Back-calculation of sediment transport during flood events with a bedload transport simulation model for steep channels
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Files R1qsraw.txt to R7qsraw.txt: Bedload rate (g/s) time series for total bedload (column 1) and 14 grain size fractions over seven experimental runs (R1 to R7). Fractional bedload rates go from column 2 to 15 as follows:
0.5-0.7 mm (column 2)
0.7-1 mm (column 3)
1-1.4 mm (column 4)
1.4-2 mm (column 5)
2-2.8 mm (column 6)
2.8-4 mm (column 7)
4-5.6 mm (column 8)
5.6-8 mm (column 9)
8-11 mm (column 10)
11-16 mm (column 11)
16-22 mm (column 12)
22-32 mm (column 13)
32-45 mm (column 14)
45-64 mm (column 15) File SurfaceDg.txt: Contains the geometric mean size for the bed surface in mm (column 2) for different experimental times (time in h, column 1) File BedloadDg.txt: Contains the geometric mean size for the bedload in mm (column 2) every 1 h (time in h, column 1) File slope.txt: Contains the bed slope at the thalweg in m/m (column 2) for different experimental times (time in h, column 1)
0.5-0.7 mm (column 2)
0.7-1 mm (column 3)
1-1.4 mm (column 4)
1.4-2 mm (column 5)
2-2.8 mm (column 6)
2.8-4 mm (column 7)
4-5.6 mm (column 8)
5.6-8 mm (column 9)
8-11 mm (column 10)
11-16 mm (column 11)
16-22 mm (column 12)
22-32 mm (column 13)
32-45 mm (column 14)
45-64 mm (column 15) File SurfaceDg.txt: Contains the geometric mean size for the bed surface in mm (column 2) for different experimental times (time in h, column 1) File BedloadDg.txt: Contains the geometric mean size for the bedload in mm (column 2) every 1 h (time in h, column 1) File slope.txt: Contains the bed slope at the thalweg in m/m (column 2) for different experimental times (time in h, column 1)
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Abstract The effect of sediment supply on sediment mobility is analyzed for a poorly sorted (0.5–64 mm) experimental bed. Water discharge was held constant over a sequence of seven runs, and 300 kg of sediment was supplied during each run in different magnitudes and frequencies. In runs with constant feed bedload transport rate increased gradually. In contrast, runs that received large sediment pulses showed pronounced increases in bedload rate as the bed surface got finer, followed by monotonic declines as the bed surface coarsened. We studied the temporal scales of bedload fluctuations by means of sample autocorrelation coefficients and the rates of decrease in bedload fluctuation with sampling time scale. The significant trends caused in bedload rate by large occasional sediment pulses increased long‐term autocorrelation in bedload rate time series relative to runs with constant feed. Bed evolution and local changes in sediment storage caused multiple scales of variability in bedload rate, which increased autocorrelation and caused long‐term persistence in bedload series over periods with a nearly constant mean. The scaling statistics of bedload transport fluctuation depended on grain size, and those for total bedload were similar to those for fine gravel (2–8 mm), which was fully mobile and dominated bedload transport. Grain size dependence of bedload fluctuation was not affected by changes in sediment feed because water discharge and sediment texture were held constant.
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Understanding bedload transport fluctuations in rivers is crucial for complementing the existing knowledge on sediment transport theory. In this contribution, we use a natural-scale laboratory flume to analyse bedload transport fluctuations in non-uniform sand under normal flow conditions. Based on the significance of downward seepage, we incorporate the seepage effect on bedload transport over a non-uniform sand bed channel. The weight of the dry material was measured, and the volumetric transport rate per unit width (bedload transport rate) was estimated. An important observation is that the bedload transport rate initially rapidly increases with time and reaches a maximum value. Based on experimental data, we propose an empirical expression to estimate temporal bedload transport. In addition, an empirical model for bedload transport is proposed by incorporating downward seepage among other variables. The performance of several existing bedload transport formulae was also taken into account by the experimental datasets.
