Scientific paper

WATER RESOURCES MANAGEMENT…..………………………………………………………………………………………….……….1

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Water resources management in the area of wadis Gasus and Gawasis (south

Safaga-Red Sea) through the geophysical and hydrological studies Galal H. Galal* and Mohamed I. Gad **

* Geophysical Department, Desert Research Center, Cairo, Egypt.

** Hydrology Department, Desert research Center, Cairo, Egypt.

ABSTRACT

To obtain the maximum quantity of water with good quality to meet requirements at least cost,

especially in arid zones, is considered the desired goal in any water management study. In this

paper, water resources management of two selected ungauged drainage basins with different

drainage characteristics (Wadi Gasus and Wadi Gawasis south Safaga-Red Sea) was highlighted

based on the geophysical and hydrological studies.

Methodologies adopted in this paper include both geophysical tools and hydrological tools.

The parametric electrical resistivity measurements for the different formations are carried out in the

field through 11 VES and the quantitative interpretation for the field curves was carried out

applying Zohdy and Vander Velpen models. Hydrological study includes the estimation of rainfall

recurrence period, the delineation of the watershed boundaries and their drainage characteristics

applying WMS model, the estimation of the rainfall-runoff relationship and the generation of the

basin hydrograph using (HEC-1) model.

The geoelctric study along the main channels of the two basins revealed that the study area

is affected by a group of step faults parallel to the Red Sea Coast. Consequently, they consider as

good groundwater conduits. The upstream portion of these wadis composed of three geoelectrical

layers that correlated to Wadi fill, fractured basement and fresh basement with good potentiality.

While the middle part are Wadi fill and sedimentary layers cover the fractured basement (high

potentiality). At the downstream portion the fractured basement was not reached due to the effect of

the normal fault with downthrown side towards the Red Sea coast and due to sea water intrusion.

The salinity of the groundwater increases with depth. The presence of Quseir shale over Nubia

sandstone restrict the recharge and affect its potential.

The results show that the maximum rainfall in one day of 28.5 mm or more can only be

expected in one year out of 20. The drainage characteristics created by WSM model show that Wadi

Gasus chatchment has great tendency to receive flash floods with peak runoff more than that of

Wadi Gawasis catchment. The weighted CN value for the catchments upstream was estimated to be

78 for the AMC-II, and 60 for AMC-I. The runoff volume resulted from maximum rainfall in one

day storm of 28.5 mm reaches 1.44 x 106 m

3 (Wadi Gasus) and 0.15 x 10

6 m

3 (Wadi Gawasis). The

good management of this water resource helps the proposed strategy for Bedouins' development

based on small irrigation communities (2 Feddan/family x 50 family x 3000 m3/Fed x 5 years = 1.5

x 106 m

3). Accordingly, to support future studies, an adequate rainfall-runoff monitoring system

covering the watershed of the two basins should be installed. Flood-recording stations should be

installed and rehabilitated at the three proposed points in the watershed of both two basins near the

proposed sites of storage dams. Detention dams were suggested in areas of high infiltration in order

to replenish the groundwater reservoir, while storage dams and cisterns are recommended in

localities of low infiltration rate for water and soil conservation.

INTRODUCTION Red Sea coastal zone suffers from fresh water scarcity although it is seasonally subjected to

flash floods. These floods cause disasters to the infrastructure. The aim of this paper is to construct

chachment hydrograph as essential tool for water resources management in the drainage basins.

Two ungauged drainage basins that occupy the western side of Red Sea are selected for this study.

The studied basins are restricted to the east by the Red Sea, and to the west by the mountainous

ridges which are the main water divide separating the River Nile drainage system from the Red Sea


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drainage system. The study area is bounded by latitudes 26º 22′ and 26º 35′ N and longitudes 33º

45′ and 34º 05′ E with total surface area of about 300 km2 (Fig.1).

Fig.1 : Location map of the studied drainage basins showing the location of VES Climatic conditions

The Red Sea coastal zone is characterized by long summer months (April-October) with

annual mean of maximum air temperature of 28º C (Quseir station) while the minimum value is

recorded in the short winters (21º C at Quseir station). The mean annual rainfall increases in the

northeast and approaches 20 mm at El Quseir station. The minimum and maximum mean daily

evaporation value varies from 5 mm to 28 mm (in January and June respectively). The recorded

maximum relative humidity at the coastal zone reaches 47 % at Quseir station.

Geomorphologic setting

Based on the studies carried out by Abu El-Izz (1971), Gawish (1981), El-Shamy (1985,

1988 &1992), El-Rakaiby (1989), Philip et al. (1992), Ashmawy (1994), Gomaa (1992) and Aggour

(1997), the study area can be subdivided geomorphologically into the following three units:

i-The coastal plain, which occupies the low land areas parallel to the Red Sea shore. Its

width ranges between few meters at Safaga City and 20 km at Ras Abu Soma and its altitude varies

from sea level up to 90 masl. The relatively small width decreases the possibility of runoff control.

ii- The watershed areas, which can be divided to the Red Sea mountainous terrain, the high

plateaux and the hilly areas. The Red Sea mountainous terrain runs parallel to the Red Sea coast and

surrounds the coastal hilly area to the west (Gabel Abu Furad 1032m, Gabel Waira 1035m and

Gabel Abu Aqarib 775m). Many wadis cut this high land area along the Red Sea in NE-SW

direction (Wadi Safaga, Wadi Gasus and Wadi Gawasis). The mountain area is built up of Pre-

Cambrian basement comprising igneous, volcanic and metamorphic rocks. These rocks with sharp

height and low infiltration rate increase the predicted runoff component of any flood.

