Flash floods occur periodically in Makkah city, Saudi Arabia, due to several factors including its rugged to-pography and geological structures. Hence, precise assessment of floods becomes a more vital demand in development planning. A GIS-based methodology has been developed for quantifying and spatially mapping the flood characteristics. The core of this new approach is integrating several topographic, metrological, geological, and land use datasets in a GIS environment that utilizes the Curve Number (CN) method of flood modelling for ungauged arid catchments. Additionally, the computations of flood quantities, such as depth and volume of runoff, are performed in the attribute tables of GIS layers, in order to assemble all results in the same environment. The accomplished results show that the runoff depth in Makkah, using a 50-years re-turn period, range from 128.1 mm to 193.9 mm while the peak discharge vary from 1063 m<sup>3</sup>/s to 4489 m<sup>3</sup>/s. The total flood volume is expected to reach 172.97 million m<sup>3</sup> over Makkah metropolitan area. The advan-tages of the developed methodology include precision, cost-effective, digital outputs, and its ability to be re-run in other conditions
Global Digital Elevation Models (DEMs) have been utilized in various geomatics activities worldwide. Recently, there exist several available DEMs vary significantly in terms of spatial resolution and release dates. This paper examines the reliability of eight recent global DEMs, namely the EarthEnv-D90, SRTM 1, SRTM 3, ASTER, GMTED2010, GLOBE, GTOPO30, and AW3D30, in two study areas in Egypt and Saudi Arabia representing different topography patterns. Known ground control points with measured accurate coordinates and precise elevations have been utilized in evaluating the performance of those DEMs. It has been concluded that such a judgment procedure should not be carried based on a single statistical measure. First, five statistical measures, specifically the range, standard deviation, correlation, kurtosis, and skewness, have been evaluated separately for each DEM's errors. Then, a new reliability index is introduced based on the weighted average concept. The accomplished results show that global DEMs perform differently in different topography patterns. It has been concluded that the EarthEnv-D90 and SRTM1 models attain high reliability indexes in the Nile delta region that represents a flat topography, while the GMTED2010 and EarthEnv-DEM90 models came in the first places for the second study area, Makkah, which represents mountainous topography.
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