| چکیده انگلیسی مقاله |
Extreme Rill erosion is an important type of water erosion. It happens at high runoff rates, especially on steep slopes, where the flow erosive force exceeds soil resistance causing the detachment and transport of soil particles. The development of rills should be taken into account in soil erosion modelling because using time independent model parameters results in incorrect estimation. Therefore, it is important to understand the dynamic mechanisms of rill erosion process that has a high spatial and temporal variability. This is particularly challenging under climate change-driven shifts in precipitation and hydrological cycle. The laboratory studies can help to simplify understanding of the stochasticity of rill processes and make it easy to understand.In this study, laboratory erosion experiments were conducted to simulate rill erosion under controlled and variable conditions. Whereas the influence of low inflow rates and soil type were investigated on the time variability of sediment concentrations, different lengths of rill were evaluated as well. Particle size distribution of sediment is determined over time to understand the eroded materials behaviour of runoff processes.A flume, 11 m long and 0.2 m wide, adjusted to 3% slope was used to provide construction of rectangular rills of 0.05 m width and maximum length of 10 m. The regulated inflow rate was produced in the flume by a calibrated rectangular weir. Experiments were carried out on two soil types: a clayey soil under three low inflow rates (20, 30, 40 ml.s-1), and a loam soil under one inflow rate (30 ml.s-1). The rill lengths tested were 1, 2, 3, 5, 7, 9, 10 m. For each experiment, the eroded soil materials in runoff at the flume outlet were periodically sampled during 30 to 50 minutes to determine sediment concentration. Two separate samples were taken within two time periods of the experiment to determine the sediment size distribution. The results indicated that the flow regime in rills was transitional except those tested under 20 ml.s-1, which was laminar. The sediment concentration over time showed different trend depending on the soil type. For the clayey soil, sediment concentration showed high changes with time for short rills (1, 2, 3, 5 m) under the three inflow rates, in which an initial peak was measured in sediment concentration just at the commencement of the experiment. However, after a certain period of time (some 10 - 15 min), it reached a relatively steady state. This change in sediment concentration decreased when the rill length was increased to 7, 9 and 10 m. For the loam soil, the sediment concentration showed a peak at a specified time during the experiment, which the time to peak decreased when the rill length was increased. Particle size distribution (PSD) of the eroded sediments were influenced by the particle size distribution of the original soil, the time, and the inflow rate. These effects were contrasted among long and short rills. In overall, PSD of sediment was generally coarser in comparison to the original soil, but in some cases, signs of size selectivity of particles were observed for the particle diameters of 0.2 to 0.4 mm. Furthermore, the PSD at the second sampling time was coarser than that of the first sampling time. This study showed the importance of the dynamic changes in characterizing concentration and particle size distribution of sediment under various conditions. This calls for thinking in temporal scale to understand the mechanisms of erosion processes and its potential use in modelling. |