In addition, the high plateaux include the Nubia Sandstone Series (El-Ababda Sandstone

Plateau with elevation varing between 400 and 500 masl) and El-Maaza limestone plateau. The

coastal hilly areas constitute the most scenic land feature in the Res Sea coastal zone. Their

elevation ranges from 155m (Gabel Abu Shgili) to 212m ( the outlet of Wadi Gasus). They act as

local watersheds draining by the downstream portions of huge wadis. These sedimentary hills

increase the infiltration capacity and decrease the flow rate of the runoff component.

iii- The water collectors, which are composed of morphotectonic depressions (initiated as a

result of the uplifting of the surrounded basement rocks from east and west directions) and

hydrographic basins (Aggour 1997). The hydrographic basins such as Wadi Gasus and Wadi

Gawasis are classified into sub-basins, according to their catchment areas. They enclose part of the

mountain as well as the coastal hills.

30 32 34 36

22

24

26

28

30

SINAI

RE

D S

EA

Ras

Banas

Quseir

Safaga

Hurgada

SUDAN

Suez G

ulf

Aqab

aG

ulf

MEDITERRANEAN SEA

Study Area

Scal0 85 170 255 km

Qena

Qeft

Nas

er

La

ke

Cairo


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Geologic setting

The rock exposures in the study area have a wide range of geologic time from Precambrian

to Quaternary (Fig. 2). According to Attia (1955), Mansour et al. (1956), Said (1962), El-Tarabili

(1964), El-Akkad and Dardir (1966), Bassyouni (1970), Issawi et al. (1971), El Ramly et al. (1972),

Sabit and Bykov (1976), El-Shazly (1977), Morcoss (1977), El-Akaad & Noweir (1980), Misak and

Abdel-Baki (1991), Saleh (1993), Abdellah (1995), Tolba (1997), Tolba and Amin (1998) and

Hammad (1998), Precambrian age is represented by igneous and metamorphic rocks where the

granitic rocks are the dominant rock type among these rocks. The Upper Cretaceous age is

represented by Nubia Formation (sandstone beds with mudstone intercalations), Quseir variegated

shale Formation (Yussef 1957), Duwi Formation (phosphate, marl, shale and limestone) and Dakhla

Formation (shale & marl). Paleocene–Eocene age is represented by Tarawan Formation (carbonate

beds), Esna Formation (shale) and Thebes Formation (limestone with flint beds). Oligocene age is

represented by Nakheil Formation (breccia, limestone, clays and sandstone). Miocene and later

sediments are represented by eight formations which are: Ranga Formation (conglomerate &

sandstone), Um Mahara Formation (sandy, gypsiferous limestone), Abu Dabbab Formation (gypsum

with rare shale & limestone intercalation), Um Gheig Formation (dolomite), Samh Formation (shale,

sandstone & limestone), Gabir Formation (sandstone, reefal limestone & gravel beds), Shagra

Formation (sandstone minor marl) and

Reefs & raised beaches (reefs &

gravel beds).

Fig. 2: Geological map of the study area (after EGSMA 1978)

Structurally, the azimuth frequency diagram of the studied area (Fig.3) indicates that the

study area is mainly affected by three main trends. These trends can be arranged in abundance into

NW-SE set of faults, parallel to the Red Sea coast, which affect the flood water flow less than NNE-

SSW set (remarkable at Wadi Gawasis) since they are perpendicular to the direction of flow. NE-

SW is visible in geosynclinal metasediments and affect the runoff component with their high

infiltration capacity. The statistical analysis of the drainage pattern of every ungaged studied

catchment was used in estimating the azimuth frequency diagram. It is clear that the two wadis are

structurally and lithologically controlled with different degree. The comparison between them (Fig.

3) shows that Wadi Gasus is more structurally controlled than lithologically, while the effect of

lithology and structure is more pronounced in case of Wadi Gawasis than in Wadi Gasus.


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Accordingly, the high initial abstraction of the soil cover in Wadi Gasus is expected.

Fig. 3: Azimuth frequency diagram of drainage lineation pattern for Wadi Gasus (a) and Wadi

Gawasis (b)

MATERIALS AND METHODS

Two field trips were achieved in both Wadi Gasus and Wadi Gawasis during 2006 to collect

the materials used in this paper. Eleven Vertical Electrical Sounding (VES) were carried out in both

Wadi Gasus (7 VES's) and Wadi Gawasis (4 VES's). The basic hydrologic data of the two studied

basins were obtained during these field trips. In addition, the archival data such as long term rainfall

records were collected from the DRC library beside the recent rainfall records from the world Wide

Web through NASA website. The methodological approach used in this paper is based on the

geophysical study and the hydrologic study.

