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

Fourth Environmental Conference, Faculty of Science, Zagazig University, 2009, 17 - 35FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS,RED SEA, EGYPTM. A. AzabArt Faculty – Zagazig UniversityE-mail: mohamed_azab28@hotmail.comABSTRACTTwenty three drainage basins with outlets to the Red Sea were defined in the areabetween Marsa Alam and Ras Banas. They range in area between small (11.52 km 2 ) and large(1476.7km 2 ) and in drainage density between 1.4 and 3.8. The drainage net of the study areais well developed, integrated and fairly dense but is not consistent all over the area. WadiGhadir and Wadi Radi have high values of relief and ruggedness number and these this givesshort time of concentration of runoff. Therefore, their probability for flooding is very high.Wadi Gemal has the highest value of maximum runoff, followed by wadi Lahmi and WadiGhadir. This reflects how dangerous these drainage basins are. However, the spread ofunplanned settlement and mis-management of the land and water resources in the downstream and catchment areas along the coast are threatened by flood hazards. Under suchconditions a strategy is suggested with the purpose of flood mitigation and control. Openembankments or boulder dams at the upstream parts of the drainage basins will minimizeflood hazards and give more chance for groundwater recharge. According to the climaticconditions the study area receives an average rainfall quantity equals to 98.75 million cubicmeters/year. Therefore, a huge amount of rainwater be percolated and consequently rechargethe shallow aquifers if we succeeded to control its runoff components. Basins of WadiKhashir, Wadi El Ranga and Wadi Um Abbas can be classified as less dangerous basins andconsequently, they can be considered as good locations for groundwater collection.ITRODUCTIOAlong the Red Sea coast, flash flooding represents the main natural hazard posing greaterthreat on life, constructions and even marine life. The coastal area is dissected by numerousparallel-pronounced valleys (wadis) that initiated from the mountainous country and runtowards the Red Sea following the general eastward slope. It is occasionally subjected heavyshowers during winter times, followed by sporadic torrential floods that may cause disastrousdamage to roads and the sporadic settlements. In October, 1979 the area was affected bydangerous flood that caused damage along Idfu- Marsa Alam, Marsa Alam – Ras Banas, andQift-Qusier roads, destruction of about 500 houses and killing about 19 persons. In 1980,1985, 1991, 1994 and 1996, floods were took place, but less effect than that occurred during1979. However, as the Egyptian government tries to deal with the short term consequences offlood, it has to think seriously about long-term strategy regarding flood and groundwater,especially after spreading the touristic developments along the Red Sea coast. Unfortunately,yet the government does not have a grip on the flood problem and flood control strategy. Wewill argue that unless serious steps are taken to correct our thinking direction, the floodproblem of the Red Sea coast is likely to become even worse with time and attain ultimatelycalamitous proportions.Generally, fluvial morphmetry and flash floods have been studied by Baker et al (1983)Greis (1983), Gupta (1983), Boughton and Renard (1984), Cooke (1984), Linsley (1986),

