Experimental study of the 3D breaching of a small-scale sand embankment by overtopping.

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Sandra Soares-Frazão, Sylvie van Emelen, Yves Zech

Wednesday 1 july 2015

12:30 - 12:33h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling, Poster pitches

Parallel session: Poster pitches: 8A. Sediment - Erosion

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The breach formation in dikes can create large floods and be devastating. In case of breaching by overtopping, the rapid flow digs an initial channel in the dike, which progressively deepens through flow erosion, and widens through sliding of the banks. Therefore, the flood hydrograph and the peak discharge depend on the temporal evolution of the breach shape, and a good knowledge of erosion processes is essential to evaluate the extent of the flood. This paper presents the results of a series of laboratory experiments on the breach evolution in a small-scale sand embankment by overtopping. The experimental campaign was conducted at the Hydraulics Laboratory of the Université catholique de Louvain, Belgium. A 0.2 m high trapezoidal sand embankment is built in a 10 m long and 1.2 m wide flume, with upstream and downstream slopes of 1V:3H. An initial erosion channel is dug in the middle of the crest to ensure a precise breach location. Two different uniform sediment grain sizes are used: coarse sand (d50 = 1.85 mm) and middle-sized sand (d50 = 0.61 mm). Moreover, a 5-cm thick layer of sand is placed downstream of the dike, allowing to analyse the erosion/deposition due to the flood. So this experimental campaign presents the particularity to study both the breach growth process and the bed evolution just downstream of the dike. The temporal evolution of the water levels is measured using ultrasonic probes, and the breach hydrograph is theoretically deduced from the upstream water level evolution. Moreover, an imagery technique, using a fast camera and several laser sheets, is used to monitor the temporal evolution of both the breach geometry and downstream topography. Experimental measures are then analysed and the reproducibility of the experiment is proven. The influence of the sand diameter is studied by comparing the breach evolution with both sand sizes. Results are finally compared with previous small-scale breaching experiments.