Effectiveness of straw bale check dams at reducing post-fire sediment yields from steep ephemeral channels.

Post-fire flooding and elevated sediment loads in channels can pose hazards to people and structures within the wildland-urban interface. Mitigation of these hazards is essential to protect downstream resources. Straw bale check dams are one treatment designed to reduce sediment yields in small ephemeral catchments (<2ha). This study investigated their effectiveness in five paired catchments burned at high severity during the 2010 Twitchell Canyon Fire in Utah. Rainfall, ground cover and hillslope erosion rates were also measured during the two-year study. Adjacent paired catchments were physically similar and ranged in size from 0.2 to 1.6ha across pairs. Within pairs, one catchment was an untreated control and the other treated at a rate of four straw bale check dams ha. High intensity rainfall, erodible soils and slow regrowth contributed to the observed high hillslope sediment yields (> 60Mgha). 1- and 2-yr I return period rain events early in the study quickly filled the straw bale check dams indicating the treatment did not statistically reduce annual sediment yields. First year annual sediment yields across all catchments were 19.6 to 25.7Mgha. Once the check dams were full, they had limited storage capacity during the second post-fire year, allowing 3.8 to 13.1Mgha of sediment to pass over the check dams. The mean mass of sediment trapped by individual straw bale check dams was 1.3Mg, which allowed them to trap a mean of 5.9Mgha of sediment at the given treatment rate. Straw bale check dams trapped <50% of the total mass delivered from catchments with efficiency decreasing over time. Increasing straw bale check dam treatment rate in stable channels may improve trap efficiency. Application of this treatment in areas with lower expected rainfall intensities and less erodible soils may be justifiable.