Transport time scales in soil erosion modelling

Unlike sediment transport in rivers, erosion of agricultural soil must overcome its cohesive strength to move soil particles into suspension. Soil particle size variability also leads to fall velocities covering many orders of magnitude, and hence to different suspended travel distances in overland flow. Consequently, there is a large range of inherent time scales involved in transport of eroded soil. For conditions where there is a constant rainfall rate and detachment is the dominant erosion mechanism, we use the Hairsine-Rose (HR) model to analyze these timescales, to determine their magnitude (bounds) and to provide simple approximations for them. We show that each particle size produces both fast and slow timescales. The fast timescale controls the rapid adjustment away from experimental initial conditions – this happens so quickly that it cannot be measured in practice. The slow time scales control the subsequent transition to steady state and are so large that true steady state is rarely achieved in laboratory experiments. Both the fastest and slowest time scales are governed by the largest particle size class. Physically, these correspond to the rate of vertical movement between suspension and the soil bed, and the time to achieve steady state, respectively. For typical distributions of size classes, we also find that there is often a single dominant time scale that governs the growth in the total mass of sediment in the non-cohesive deposited layer. This finding allows a considerable simplification of the HR model leading to analytical expressions for the evolution of suspended and deposited layer concentrations.

Effect of Antecedent Conditions on Soil Erosion Dynamics in a Laboratory Flume

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

The effect of antecedent conditions on precipitation-driven soil erosion dynamics through multiple rainfall events was investigated using a pair of 6-m • 1-m flumes with 2.2% slope. Three experiments denoted H6, H7-E2 and H7-E3, involved the same precipitation rate of 74 mm h-1, but using the different initial conditions. 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 (H6) only. However, the initial bulk density and moisture content were increased before the two others events using different pre-wetting followed by 22h of air-drying. Sediment concentrations at the flume exit reach steady-state conditions over time scales that increase with sediment size, and experiments were designed such that both steady and non-steady effluent concentrations were reached in H7-E1. Results showed that short-time soil erosion was sensitive to whether steady-state erosion was achieved during the preceding event, although consistent steady-state effluent concentrations were reached for each sediment class. Steady state concentrations were, however, dependent on the rainfall intensity. 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 steady concentrations were not reached, the subsequent event produced effluent concentrations that increased gradually to steady state. The results showed that the presence of rock fragments on the topsoil reduced the time needed to reach steady state compared with bare soil. Digital terrain models (DTMs) were generated before and after one of the experiments. The results revealed that the rock fragments protected the soils from raindrop detachment and retarded the overland flow, therefore decreasing its sediment transport capacity. The DTM results showed that the presence of rock fragments on the soil surface led to increased soil compaction, perhaps due to higher soil moisture content (from greater infiltration) within the rock fragment-covered flumes.

2012

Soil erosion transport through multiple rainfall events in the presence of stone cover: Laboratory flume experiments and analysis with the Hairsine-Rose model

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

Soil erosion is a major environmental problem that can lead to loss of fertility and degradation of agricultural fields. In order to develop efficient strategies to mitigate the impact of precipitation and reduce the erosion rate, a process-based understanding of the mechanisms that govern sediment transport and delivery is necessary. Soil state and physical properties prior to a precipitation event can affect significantly the erosion rate. Among the most important soil variables are moisture content, compaction and infiltration capacity. Additionally, the presence of stones on the topsoil surface retards the overland flow discharge, reduces runoff generation as well as the sediment delivery and prevents the development of a surface seal, which in turn maintains the infiltration rate. The aim of this study was to examine in detail the effect of surface stones, soil compaction and sealing for a sequence of rainfall events on soil erosion. Experiments were conducted using the EPFL erosion flume, which was divided into two identical flumes (one with stone and one without). The experiment involved four rainfall events with the precipitation rates: 28, 74, 74 and 28 mm h-1. After each 2-h event, the soil was allowed to air dry for 22 h. The total sediment concentration, the concentration of seven sediment size classes and the flow discharge were measured during each event at the outlet of each flume. Experimental results were analyzed using the Hairsine and Rose (H-R) soil erosion model. Results showed that (i) within each precipitation event, the proportion of each size class for the bare/stone-covered flume pairs at steady state were similar, whereas the initial response differed significantly; (ii) in all cases the effluent was enriched in finer particles relative to the original soil; and (iii) the effluent sediment composition was different from that of the original soil, and there was no clear trend towards the parent soil sediment size composition with time. The H-R model was able to reproduce well the events with high precipitation rate (events 2 and 3) with the same parameter set, while the match was less satisfactory for the low precipitation events. A possible explanation for this is that the initial soil compaction/sealing/development of the deposited layer combined to yield a surface that eroded similarly for identical rainfall conditions. Changes in the precipitation rate modifies the soil surface (the deposited layer in particular) and thus the erosion rates. Model application further suggested that over the course of the rainfall events, the contribution of the original soil to the eroded sediment decreased gradually, while that of the deposited layer increased.

2011

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 Antecedent Conditions on Soil Erosion Dynamics in a Laboratory Flume

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

2012

Process-Based Precipitation-Driven Soil Erosion Modeling

Seifeddine Jomaa

EPFL2012

Soil erosion transport through multiple rainfall events in the presence of stone cover: Laboratory flume experiments and analysis with the Hairsine-Rose model

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

2011