Small collapses occur on the top of the downstream side of the dam with progressive collapses occurring faster and more rapidly. When the dam failure reaches a certain point, the landslide dam can maintain relative stability because of the large particles that have not been washed away. When the water flow achieves an equilibrium state, the upstream flow is approximately balanced with the discharge, and dam failure ceases. Compared, dam break patterns for water flow rates of 0.1L/s and 0.2L/s, larger flow rates result in a rapid failure of landslide dams. If the water flow is large enough, the erosional force of the water is enormous and the failure speed of landslide dam is very rapid. In this scenario, the entire landslide dam collapses causing an enormous flood downstream of the dam. When the water flow is large enough (2.0L/s), a dyke breaches the middle of the landslide dam and is gradually expanded by the flowing water. Ultimately, the rapid water flow results in total dam failure. Figure 9 shows the evolution of the dyke breach when the water flow rate is 2.0L/s. Figure 9Evolution of the dyke breach when the water flow is 2.0L/s: (a) schematic diagram and (b) evolution of the size of dyke breach.In Figure 9(a), the depth of dyke breach is h, the top width of dyke breach is w1, and the bottom width of the dyke breach is w2. After the first failure of the landslide dam, the size of the dyke breach (including top width, bottom width, and depth) increases due to continuous water flow. In the early stages, overflow results in a small dyke breach, but the size of dyke breach increases slowly. The collapse speed accelerates until the dam achieves stability (Figure 9(b)). When the flow rate is large, the width of the dyke breach increases quickly. 3.2. Impact of Boulders on the Top of the DamDuring overflow, the resistance to erosion of the landslide dam depends on the size of dam materials and the velocity of water flow. Fine particles are easily transported downstream by water flow, whereas coarse particles are not. The resistance to erosion of boulders is larger than for fine particles, so that, for all of the landslide dams triggered by the Wenchuan earthquake, those that are composed mainly of boulders can be preserved over several rainy seasons. Most of the landslide dams made mainly of soil, however, have disappeared. The resistance to erosion of a dam’s material directly impacts the mechanisms of dam break of a landslide dam. Here, we design two comparable experiments to analyse how the mechanisms of dam break are influenced by boulders at the top of the dam (Figures 10(a) and 10(b)). These two dams are the same size, experience the same water flow conditions, and are composed of the same material. The only difference between these two dams is that one has boulders on the top of the dam and another one does not.