Effectiveness of Straw Bale Check Dams for Reducing Post-Fire Ephemeral Channel Erosion

Increased bed and bank erosion associated with higher flows, together with increased hillslope erosion rates, can lead to substantially increased sediment loads in ephemeral channels in areas recently burned by wildfires. The increased sediment loads can cause damage to roads/structures and impair downstream habitat/water quality. Channel treatments, such as straw bale check dams (SCD's), are designed to capture mobilized sediment in backwater pools upstream of the structures and reduce the total sediment yield at the watershed scale. Anecdotal evidence have found SCD's to be effective at retaining sediment, however they provide a limited perspective because they do not quantify treatment effectiveness as a function of the ratio of sediment retained to watershed-scale sediment yield. A paired watershed study approach was used after the 2010 Twitchell Canyon Fire in southern Utah to analyze SCD effectiveness at the watershed scale and quantify the sediment retention of individual structures. In-channel sediment fences capture the sediment produced in five pairs of control and SCD treated watersheds, approximately 0.4 to 1.6 hectares in size. In July 2011, two sediment producing rain events occurred at the site. The first event had 10-min rainfall intensities from 27 to 59 mm hr-1, and filled all the SCD's in the five treated watersheds to capacity. Treated watersheds produced less sediment than control watersheds; in two cases the difference was greater than 2 to 3 m3. After the second rain event which was significantly smaller than the first, there was little to no difference in sediment yields between treated and control watersheds. These initial results suggest that SCD's are effective at capturing sediment during the first sediment producing rain events after a fire. However, due to their limited storage capacity, sediment produced by subsequent events might not be retained, leading to substantially reduced treatment effectiveness for multiple events.