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

24M. A. AzabQibli, Wadi Sharm Faquri, Wadi Erier, Wadi Qulan, Wadi Radi and Wadi .Kirah El Hertawi.Drainage density is the most important measure of the areal aspects of the basins because itsimplications for runoff (Chorley 1971) and climate (Gregory 1976). Drainage density tends tobe high in semi-arid regions and in humid regions (Gregory 1976).Stream Frequency (f) is defined as the ratio of the total number of stream segments of allorders in each basin to the total area of the respective basin Horton (1954).Physiographic etwork ParametersDetermination of physiographic parameters such as basin shape, relief, slope sinuosity is of agreat importance as a reconnaissance tool to delineate the rainfall/runoff relationship and toassess the surface water capacity of any hydrographic basin.Basin Shape controls the stream discharge from a watershed. It is numeri expressed in theform of circularity and elongation ratios. Circularity ratio (Rc) is the rati the basin area to thearea of a circle with the same perimeter as the basin (Miller 19 Elongation ratio (Re) is theratio of the diameter of a circle equal in area to the basin to maximum basin length (Schumm1956). Rc values range between 0.33 and 0.98 while values range between 0.39 and 0.97(Table 2). Basins of high Rc (Wadi Samadi, Wadi Abbas, Wadi Sharm Luli, Wadi Dahise,Wadi EI Hertawi, Wadi Khashir, Wadi Um Rimari Wadi Ghadir and Wadi Khalilate EI Qibli)yield favorable conditions for shortest runoff give chance to form highly peaked floods.Relief Ratio as expressed by Schumm (I956) is the ratio between Rand L, where is theelevation difference between the basin mouth and the highest point on the b perimeter, and Lis the maximum length of the basin, measured in the same units as R alan line essential1yparal1el to the principal channel. It is an expression of the drainage basin reli in a twodimensionalform, elevation and distance, and correlates better with hydrologi characteristics.Basins with more or less unifOlm slopes result in significant relationshi between relief ratioand hydrologic vaJiables such as runoff and sediment yield. The small the relief ratio, themore is the annual sediment yield and hence more dangerous durin flooding. In theinvestigated area, the relief ratios are low (Wadi Samadi, Wadi Sharm Faqu . Wadi UmRimarim, Wadi Qulan and Wadi Satiya) and high (Wadi Ranga, Wadi Abu Ghusu and WadiKhashir) (Table 2).Ruggedness umber is the product of drainage density and basin relief (Strahle1958). The increase of the runoff discharge is the result of the increased relief and drainagedensity. In the studied area, the ruggedness number ranges from 4366 (Wadi Radi) to 365(Wadi Satayia) (Table 2). Accordingly, the probabilities offload occurrence along the streamsof Wadi Ghadir, Wadi Erier, Wadi Gemal, Wadi Abu Ghusun, Wadi Ranga, Wadi Radi, WadiKhashir and Wadi Lahmi are higher than those of the other streams. This means that the timerequired for the runoff to flow from the highest distal ridge to the mouth (concentration time)of these streams is much shorter than that required for the other streams.Slope Index is the quotient of the elevation difference between basin divide (basin relief) andoutlet and the main channel length (Taylor and Schwarz 1952). The main streams of WadiGemal, Wadi Qulan, Wadi Um RimaJim, Wadi Satayia and Wadi Sharm Luli are moresteeper than the that of Wadi Ghadir, Wadi Khalilate EI Qibli, Wadi Abu Ghusun, WadiRanga, Wadi Khashir and Wadi Urn Ghazal (Table 2). On the other hand, the upstream partsof some wadis are steeper than downstream ones and consequently, they are more favorable toyield floods, while the lower parts are suitable to store ground water.Sinuosity is defined as the ratio of the valley length to the shortest distance between mouthand source of stream (Mueller 1968). The estimated ratios indicate that streams ofWadi Ghadir, Wadi Gemal, Wadi Lahmi, Wadi Shmm Luli and Wadi Um Dahise are more