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You are here : eLibrary : IAHR World Congress Proceedings : 36th Congress - The Hague (2015) ALL CONTENT : Sediment management and morphodynamics : Experiments on the evolution of supply-limited sand bed forms on coarse gravelbeds
Experiments on the evolution of supply-limited sand bed forms on coarse gravelbeds
The advanced age and impending decommissioning of many dams have brought increased attention to the fate of
sediments stored in reservoirs. In many cases, fine sediments are reintroduced to coarse substrates that have large
volumes of pore space available for storage after having sediments removed by years of sediment-starved flow. Recent
research has found that the fine sediment elevation relative to the coarse substrate significantly alters bed surface
roughness, turbulence characteristics, the mobility of the fine sediment, and consequently sediment transport rates and
sediment bed forms that move over and through these coarse substrates (Wren et al., 2011; Kuhnle et al., 2013). The
roughness of the bed surface is an important parameter for the prediction of bulk flow and sediment transport rates. In
order to calculate sediment transport rates, bed shear stresses are typically adjusted for drag exerted by the flow on
macro roughness elements, which are related here to the protrusion of coarse substrate particles and sediment bed forms.
Hence, a proper understanding of the interactions between near-bed flow structure, sediment transport rates, and bed
surface elevation is needed to adequately determine the downstream impact of fine sediment releases from reservoirs.
Recent experiments in a sediment-recirculating flume (15 m long, 0.36 m wide, and 0.45 m deep) were carried out to
elucidate turbulence and sand transport over and through coarse gravel substrates. The median diameter of the sand was
0.3 mm, and that of the gravel was 35 mm. This paper presents results on the change in bed form types with increasing
sand elevation relative to the coarse gravel substrate. The mean sand elevation was varied between 5 cm below the mean
gravel tops and mean gravel tops.
Bed and water surface elevations were measured along the flume centerline using acoustic sensors mounted on a
carriage, which rode on rails above the flume (Figure 1). Local elevation of the bed was measured at 40 Hz using a
custom-built 1 MHz acoustic pulse echo sounder. Measurements of the water surface were collected using a Contaq® inair
acoustic pulse echo device which operated at 6 Hz. Bed surface topography was measured using close-range
terrestrial digital photogrammetry. Images were taken with a Pentax K10D digital camera with a Pentax P-DA 18-55 mm
F3.5-5.6 lens with an image resolution of 3872 x 2592 pixels. The camera was mounted on a carriage that traversed the
top of the flume. Ground control points with an average longitudinal spacing of 5 cm were placed on the gravel surface
and their X, Y, and Z coordinates measured using a point gauge with a horizontal and vertical accuracy of 0.5 mm.
ERDAS LPS® 2010 was used to extract a point cloud of the gravel topography with a mean vertical root-mean-square
error of about 1 mm. The point clouds were interpolated onto Digital Elevation Models (DEMs) with a horizontal grid size of
1x1 mm (Figure 1).
Sand was added to the flume near the downstream end over several hours using a vibrating sediment feeder. The initial
sand addition (300 kg) filled the gravel bed from the flume bottom upwards, and resulted in an equilibrium elevation of
about 5 cm below the mean tops of the gravel bed at a high discharge of 65 L/s. Eleven sand additions (in the figures
labeled ¡®Case¡¯) ranging from 10 to 40 kg were introduced with the final sand addition resulting in a mean sand elevation
that was located slightly above the mean tops of the gravel substrate (Table 1). Flow and sediment transport
measurements were then collected at four discharges (10, 30, 50, and 65 L/s) for each bed. Flume slope was adjusted as
necessary after each sand addition and for each flow rate to allow the achievement of equilibrium conditions in the
channel. Here, results are shown for a discharge of 50 L/s.
Figure 2 shows the flume width average longitudinal bed profiles obtained from the DEMs. Up to the 8th sand addition the
sand filled the gravel substrate fairly uniformly along the length of the flume. Starting at the 9th sand addition, groups of
increasingly numerous low-relief bed forms developed when increasing mean sand elevation (Figure 3). The crests of the
bed forms were located at an elevation similar to those of the higher gravel tops, and did not increase in elevation between
sand additions 9 and 11. The organization of the mobile sand into bed forms coincided with significant increases in sand
transport rate (Table 1).
Kuhnle et al. (2013) showed that sand transport rate was strongly related to the shear stress acting on the sand below the
gravel tops. This shear stress was found to be the bed shear stress (¦Óbed) scaled by the roughness geometry function (A)
of the gravel substrate. The roughness geometry function is equivalent to the cumulative distribution of the gravel bed
elevations. When the roughness function is evaluated at the mean elevation of the sand in the gravel bed (As), itrepresents the fractional surface exposure of the sand. Kuhnle et al. (2013) determined that the shear stress acting on the
sand ¦Ós = ¡ÌAs ¦Óbed. Figure 4 shows that the initial, larger increases in sand transport rate (sand additions 5 and 6) coincided
with the region where the roughness geometry function (and its square root) rapidly increases. The increasing roughness
geometry function indicates both increasing shear stress acting on the sand particles as well as increasing availability of
sand particles to be transported.
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Chapter : IAHR World Congress Proceedings
Category : 36th Congress - The Hague (2015) ALL CONTENT
Article : Sediment management and morphodynamics
Date Published : 18/08/2015
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