Assessing controls on the displacement of tracers in gravel-bed rivers

Particle travel length is one of the main dimensions of bedload and strongly influences river morpho-dynamics, particularly when exploring the interactions between sediment transport and channel morphology. This process has been traditionally studied by using tagged stones that allow tracking the movement experienced by individual grains during transport episodes. In this paper, we relate measured particle travel lengths to flow metrics and river channel parameters. First, we link the event-based bedload volumes to the active-layer dimensions, and the product between the average bedload rates and the duration of competent flows. We then hypothesize that travel length depends on channel width, surface grain-size, particle size, bed structure, flow strength, and duration of competent flow. The results from this approach are, subsequently, tested with a set of tracer observations from eight rivers that were available in the literature. The relationship between travel length and flow metrics was found to be statistically strong and has the potential to allow us to quantitatively assess the one-day dynamics of particles moving along streambeds. We also analyzed the influence of channel morphology and bed structure and identified morphological signatures for particle transport in gravel-bed rivers.The present work has been possible thanks to the financial support provided by the grant ACB17-44, co-funded by the post-doctoral ‘Clarín’ program-FICYT (Government of the Principality of Asturias) and the Marie Curie Co-Fund. This research was partly undertaken through the MorphPeak (CGL2016-78874-R) project funded by the Spanish Ministry of Economy and Competiveness and the European Regional Development Fund Scheme. Authors acknowledge the support from the Economy and Knowledge Department of the Catalan Government through the Consolidated Research Group ‘Fluvial Dynamics Research Group’ -RIUS (2017 SGR 459), as well as support from the project RIVERCHANGES-CGL2015-68824-R (MINECO/FEDER, UE)

Quantifying the Variability in Flow Competence and Streambed Mobility with Water Discharge in a Gravel-Bed Channel: River Esva, NW Spain

Streambed mobility in gravel-bed rivers is largely controlled by the rate at which particles with different grain sizes are recruited from the riverbed into the bed load. In this paper, we present a study case in which we explored this question, based on combining field observations using painted plots and the grain size analysis of a large flood sediment deposit in the River Esva, northwest Spain, and the generalized threshold model (GTM) competence model developed by Recking. The main aim was to accomplish a complete characterization of streambed mobility in this river. The obtained results suggest the large potential of the GTM model compared to previous competence models when searching for the quantitative description of particle entrainment and streambed mobility in the River Esva. We observed how the grain size of the bed load in the River Esva tended to be closer to that of the sub-armour bed material during large floods, while moderate magnitude flows tended to carry a relatively fine bed load. Additionally, we compared our results with previously published field observations on flow competence. This comparison outlined the large degree of site specificity in the links between grain size of the bed load and that of the bed material

Hazardous Processes: Flooding

This article reviews concepts and methodological approaches commonly used in fluvial geomorphology to understand and analyze flood hazards, spanning from catchment to reach spatial scales. Modern fluvial geomorphology applied to flood hazard studies has developed close links with hydrology and engineering to provide a holistic approach for flood hazard assessment. Flood geomorphology is being applied to (i) extend the flood record into the past from sediments, (ii) hydro-morphologically map channel and floodplain landforms, (iii) analyze and quantify morphodynamic processes such as channel migration and sediment transport in response to individual or sequential flooding, and (iv) understand natural processes at the landscape scale as a means of reducing flood risk by providing nature-based alternatives to conventional structural solutions. General and specific approaches on the study of flood hazards are considered here for four different fluvial environments: mountain streams, bedrock rivers, alluvial fans, and alluvial rivers. Geomorphologic and stratigraphic signatures of floods are critical to understanding the linkages among climate change, environmental change, flood hydrology, and the geomorphic development of fluvial landscapes

