One-Dimensional Hairsine-Rose Erosion Model: Parameter Consistency for Soil Erosion in the Presence of Rainfall Splash

,
2009
Discussion par affiche

Résumé

Process-based erosion modelling has proven to be an efficient tool for description and prediction of soil erosion and sediment transport. The one-dimensional Hairsine-Rose (HR) erosion model, which describes the time variation of suspended sediment concentration of multiple particle sizes, accounts for key soil erosion mechanisms: rainfall detachment, overland-flow entrainment and gravity deposition. In interrill erosion, it is known that raindrop splash is an important mechanism of sediment detachment and therefore of sediment delivery. In addition, studies have shown that the mass transported from a point source by raindrop splash decreases exponentially with radial distance and is controlled by drop characteristics and soil properties. Here we test experimentally and numerically the HR parameter consistency at different transversal widths for soil erosion in the presence of splash. To achieve this, soil erosion experiments were conducted using different configurations of the 2 m × 6 m EPFL erosion flume. The flume was divided into four identical smaller flumes, with different widths of 1 m, 0.5 m, and 2 × 0.25 m. Total sediment concentration and the concentrations for the individual size classes were measured. The experimental results indicate that raindrop splash dominated in the flumes having the larger widths (1 m and 0.5 m). This process generated a short time peak for all individual size classes. However, the effect of raindrop splash was less present in observed sediment concentrations of the collected data from the smaller width flumes (0.25 m). For these flumes, the detached sediment was controlled by the transversal width of the flume. An amount of detached sediment adhered to the barriers instead of being removed in the overland flow. Moreover, the experimental results showed that the boundary conditions affect the concentration of the mid-size and the larger particles. The one-dimensional Hairsine-Rose model was used to fit the integrated data and to provide parameter estimates for each flume. The analytical results agreed with the total sediment concentrations but not the measured sediment concentrations of all individual size classes. The observed sediment concentrations for the individual size classes could be predicted only when the initial sediment concentration was adjusted and a new calculation of the settling velocities was used. This new settling velocity calculation was conducted by taking the effect of raindrop splash on the deposition force of the particles into account.

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,
2009
Discussion par affiche

Résumé

Official source

À propos de ce résultat

Concepts associés (12)

Concepts associés

Chargement

Publications associées (48)

Publications associées

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Experimental assessment of riverbed sediment reinforcement by vegetation roots

Paolo Perona

Vegetation roots are known to increase soil stability on hillslopes, crops and river banks, thus reducing slide hazards, surface erosion and general processes affecting lateral channel migration. Many studies have tackled this classical engineering problem from either experimental or modelling viewpoints, in general describing the role of roots entanglement of soil particles as an additional cohesion, though non-physical. On river bedforms, the alluvial material on which pioneering plants grow is often non-cohesive. Moreover, both local and non-local erosion processes drive the mechanisms of vegetation uprooting. While the first are influenced by the ratio between non-porous obstacles diameter vs sediment grain size, the second have rather a morphodynamic origin, and are responsible for the onset and migration of alluvial bedforms. In both cases, ablation of particles does not occur as the failure of a volume along preferential sliding planes, but rather gradually when local hydrodynamic bed shear stress conditions exceed some critical value. Hence, classic soil stability models are inadequate to assess the role of roots on stabilizing riverbed sediment, and experimental methods are also not well established. This study proposes an analysis of the data available from the experimental setup of (Pasquale et al. 2011, Pasquale et al. in press), in order to inquire the effect of root reinforcement of river bedforms alluvial material. We compared the amount of sediment scouring recorded inside square plots (4 m(2)) with growing Salix cuttings with that occurring around them and caused by pure morphodynamic erosion, i.e. under the absence of obstacles inducing either deposition or erosion. We show that for those cases where the aforesaid assumptions hold, then bedload transport models can be used in an inverse way in order to explain the reduced erosion in the presence of roots as an increased critical shear stress condition, which might be more useful to morphodynamic modelling purposes. We discuss the accuracy of the proposed methodology in relation to data availability, and propose improvements to design more effective laboratory experiments.

Crc Press-Taylor & Francis Group2014

Chargement

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

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Morpho-dynamics of mountain streams: from laboratory experiments to field observations.

Christophe Ancey, Stuart Nicholas Lane, François Mettra

Due to the complexity of mountain stream evolution and its causes, it is judicious to conduct flume experiments to better understand the morpho-dynamics of steep river channels. A series of long-term experiments under steady conditions revealed the high internal variability of transport processes (including erosion and deposition events). In particular, high fluctuations over time of the sediment volume stored in the channel (or released from the channel) were observed for the steepest channel experiments at intermittent transport conditions. These conditions imply an autogenic regulation of the sediment transport process. When these were extended to non-steady conditions, repeated flood events suggested a hysteresis effect on sediment transport but where the forcing frequency (of the repeated floods) plays an important role in the response of the channel system. The results are compared with preliminary field observations from continuous measurements in Swiss Alpine rivers. Simple mechanisms of steep river channel behavior are proposed.

2017

Chargement

2009

Experimental assessment of riverbed sediment reinforcement by vegetation roots

Paolo Perona

Crc Press-Taylor & Francis Group2014