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

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.

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

Effect of stone cover on rainfall-driven soil erosion with negligible surface roughness

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

Numerous studies have shown that, for soils with sparse vegetation cover, stones/rock fragments are an important factor controlling the hydrological response and soil erosion yields. Additionally, it has been observed that this effect is complex and ambivalent depending on the features of the stones (fraction of soil surface covered, stone size, shape and emplacement, i.e., resting on the soil surface, partially or totally embedded in the soil). The aim of this study was to ascertain whether cumulative soil erosion is proportional to the area of soil exposed under the condition of constant and uniform precipitation onto an initially smooth soil surface. Precipitation-driven soil erosion with surface stone cover and negligible roughness was tested in laboratory flume experiments under controlled conditions. The surface coverage was adjusted by placing stones onto identically prepared surfaces in laboratory flumes. Three experiments were carried out with different stone coverage (20 and 40%), rainfall intensity (28 and 74 mm h-1) and initial moisture content (6.52-24.30%), but with a fixed slope (2.2%). The laboratory results showed that the cumulative mass eroded depends on the cumulative runoff, and that soil erosion is proportional to the soil surface area exposed to raindrops. Additionally, the results showed that this relationship is controlled to a smaller extent by the soil’s initial moisture content and bulk density. Three sets of previously published field data on precipitation-driven soil erosion were analyzed to ascertain whether the same behavior occurs under field conditions. For this case it was found that precipitation-driven erosion is not proportional to the area exposed. The data sets showed consistently that prediction of cumulative eroded mass using only the stone coverage over predicts the measured eroded sediment (when plotted as a function of cumulative runoff). In contrast to the laboratory experiments, the field experiments are characterized by non-uniform initial surface roughness, and heterogeneous stone size and spatial distribution. The data are consistent with the conclusion that soil erosion is controlled by several factors including area of soil exposed, surface roughness, stone size and spatial distribution, and surface soil aging. However, the presented laboratory results show 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 (iii) that it is only slightly dependent on other soil variables.

2011

Laboratory flume experiments to study the influence of initial conditions and rock fragment cover on soil erosion dynamics

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

The influence of the initial soil surface characteristics and rock fragment coverage on precipitation-driven soil erosion was investigated through laboratory experiments on a 6-m × 1-m soil parcel with mild slope. A sequence of four experiments, consisting of 2-h precipitation followed by 22 h of air drying, was conducted. The precipitation rates used were 28, 74, 74 and 28 mm h-1. In each experiment, one flume was bare while the other had 40% rock fragment coverage. The soil was hand cultivated and smoothed before the first event (named H7-E1). Experiments were designed such that for H7-E1 steady state concentrations were obtained at the end of the erosive event only for the finer sediments. Results suggest that steady state concentrations were mainly controlled by the rainfall intensity. The sediment yields during the transient phase at the beginning of each erosive event were instead sensitive to the initial soil condition, in particular to whether steady-state was reached in the previous event. If steady-state concentrations were reached for a particular size class, that class’s effluent concentration peaked rapidly in the next rainfall event, then declined gradually to its steady-state value. When the stationary behaviour was not reached, the subsequent event produced effluent concentrations that increased gradually to steady state. Steady state concentrations were achieved faster in the presence of stones on the soil surface compared with bare soil condition. The suitability of the Hairsine and Rose model to analyse the sediment concentration of the individual size classes was evaluated. The model could not predict in detail the erosion yields when topsoil was heavily modified by rainfall (H7-E1 and H7-E2), while a satisfactory comparison was observed for the last two experiments (H7-E3 and H7-E4), in which the soil was less modified by raindrop impact. Model application indicates that the contribution of the original soil to the eroded sediment decreased gradually during the sequence of experiments, while that of the deposited layer increased.

2012

Effect of stone cover on rainfall-driven soil erosion with negligible surface roughness

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

2011

Process-Based Precipitation-Driven Soil Erosion Modeling

Seifeddine Jomaa

EPFL2012

Laboratory flume experiments to study the influence of initial conditions and rock fragment cover on soil erosion dynamics

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

2012