Filtration capacity of a gravel inlet system under low accumulation conditions

Filtration models were applied to a gravel inlet system to estimate attachment and/or detachment of particles onto collectors (gravel grain). Two methods were used to estimate total solidstrapping efficiency at the gravel inlet: mass concentration and particle count. The first method provided trapping estimate between 11% and 22% based on two averaging computations. The second method, particle count, showed that detachment of total solids occurred mostly with the clay size category and early duringrainfall events. Detachment reveals the quality of effluent and can be interpreted as particles being detached either from previous total solids deposit or not being retained by the collector. Based on a model by Rajagopalan and Tien, trapping ability of gravel inlet was expected to be relatively low (<50%) for particles and aggregates smaller than 100 μm. Five rainfall events in 2002 were analyzed and showed that the first event had a retention capacity of 32% with a significant statistical difference between pairs of samples from “above” and “below” the gravel, based on a paired t-test. The following rainfall events had not seen any significant difference based on the same statistical test between the above and below water samples; however, the pattern of retention within pairs of samples showed that large filtration values were associated with incoming large solids concentrations, which, in turn, are related to rainfall bursts. The laser diffractometer technique allowed the particle count method to estimate number of particles retained or detached with respect to the gravel media. Particle count was obtained by direct measurement in the fine silt and clay size region and by extrapolation of measured data for large size in the silt-sand region including small particles and aggregates. Two rainfall events (August 3 and 21) showed important detachment based on particle counting method.