Ability of One-Dimensional Hairsine-Rose Erosion Model to Predict Sediment Transport over a Soil with Significant Surface Stones

Surface stones affect erosion rates by reducing raindrop-driven detachment and protecting the original soil against overland flow induced-hydraulic stress. Numerous studies have shown that the effect of surface stones on erosion depends on both the stone characteristics (e.g., size, distribution) and the soil properties. The aim of this study was (i) to quantify how the stone characteristics can affect the total sediment concentration and the concentrations of the individual size classes, (ii) to test if stones affect preferentially a particular size class within the eroded sediment and (iii) to determine whether the 1D Hairsine-Rose (H-R) erosion model can represent the experimental data. A series of laboratory experiments were conducted using the 2 m × 6 m EPFL erosion flume for a high rainfall intensity (60 mm h-1) event on a gentle slope (2.2%). The flume was divided into two identical 1-m wide flumes. This separation was done to allow simultaneous replicate experiments. Experiments were conducted with different configurations and scenarios (stone coverage, size and emplacement). Three coverage proportions (20%, 40%, and 70%), two stone diameters (3-4 and 6-7 cm) and two emplacement types (topsoil and partially embedded) were tested. For each experiment, the total sediment concentration, the concentration for the individual size classes, and the flume discharge were measured. Infiltration rates were measured at different depths and locations. A high resolution laser scanner provided details of the surface change due to erosion during the experiments. This technique allowed us to quantify the spatial distribution of eroded soil and to understand better if sediment transport is 1D or rather 2D over the flumes. The one-dimensional Hairsine-Rose (H-R) erosion model was used to fit the integrated data and to provide estimates of the parameters. The ability of the 1D H-R model to predict the measured sediment concentrations in the presence of stones in the soil matrix with different configurations was tested, with reasonable results.

Process-Based Precipitation-Driven Soil Erosion Modeling

Seifeddine Jomaa

Soil erosion due to rainfall is a complex phenomenon. Factors influencing erosion and subsequent transport include rainfall intensity, soil properties, topography, land cover and antecedent conditions. These factors and their interactions can be approximated in physically based models. This thesis aimed to develop new physical understanding of precipitation-driven soil erosion processes on the laboratory flume scale. Experiments were carried out using the 6-m × 2-m EPFL erosion flume. In all experiments, different surface treatments were employed, with sediment concentration data collected from the flume effluent. The experimental data were analyzed using a mechanistic process-based soil erosion model, known as the Hairsine-Rose (H-R) model, with the goal of assessing the model's ability to describe the data. The H-R model simulates an arbitrary range of sediment size classes, a feature that is crucial to describe flume-scale soil erosion in detail. A range of related phenomena was investigated, as outlined below. First, the effect of raindrop splash and transversal width of the experimental flume on soil erosion was examined. In addition, the parameter consistency of the H-R model under these conditions was tested. Results showed that the H-R model represented well the total sediment concentrations as well as those of the fine and medium sediment size classes only when the transverse width of the flume experiment was below/around a threshold value related to lateral splash length. It was found, however, that boundary condition-induced asymmetry markedly reduces the applicability of the H-R model (and likely any other erosion model). Furthermore, in common with some existing reports, it was confirmed that the H-R model does not predict well the breakthrough of larger sediment size classes, possibly because for those sediments the transport mechanism involves rolling on the bed, which is not accounted for in the model. Second, the proportionality between soil erosion and area-exposed to the raindrop splash was tested. Results revealed that under carefully controlled laboratory conditions, the cumulative eroded mass depends on the cumulative runoff, and that soil erosion is proportional to the area exposed to raindrops. However, this relationship was controlled to a smaller extent by the initial soil conditions such as moisture content, bulk density and soil surface characteristics. The sediment concentrations in the flume effluent during the initial phase of the erosion event were sensitive to the initial state of the soil surface. At steady state, it was observed that the concentration of eroded sediments were controlled mainly by the effective rainfall and area exposed to raindrops. In contrast, using published data it was shown that rain splash erosion in the field is, in general, not proportional to the area exposed. Unlike the controlled laboratory experiments, the field experiments were characterized by a non-uniform initial surface roughness, surface soil aging and heterogeneous rock fragment size and spatial distribution. The third part of this study was dedicated to investigation of the effect of rock fragments on flume-scale hydrological response and soil erosion delivery. An area-based modification of the H-R model that accounted for the rock fragment coverage was employed to analyze the experimental data. The results revealed that the rock fragments protected the soils from raindrop splash and retarded the overland flow, therefore decreasing its sediment transport capacity. Individual rock fragments prevented surface sealing beneath them, which in turn increased the overall infiltration rate and affected the development of water depth compared with bare soil conditions. At short times, rock fragments were found to affect selectively the different size classes. This is due to the delayed (compared to a bare soil) development of the overland flow depth at the commencement of the erosion event. At steady state, the rock fragments led to decreased concentrations of the individual size classes in proportion to effective rainfall and the area exposed to raindrops. It was found that the modified H-R model predictions agreed well with measured sediment concentrations when high rainfall intensity and low rock fragment coverage were used. On the other hand, the H-R model could not reproduce the details of measured sediment breakthrough curves for conditions of low rainfall intensity and high rock fragment coverage. Finally, the effects of initial soil conditions and rock fragment coverage on particle size distribution and eroded mass were investigated using multiple rainfall events. Results revealed that short-time soil erosion is sensitive to the erosion history, in particular to whether steady-state conditions were reached in the preceding erosion event. Because the largest sediment sizes take longer to reach steady state, this sensitivity was more pronounced for the larger size sediment classes. The results also showed that the presence of rock fragments on the topsoil surface reduced the time needed to reach steady state compared with bare soil. This reflects the reduction in rain splash erosion caused by rapid development of the overland flow depth (which results from the reduction of the flow cross-sectional area by rock fragments).

