Flood Risk Mapping in Low Coastal Regions considering Watershed Delineation Uncertainty

Low-lying flat regions with a mountainous hinterland are prone to flash floods. Reports of flooding events are reported more frequently, coming from all parts of the world. Measures to mitigate hazardous effects from flash floods are discussed and put in operation at many places, among others the identification of flood prone zones in flood maps. These maps are drawn on basis of the knowledge of watersheds and the main watercourses therein. However, in lowlands on alluvial fans or on sedimentary terrain the delineation of catchments and streams is uncertain for several reasons. Streams may change their course as effect of a flood event. Due to small topographic gradients measurement inaccuracies amount to a higher relative error in flow calculations. We take the Batinah coast of Northern Oman as an example to demonstrate the deviations in delineations of downstream watersheds, resulting from different digital elevation maps. Finally we come up with recommendations concerning the construction of flood maps in low-lying coastal regions and the use of watershed delineation tools available in GIS software.