Secondary effects of in-stream natural flood management (NFM) projects in upland environments

Natural Flood Management (NFM) refers to flood risk reduction techniques that restore the natural functioning of catchments. We should not only put our focus on the hydraulic and flood reduction effects of NFM. It is also necessary to understand how NFM interacts with other natural processes and materials. The overarching aim of this research project was to evaluate the secondary effects of in-stream woody dams designed for NFM. Specific objectives included: (i) quantifying the effects of NFM woody dams on stream channel morphology; (ii) evaluating the influence of NFM woody dams on benthic macroinvertebrate community structure; and (iii) assessing the impact of NFM woody dams on benthic metabolic activities.

These objectives were achieved by conducting a field experiment at the upper River Cover catchment, North Yorkshire, United Kingdom. A full Before-After Control-Impact research design was adopted. Baseline data were collected between May and July 2018 (two months prior to the construction of woody dams), and three monitoring fieldwork campaigns were conducted in autumn 2018, spring 2019, and autumn 2020. Results indicate that the NFM woody dams monitored led to small geomorphic changes, including localised bank erosion, pool formation only after high flows, and sediment storage only when the dams had small clearance heights and remained stationary in place. These geomorphic effects did not translate into substantial changes in benthic macroinvertebrate communities as they did not appear to be governed by local physical habitat features. However, the woody dam reaches were less dominated by generalist taxa than the control reaches. Rates of benthic respiration and photosynthesis were both negatively related to local grain sizes. Since the dams had not resulted in large-scale stream bed fining, they had no observable impact on rates of benthic photosynthesis, and rates of benthic respiration were only slightly higher in the woody dam reaches.