Erosion Barriers for Postfire Rehabilitation
Straw wattles, contour-felled log erosion barriers, and other natural and engineered structures have been used to provide mechanical barriers to overland flow, promote infiltration, trap sediment, and thereby reduce sediment movement on burned hillslopes. Contour-felled log erosion barriers have been widely used in areas where fires leave dead trees that can be felled, placed along the contours of burned hillslopes, staked in place, and the gaps between the logs and soil surface filled with additional soil to prevent underflow (Robichaud and others 2000). Some recent installations have included the construction of soil berms at the end of the logs to increase their storage capacities. Straw wattles, 12 in (0.25 m) diameter nylon mesh tubes filled with straw, are permeable barriers used to detain surface runoff long enough to reduce flow velocity and provide for sediment storage. Turning 2 ft (0.6 m) at each end of the wattle upslope can increase the sediment holding capacity. Straw wattles are a good alternative in burned areas where logs are scarce or poorly shaped.
The potential volume of sediment stored on a hillslope treated with erosion barriers is highly dependent on slope, barrier diameter and length, construction of berms at the ends of the barriers, and ‘quality of installation’—i.e., the degree to which the barriers are adequately staked, aligned perpendicular to the flow path, and maintain ground contact throughout their length. The length of time the treatment remains functional depends on post-installation rainfall parameters (especially rainfall intensity), density of barrier installation (i.e., volume of sediment holding capacity per acre), soil erodibility, and topography. In some instances contour-felled log erosion barriers have filled with sediment following the first several storm events after installation, while others have taken 1-2 years to fill (Robichaud 2000).
Erosion barriers do provide immediate benefits after installation, in that they trap sediment during the first postfire year, which usually has the highest erosion rates (Robichaud and Brown 2000). Dean (2001) found that plots treated with both contour-felled log erosion barriers and straw mulch with seed yielded 77% less sediment in the first postfire year and 96% less in the second year; however, these results were not significantly different from the straw mulch with seed treatment alone. Preliminary data from on-going studies suggests that contour-felled log erosion barriers can be effective for low to moderate intensity rainfall events. However, during high-intensity rainfall events, their effectiveness is greatly reduced (Table:Preliminary Results from Paired Catchment Studies of Contour-Felled Log Erosion Barrier (LEB) Treatment Effectiveness). The effectiveness of contour-felled log erosion barriers also decreases over time as the sediment storage areas behind the logs become filled and the barrier can no longer trap mobilized sediment (Robichaud 2000; Wagenbrenner 2003).
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