Author(s): Gerardson Mathieu; Sandra Soares-Frazao; Yves Zech; Adermus Joseph; Nyankona Gonomy
Keywords: Flood; Numerical modelling; Ravines; Mesh
Abstract: In flood modeling, the emphasis is generally placed on the flows brought by permanent rivers whose flow exists all year long, and that inundate surrounding areas in case of high discharges. Ravines are temporary rivers which can also make an important contribution to the flow, especially during great rainy episodes but their input is often overlooked. As a consequence, modelling results sometimes clearly underestimate what is really happening. This work proposes to highlight this difference by comparing the results of two different models of the Cavaillon River located in the Southern Department of Haiti: a first model without the ravines and a second with the ravines. These two models are produced using a triangular mesh which is finer in the minor bed for a better accuracy, and coarser on the floodplains which generally have a more or less regular topography. Topographic data for the study area were obtained from a digital terrain model (DEM 1.5 m × 1.5 m) available at CNIGS (Centre National D’information Géospatiale, Haiti). The triangular mesh was created in the entire flow domain using GMSH. The superposition of the mesh on the DEM provides an altitude value at the different nodes representing the different vertices of the triangles or cells. The finite volume method implemented in a calculation code developed at UCLouvain is used for the discretization of the Saint-Venant equations. The severe flooding of the Cavaillon city after hurricane Matthew was simulated. The upstream hydrograph in the Cavaillon River was reconstructed from measured water levels, while the hydrographs of the various ravines were obtained with Athys software using rainfall data from the Satellite Giovanni. The computed results of water levels and flow velocities are used to produce alea maps related to the flooding of the city. The results obtained from the two models were compared with what actually happened in the Cavaillon River with Hurricane Matthew October 2016. This comparison highlights the significant role played by the ravines in this system.