Sediment retention by forested filter strips in the Piedmont of Georgia

ABSTRACT:

The capacity of forested filter strips to retain sediment mobilized during harvesting and regeneration of southern pine plantations and the relationship between sediment retention and filter strip characteristics in these systems are not well known. Simulated runoff containing particle sizes ranging from colloidal-size clay to sand-size particles and aggregates was applied to forested filter strips in two conditions (undisturbed forest floor and forest floor removed by hand raking) across five slope classes (0% to 2%, 5% to 7%, 10% to 12%, 15% to 17%, and 20% to 22%) in the Piedmont of Georgia. Total sediment retention ranged from 53% to 96% and averaged 72% across all slope classes and conditions. Particles >20 μm (>0.00079 in) in diameter were largely retained in the first 2 m (6.6 ft) of the filter strip by settling. The runoff concentration of particles <2 μm (<0.000079 in) in diameter was not affected by the filter strips, but some retention occurred through infiltration. Retention of the 2- to 20-μm size fraction was correlated to flow distance within the filter strip. In areas of undisturbed forest floor, there was a correlation between forest floor depth and sediment retention. Forest floor removal decreased retention, but even where forest floor was removed, a majority of the applied sediment was retained. Our results indicate that narrow filter strips will effectively remove coarse-textured sediments >20 μm in diameter, and a 16-m (52-ft) wide filter strip should remove most 2- to 20-μm size sediments from runoff water.