18M. A. AzabBaker and Pickup (1987), Cunnane (1987), Klemes (1987), Tasker (1987), Patton (1988) andSummerfield (1991). Moreover, studies in fluvial geomorphology and morphometry of someflood vulnerable areas of Egypt such as Sinai and the Eastern Desert and have receivedparticulars consideration by some workers (e.g. Mobarek et al. 1981; EI Kassas and ElRakaiby 1983; Salem 1985; El Shamy 1985, Schick and Lekach 1987; Saleh 1989 a, b, c; ElRakaiby 1989; EI Etr and Ashmawy 1993; Ashmawy 1993; Yahia et al. 1999 a, b; EI-Etr etal. 1999; Ashmawy et al. 2000; Ashmawy 2001,2002 and Kamh 2002).The study area is located in the central Eastern Desert of Egypt and lies entirely within thecatchment area of the Red Sea. It is bounded from the east by the Red Sea and from the westby the River Nile-Red Sea water divide, and is limited by maximum latitudes 24° and 25° N(Fig. 1). It is of elongated and covers an area of about 6000 km2.Geomorphological studies may give information about the areas affected by flooding.Studying the morphometric parameters of drainage basins is very important for determinationof the intensity of floods, and provides suitable solutions for avoiding or reducing as much aspossible, hazards that occur, in addition to increasing groundwater recharge. The study of thehydro-geochemical characters of groundwater in Wadi EI-Gemal basin (at the middle part ofthe study area) is important for domestic, livestock and irrigation uses. Beside deciphering thedifferent landforms and the drainage courses and their basins, this study helps to define thelocations and distribution of the Plio-Quaternary fans of probably great importance to thegroundwater recharge.Methods Of StudyQualitative and quantitative geomorphic analyses of the drainage basins in the area betweenMarsa Alam and Ras Banas were measured and analyzed for flash flood (Fig. 1).These parameters give a better understanding for surface water potentialities in the definedhydrographic basins. Drainage networks of each drainage basin were delineated from photomosaic(1 :50,000), topographic maps (24 maps at scale 1 :50,000) and landsat images ofscale 1:50.000, as well as, field observations. Twenty-three drainage basins were defined andmapped.The geomorphological parameters of each basin were defined in terms of stream order,bifurcation ratio (Horton 1945 and Strahler 1952), basin shape (Miller 1953 and Schumm1956), sinuosity (Mueller 1968), relief ratio (Schumm 1956.) and ruggedness number (Melton1957). The rainfall and surface runoff were defined according to the equation given by Ball(1937).Topography And GeomorphologyThe study area comprises essentially high and rugged mountains, built up of a seriesmountain our range, more or less coherently trending parallel to the coast and interrupted by anumber of detached masses and peaks (Fig. 2). The high peaks are concentrated in thesouthwestern corner where they rise to elevations up to 1975m (Gaba1 Hamata). The RiverNile-Red Sea water divide runs over the high peaks of the Red Sea Mountains with anaverage elevation of 650m. A large number of wadis initiate from the mountainous terraindissecting the area and run toward the Red Sea following the general eastward slope. They aremostly oriented E-W, WNW-ENE, NNW-SSE and NE-NW. The main wadis are veryimportant sutable areas for extending net of roads crossing the area and as the main arteriesfor groundwater. Slope gradient is mostly steep in the upper reaches of drainageways andtends to be gentle to the east.

FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS 19Fig. (1) Location Map of the Study AreaThe Red Sea high mountains form the backbone of the Eastern Desert and extend parallelto and at a relatively short distance from the coast. They are flanked to the north and west byintensively dissected sedimentary plateaux. In the meantime, they do not form a continuousrange, but rather a series of mountain groups disposed in a linear direction approximating thatof the coast, with some detached masses and peaks. The elongated massive block ofultramafic rocks forms the highest mountainous in the area and the watershed that separatesbetween the Nile and the Red Sea basins. However,the coastal hilly area and the lower mountains form conspicuous topographical featuresbetween the coast and the main Red Sea mountains. The granitoid rocks forming low tomoderate relief but the gabbroic ones forming relatively higher hills.The coastal plain is a low topographic feature of a variable width ranging between 0.6km inthe north (Wadi Ghadir) to more than 12km in the south at Hamata. Sedimentary depositesextending from the piedmont to the Red Sea coast, comprise alluvial fans, wadis and littoral(reef) deposits. Modem fringing coral reef (50-100m wide) is extended along the coast. Coralreefs disappear at the mouth of some wadis as a result of sediment (sand and pebbles)accumulation during floods. However, the coastal line character by presence of embayment(marsas).Several terrace steps were formed in alluvial fans, sometimes occur as a single featureflanking the sides of the wadis but on occasions they may be arranged in vertical successionsforming flights along the sides of the wadi channels. They were recorded along the coast ofthe Red Sea and the Gulfs of Suez and Aqaba and were considered to be closely related to thePlio-Pleistocene peripheral uplift of the hinterland areas (Sellwood and Netherwood 1984).

