A Flume Flushing Method Based Study On Process Of Rill Erosion On Loess Slope

Rill erosion is a serious environmental problem threatening the agricultural production safety and sustainable societal development in the Loess Plateau, China. It is, therefore, very important and urgent to quantify dynamic process of the rill erosion. To that end, an indoor simulation experiment was carried out using a PVC trough with loess and the scouring and backfilling method. Erosion process was analyzed through observing variation of the shape of rills and measuring volume of erosion along the rill segment by segment. Cumulative amount of erosion along the rills was worked out by refilling the separated rill segments with water to their original elevation. The sediment concentration was calculated by dividing the cumulative amount of erosion at each segment by the total volume of the water flow during the erosion period. A typical silty-loam soil was sampled from the Ansai Research Station of Soil and Water Conservation on the Loess Plateau of China (109° 19’23" E,36° 51’30" N) and used as experimental soil in this study. The soil contains 16.31% clay (<0.005 mm),61.35% silt (0.005 to 0.05 mm), and 22.34% sand (>0.05 mm). The soil was cleared off grass roots and rocks, air-dried and crushed immediately after it was transported to the experimental site, and then passed through an 8 mm sieve. In order to minimize boundary effect of the PVC boards and get a complete continuous erosion process, an experimental trough,3.0 wide and 12.0 m long, filled with loess, was used. In the trough six experimental rills,0.1 m wide and 12 m long, were built and separated with PVC boards inserted deep to the plow pan to simulate well-developed rills and to keep water from flowing between the rill rows. The trough was put at horizontal position for soil packing. Soil of the same nature was glued onto both sides of the PVC boards, to guarantee the same roughness of the PVC boards as the soil surface, so as to minimize boundary effect of the PVC boards on the rill erosion process. At the bottom, the trough was densely packed with a 5cm thick layer of local clay soil, made compact with a hammer to simulate the plow pan layer,15 g cm-3 in bulk density. The upper 20 cm of the trough was packed with loess in layers, about 5 cm each, to make the soil even in bulk density, ranging approximately between 1150 to 1200 kg m-3. The soil surface was raked to make it as rough as natural leaving the soil near the side boards of the trough and the PVC separators packed slightly higher than in the middle so as to converge the water flow into the middle and to further minimize boundary effect. Cumulative volume of erosion of a rill was the sum of erosion volumes of the eleven rill segments (0～0.5 m,0.5～l m,1～2 m,2～3 m,3～4 m,4～5 m,5～6 m,6～7 m, 7～8 m,8～10 m,10～12 m) of a rill. Total amount of erosion sediment was measured volumetrically by a collecting vat at the outlet of each rill during the experiment. The experiment was designed to have five slope gradients, i.e.5°,10°,15°,20° and 25° for the trough, three flow rates, i.e.2,4 and 8 L min-1 and three replicates. Results indicated that:1. The process of rill erosion was not constant along a rill, with cumulative rill erosion and sediment concentration increasing exponentially and margin of the increase declining with rill length till the extreme in the end, which means that cumulative amount of erosion and sediment concentration increases rapidly at the initial segments of the rills, and the increase rate (the slope of the curve) attenuates gradually to approach zero along the rill. Flow rate and slope gradient were the two major factors affecting rill erosion, but the former seemed to have more influence than the latter. In addition, there is a critical slope gradient between 20°～25°.2. The comparison of the cumulative amount of erosion measured with the volumetric method with the result of direct measuring at the rill outlet validated the accuracy of the former, and the comparison of the sediment concentration measured with the water backfilling method with the result of direct measuring demonstrated that the former was quite accurate in and applicable to investigating distribution of erosion sediment along a rill.3. Rill detachment rate decreased linearly with sediment concentration and exponentially with rill length (under slope gradients of 15°,20° and 25°) for all flow rates and slope gradients. That is to say, the soil detachment rate and sediment concentration moved in the opposite direction. This variation trend was more obvious at steeper slope gradient and higher flow rate. And both slope gradient and flow rate had a positive influence on detachment rate. The detachment rate obtained from previous experiment and this study were compared. The correlation coefficient of the two data sets is 0.917 which indicated that detachment rate could be reliably estimated by the two methods.4. Additionally, an analytic result was suggested to calculate analytic values of soil detachment rate. And the analytic data was compared with experimental data, a high correlation was found in comparison result which indicating that this new analytic method is accurate and provide a new aspect for estimation of soil detachment rate.5. Sediment transport capacity of rill flow was estimated. Sediment transport capacity increased with slope gradient and flow rate, but flow rate effects more significantly than slope gradient; and the critical rill length for loess soil ranges from 17.91 m to 4.41 m. Also, sediment transport capacity increased logarithmic with water power.6. Sediment transport yield increased with rill length, flow rate and slope gradient; detachment decreased with rill length, its maximum value increased with slope gradient and flow rate, so did the decreasing rate, the values of transport capacity equals to the sum of the value of sediment transport yield and the value of detachment rate indicating transport capacity should be fixed while the soil type, slope gradient and flow rate was clear. Additionally, transport capacity is a characterization of the potential of sediment transport power of rill flow. This "potential of sediment transport power" was used mostly to detach and move sediment particles at the rill inlet, and this energy would have to use to transport the detached sediment particles, and at the rill outlet almost all the power was used to transport sediment.