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

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.

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

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

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

2012

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

David Andrew Barry, Seifeddine Jomaa

2009

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