Abstract
Following the construction of a reservoir, sediment is intercepted, which greatly reduces the sediment concentration in the discharged flow. This reduction causes riverbed scouring and flow structure adjustments downstream, thereby impacting the river habitat. This study used the generalized flume test with different bed sand compositions and discharge rates to simulate the scouring adjustment process of a sand pebble riverbed channel downstream of a reservoir. The results show that the bed sediment composition affects the water surface gradient, scour depth, turbulence intensity, and sand resistance directly after final scouring. Coarse-grained bed sediment demonstrated the greatest final turbulence intensity and sand resistance, while bed sediments with reduced coarseness exhibited a greater scouring degree; the resistance for sand grains of moderate coarseness showed the greatest change. Sand resistance was exponentially and positively correlated with the median grain size and the fractal dimension of bed sediment mass. The mass fractal dimension expression was suitable for the analysis of bed sand grain-size distribution; it contributed to the calculation of grain resistance with fewer hydraulic parameters. The relationship between the mass fractal dimension and the adjusted grain resistance was also established, which can aid the calculation of the resistance changes in sandy gravel-bed river reaches downstream of reservoirs, enabling the prediction of their effects on aquatic habitats.
Highlights
Reservoirs are built for a range of benefits such as flood control and runoff storage; they inevitably alter the natural sediment transport equilibrium of rivers (Zhou et al, 2020)
The greatest change in the grain size of the set B bed load with a sand content of 42% was d60, and it changed as follows: 1.5 → 7 → 1.6 → 1.8 → 5 mm
The degree of coarsening in set A was low because it contained more gravel and had a coarser bed load and greater resistance, while that of set C was less than that of set B
Summary
Reservoirs are built for a range of benefits such as flood control and runoff storage; they inevitably alter the natural sediment transport equilibrium of rivers (Zhou et al, 2020). The construction of reservoirs often causes riverbed scouring adjustments due to a drastic reduction in incoming sand. This is especially true downstream of large reservoirs; this, in turn, affects the riverbed microtopography, riverbed resistance, and flow structure. These changes have a significant impact on economic production and aquatic habitats, wherein the key factor for riverbed deformation and flow structure is the change in riverbed resistance. For this reason, research on channel resistance downstream of dams has become an area of interest to predict habitat changes Research on channel resistance downstream of dams has become an area of interest to predict habitat changes.
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