Geomorphic effects of natural flood management woody dams in upland streams

Abstract

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 instream 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.

Effects of natural flood management woody dams on benthic macroinvertebrates and benthic metabolism in upland streams: Importance of wood‐induced geomorphic changes

“…It may therefore be argued that the lack of new pools formed by the NFM woody dams in the current study was one of the reasons why diversity did not increase. As discussed in Lo et al (2022), bank erosion was more prominent than pool formation as the bank was much softer than the stream bed in the study catchment. The flow during the first half of the study period was also not strong enough to form pools.…”

“…Moreover, even if one patch contained a lot of sand and had a small overall grain size, the amount of sand was insignificant at the reach scale (as reported by Lo et al, 2022). New taxa preferring sand-sized sediments would not be attracted, and the patch would still be dominated by macroinvertebrates preferring coarse grains migrating from neighbouring coarse patches (Beisel et al, 2000).…”

“…The reason why the current study did not detect an impact of NFM woody dams on benthic metabolism at the reach scale was because of the small textural changes that they caused. As described by Lo et al (2022), only dams with clearance heights <0.3 m from the stream bed could result in significant sediment storage. If greater amounts of small gravels had accumulated upstream of the dams, there might have been higher…”

“…During each of these three fieldwork campaigns, benthic macroinvertebrates were collected from the three 10 m control reaches on West Gill and the 10 m reaches upstream and downstream of the six closely-monitored woody dams. Attention was given to the 10 m within the NFM woody dams as most physical habitat changes were identified within this range (Lo et al, 2022).…”

“…These patches could be biogeochemical hotspots (Krause et al, 2014), but they did not occur extensively in the study site. For example, although finer gravels accumulated immediately upstream of the seventh dam on Downs Gill, they only occupied less than 2 m of the 10 m study reach (Lo et al, 2022).…”