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Bedform
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Abstract A comprehensive monitoring programme focusing on bedload transport behaviour was conducted at a large gravel‐bed river. Innovative monitoring strategies were developed during five years of preconstruction observations accompanying a restoration project. A bedload basket sampler was used to perform 55 cross‐sectional measurements, which cover the entire water discharge spectrum from a 200‐year flood event in 2013 to a rare low flow event. The monitoring activities provide essential knowledge regarding bedload transport processes in large rivers. We have identified the initiation of motion under low flow conditions and a decrease in the rate of bedload discharge with increasing water discharge around bankfull conditions. Bedload flux strongly increases again during high flood events when the entire inundation area is flooded. No bedload hysteresis was observed. The effective discharge for bedload transport was determined to be near mean flow conditions, which is therefore at a lower flow discharge than expected. A numerical sediment transport model was able to reproduce the measured sediment transport patterns. The unique dataset enables the characterisation of bedload transport patterns in a large and regulated gravel‐bed river, evaluation of modern river engineering measures on the Danube, and, as a pilot project has recently been under construction, is able to address ongoing river bed incision, unsatisfactory ecological conditions for the adjacent national park and insufficient water depths for inland navigation. Copyright © 2017 John Wiley & Sons, Ltd.
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Steep streams occupy a large fraction of mountainous drainage basins and partially control the sediment supplied to downstream rivers. In these channels, sediment transport equations typically over‐predict bedload flux by several orders of magnitude because they do not account for sediment‐supply limited conditions. Thus, accurate predictions of bedload flux require an estimate of the sediment available for transport in a given event. We demonstrate through field measurements that boulder step protrusion is a proxy for sediment availability. Protrusion is also a function of the time elapsed since an extreme event and this simple relationship can be used to estimate the relative sediment availability at any given time. In addition, bedload transport predictions in a steep channel were only accurate if they included this variable protrusion. Predictions of sedimentation hazards, water quality, river restoration success, long‐term channel network evolution, and channel stability may therefore require estimates of sediment availability for transport.
Sedimentation
Sedimentary budget
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In Ecuador, steep rough-bedded channels constitute the main component of mountainous drainage system. They provide sediment to milder-slope downstream channels. Thus, sediment transport represents a driving process in natural drainage system. Namely, it defines river morphology evolution. To quantify and to understand the magnitude and effect of this process in the surrounding environment, the understanding and knowledge of bedload transport must be improved. The study of sediment transport in steeper channels with coarser material is a complex process. The continuously changing environment results in a high uncertainty in the quantification of sediment transport rates. Some equations have been proposed to quantify the rates. However the lack of actual measured data does not allow proper quantification and verification. On the other hand, hydraulic geometry (HG) theory has been applied to generate elements for a consistent monitoring of rivers behavior. Dimensionless HG relations that replicate what is observed in rivers have been obtained. Parameters such as top width, mean flow depth, mean velocity, and suspended sediment load of several gravel-bed rivers have been related with liquid discharge. The present study proposes the characterization of bedload sediment transport of steep gravel-bed rivers in terms of dimensionless HG relations. Measurements in various reaches along a river of bedload transport rate are performed to determine the parameters (exponents and coefficients) of the HG relations. The results represent a contribution that allows the reduction of the lack of field-measured data as well as the application of a theory generally used to characterize hydraulic-geometric parameters to characterize bedload sediment transport.
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Abstract Understanding how river bedload responds to climate and land use changes and water resource management initiatives is critical in developing sustainable approaches to river management. Passive monitoring techniques permit investigation of interannual dependencies in bedload transport in high resolution, including sediment supply factors. Here, seismic impact plate records are processed using a probabilistic model BedLoad from Impact Plates model to derive a 5‐year bedload data set at 5‐min intervals for the lower River Avon, Devon, UK. For water years that range from very dry to very wet, annual coarse bedload yields are estimated to vary through two orders of magnitude with wide prediction intervals. The most effective discharge occurs consistently at about one‐third of bankfull flow, morphologically at “subbarfull” stage, the result of hysteretic trends and falling limb transport in this non‐threshold channel. A two‐phase sediment rating curve is revealed with a variable supply related “bulge” during in‐bank flows, giving way to a near‐linear trend during overbank flows. The supply related component is predicted well using a sensitivity style metric that combines the cumulative duration of competent flows with the magnitude‐duration product of near‐threshold flows, defining a field‐scale exemplar of “stress history.” Further, the relative proportion of supply related coarse bedload yield relates strongly to the relative wetness of the previous year. High resolution, multiyear data reveal that controls on bedload dynamics are unique to a site's hydrogeoclimatic context and position in the river basin. Passive monitoring holds promise for generating “type sites” of bedload behavior critical for use in improving aquatic biodiversity and the sustainability of river management.
Hyperconcentrated flow
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Hyperconcentrated flow
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