20M. A. AzabFig. (2) Topography of the study area (based on Egyptian MilitarySurvey topographic map 1986 at scale (1 : 250000).Geomorphic Characteristics of the Plio-Quaternary Alluvial Fans:From north to south, there are six major fans in the study area: Samadi, Nakari, Khalilate EIBahri, Rimarim, Khashir and Ras Banas. They range in area between 10km 2 (Khalilate EIBahri) and 190km 2 (Khashir), and their altitudes range between 79m a.s.l (Khalilate El Bahri)and 172m a.s.l (Nakari). The lower limit of the studied fans varies from 6.0 m a.s.1 (WadiSamadi) to 10.0m a.s.l (Wadi Rimarim). The apex of fan area are arranged adjacent to alongthe main faulted boundary extending at the mountain front has relatively higher slope. Itvaries from 5° (Wadi Rimarim) to 16° (Wadi Nakari). The middle fan areas one of gentleinclination, and varies from 3° at Wadi Gemal and Ras Banas to 8° at Khalilate EI Bahri. Thelower fan has a slope of less than 2°.These fans are drained by number of channels originating from the high relief mountain.These channels are generally parallel at the lower and middle parts of the fan zone. They arehigher dissected and have deep gullies in their lower parts. Most extensive terraces withdifferent levels are developed around the channels of these fans.Drainage CharacteristicsThe study area represents a part of the Red Sea drainage submegabasin (El Shazly et al.1991). The water divide follows the high peaks of the Red Sea Mountains. It is crooked andtrends roughly in a NNW direction parallel to the regional trend of the Red Sea. The finalshaping of the drainage network is an outcome of the last fluvial periods. The present dryvalleys and tributaries were already engraved during these fluvial periods. They are now filledwith surficial deposits of gravels, sands and clays. Twenty three drainage basins with outletsto the Red Sea were defined and sorted out, area-wise into three distinct categories (large>

FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS 21200 km 2 , medium 200-60km 2 , and small

22M. A. AzabFig. (4) Drainage net of the area between Wadi Samadi and Wadi Sherm LuliFig. (5) Drainage networks of the area between Wadi Um Abbas and Wadi Qulan

FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS 23Fig. (6) Drainage networks of the area between Wadi Radi and Ras Banas.Drainage Basin MorphometryQuantitative analysis of hydrographic basin can be achieved through measuring thegeomorphological parameters including stream order (U), bifurcation ratio (Rb), streamfrequency (F) and drainage density (D).Stream Orders (U) of the studied basins range between 7 (Wadi EI Gemal and Wadi Ghadir)and 4 (Wadi Samadi, Wadi Nakari Wadi Khalilate El Bahri, Wadi Khalilate EI Qibli, WadiUm Dahise, Wadi Urn Ghazal, Wadi Stayai, Wadi Kirah EI Hertawi and Wadi Um Rimarim).(Table 1).Bifurcation Ratio (Rb) defined as the ratio between the number of stream order (Nu) to thenumber of the next order (Nu+1)(Horton 1945). Basins with low Rb values tend to produce asharp peak flow while the high Rb reflects low extended peak flow. The estimated Rb valuesof the studied basins (Table 1) ranges between 2.82 and 4.85. Low values were recorded forWadi Satayia, Wadi Khalilate El Bahri, Wadi KhaIilate El QibIi, Wadi Sharm Luli, WadiGhadir and Wadi Um Abbas, and high values were recorded for Wadi Samadi, Wadi Nakari,Wadi Ranga, Wadi Radi, Wadi Um Rimarim, Wadi Kirah EI Hertawi, Wadi Lahmi and WadiQulan. The variation in Rb values reflects the difference in stages of geomorphic developmentand topographic variations. Basins of high bifurcation ratio are elongate in shape and permitthe passage of runoff over an extended period of time to feed the groundwater. Basins of lowbifurcation ratio are circular in shape and allow the runoff to pass in a short time (Saad et al.1980).Drainage Density (D) reflects the effectiveness of the surface water flow,as well as, theinfiltration capacity. It is a function of slope, rock resistance, rock' permeability, andvegetation cover. In the study area, the drainage density (1.4-5.02) is a direct expression ofrelief as the vegetation is not represented and the rocks are more or less impermeable andresistant. Drainage density is low in Wadi Samadi, Wadi Um Abbas, Wadi Abu Ghusun,Wadi Ranga, Wadi Khashir, Wadi Lahmi and Wadi Sataiya, and high in Wadi Khalilate El

