The hydro-geomorphic effects of wooden leaky barriers in a steep headwater stream

Abstract

Purposely placed in-stream wooden leaky barriers (LBs) with lower gaps to facilitate flow during average conditions have become a popular type of nature-based intervention in catchments for flood risk mitigation. Empirical evidence on their effectiveness remains rare, however. The hydro-geomorphic effects of 16 LBs were monitored over three years on the Elm Sike, a small upland stream (catchment area: 0.33 km²) in the Scottish Borders, UK. The aims of the structures were to: (1) increase water storage and roughness and, in turn, attenuate and delay flood peaks; and (2) trap and store coarse sediment to reduce sediment-related problems downstream. Annual topographical surveys were conducted to assess geomorphic changes, and continuous 5-minute water levels were recorded to assess changes in peak flow travel times. Two-dimensional hydraulic modelling (HEC-RAS 2D) was undertaken to assess the hydraulic impact of the structures and geomorphic changes. The LBs had no effect on peak flow travel time and rate of stage rise or fall based on 22 events with an Annual Exceedance Probability (AEP) of ≥ 3.7%. Hydraulic modelling showed limited velocity reduction and expansion of inundation extent due to the LBs for 50% AEP flows, but slightly greater effects for larger 5% AEP flows. However, for 50% AEP flows, inundation extent increased and velocities decreased more significantly due to morphological changes initiated by the structures. The limited hydrological and hydraulic impacts were related to the small size of the structures, close spacing and the steep, confined valley setting that limited water storage capacity. At the end of the three-year period, a reach-wide net deposition response of 3.49 ± 0.36 m³ and a 5.3% loss of total LB backwater storage were observed, but local geomorphic response due to the LBs was spatially variable. Backwater capacity at 10 structures was reduced, and the lower gap increased at eight structures due to bed scour, indicating accelerated underflow. Multivariable analysis showed that gap height change was inversely related to backwater shear stress, channel cross-section area and channel slope. No factors explained backwater sediment deposition, but a threshold lower gap height of ≤0.4 m for initiating deposition was evident. These hydro-geomorphic observations have implications for the design and placement of LBs in river restoration and flood risk management projects.