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Breaching of fluvial dykes: Influence of the dyke geometry

Fluvial dyke breaching is a major concern for flood risk management; but the mechanisms of dyke breaching remain incompletely understood. We report here on forty (40) laboratory tests performed to assess the influence of the geometry of dykes made of non-cohesive sediments on the evolution of the breach discharge. Three parameters were systematically varied: the side slopes of the dyke and its crest width. Each geometric configuration was tested for several main channel inflow discharges. Results show that the breach discharge grows faster when the dyke volume becomes smaller and when the inflow discharge is increased. All geometrical parameters were then lumped into a single non-dimensional parameter and a characteristic time was defined to quantify the growth rate of the breach discharge. For a given inflow discharge, the proposed characteristic time increases linearly with the non-dimensional parameter. 2 EXPERIMENTAL SETUP The laboratory setup is the same as that used by Rifai et al. (2017, 2018). It consists of a horizontal, straight main channel (10 m x 1 m) with a 3-meter-long lateral opening toward a floodplain (4.3 m x 2.5 m). Along the side opening, sand was gathered and compacted to build a trapezoidal shaped fluvial dyke, whose dimensions were systematically modified while keeping the dyke height constant (0.3 m). The dyke geometry is characterized by the upstream slope Su, the downstream slope Sd and the crest length Lk (Figure 1). A specific coating was applied on the surfaces of the main channel and the floodplain to ensure roughness continuity between those surfaces and the dyke. The bottom of the main channel and the floodplain were at the same level and elevated by 0.7 m compared to the outflow tanks. To avoid seepage flow, a drainage system was installed at the dyke bottom. Finally, a perforated plane followed by a tank were placed at the end of the main channel to control the downstream water level and to collect the main channel outlet flow. Each geometrical configuration was tested with three different main channel inflow discharges, i.e. Qin = 25 l/s, 40 l/s and 55 l/s. The inflow discharge was kept constant and supplied at the upstream end of the main channel. During the filling phase, the water level rose in the main channel. The perforated plate was tuned such that at equilibrium the water level corresponded to the dyke height. Before starting the tests, a 2-cm-deep and 10-cm-wide notch was created in the dyke crest to trigger breaching. Once the water level overtopped the notch, erosion and breaching developed. The water level in the main channel was measured and recorded in three locations using ultrasonic sensors (accuracy of ± 1 mm). Similar sensors were used to monitor the water level in the outflow tank and in the drainage tank. The inflow discharge was measured using an electromagnetic flowmeter (accuracy of ± 4%). The breach discharge was determined from mass balance considering the main channel inlet and outlet flows, storage in the main channel, as well as the drainage flow. Figure 1. Sketch of a dyke vertical profile, highlighting the main geometric parameters: upstream and downstream slopes (Su and Sd), dyke height (h) and crest length (Lk). The main channel width b is 1 m.