevations (WSE). Theoretical LW blockages were added to the model and resulted in a maximum of 0.3 m rise in upstream WSE for the most severe channel blockage scenario of 75%. Sensitivity analysis showed that the impact of LW blockage is greatest for steeper streams with narrower floodplains, although backwater conditions propagated only short distances upstream compared to streams with lower slopes. Overall, we recommend moving away from blanket removal of LW and toward limited strategic removal that will result in the desired outcomes. A strategic approach will require a comprehensive evaluation of proposed LW removal projects, including documenting flooding concerns, the channel and floodplain morphology, and adjacent infrastructure. The results of this study will help quantify the potential flood reduction of LW removal and provide guidance for when the removal of LW from rivers may or may not provide flood reduction benefits.