Hydrodynamic Forces on a Group of Three Emerged Cylinders by Solitary Waves and Bores: Effect of Cylinder Arrangements and Distances

Abstract

In tsunami hazard assessment, usually depth-averaged flow models are applied which use the quadratic friction law with Manning's coefficients to describe the surface roughness of the bottom. Large roughness elements such as buildings and tree vegetation, which are too small to be resolved by the grid of the bottom topography, are mainly considered by using purely empirical Manning coefficients. This approach, however, is not physically sound and may thus result in very large uncertainties in inundation modeling. A more physically-based approach is to determine prediction formulae for the hydraulic resistance of large roughness elements, considering for example different shapes, sizes and types of arrangement which can then be directly implemented in depth-averaged models such as nonlinear shallow water (NLSW) models. Such prediction formulae can be determined on the basis of systematic simulations using a well-validated 3D numerical model. To better understand complex flow phenomena involved in tsunami inundation, three vertical emerged cylinders have been arranged in four different configurations with four different distances between each other and subject to a solitary wave and to a bore. A validated three-dimensional two-phase Reynolds-averaged Navier–Stokes (RANS) model with the volume of fluid (VOF) method has been used to assess flow velocities and water levels near the cylinders. In this study, the validation of the numerical model by data obtained from large-scale model tests in the Large Wave Flume (GWK) Hanover, the flume at the Leichtweiss Institute for Hydraulic Engineering and Water resources (LWI) and the wave tank of the University of Washington is presented and the effects types of cylinder arrangement and distances between the cylinders on the flow induced by a solitary wave and a bore in the near field are discussed.