EPFL2012

Parameter Changes from Upscaling of a Local Scale, Process-Based Erosion Model

David Andrew Barry, Seifeddine Jomaa

Soil erosion affects agricultural productivity, the natural environment and infrastructure security. Soil loss and its associated impacts are important environmental problems. Consequently, model-based predictions of erosion are beneficial for a variety of applications. Process-based erosion models are used to forecast sediment transport concentration as it varies temporally and spatially. Of these, the one-dimensional Hairsine-Rose model describes multiple particle size classes, rainfall detachment, flow-driven entrainment and deposition. This model has been evaluated for different experiments, and has been shown to reliably explain experimental data in a consistent manner. It is common on both the hillslope and laboratory scales to apply one-dimensional erosion models even though the overland flow and sediment transport is two-dimensional. One-dimensional parameter determinations, which are based typically on outflow data, implicitly average the two-dimensional flow. Here we compare experimentally and numerically this averaging process for the Hairsine-Rose model. For this purpose, laboratory experiments were performed using different configurations of the 2 m × 6 m EPFL erosion flume. The flume was divided into 4 smaller flumes, with widths of 1 m, 0.5 m, and 2 × 0.25 m, but otherwise identical. A series of experiments was to provide data sets for analysis by the Hairsine-Rose model. After running the experiments, the amount of the eroded sediment in each subplot was assessed by comparing the temporal variation of eroded mass to evaluate the effect of, and sensitivity to, transverse width on erosion dynamics. The surface elevation changes due to erosion were examined to provide further understanding of the erosion data. A high resolution laser scanner provided details of the soil surface in the form of digital terrain maps before and after the experiment. This method presents a promising way for identification of spatial distribution pattern of eroded soil. In addition, we ran simulations using a fully two dimensional implementation of the Hairsine-Rose model for erosive flows with varying topography with spatially dependent flow and erosion input parameters to produce both outflow hydrographs and suspended sediment graphs. The data were integrated transversely and, as for the experimental data, the one-dimensional Hairsine-Rose model was used to fit the integrated data and so provide parameter estimates to compare with the two-dimensional input values.

2009

Rain splash soil erosion estimation in the presence of rock fragments

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

Rain splash soil erosion in the presence of rock fragments and different initial conditions was tested in laboratory flume experiments under controlled conditions. The aim of the experiments was to ascertain whether cumulative soil erosion is proportional to the area of soil exposed to raindrop detachment under the condition of constant and uniform precipitation. The surface area exposed to rain splash erosion was adjusted by placing rock fragments onto the surface of identically prepared soil in laboratory flumes. The laboratory results showed that the eroded cumulative mass depended on the cumulative runoff, and that soil erosion was proportional to the soil surface area exposed to raindrops, in situations where an initially dry, ploughed and smoothed soil surface were ensured. The results showed that this relationship was controlled to a smaller extent by the soil’s initial moisture content, bulk density and soil surface characteristics. When the initial conditions were more complex, soil erosion was proportional to the area exposed only at steady state. Then, sediment concentrations during the first part of the erosion event were instead more sensitive to the initial state of the soil surface, whereas at steady state it was observed that the concentrations of eroded sediments were controlled mainly by the effective rainfall and area exposed to raindrops. Previously published field data on rain splash soil erosion were analyzed to ascertain whether the same behavior was evident under field conditions. For this case it was found that rain splash erosion is in general not proportional to the area exposed. In contrast to the controlled laboratory experiments, the field experiments were characterized by non-uniform initial surface roughness, surface soil aging and heterogeneous rock fragment size and spatial distribution. However, the presented laboratory results showed clearly that, for soils with negligible surface roughness, erosion depends on (i) the area of soil exposed to rainfall and (ii) the cumulative runoff, and that it is only slightly dependent on other soil variables.

2012