To analyze the flash flood hazards in our study regions Amman and Wadi Musa hydraulic modelling was performed.
The hydraulic model and the simulations were realized with the software package "Urban Flash Floods" (HE2D/FOG2D) from itwh GmbH in version 8.6.2. HE2D uses the complete system of equations of the two-dimensional shallow water equations to calculate the surface flooding processes. The flow equations are solved in space with cell-centered finite volumes and in time with the explicit Euler approach.
HE2D performs discharge calculations on an irregular triangular grid. This is generated on the basis of the DEM (1 m grid) and the buildings using the FOG2D model generator. To ensure that the simulation of the model runs is as efficiently as possible, the size of the triangles is defined differently depending on the location. Each triangular element has a constant height value (Z-value), which is determined using a smoothing process. The building polygons are considered as non-flow gaps in the 2D calculation grid.
Fig.1: Triangular calculation grid for Downtown Amman
For the rainfall-runoff calculations, a distinction is made between paved and unpaved areas. For infiltration on unpaved surfaces, infiltration losses are considered with the Horton approach. HE2D uses the Manning-Strickler roughness approach. The roughness coefficients (kSt) are determined on the basis of the LULC.
The catchment area determined in a topographical analysis is used as the model boundary. As the hydraulic model has much longer computing times, for Amman a smaller study area of around 120 km² is simulated. The triangular grid consists of over 4.5 million triangles in this area, whereby the average triangle size is approx. 20 m². According to the rainfall scenario analysis, the simulations were carried out with the four selected scenarios.