MODELING DEPOSITION PROCESSES IN VEGETATIVE FILTER STRIPS

Vegetative filter strips have been widely used for controlling soil erosion from agricultural land, road sides, construct sites, and other disturbed lands. The most importan characteristic of a filter strip is the sediment trapping efficiency. Yet, not many studies have been conducted to quantitatively describe the trapping process and trapping efficiency. In this paper, the results of a laboratory study are presented. In the study vegetative filter strips were simulated with polypropylene broom bristles of different densities.Results show that the deposition process in filter strips is described by a time decreasing, three-parameter exponential relationship. The parafneters vary with both bristle density and nume slope steepness. Bristle density and fiume slope steepness are two impobot factors in derermining the trapping efficiency. As bristle density increased from 2,500 bunches/m2 to 10,000 bunches/m2, trapping efficiency increased to about 45%. When slope steepness increased from 2% to 4%, trapping efficiency decreased from 50% to 5% for the low bristle density and from 90% to 50% for the high bristle density. Over 80%~90% of the sediment deposited upstream from the entrance of the filter strips. Most of the sediment trapped had a particle size greater than 150 pm. As the flume slope steepness increased, deposition occurred further down into the filter strips and the sediment passing through the filter strips became largr in size. Flow rate and sedimtnt concentration in the studied ranges (1 .45~7.0×10-3 m3/s for flow rates and 1.71~7.0 kg/m3 for sediment concentrations) hardly impafted the trapping efficiency.