Evaluation of interrill component of WEPP model for three contrasting soil types

Abstract

The ability to predict the movement of sediment and associated pollutants from catchments into waterways is crucial for water quality management of the future. Vegetative buffer strips are frequently used in catchments to intercept runoff from hillslopes and reduce fluxes of eroding sediment into waterways. However, the prediction of this sediment removal is difficult, as complex physical processes are involved and the buffer strip efficiency is time-dependent, changing as sediment deposition builds up. Our research focuses on settling of sediment in front of stiff grass buffers due to the induced backwaters under subcritical flow conditions. A new model is introduced, the Griffith University Soil Erosion & Deposition Model (GUSED-VBS), which couples the hydraulics, deposition and subsequent adjustment to bed topography, to simulate the build-up of sediments in the backwater zone and its effect on flow conditions. The model thus predicts changing water, sediment and nutrient profiles in front of buffers as well as sediment concentrations and size distributions of the outflow. Experiments to test the model were carried out in the Griffith University Tilting-Flume Simulated Rainfall facility. Water profiles were recorded, then sediment comprising a sandy soil (Podzol), a red clay (Ferralsol) and a black clay (Vertisol) was introduced into flow upstream of the buffer and sediment deposition and outflow characteristics were measured. The buffer caused a region of enhanced flow depth (backwater) upstream of the buffer. Deposition started at the beginning of the backwater but subsequent deposition varied with sediment type. Sediment loads in the outflow increased slightly with time for the Vertisol due to sediment movement into the buffer but were static for the other two soils and buffering reduced sediment to <11.3 % of the inflow concentrations, with a significant difference between the Podzol and Ferralsol. In contrast to other buffer research where deposited sediments were coarsest upstream of the backwater, we found the coarsest particles at the downstream end of the deposit for the Podzol and Ferralsol, indicating that bedload movement occurred in addition to the deposition, entrainment and re-entrainment processes that dominate supercritical flow. Sediments in the runoff were primarily in the 0.002 - 0.2 mm size range and the greatest enrichment of fines occurred in the Podzol. Particulate-carbon, -nitrogen and -phosphorous levels in the outflow were reduced by more than 60% compared to the inflow. Measured data from the flume experiments were compared to simulated data from GUSED. Water profiles and total sediment deposition were simulated well with low root mean square errors and coefficients of efficiency approaching 1. The sediment concentration/particle sizes in the outflow and reductions of the inflow nutrient loads were moderately well simulated. Further work is underway to test the model using field data.