Experimental evidences and numerical modelling of runoff and soil erosion in flume tests

Rainfall causes runoff and soil erosion in artificial and natural slopes and important insight may derive from quantitative physically-based models or laboratory tests. Measurements of runoff and sediment discharges in small-scale experimental flume tests have become popular in recent years and a wide range of slope angles, soil grain-size-distributions and rainfall characteristics has been tested so far. In the literature, there are numerous studies dealing with a comparison between experimental data and model predictions using appropriate assumptions. However, there are still scientific gaps under complex experimental circumstances. The main goal is to discuss the performance of a physically-based numerical model in simulating well-documented runoff-erosion laboratory flume tests, also highlighting the uncertainties one may expect for real cases when applying numerical modelling of runoff and soil erosion to a real catchment. The paper deals with the numerical analysis of four experimental flume tests available in the literature, which investigate the erosion of bare gentle slopes due to constant-intensity rainfall; the behaviour of a steeper slope, bare or vegetated, under constant rainfall larger than in the previous experiments; the role of a sequence of different rainfall intensities (with the same cumulated rainfall), and different surface roughness in gentle slopes. Those experimental tests were simulated through LISEM, and the numerical results reproduce satisfactorily the global behaviour of the experimental plots eroded by artificial rainfall in all the four flume tests. As far as the ratio of the observed to the predicted peaks of water discharge and sediment concentration, the simulated peaks are very close to those observed in the laboratory experiments, except for low slope angle conditions where water discharge peak is overestimated and for one flume where sediment concentration peak is underestimated in two out of three cases. This analysis highlights that LISEM allows reasonably estimating the peak values of water and sediment discharge, which are generally used as design parameters of erosion control works. With reference to peak times of water discharge and sediment concentration, this paper highlights that LISEM has limitations in properly assessing the peak times of water discharge and sediment concentration; better results are, instead, expected when LISEM is used to simulate erosion and runoff on vegetated slopes. Globally, the results allow the assessment of the overall performance of the selected erosion model to correctly interpret the experimental evidences. As well, the discrepancies among the laboratory evidences and numerical results are discussed in relation to slope geometry and soil properties. (C) 2016 Elsevier B.V. All rights reserved.