Boulder-Faced Log Dams as an Alternative for Gabion Check Dams in First-Order Ephemeral Streams with Coarse Bed Load in Ethiopia

Abstract

Although thousands of gabion check dams have been installed to control gully erosion in Ethiopia, several challenges still remain, such as the issue of gabion failure in ephemeral streams with coarse bed load, which abrades at the chute step. As an alternative for gabion check dams in torrents with coarse bed load, boulder-faced log dams were conceived, installed transversally across torrents, and tested (n=30). For this, logs (22–35 cm across) were embedded in the banks of torrents, 0.5–1 m above the bed, and their upstream sides were faced with boulders (0.3–0.7 m across). Similar to gabion check dams, boulder-faced log dams lead to temporary ponding, spreading of peak flow over the entire channel width, and sediment deposition. The results of testing under extreme flow conditions (including two storms with return periods of 5.6 and 7 years) show that 18 dams resisted strong floods. Beyond certain flood thresholds (represented by proxies such as Strahler’s stream order, catchment area, D95, or channel width), 11 log dams were completely destroyed. Smallholder farmers see much potential in this type of structure to control first-order torrents with coarse bed load because the technique is cost-effective and can be easily installed.