A laboratory experiment on the evolution of a sand gravel reach under a lack of sediment supply

Clara Orru, Victor Chavarrías, Velia Ferrara, Guglielmo Stecca, Astrid Blom

Tuesday 30 june 2015

17:45 - 17:48h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics, Poster pitches

Parallel session: Poster pitches: 7B. Sediment - River

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.