Composite modelling of non-cohesive homogeneous spatial dam breaches with varied grain size distributions

The failure of embankment dams due to overtopping can produce floods that inundate vast downstream regions. Flooding extents are related to the breach formation process, which depends on numerous properties of the dam-reservoir system, including the embankment material grain size distribution (GSD). The effect of GSD on spatial breaching in non-cohesive homogeneous dams is studied here through composite modelling. A numerical model that applies a multi-grain morphodynamic solver was systematically calibrated using a previous laboratory hydraulic model parameter investigation of constant-head overtopping with mixed-size sediment dams with varying GSDs. The numerical model is applied to extend the parameter investigation by simulating longer dams with larger reservoirs, and two additional dams with different GSDs. Dimensionless equations for breach evolution, and a proposed empirical-analytical model for estimating breach discharge and sediment erosion, show good agreement with experimental results. Relations are proposed for determining the hydraulic roughness coefficient and the number of grain classes needed for multi-grain morphodynamic numerical modelling, as a function of GSD.