Sediment Production in French Alpine Rivers Pitlick, John; Recking, Alain; Liebault, Fred ...
Water resources research,
December 2021, 2021-12-00, 20211201, Letnik:
57, Številka:
12
Journal Article
Recenzirano
Global compilations of river sediment loads show that mountainous areas produce a high proportion of the sediment transported to the oceans. However, because of the effort involved in measuring ...sediment fluxes in mountain river systems, the loads of these rivers are generally unknown. Here, we present estimates of contemporary sediment loads of 16 gravel bed rivers draining the Ecrins‐Pelvoux Massif in southeast France. Sediment production in this part of the Alps is relatively high and many river segments are either wandering or braided. We model sediment fluxes and annual sediment yields by coupling hydraulic‐based relations for sediment transport with hydrologic‐based relations for flow frequency. Bed load fluxes are modeled for a range of discharges using a function that relates transport rates to excess shear stress. Fluxes are then weighted by the frequency of individual discharges and summed to get the annual bed load for each site. The suspended load is estimated empirically as a fraction of the bed load. Results suggest that bed load fluxes at channel‐forming flows scale almost linearly with downstream increases in discharge. In addition, it appears that annual sediment loads (bed load + suspended load) scale linearly with drainage area. A complementary relation for specific sediment yield suggests that the load per unit drainage area is constant across the range of basins studied. The modeled sediment yields are comparable to previous field‐based estimates of modern sediment yields, and generally lower than estimates developed from analyses of cosmogenic radionuclides.
Key Points
Relations for flow frequency and sediment transport are used to model sediment fluxes of 16 rivers in southeastern France
Annual sediment fluxes (t/year) scale linearly with drainage area, specific sediment yields (t/km2/year) exhibit no trend with drainage area
Contemporary denudation rates are roughly half the long‐term denudation rates determined from analyses of cosmogenic nuclides
River rehabilitation and ecological engineering are becoming critical issues for improving river status when ecological habitats and connectivity have been altered by human pressures. Amongst the ...range of existing rehabilitation options, some specifically focus on rebuilding fluvial forms and improving physical processes. The aim of this contribution is to illustrate how geomorphological expertise and process-based thinking contribute to river rehabilitation success. This semantic contribution is intended to feed the rehabilitation debate, particularly concerning the design of actions and the proposed references for monitoring target reaches and evaluating rehabilitation effects empirically. This article is also based on lessons learned from practical cases, mainly in gravel-bed rivers. Geomorphic understanding is needed at a local level to achieve an adequate diagnosis of river functioning, estimate human impacts and potential remnant river responsiveness, and to assess the gains and risks from rehabilitation, as well as to appraise success or failure through several pre- and post-project assessment strategies. Geomorphological studies can also be upscaled in a top-down manner (from high-order controls to small-scale processes, understanding detailed processes in their regional or basin-wide context), providing large-scale information at the regional, national, or even global level, information that can be used to diagnose the health of riverscapes in relation to local site-specific contexts. As such, geomorphological studies support strategic planning and prioritization of rehabilitation works according to specific contexts and river responsiveness, so as to move from opportunistic to objective-driven strategies.
An innovative scour chains device composed of active UHF RFID tags is proposed to survey the bed active layer. This device is tested on three cross-sections deployed along two large Alpine braided ...rivers: the Drac and the Vénéon. A specific field deployment procedure is developed, using a technique of drilling with a tube and a jack-hammer. After each flood, the device allows recording presence/absence of active tags inside a scour chain column. This provides the maximum scouring depth of the bed at the position of the column, and the topographic resurvey of the channel provides the net fill depth. Results show that bed active layer can reach up to 1.43 m during high flow conditions and remains <0.1 m during low and moderate flows. The main advantage of this technology against traditional scour chains is the rapidity and easiness with which the scouring depth is obtained after a flow event, notably under conditions of massive net deposition after the flow. This technology is therefore particularly suitable for large braided rivers, where traditional scour chains necessitate too much field efforts for their relocation, and can be rapidly lost under deep active layers.
During the last 20 years, flume and field experiments have shown that grain sorting contributes to bed‐level fluctuations and bedload pulses. In this work, we propose a new analysis of these ...experimental data. From the flume data, we derive a model for gravel‐bed rivers where both local (bedform‐scale) slope and bedload are known to fluctuate through space and time, in the so‐called ‘bedload active layer’. The model uses standard concepts and empirical tools with reach‐averaged data for the hydraulics and sediment transport. It considers a maximum slope for local armouring equal to the mean bed slope (reach scale) affected by a coefficient which expresses the difference in mobility of the coarse fraction considered alone or in a mixture. The minimum local slope for bed erosion is the mean bed slope corrected by a coefficient that depends on the armour ratio Ar (ratio of the surface to the subsurface grain diameter) and the reach‐averaged transport rate. The model is compared with a compilation of scour–fill depths measured in the field. Results suggests that the slope fluctuations in 1D flume experiments are consistent with in‐channel bed‐level fluctuations associated with scour–fill processes in the active layer. The model also suggests that although the length scale of the maximum scour depth δ is on the order of the bed surface D90, it is well explained by the product between the mean bed slope S and the active channel width W, with δ ≈ 1.4SW. For the pulse intensity, we provide a justification for the simplified squared slope equation for solid concentration C = Qs/Q ∝ S2 (with Qs the solid discharge, Q the water discharge and S the slope), which has often been used in place of standard bedload equations for modelling highly concentrated bedload transport events in mountain streams.
What happens at the flow–grain interface is complex, and for modelling purposes it has long been represented by a conceptual layer called the bedload ‘active layer’. In this work we re‐analyse existing flume results aimed at unravelling the physical mechanism acting within the streambed, both in the granular zone and in contact with the flow. We propose a model for the bed fluctuations, which is then successfully tested against available field measurements.
