Sediment delivery through a vetiver buffer strip as affected by the interaction between flow hydraulics and surface relief of the deposited sediment.

Abstract

Riparian buffer strips are widely employed to reduce fluxes of eroding soil and associated chemicals, from hillslopes into waterways. However, there are a number of inconsistencies in reported research and the associated recommendations for use of buffer strips. The strip efficiency is time-dependent and changes as sediment deposition builds up, adding to the complexity of the situation. Our research focuses on settling of sediment in front of stiff grass buffers due to the induced backwaters under subcritical flow conditions and a new model is introduced to predict water, sediment and nutrient profiles in front of buffers. Experiments to test the model were carried out in the Griffith University Tilting-Flume Simulated Rainfall facility. Replicate experiments were conducted at 5% slope using a dense vetiver strip inserted into the flume. Water profiles were recorded, then asediment comprising either a sandy soil (Podzol), a red clay (Ferrosol) or a black clay (Vertosol) was introduced into flow upstream of the buffer and sediment deposition and outflow characteristics were measured for each soil. Organic and inorganic nitrogen (N) and phosphorous (P) levels were determined in the sediment and outflow. 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, moving downstream for the Vertosol, upstream for the Podzol, with the Ferrosol intermediate. 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. Buffering action reduced sediment in the outflow to 3.2, 6.0 and 11.3 % of the inflow concentration for the Podzol, Ferrosol and Vertosol respectively, with a significant difference (P<0.01) between the Podzol and Ferrosol. Sediments in the runoff were primarily in the 0.002 - 0.2 mm size range and the greatest enrichment of fines (silt size or smaller) occurred in the Ferralsol and Vertisol. Nitrate concentration in the runoff was increased by 40% compared to the inflow concentration for the Vertosol but inorganic N and P concentrations were similar to the inflow for the other 2 soils. Measured data from the flume experiments were compared to simulated data from the model. Water and sediment profiles were simulated reasonably well with low root mean square errors and coefficients of efficiency approaching 1. However, further work is required to assess the simulation of sediment concentration/particle sizes in the outflow from the model. Further work is also underway to test the model using field data from south-east Queensland.