Experimental assessment of riverbed sediment reinforcement by vegetation roots

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.

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

David Andrew Barry, Seifeddine Jomaa

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.

2009

Experimental study of suspension flow in open channels

The study of the sediment transport in open-channel as well as in river flow is of great importance for fluvial hydraulics. While the transport of sedimentary particles at the bed, as the bed-load, has been the subject of much research, less attention has been paid to the transport of sediments in suspension. This thesis is a contribution to our knowledge of the transport of sediments in suspension. The most important theory on the vertical distribution of the mean sediment concentration in suspension flows is the diffusion-convection theory, given by the Rouse equation. This equation is rather simple and contains few parameters. About one of these parameters, the β-value, there is little known, despite its importance and physical meaning. One of the main aims of this thesis is to experimentally measure the β-values for different suspension flow configurations. The major objective was to investigate suspension flows over moveable beds without bed forms and this in capacity condition. Since flows are at times not in capacity, i.e. the flows are not saturated with sediments, an additional study (see Appendix C) was also performed. Furthermore, two special runs were done (see Appendix D), where the bed was covered with artificial bed forms. In order to evaluate the β-values, the vertical profiles of the mean velocity and its fluctuations as well as the mean concentration and its fluctuations had to be measured. This could only be done by using the new ultrasonic instrument (Acoustic Particle Flux Profiler, APFP), developed and assembled at the Laboratoire de Recherches Hydrauliques (LRH). Suspension flows were investigated focusing on the possible modification of the clear-water turbulence by suspended sediments. The concentration was carefully measured and interpreted by the Rouse equation. In particular, the ratio of the sediment, εs(y) , and momentum, εm(y) , diffusion coefficients, being the β(y) -value, was for the first time, directly measured. The strongest effect of particles on the flow was noticed on the vertical component of the turbulence intensity, , which was considerably suppressed, when compared to clear-water flow. The longitudinal velocity, u(y) , its turbulence intensity component, , and the Reynolds stress, – ρu'v' , were only slightly affected. By using the APFP instrument, the instantaneous sediment concentrations, cs, were, for the first time, directly measured. The calibration of the APFP instrument was achieved by measuring the mean concentration profiles, cs, with the suction method. The largest measured fluctuating concentrations, , and sediment flux, profiles were observed close to the bed, where the mean concentration is also very large. The sediment, εs, and the momentum, εm, diffusion coefficients, which represent the "ability" of sediment and fluid particles to be diffused in the flow by turbulence, were computed from the APFP measurements. For suspension flows over plane-bed, the sediment diffusion-coefficient profiles are always smaller than the momentum ones, εs < εm. This indicates that sediment particles undergo less diffusion than fluid particles; consequently the β-values are less than unity, β < 1. Thus, the usual assumption of εs = εm, that leads to β = 1, is not justified for the present measurements. The Rouse equation gives a better agreement to the concentration distributions measured with the suction method, by using the experimental obtained β-values, , rather than by using β = 1. For practical use, the experimental values and the ones obtained by a best-fit (least-squares method) of the Rouse equation to some concentration distributions taken from the literature are summarized in a plot. It seems that the β-values increase with either scaling parameters, vss/u*, and (vss/u*) · (Cs/csa). The effects of suspended particles on the clear-water turbulence were also investigated in non-capacity suspension flows having increasing concentrations. The measurements show that, the suppression (damping) of the vertical component of the turbulence intensity – caused by suspended particles – increases with the depth-averaged concentration. The tendency of the - values to decrease approaching the capacity condition, i.e. increasing the concentration, was also found. Suspension flows over bed forms were also investigated; in this study special attention was paid to the evolution of the flow structure behind the bed-form crest as well as the effect of bed forms on the β-values. The bed-form crest seems to generate a high-turbulence region with consequent peaks in both the longitudinal and vertical turbulence-intensity profiles as well as in the Reynolds-stress profiles. This high-turbulence region enhances the sediment diffusion coefficient but suppresses the momentum one. As a consequence, we found that for suspension flows over beds with bed forms, εs > εm, leading to β > 1. The most important result of this thesis is a recommendation that the Rouse equation with an improved β-value – itself to be estimated from the experimental plots obtained in this study – can be used to establish the dimensionless concentration profile of suspension flows. For beds without bed forms the β-values are β < l, while for beds with bed forms the β-values are β > 1. The correlation between the velocity fluctuations, associated with coherent structures (burst cycle), and the concentration was also investigated. From this study it becomes evident that the ejection event, being the most important phase of a burst cycle, is responsible of the erosion of sediment on the bed, behaving as an "injector" of sediments into the main flow.

EPFL1998

Experimental Study on Bedload Transport and Bedforms: Behaviour and Interplay in Steep Turbulent Streams

Ivan Pascal

In mountain regions, steep streams play an important role in water and sediment connectivity. In these highly dynamic systems, water flow features, sediment fluxes and stream morphologies are tightly interlinked over a broad range of temporal and spatial scales. Understanding their interactions is key to forecast, prevent and mitigate the impacts of stream-related hazards on human population and facilities as well as to manage the resources connected to the rivers. Sediment clasts at rest on the bed surface may be eroded, transported and deposited by the water flow. Grains that move in rolling and saltation regimes close to the bed compose the 'bedload', which is considered the main mode of sediment transport when studying morphodynamics of mountain rivers. The fate of these particles is of primary interest for river scientists and engineers as bedload predominantly drives the river morphology evolution and responds to it in a complex manner. Therefore, measuring and predicting bedload transport rates are fundamental concerns for many works of research and applications. Bedload transport rates may exhibit large non-Gaussian fluctuations, even under steady-state conditions, especially when bedload transport intensity is weak. These fluctuations may originate from different phenomena and affect the accuracy of bedload transport rate estimates. Particles transported by the water flow can destabilise other particles at rest on the bed surface. Under weak bedload transport conditions, this positive feedback can lead to large bedload pulses. When grains settle, they often organise in small clusters or larger bedforms. Bedform migration has been identified as one of the potential causes of bedload transport fluctuations. Trains of upstream-migrating antidunes may develop in steep coarse-bedded streams when the water discharge is sufficiently high to ensure fully supercritical flow conditions. In this dissertation, I present an experimental investigation on the behaviour of bedload transport fluctuations and antidunes in steep supercritical flows. The experiments were conducted in narrow flumes under well-controlled conditions with high-resolution image-based bedload transport and bed topography monitoring. The dependence of mean bedload transport rate uncertainty on the averaging time interval was analysed. Furthermore, I performed experimental runs with increasing transport intensity and a nearly constant mean bed slope angle to study how antidune sequences composed by bedforms of variable shape typically responded to the mean bedload flux imposed under steady-state conditions. Spectral analysis of the bed elevation perturbation in time and space allowed us to quantify the variability ranges of antidune shape and celerity. Scaling the spectra with opportune reference scales made it possible to identify a consistent dimensionless relationship between antidune geometry and migration celerity, which includes the dependencies on flow and sediment supply forcings. I studied how the bedload transport rate fluctuates in presence of migrating bedforms. The effects on the bedload transport rate periodicity due to non-uniformities in the antidune migration period and to deviations from the steady-state transport conditions were also investigated. This work is a further step on the way towards a complete understanding of the interplay between bedload transport fluctuations and bedforms in mountain streams.

EPFL2022