The overall dam-break phenomenon is currently well understood for reservoirs filled with water. However, many dams face serious problems of reservoir sedimentation. Thus, regarding risk management, the influence of a large amount of sediment in the reservoir on dam-break wave propagation is of great importance. Experimental investigations were performed to investigate the influence of the silting degree of reservoirs and grain size distribution on wave heights and velocities. Dam-break waves in silted-up reservoirs are the result of a two-phase flow formed by a layer of water over a layer of saturated sediments in which complex phenomena occur simultaneously. Sediments flow such as a viscous fluid from the dam breach until complete stoppage, and clear water flows over the sediments creating bed load transport. The final sediment deposition cone, the flow of the positive front, and maximum wave heights were studied. An empirical formula is proposed to describe the celerity of the positive wave front in the case of silted-up reservoirs