Numerical Study of the Effect of Floodplain Obstacles on the Flow Induced by Dam Break using MPS method

The floodplains are relatively flat lands in the vicinity of the rivers with the residential, industrial or agricultural usage. The sudden breakdown of large dams leads to the formation and propagation of devastating flood waves over the downstream. The flood propagation occurs over the floodplains due to the topographic variations and in-stream obstacles such as bridges. These waves are developed one and two dimensional over a small reach of the river and floodplains, respectively. In the hydrodynamic simulations, two-dimensional characteristics of the dam-break flow over the floodplains have been studied scarcely. Therefore, the combined effects of the floodplains obstacles, constriction and the bottom barriers are calculated over the dam-break flow characteristics.

In the present study, the moving particle semi-implicit (MPS) method was used to the numerical study of the dam-break flow characteristics over the floodplain. From advantages of this method are including the incompressibility, the particle-based and using of powerful models of gradients and Laplacian in velocity-pressure corrections without any complex smoothing functions. Hence, the effects of the reservoir initial water level, the shapes of the obstacles - lateral transitions as well as the bottom barriers on the hydraulic parameters were studied in 15 various cases. The floodplain obstacles are cylindrical, cubic, rhomboidal and asymmetric, and the bottom barriers are cubic. At first, the sensitivity analysis was carried out on three particles diameters including the 0.01, 0.015 and 0.20 m. Finally, the diameter of the particles equal to 0.015m was adopted as the water particles size in the model. The simulations carried out through more than 280000 spherical particles, with the second order spatial and temporal accuracy.

The precision of the numerical results was calculated using the NRMSE normal error through the comparison with the previous experimental one. The results demonstrated that evacuation of the reservoir occurs in numerical solutions faster than the experimental. Therefore, the MPS model under and overestimates the values of the free-surface profile height and the flow propagation velocity, respectively. The impact of flow to the floodplain obstacles leads to the rising up and the formation of a three-dimensional flow at the obstacles place, the lateral constrictions and the bottom obstacles. Further, the shape of the obstacles represents a crucial factor in the free surface profile deformations, the horizontal component of the surface velocity, and the drag resistance forces applied to the flow.

Normal error values showed that the accuracy of the MPS method in the calculation of the free surface longitudinal profile deformations are variable between 88 and 91 percent.