flood hazard betwee marsa alam - ras ba as, red sea, egypt abstract ...

28M. A. Azabrunoff and consequently increase its velocity. These parameters increase the flash floodpotentiality.Wadi El Ranga Basin is fifth order and has an area of about 310 km2. It is relatively denseand has variable angles of juncture. Its drainage network pattern is denderitic to subdenddriticin parts and parallel to subparallel in others. The trunk wadi (wadi Ranga) flows NNE, NNWand E-W substantial segments of the main channel and large tributaries are guided by faults.Lithology is dominated by basement rocks with only small outcrops of Neogene andQuaternary sediments exposed near the mouth. This basin is characterized by ruggedmountainous relief (up to 1976 m high), low bifurcation ratio and very steep slope in theupper and middle reaches, which indicates high potential for flood hazards at these parts.Wadi Khashir Basin is a sixth order and occupies an area of 464 km 2 . Its drainage net iswell developed, relatively dense and has variable angles of juncture. The basin drains highmountains (up to 1658 m high) with very steep slopes. The relatively large channels arearranged in parallel pattern. Trellis pattern is also developed. Wadi Khashir is oriented E-W inthe upper part and NE in the middle and lower pat1s. Large tributaries are mostly directed EWand NE and less significantly at north. Metavolcanics, metasediments and granites are thewidespread lithologic units exposed in the middle and upper parts. Neogene and Quaternarysediments are exposed at the mouth towards the Red Sea. Local fans are common at the footslops of the mountainous relief. The basin is characterized by very steep slope and low valuesof Rb in the upper part which leading to produce high peak flow. It is also characterized bylow drainage density and low stream frequency, which tends to produce moderate floodintensity especially, along the permeable downstream part. The basin tends to be elongated inshape and therefore, can be classified as a low danger basin. Consequently, it is considered asa good location for groundwater harvesting.Wadi Lahmi Basin is of sixth order and has an area of about 967km 2 (Table 3). It is roughlyrectangular, well integrated; dense and has variable angles of juncture. Its drainage patternsare trellis to sub trellis, radial in parts, and sub parallel in others. The trunk wadi (WadiLahmi) flows NNE to W-E and again NNE. Its large tributaries are mostly oriented E-W,NNW and NNE. Substantial segments of these channels are controlled by faults. Lithology isdominated by basement rocks with small outcrops of Neogene and Quaternary sedimentsexposed near the mouth. This basin is characterised by rugged mountainous relief (up to 1590m high), steep slope, low bifurcation ratio and circular shape. This indicates that there is ahigh potential for flood hazards at the upper reaches of the basin and the downstream part ofthe trunk wadi.Rainfall And Surface RunoffThe study area is characterized by arid climate and dominated by hot temperatures, rainlesssummer and mild winter. The average annual precipitation rate is about 17.4 mm(meteorological stations of Ras Banas). Most of precipitation occurs as heavy showers withshort duration, results in flash floods during the winter season between October and February.The maximum amount of precipitation recorded in one day was 64 mm (24 November 1966).The monthly mean temperature varies between 24-38 °C during summer and 12-26 °C duringwinter. The relative humidity varies between 28% in summer and 59% in winter. The averageevaporation, transpiration varies between 8.7 mm/day in winter and 28 mm/day in summer.According to the above-mentioned climatic parameters, the study area receives an averagerainfall quantity equals to 98.75 million cubic meters/year (Table 3). It is a huge amount ofdownstream outlet. However, a considerable amount will be evaporated.