Experiments on measuring and verifying sediment trapping capacity of grass strips

Abstract

Vegetation-restored hillslopes not only reduce erosion but also trap sediment from uphill silt-laden inflow. To investigate the sediment trapping mechanism of grass strips, a series of crossed experiments with various factors, including slope (5–20°), sediment concentration (40–160 g L−1), and unit flow rate (7.5–45.0 L min−1 m−1) were conducted using a 10-m-long and slope adjustable soil plot. The duration of each experiment was longer than required time to reach the stable state of sediment trapping, which ensures that the whole process of sediment trapping can be monitored and the maximum sediment trapping yield (sediment trapping capacity, Rm) can be obtained. The results showed that gentler slopes generated higher instantaneous sediment trapping efficiency (ISTE) and greater Rm. Higher sediment concentration or larger unit flow rate led to lower ISTE but greater Rm. The impact of slope on Rm amplified when the sediment concentration increased, implying that the interaction among these factors affects sediment trapping process. The sediment trapping effect of the first 2-m width grass strips performed better than the subsequent strips. 90% of sediment deposition occurred within first half of the time needed to reach the stable state. Slope had similar effect intensity on sediment trapping in each section of grass strips, so did flow rate, whereas the effect of sediment concentration was concentrated primarily in the first 5-m width. The standard regression coefficients of the multiple curve regression equation showed that the intensities of the impacting factors on Rm were as follows: slope > grass strips width > unit flow rate > sediment concentration. It indicated that slope plays a key role in controlling Rm. These results are helpful in assessing the effects of vegetation restoration on sediment transport at a hillslope scale.