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You are here : eLibrary : IAHR World Congress Proceedings : 36th Congress - The Hague (2015) ALL CONTENT : Sediment management and morphodynamics : A laboratory experiment on the evolution of a sand gravel reach under a lack of sediment supply
A laboratory experiment on the evolution of a sand gravel reach under a lack of sediment supply
A flume experiment was conducted to examine the evolution of a sand-gravel reach under a lack of sediment supply. A
bed composed of a bimodal sediment mixture was installed with a uniform slope and an gradual fining pattern. At the
upstream end of the flume the initial bed consisted of 100% gravel, which decreased stepwise (10% steps) in
downstream direction until the location where the initial bed consisted of 100% sand. The water discharge and
downstream water level were constant and the sediment feed rate was equal to zero. New image analysis equipment
was used to frequently measure the grain size distribution of the bed surface during the experiment over the entire length
of the flume. The experiment was governed by bedload transport and subcritical flow. The flow rate was such that only
sand was mobile (partial transport). This led to the washing out of sand from the upstream bimodal reach and its
gradual coarsening. In this reach we observed the formation of a static armour layer, which resulted in a more abrupt
transition in the mean grain size of the bed surface. The total amount of bed degradation slightly increased in
streamwise direction over the bimodal reach, which was due to the streamwise increase of the sand fraction in the initial
bed. The sand reach adjusted to the lack of sand supply by decreasing its slope and so its sand transport capacity (i.e.
through decreasing the flow velocity). Consequently, the sand transport rate at the downstream end of the flume
gradually reduced during the experiment. A morphodynamic steady state over a movable bed is generally governed by
normal flow. In the present case, governed by partial transport conditions, the morphodynamic steady state is dominated
by a backwater curve. This happens because under partial transport conditions the bed cannot (fully) adjust to the
upstream water and sediment discharge. These experimental results were reproduced using a one-dimensional
numerical model based on the St Venant-Hirano equations, although the effects of the temporal change in bed porosity
were not considered in the model.
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Chapter : IAHR World Congress Proceedings
Category : 36th Congress - The Hague (2015) ALL CONTENT
Article : Sediment management and morphodynamics
Date Published : 27/08/2015
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