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

FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS 25sinuous than the others in the area (Table 2). The straight shape of the streams of Wadi UmTendeba, Wadi Khalilate El Qibli, Wadi Ranga, Wadi Qulan, Wadi Radi and Wadi UmRimarim is favorable to yield floods, but the other physiographic parameters tend to decreasethis effect.Main Characteristics Of The Most Significant Basins: Detailed study was carried out onselected eight basins. The selection was based on basin shape and size, relief and gradient,rock type, geological structure and flash flood vulnerability. Accordingly, Wadi Ghadir, WadiErier, Wadi Gemal, Wadi Um Abbas, Wadi Abu Ghusun, Wadi Ranga, Wadi Khashir, andWadi Lahmi basins were chosen. Rocks exposed in these basins belong mainly to thePrecambrian basement complex and the structures are mainly faults. Sediments andsedimentary rocks occupy the lower part of these basins. Their lithologies favoured lateralerosion of the main channel, and the development of an alluvial bed through which the streamflows.Wadi Ghadir Basin is fan shaped, reaches seventh order and has an area of about 555km 2(Table 3). The basin is characterized by rugged mountainous of high relief (up to 1366 m)with very steep slopes, particularly along and near the water divide. It is asymmetric, withlarge catchment area, and discharge on the southern side of the trunk Wadi. Wadi Ghadir isthe main channel of basin, its flows. Segmental changes in wadi width are agree to change inrock type. Due to the effect of structures (mainly faults) the large tributaries are possesseslineament characters. The short distance between the head and the mouth of the main channel,ruggedness of relief and steepness of slope are factors that tend to decrease the time of runoff,and consequently increase its velocity. This, in turn, increases the flash flood potentiality. Thebasin mouth is crossed by a short segment of the Red Sea coastal road, which is stronglyvulnerable to flooding.Wadi Erier Basin of the sixth order, and has an area of about 261km2. It is roughlytriangular shape, dense with variable angles of juncture. Its pattern is dendritic to subdendriticin parts and parallel to subparallel in others. Wadi Erier flows W-E, and its large tributariesare mostly oriented NNW, NNE to N. Substantial segments of these channels are controlledby faults. The basin is characterized by steep slope in the upper reaches of the main wadi andits large tributaries. This is mainly ascribed to relief ruggedness (up to 771 m high), slopesteepness (due to faulting) and to its lithologic characteristics (in decreasing order ofsignificance). Only small outcrops of Neogene and Quaternary sediments are exposed near themouth of the main wadi. Local fans are common at the foot slopes of the mountains. A smallpart of the Red Sea coastal road is crossing the basin mouth area. This part of the road isstrongly vulnerable to torrential floods. Besides, the settlements of Marsa Alam are stronglyvulnerable to torrential floods (ex. the flash flood on November 1991, 2-3 November 1994and November 1996).

26M. A. AzaboI234567891011121314151617181920212223Table (3) Estimated rain fall and runoff quantities in the studied drainage basinsBasin name Area Annual Max. run off Max. run off in(Km 2 ) rainfallquantityin one day(l0m 3 )one day (l0m 3 ) Min. run off inone day (l0m 3 )W.SamadiW.Um TundibaW. AmboutW. NakariW. GhadirW. Khalilat El BahriW. Khalilat El QibliW. Sharm FaguriW. ErirW. El-GemalW. Sherm LuliW. Um DihesiW. Um AbbasW. Abu GhusunW. RangaW. QulanW. RadiW. Um RimarimW. KhashirW. LahmiW. Um GhazalW. HirtawyW. Sataiya63.2667.0790.4748.34554.2413.2811.5273.05261.321476.66337.97495.29309.6357.33156.7748.83464.01967.0816.4237.2139.0660.1524.121.11.171.570.849.640.230.21.274.5525.75.98.625.3912.730.858.0716.830.290.650.681.080.4298.784.054.295.793.0935.470.850.744.6816.7294.5121.6331.719.823.6710.033.1329.761.891.052.382.53.851.54363.082.662.823.82.0323.280.560.483.0710.9862.0214.1920.8132.416.582.0519.4940.620.691.561.642.531.01238.270.240.250.340.182.080.050.040.270.985.541.271.861.160.210.590.181.743.630.060.140.150.0230.0921.073Wadi Gemal Basin occupies a rather large area (about 1477 km 2 ) and its main trunk wadiassumes a seventh order. The drainage net is well developed, integrated, dense, oriented andhas variable angles of juncture (Fig. 7). The basin drains high mountains with steep slopesand elevations up to 1508 m. The significant large channels are mostly arranged in parallel tosubparallel patterns with dendritic to subdendritic tributaries. Trellis and subtrellis drainagepatterns are locally developed. The main wadi is oriented NNW in the upper part, EW in themiddle part and NE near its the mouth. Its large tributaries are mostly directed NW and NNE,mostly due to fault control. It is characterised by low (Rb), low drainage density and nearlycircle shape. Metamorphic rocks, metasediments and granites are the most widespreadlithologic units exposed in this basin. Neogene sedimentary rocks and Quaternary sedimentswith local fans are located at the mouth of the master wadi. The main trunk wadi at the lowerpart is narrow. This indicates a high potential for flood hazards at the upper reaches of thebasin and the downstream part of the trunk wadi. The basin mouth is crossed by a shortsegment of the Red Sea coastal road.

FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS 27Fig. (7) Drainage network of Wadi Gemal area (Ashmawy 2001).Wadi Um Abbas basin is of sixth order and has an area of about 338km.It is roughlyrectangular, well integrated, relatively dense, and has variable angles of juncture. Its pattern isdenderitic to subdenderitic in the upper part and parallel to subparallel in the middle part. Thetrunk wadi (Um Abbas) flows N, NE in the upper and lower parts to N, NW in its middle part.Its large tributaries are oriented N, NW, NNE, E-W to N. substantial segments of thesechannels are guided by faults. The lithology is dominated by basement (granitic rocks), whileoutcrops of Neogene and Quaternary sediments are exposed near the middle part and at themouth of the waid. The basin is characterized by middle drainage denisity, middle relief (upto 748 m high), high contribution for groundwater and classified as middle dangerous basin.Its shape is nearly circular and reflects the most favorable conditions for the short runoffdistance.Wadi Abu Ghusun Basin is fan shaped of sixth order and has an area of about 495km. Thebasin is characterized by rugged mountainous relief (up to 1468 m high) with very steepslopes particularly along and near the water divide. Wadi Abu Ghusun is the main channel,which flows NW to NE at the upper part and W-E, NW to NE at the lower part toward theRed sea. The long tributaries are clearly oriented and run NE and SE. The change of wadisegments in width, degree of channel incision and channel trends is generally due to thestructural control (mainly faults) and/ or to change in rock type. The large channels are mostlyarranged in parallel to subparallel patterns with dendritic to subdendritic arrangement for thesmaller tributaries. The lithology is dominated by basement rocks (granites and metavolcanicrocks) in the upper and middle parts. Towards the outlet, a few outcrops of Neogene andQuaternary sediments are exposed. This basin has low bifurcation ratio, nearly circular inshape. Ruggedness of relief and slope steepness are factors which tend to decrease the time of

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.

FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS 29The exposed maximum volume of runoff (V) can be calculated according to the equationdeveloped by Ball (1937) as follows V=750A (R-8) m 3 . V, the average weighted maximumvolume of runoff in the basin can reach up to 238.27 million m 3 (Table3). The minimumrunoff on the hydrographic basins of south Marsa Alam is expected to be 21.28 millionm 3 /day; Table 3; if the rainfall in one storm exceeds 13 mm/day (Ras Banas meteralogicalstation).Flash Flood VulnerabilityThe occasional heavy showers along the basement highland during winter in the present timemaintain short period floods. In fact, desert floods are inherently of low duration (few hours tosome days) and are commonly characterized by sharp peak of discharges. Peak of duration isvariable and may be in the range of 10-30 min. Torrential floods cause serious desertproblems and excessive life and property losses, especially along the coast. They may hamperdevelopment and resource exploitation activities. Although their influence is locally verysignificant, sporadic fluvial (flash flood) processes have not been considered in the way thatthey should be during planning of new settlements and also in the major developmentprojects. The study area includes sector of the Red Sea coastal road parallel to the shoreline.Substantial parts of this high-way are crossed from west to east by the lower reaches of theRed Sea drainage basins and consequently subjected to flash flood hazards (Fig. 8).Vulnerability is differentiated into three categories: slight, moderate and high. Thisassessment was based on detailed analyses of satellite images (scale, 1: 50.000), topographicmaps and morphometeric characteristics of the effected drainage basins. Flood hazard doesnot only affect the traffic ability of these roads but extends also to existing and planned urbansites and development projects along and close to them. It also affects the marine life at themouth of wadis, coral reef is completely absent in the torrent pass way and starts to appearonly on the margins of sharms which increase in density and size seawards (Mansour 1999).The present assessment of flash flood vulnerability of the drainage basins in the study areahelps in the selection of the most suitable sites for developmental land use (e.g, establishmentof touristic villages), particularly along the Red Sea coastal zone (Fig. 10). These sites weredistinguished into 4 divisions from A (the least suitable) to D (the best suitable sites). Class Dsites are commonly situated in the interspaces between the drainage basin outlets.Fig. (8) Flash flood vulnerable sites along Marsa Alam-Bernice coastal highway