This paper explores the potential of unmanned aerial system (UAS) optical aerial imagery to characterize grain roughness and size distribution in a braided, gravel-bed river (Vénéon River, French ...Alps). With this aim in view, a Wolman field campaign (19 samples) and five UAS surveys were conducted over the Vénéon braided channel during summer 2015. The UAS consisted of a small quadcopter carrying a GoPro camera. Structure-from-Motion (SfM) photogrammetry was used to extract dense and accurate three-dimensional point clouds. Roughness descriptors (roughness heights, standard deviation of elevation) were computed from the SfM point clouds and were correlated with the median grain size of the Wolman samples. A strong relationship was found between UAS-SfM-derived grain roughness and Wolman grain size. The procedure employed has potential for the rapid and continuous characterization of grain size distribution in exposed bars of gravel-bed rivers. The workflow described in this paper has been successfully used to produce spatially continuous grain size information on exposed gravel bars and to explore textural changes following flow events.
This study was aimed at untangling the relative impacts of successive phases of human modifications on changes in bedload transport along a 430 km-long river reach: the Rhône River from Motz dam to ...the sea. We used a 1D hydraulic model to solve for water lines across a range of discharges and all along the reach. Next, using grain sizes measured in the channel, we estimate flow competence and mean annual bedload transport capacities using the Recking (2013) bedload transport equation. In addition, we used the Generalized Threshold Model to estimate the relative fine and coarse fractions of the load. Bedload transport estimates were carried out under present-day hydraulic conditions and compared to estimates based on model runs simulating an unimpeded flow regime and using grain sizes measured in bars as a proxy for conditions prior to armouring.
Our results show that present-day bedload transport along the Rhône is significantly fragmented by multiple closely spaced dams. Mean annual bedload capacity varies between 2500 and 16,300 m3/year over all the reaches, with an average of 4700 m3/year. Results of the GTM analysis suggest that this load is composed of 89% fines. We find bed sediment mobility to be very low in most reaches, and that potentially mobile sediments are finer than the median grain size in the riverbed even at high flows. Our results suggest that bedload capacities were 25–35 times higher prior to bed armouring and flow modifications; dams had an impact 2–3 times more important on transport capacities than channel embankments, and bed armouring was foremost a response to channel embankments. Based on an analysis of the ratio of sediment yields to transport capacities, we propose a conceptual scheme illustrating how bedload supply, channel morphology, and surface texture coevolved in the Rhône over the past century and half.
Display omitted
•Mean annual bedload capacity is 4700 m3/year with strong intra-reach variability.•Bed mobility is very low and the majority of transported sediments are fines.•Bedload transport rates were up to 35 times higher prior to embankments and dams.•Dams had an impact 2–3 greater than changes in grain-size on BL transport capacities.•The effects of successive dams show strong overlap.
Modeling transport, erosion, and deposition of nonuniform sediment over temporal intervals that are short compared to those characterizing channel bed aggradation and degradation remains an open ...problem due to the complex quantification of the sediment fluxes between the bed material load and the alluvial deposit. Parker, Paola, and Leclair in 2000 proposed a morphodynamic (PPL) framework to overcome this problem. This framework is used here to model the dispersal of a patch of gravel tracers in three different settings, a laboratory flume, a mountain creek, and a braided river. To simplify the problem, (a) the bed slope, bedload transport rate, and bed configuration are assumed to be constant in space and time (equilibrium), (b) sediment entrainment and deposition are modeled with a constant step length formulation, and (c) the PPL framework is implemented in a one‐dimensional (laterally averaged) model. Model validation against laboratory experiments suggests that, as the transport capacity of the flow increases, the maximum elevation‐specific density of sediment entrainment may migrate downward in the deposit. The comparison between model results and field data shows that the equilibrium solution can reasonably capture tracer dispersal. The equilibrium model can also reproduce subdiffusion and superdiffusion of a patch of tracers in the streamwise direction, depending on the magnitude of the short‐term bed level changes. Finally, the average tracer elevation in a cross‐section decreases in time because particles that are buried deep in the deposit are only rarely reentrained into bedload transport.
Key Points
A vertically continuous model reproduces tracer dispersal in the field and in the laboratory
The model predicts superdiffusion and subdiffusion of tracer stones while modeling entrainment with a constant particle step length
The average tracer elevation decreases in time as tracers emplaced deep in the deposit are hard to mobilize
Abstract
Monitoring bedload transport in rivers is a challenging research domain teeming with technical innovations and methodological developments aimed at improving our knowledge and models of ...bedload processes at different spatial–temporal scales. Radio frequency identification (RFID) technology has improved sediment tracking, allowing the characterisation of transport processes of individual particles at flood‐event scales. Meanwhile, geophone sensors have enabled the long‐term continuous monitoring of seismic signals that can provide surrogate measures of bedload fluxes at local scales, during flood events and at sediment‐pulses. The combination of these two techniques could allow sediment transport processes to be linked with both flood events and sediment pulses. In this study, we used a combination of active ultra‐high frequency RFID technology and geophone monitoring stations to link the virtual velocity of tracers with seismic activity, hydraulic forcing, and the properties of the tracked particles. Single and multiple regression models show that seismic activity best explained the observed variance (81%) of the virtual velocity of particles, in comparison with discharge (58%) and stream power (63%). Furthermore, when several control variables (seismic activity and particle properties) were combined in an empirical model, the model explained 89% of the variance and allowed quantification of the portions of the variance explained by hydraulic forcing, geophonic activity and tracked particles. These results show the high potential of these combined monitoring techniques for future in‐field experiments to investigate bedload processes at different spatiotemporal scales in rivers of different morphologies.
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