Assessing controls on the displacement of tracers in gravel-bed rivers

Particle travel length is one of the main dimensions of bedload and strongly influences river morpho-dynamics, particularly when exploring the interactions between sediment transport and channel morphology. This process has been traditionally studied by using tagged stones that allow tracking the movement experienced by individual grains during transport episodes. In this paper, we relate measured particle travel lengths to flow metrics and river channel parameters. First, we link the event-based bedload volumes to the active-layer dimensions, and the product between the average bedload rates and the duration of competent flows. We then hypothesize that travel length depends on channel width, surface grain-size, particle size, bed structure, flow strength, and duration of competent flow. The results from this approach are, subsequently, tested with a set of tracer observations from eight rivers that were available in the literature. The relationship between travel length and flow metrics was found to be statistically strong and has the potential to allow us to quantitatively assess the one-day dynamics of particles moving along streambeds. We also analyzed the influence of channel morphology and bed structure and identified morphological signatures for particle transport in gravel-bed rivers

Can bed-load help to validate hydrology studies in mountainous catchment? The case study of the Roize (Voreppe, France)

Larges uncertainties are attached to hazard prediction in mountain streams, because of some limitations in our knowledge of physical processes, and overall, because of the lack of measurements for validation. This is particularly true for hydrological data, making the hydrology assessment of a mountain river a very difficult task, usually associated with large uncertainties. On the other hand, contrarily to lowland rivers, bed-load in mountain streams is often trapped in mitigation-structures, such as open check dams. This study aims to take advantage of these additional information for compensating the general lack of hydrological data, in order to converge toward a comprehensive diagnosis of the catchment hydrological behavior. A hydrology and sediment transport study has been done on the Roize torrent (16.1-km2 - Voreppe - 38-FR). After a classical historical study, a regional analysis of raingauges and water-discharge-stations situated in the calcareous north Pre-Alps massifs of the Vercors, Chartreuse and Bauges has been done. A catchment geomorphology study has been performed to get insight about the Roize torrential activity and sediment transport. The volumes of bed-load transported each year on average and during extreme floods have been computed using the estimated hydrology. The good bed-load predictions compare to the volume dredged in the Voreppe sediment trap are considered an indirect validation of the hydrology study

Assessment of bedload equations using data obtained with tracers in two coarse-bed mountain streams (Narcea River basin, NW Spain)

International audienceThis paper evaluates the predictive power of nine bedload equations, comparing the results provided by the equations with the bedload rates obtained in a previous field-based tracer experiment accomplished in River Piguena and River Coto, two coarse bed streams from NW Spain. Rivers from NW Spain draining the northern watershed of the Cantabrian Mountain range flow into the Bay of Biscay in a short path (50-60 km). In this region, they are developed forested catchments featured by fluvial networks with relatively steep slopes, single-thread sinuous channels, and where bed sediment is typically coarse (cobble and gravel). Tagged stones were used to trace bed sediment movement during flood events in River Piguena and River Coto, the two main tributaries of the Narcea River basin. With the tracer results, bedload transport rates between 02 and 4.0 kg/s were estimated for six flood episodes. The tracer-based bedload discharges were compared with the bedload rates estimated with the bedload formulae (DuBoys-Straub, Schoklitsch, Meyer Peter-Muller, Bagbold, Einstein, Parker-Klingeman-McLean, Parker-Klingeman, Parker and Wilcock-Crowe). Our assessment shows that all of the bedload equations tend to overestimate when compared with the tracer-based results, with the Wilcock and Crowe (2003) equation the only exception in River Piguena. We linked these results to the particular geomorphology of coarse-bed rivers in humid and forested mountain environments. Within these rivers, armored textures and structural arrangements in the bed are ubiquitous; these features, together with a low sediment supply coming from upstream forested reaches, define a supply-limited condition for these channels limiting the potential use of bedload equations. The Wilcock and Crowe (2003) equation introduces complex corrections into the 'hiding function', and this could explain why it performs better

The contribution of grain sorting to the dynamics of the bedload active layer

International audienceAbstract 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 A r (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 D 90 , it is well explained by the product between the mean bed slope S and the active channel width W , with δ ≈ 1.4 SW . For the pulse intensity, we provide a justification for the simplified squared slope equation for solid concentration C = Q s / Q ∝ S 2 (with Q s 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