30M. A. AzabFig. (9) Developmental land use Assessment along the coastal zoneof the Red Sea. Assessment as evaluated on a scale of 4 divisions(A throught D), D for the best sites and A for least sutiable sites.Mitigation Of Flash Flood HazardsThe spread of unplanned settlement and mis-management of the land and water resources inthe downstream areas along the coast will be responsible for the flood damages. The floodhazard risk depends upon exposure to floods, which has to be measured in terms of populationand land use activities (Chan and Parker1996) and the demands on natural resources exertedin either an unsustainable or sustainable manner (Woube 1999). In thinking about the floodproblem, the volume of fresh water lost in the sea needs to be taken into account. This wateris urgently needed for the development projects in the area. Flood also helps in recharginggroundwater aquifers. To avoid or alleviate flash flood damages and possibly to aid inexploitation of floodwater for recharging shallow aquifers the following preventive andcontrol measures should be considered. These measures can be summarized as follows:-1- Construction of successive incomplete rocky dams using the available locale materials atleast along the elongated main course of large wadis as Wadi Gemal, Wadi Lahmi and WadiErier (EI Shamy 1992).2- Construction of some loose boulder dams along the small tIibutaries of the main wadis todecrease the velocity of flood waters and increase their percolation into the wadi bed ( EIShamy 1992 ).3- Using the artificial recharge techniques may offer an important method for waterconservation to reduce the effect of evaporation in the area of study. Also, the onstruction ofsubsurface dams may create subsurface reservoirs (Hanson and Nilsson 1986).4- Decrease the elevation of the dual caring road and the main paved road to a lower level

FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS 31than the level of the crossing wadis floor in the narrow sectors to allow water flow directlyover these roads.COCLUSIOMorphometric parameters of most drainage basins in the area between Marsa Alam and RasBanas indicate low bifurcation ratio, and high values of drainage densities and frequenciesleading to the high possibilities for flash floods and low contribution for groundwaterrecharge. However, drainage basins of Wadi Samadi, Wadi Gemal, Wadi Um Abbas, WadiAbu Ghusun, Wadi Um Rimarim, Wadi Lahmi and Wadi Staiya are characterized by widedrainage channels and low density and thus their contribution to groundwater recharge isexpected to be high. Drainage basins of Wadi Samadi, Wadi Ghadir, Wadi Sharm Luli, WadiUrn Abbas and Wadi Khalilate EI Qibili are nearly circular in shape and thus reflect the mostfavourable conditions for the shortest runoff distance. The other drainage basins have lowelongation ratio and were classified as less dangerous basins. Probability of flash flooding isvery high for drainage basins of Wadi Radi, Wadi Ghadir, Wadi Ranga due to their highvalues of relief and ruggedness numbers. Wadi Gemal and Wadi Lahmi have the highestvalues of maximum runoff, which reflects the dangerous of their drainage basins. Under suchconditions, the construction of alternative barriers of boulder dams at the upstream parts of thedrainage basins will minimize flooding hazards and give more chance for groundwaterrecharge. The majority of wadis that display abnormal geomorphic features are related to thepresence of structural lines rather than any other surface or lithological features. To mitigatethe flash flood hazards and to increase the groundwater recharge, precautionary measuresneed to be taken into consideration at least at the high risk sites.REFERECESAshmawy, M. H., 1993: Assessment of flash flood potential of the Red Sea drainage basinsalong the Qena-Safaga highway, Eastern Desert, Egypt. ITC Journal 1994-2, pp.119-128, Netherlands.Ashmawy, M. H.; Swedan, A. H. and Abdel Fattah, T.,2000: Flash flood hazards ofdrainage basins of Sir" Peninsula, Egypt. Ann. Geol. Surv. Egypt, V. XXIII , pp.467-489.Ashmawy, M. H., 2001: Flash flood potential of Wadi Al Gemal catchment area southEastern Desert, Egypt. M. R C. Ain Shams Univ. Ser., V. 15, pp. 79-93.Ashmawy, M. H., 2002: Surface runoff potentiality and hazards of drainage basins alongthe eastern side of the Nile Valley between Asyut and Al-Minya, Egypt. Egypt. J. Geol., V.46/1 , pp. 283-300.Baker, V.R; Kochel, R.C.; Patton, P.C. and Pickup, G., 1983: Paleohydrologic analysis ofHolocene flood slack water sediments: Spec. Pub. Int. Assoc. Sed. V. 6, pp. 229-239.Baker, V.R and Pickup, G., 1987: Flood geomorphology of Katherine Gorge. NorthernTerritory, Australia. Geol. Soc. Am. Bull. V. 98, pp. 635-646.Ball, J., 1937: The water supply of Mersa Matruh: Survey and mines deposits. Paper No. 43,Cairo, 41 p.Boughton, W.C. and Renard, K.G., 1984: Flood frequency characteristics of some Arizonawatersheds. Water Rescuer. Bull. V. 20 (5), pp. 761-769.Chan, .W. and Parker, D., 1996: Response to dynamic flood hazard factors in PeninsularMalaysia: Geographical Journal, V.162(3), pp. 313-325.Chorley, RJ., 1971: The drainage basin as a fundamental geomorphic unit. In: Chorley, R.

FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS 33M., 1987: Pan fraction crustal evolution in the Nubian Sediment of NortheastAfrica. In : A Kroner (Ed.) proterozic Lithospheric evolution Mid-term Report ofworking group 3 of the international lithospere project. Am. Geophys. Union,Geodynamic Sec., V. 17: pp. 237-257.Linsley, R.K., 1986: Flood estimates. how good are they? Water Resour. Res., 22, pp. 159.164.Mansour, A.M., 1999: Changes of sediment nature by environmental imacts, Sharm AbuMakhadeg area, Red Sea, Egypt. Sedimentology of Egypt. V. 7, pp. 2536.Melton, M.A., 1957: An analysis of the relation among elements of climate, surfaceproperties and geomorphology. Technical Office of National Research, project NRColumbia University. pp 389-642.Miller, V.C., 1953: A quantitative geomorphic study of drainage basin characteristics in theClinch Mountain area; Virginia and Tennessee, Project NR3 P9-042, Tech Rept. 3,Columbia Univ., Geology Dept., ONR, Geography Branch, New York, 30p.Mobarek, I.E:-; Salem, M.H. and Dorrah, H.T., 1981: Flash floods hazard prevention inUpper Egypt villages: Case study of Aulad Salama Villages. Cairo Univ. / MIT,(TPP).Mueller, J.E., 1968: An Introduction to the hydraulic and topographic sinuosity indexes:Ann. Ass. Am. Geol, V. 58, pp. 371-385.ational Academy of Science, 1974: Substances in water for livestock and poultry U.S.Environmental protection Agency. Washington. D.C.Omara, S.M.; Philobbos, E.R. and Hanaa, S.S., 1973: Contribution to theGeomorphology and Geology of the area east of Mini a: Bull Soc. Geogr.Egypt, pp. 125-147.Patton, P.c., 1988: Drainage basin morphmetry and floods. In : Baker, V.R., Kochel, R.c. andPatton, P.C. (Eds): Flood Geomorphology, John Wiley and Sons, New York, p.51-64.Saleh, A.S., 1989a: Geomorphological effects of a torrential floods in Wadi EI-Atfeehy, theEastem Desert of Egypt: Bull. Soc. Geogr. Egypt (in press).Saleh, A.S., 1989b: Natural hazards on the eastern sector of Nuweiba. Tunnel nationalhighway, Sinai, a geomorphologic study (in arabic). Soc. Geogr. Egypt, Tom- V. 62,pp. 143-176.Saleh, A.S., 1989c: Flash flood in Deserts. A geomorphic study of desert wadis: specialstudies series. Inst. of Arab studies and Research, 51, Cairo, 93p. (In Arabic).Salem, M.H., 1985: Collapse of Khuzam Bridge, Qena Governorate, as a result of April,1985 flash flood Technical report: Dev. Res. And Tech. Plan; Center, Cairo Univ.Schick, A.P. and Lekach, J., 1987: A high magnitude flood in the Sinai Desert. In : Mayer,L. and Nash, D. (Eds) : Catastrophic flooding. Allen and Unwin, Winchester,Massachussettes, pp. 381-410.Schumm, S.A, 1956: Evolution of drainage systems and slopes in badlands at PerthAmboy, New Jersey: Geol. Soc. Am. Bull., V. 67, pp. 596-646.Sellwood, B. w. and etherwood, R.E., 1984: Facies evolution in the Gulf of Suez area:Sedimentation history as an indicator of rift initiation and development: ModemGeol., V. 9, pp. 43-69.Strahler, A., 1952: Hypsometric (area altitude) analysis of erosional topography: Geol.Soc. Am. Bull., V. 63, pp. 1117-1142.Summerfield, M.A, 1991: Global geomorphology: An introduction to the study oflandforms. Longman, 537 p.

