Geomorphic effects of natural flood management woody dams in upland streams

One popular Natural Flood Management (NFM) technique involves the construction of channel-spanning woody dams in low-order streams that maintain a clearance height above base flows. While extensive research has examined the geomorphic effects of natural wood accumulations, little has been documented of NFM woody dams, which are structurally distinct from natural accumulations and may produce different patterns of erosion and deposition. This consideration is crucial because changes in physical habitat characteristics have implications for flood management objectives as well as ecosystem structure and functioning. This study adopted a Before-After Control-Impact (BACI) design to assess the geomorphic effects of NFM woody dams in the upper River Cover catchment, United Kingdom. One baseline survey prior to and three monitoring surveys up to 2 years following dam construction were conducted. Structure-from-Motion (SfM) photogrammetry was employed to capture topographic change, supplemented by bathymetric surveys. Results highlight that where the dams remained secure in place, they promoted in-stream habitat diversity by creating underflow pools. Sediment storage was observed only where the dams had clearance heights <0.3 m from the stream bed. Additionally, the dams commonly led to bank erosion, likely enhanced by inherent bank instability in the study catchment as observed along the control reaches. However, volumes of sediments eroded and deposited were not statistically different between the control and woody dam reaches. Longer monitoring is required to determine whether these effects on channel morphology and habitat diversity will persist, amplify, or diminish over time, and to better understand the longevity of NFM woody dams.