Hysteretic sediment fluxes in rainfall-driven soil erosion

Hysteresis patterns of different sediment particle sizes were studied via a detailed laboratory study and modelling. Seven continuous rainfall events with stepwise- varying rainfall intensities (15, 30, 45, 60, 45, 30 and 15 mm,h\textsuperscript{-1}, each 20 min duration) were conducted using a 5-m, $\times$ ,2-m erosion flume. Flow rates and sediment concentration data were measured using flume discharge samples, and interpreted using the Hairsine and Rose (HR) soil erosion model. The total sediment concentration and concentrations of seven particle size classes (< 2, 2-20, 20-50, 50-100, 100-315, 315-1000 and > 1000 $\mu$ m) were measured. For the total eroded soil and the finer particle sizes (< 2, 2-20 and 20-50 $\mu$ m), there was a clockwise pattern in the sediment concentration versus discharge curves. However, as the particle size increased, concentrations tended to vary linearly with discharge. The HR model predictions for the total eroded soil and the finer particle size classes (up to 100 $\mu$ m) were in good agreement with the experimental results. For the larger particles, the model provided qualitative agreement with the measurements but concentration values were different. In agreement with previous investigations using the HR model, these differences were attributed to the HR model’s assumption of suspended sediment flow, which does not account for saltation and rolling motions.

Hysteretic sediment fluxes in rainfall-driven soil erosion: Particle size effects

David Andrew Barry, Mohsen Cheraghi, Seifeddine Jomaa

A detailed laboratory study was conducted to examine the effects of particle size on hysteretic sediment transport under time-varying rainfall. A rainfall pattern composed of seven sequential stepwise varying rainfall intensities (30, 37.5, 45, 60, 45, 37.5 and 30 mm h‑1), each of 20-mins duration, was applied to a 5-m × 2-m soil erosion flume. The soil in the flume was initially dried, ploughed to a depth of 20 cm and had a mechanically smoothed surface. Flow rates and sediment concentration data for seven particle size classes (< 2, 2-20, 20-50, 50-100, 100-315, 315-1000 and > 1000 µm) were measured in the flume effluent. Clockwise hysteresis loops in the sediment concentration versus discharge curves were measured for the total eroded soil and the finer particle sizes (< 2, 2-20 and 20-50 µm). However, for particle sizes greater than 50 µm, hysteresis effects decreased and suspended concentrations tended to vary linearly with discharge. The Hairsine and Rose (HR) soil erosion model agreed well with the experimental data for the total eroded soil and for the finer particle size classes (up to 50 µm). For the larger particle size classes, the model provided reasonable qualitative agreement with the measurements although the fit was poor for the largest size class (> 1000 µm). Overall, it is found that hysteresis varies amongst particle sizes and that the predictions of the HR model are consistent with hysteretic behavior of different sediment size classes.

American Geophysical Union2016

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

Statistical and physical characterization of rainfall-driven overland flow morphologies

Mohsen Cheraghi

Environmental transport processes are highly coupled with the shape of landscapes. Modern catchment analysis with high-resolution data and huge computational powers demand more detailed within-cell metrics for surface evolution modeling. This dissertation involves experiments and numerical simulations of rainfall-driven sediment transport at laboratory scales of which areas were less than the common computational cell sizes at catchment scales. The objective was a stochastic and physical study of unchanneled overland flow morphologies. In the first step, a detailed laboratory study was conducted to highlight the effects of morphological changes on hysteretic sediment transport under a time-varying rainfall. A rainfall pattern composed of seven sequential stepwise varying rainfall intensities was applied to a 5-m×2-m soil erosion flume. Clockwise hysteresis loops in the sediment concentration versus discharge curves were measured for the total eroded soil and the finer particle sizes. However, for larger particle sizes, hysteresis effects decreased and suspended concentrations tended to vary linearly with discharge. Overall, it is found that hysteresis varies amongst particle sizes and that the predictions of the HR model are consistent with hysteretic behavior of different sediment size classes. After demonstrating the role of morphological changes (generation of a shield layer on the topsoil) on erosion patterns, overland flow morphologies were statistically characterized. To do so, the catchment-scale network analyses were applied on the micro-roughnesses of unchanneled surfaces at the laboratory scale (2-m×1-m flume). The scaling relation between the drainage area and stream length (Hack¿s law), along with exceedance probabilities of drainage area, discharge, and upstream flow network length, is well known for catchment-scale channelized fluvial regions. It was found that the relationships for the overland flow network were the same as those found for large-scale catchments and for laboratory experiments with observable channels. In addition, the scaling laws were temporally invariant, even though the network dynamically changed over the course of the experiment. In the next step, we tested the applicability of a physically-based catchment scale landscape evolution model (LEM) at laboratory scale and in absence of rills. We modeled the overland flow as a network that preserves the water flux for each cell in the discretized domain. This network represented the surface flow and determined the evolution direction. The spectral analysis confirmed that the model predictions capture the main characteristics of the measured morphology. However, the model could not reproduce the experimental scaling relation as the micro-roughnesses of the surface were not produced by the model. In order to modify the LEM, it was solved as a stochastic partial differential equation. The results showed that an extra term for roughness was necessary to simulate more details of the morphology. Furthermore, a new deterministic approach (diffusion coefficient as a step function of curvature) was proposed and tested to improve the model predictions in the statistical sense.\ Besides the scientific contributions, useful modeling, optimization and data analysis tools (C++ and Python codes) were provided for future geomorphological studies.