34M. A. AzabTasker, G.D., 1987: Regional analysis of flood frequencies. In :Singh, V.P (Ed): Regionalflood frequency analysis. D. Reidal, Boston, pp. 1-9TaylorYehia, M.A.; Ashmawy, M.H.; EI-Etr, H.A.; Abdel Monsef, H.; El Shamy, I.Z.; Hermas,E.A.; Hegazy, M.. and Hassan, S.M., 1999a: Flash flooding threat to the Red Seacoastal towns of Safaga, Quseir and Marsa Alam, Egypt. Egypt. J. Remote Sensingand Space Sciences, V. 2, pp. 69-86.Yehia, M.A; Ashmawy, M.H. and EI-Etr, H.A., 1999b: Flash flooding threat to the RedSea coastal towns of Ras Gharib and Hurghada, Egypt. Egypt. J. Remote Sensingand Space Sciences, V. 2, pp. 87-106.Woube, M., 1999: Flooding and sustainable land-water management in the lower BaroAkoboriver basin, Ethiopia: Applied Geography, V. 19, pp. 235-251.

FLOOD HAZARD BETWEE MARSA ALAM - RAS BAAS 35مخاطر الفيضانات بين مرسى علممحافظة البحر األحمر– ورأس بناس– مصرالملخص العربيھناك ٢٣ حوض تصريف تصرف مياھھا على البحر األحمر مابين مرسى علم ورأسبناس بمعدل مساحات لكل حوض من األصغر ‎٥٢‎و‎١١‎ كم ٢ إلى األكبر بمساحة ١٤٧٦كم‎٢‎ وبكثافة تصريف تتراوح من ‎٤‎و‎١‎ إلى ‎٨‎و‎٣‎ م/ث.‏ فنجد أن وادي غادير ووادي راضىلھم أعلى قيم في التضاريس وھذا يعطيھم أقل وقت في تركيز مياه األمطار بينما نجد أنوادي الجمال تسقط عليھم أعلى قيمة من معدل األمطار يليه وادي لحم ثم وادي غادير وھذايعكس مقدار المخاطر في ھذه األودية.‏ولذلك فإن المنشآت والقرى السياحية التي تنشأ بدون تنظيم في مواجھة مصبات تلك األودية،‏تحت ھذه الظروف يجب أن توضع إستراتيجية ھدفھا التحكم وتخفيف مخاطر الفيضاناتوذلك إقامة السدود عند األجزاء األولى من منابع األودية للتقليل من حدة اندفاع المياه وتقليلمسارھا.‏وبناءاً‏ على الظروف المناخية للمنطقة نجد أن متوسط سقوط األمطار يساوى‎٧٥‎و‎٩٨‎ مليونمتر مكعب في السنة ومن أجل ذلك علينا استغالل كمية المياه ھذه في تغذية الخزاناتاألرضية الموجودة بالمنطقة ومن النظر على أحوال المنطقة نجد أن حوض وادي خاشيرووادي رانجار ووادي أم عباس تصنف على أنھا أحواض تصريف أقل خطورة نسبياً.‏