With the help of the topographical analysis, potential (main) flow paths and terrain sinks can be easily localized and catchment area boundaries visualized. This analysis of the topology can serve as a preliminary stage of hydraulic modelling in order to obtain an overview of potential hazards and to delineate hydrological (sub-) catchment areas. The analysis is performed based on elevation information from a digital elevation model (DEM) with highest possible resolution (1 - 2 m) using the so-called D8 method, processed with a geo information system software. This method identifies the flow paths in the direction of the largest gradient of the surrounding grids. In order to consider buildings as flow obstacles in urban and densely built-up areas, the elevation value of the DEM is increased in the area of building polygons. The catchment area between watersheds can be determined based on the identified flow paths and a chosen outlet (pour point). The potential flow paths can be classified and visualized according to the size of the catchment area. A sink is a topographically delimited area whose surface slope leads to a local low point and in which the inflowing runoff is completely or partially retained.
Fig.1: Flow paths in the Wadi Musa catchment derived from the DEM 2m (PDTRA).