Water resources management The surface water resources management of any hydrographic basin needs the estimation of

rainfall probability and recurrence period, the delineation of the watershed boundaries and their

drainage characteristics, the estimation of the rainfall-runoff relationship and the generation of the

basin hydrograph.

Probability of exceedence and return period The rainfall data obtained from the ground rain gauge (Qusier meteorological station) are used to

generate probability of exceedence (Table 3 and Fig. 4, Gad and Abdel-Latif 2003) while the Tropical

Rainfall Measuring Mission (TRMM) data are obtained and used to generate the isohyets map (Fig. 4). TRMM is an international project jointly sponsored by the Japan National Space Development Agency

(NASDA) and the U.S. National Aeronautics and Space Administration (NASA) Office of Earth

Sciences. The rainfall data can be obtained as mm/hr by running the algorithm 3B42-V6, which produce

TRMM merged high quality infrared precipitation and root-mean-square precipitation-error estimates.

These girded estimates are on 3-hour temporal resolution and 0.25-degree by 0.25-degree spatial

resolution in a global belt extending from 50 degrees south to 50 degrees north latitude. The relationship between the probability of exceedence G (x) and return period T is given by the

following formula (Ponce, 1989);

G (x) = 1/T……………………………………………………..………………1

While the most universally used formula for calculating the return period (T) for a particular

flood peak is that of Weibull (Brook, 1986);

T = (N+1) /m…………………………………………………….……………..2

Where N is the number of events in the series (dimensionless) and m is the rank (from largest to

smallest) of each events in the series (dimensionless). Applying these formulae, the return period

and the probability of exceedence are obtained (Table 1 and Fig. 4).

Delineation of the basin characteristics Delineation of the watershed boundaries and their characteristics is carried out through the

use of a Digital Elevation Model (DEM) file and the program TOPAZ (Martz and Garbrecht 1993).

In order to obtain the basin characteristics, Watershed Modeling System (WMS) package has been

used. WMS is a comprehensive environment for hydrologic analysis. It was developed by the

Environmental Modeling Research Laboratory of Brigham Young University in cooperation with

the U.S. Army Corps of Engineers Waterways Experiment Station. WMS is used to delineate the

catchment and Wadi streams (Nelson et al 2000). The input data to WMS model were obtained

from SRTM3 (Shuttle Radar Topography Mission). SRTM3 data give the elevations in reference to

the mean sea level in the center of a grid of 90mx90m spacing. Table 2 summarizes the terrain


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P = 9.949 ln (T) - 1.554

R² = 0.979

0

5

10

15

20

25

30

1 10 100

Ma

xim

um

rain

fall

de

pth

in o

ne

day

P

(mm

)

Recurrence period T (year)

Gwasis Basin

Gasus Basin

RED

SEA

33 50 34 00

2635

2630

Legend1 st Order2 nd Order3 rd Order4 th Order5 th OrderWater divide

0 2 4km

50

52

54

56 58

characteristics of the two studied wadis while Fig. 5 shows the delineated Wadi catchment and

streams overlaid the satellite image.

Fig. 4: Isohyets map based on TRMM data (1998-2010, left map) and the relation between

recurrence interval of events (years) and max. rainfall depth in one day in mm (left map)

Table 1: Return period and probability of exceedence for maximum rainfall in one day during the

period (1980 – 1998)

Date Max.

Rainfall

in one

day

(mm)

Max. rainfall

(descending)

Rank

(m)

Recurrence

period (T)

(year)

Probability of

exceedence

(G (x)) (%) 1980 14 28.5 1 20 5%

1981 1.5 20 2 10 10%

1982 0 19.5 3 6.67 15%

1983 20 14.2 4 5 20%

1984 4.3 14 5 4 25%

1985 3.2 12 6 3.33 30%

1986 0.1 7.2 7 2.86 35%

1987 0.1 6.5 8 2.5 40%

1988 1.8 5.4 9 2.22 45%

1989 0 4.3 10 2 50%

1990 0 3.2 11 1.81 55%

1991 0.2 2.8 12 1.67 60%

1992 19.5 1.8 13 1.54 65%

1993 28.5 1.5 14 1.43 70%

1994 14.2 1.2 15 1.33 75%

1995 6.5 0.9 16 1.25 80%

1996 5.4 0.5 17 1.18 85%

1997 7.2 0.2 18 1.11 90%

1998 2.8 0.1 19 1.053 95%

The drainage characteristics of terrain surfaces of the two basins required for water

resources management are automatically computed applying WMS. These parameters include basin

area (A), basin slope (BS), average overland flow (AOFD), basin length (L), sinuosity factor (SIN),

mean basin elevation (AVEL), mean flow distance (MFD), maximum flow slope (MFS), maximum


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stream length (MSL), stream segment slope (CSS), bifurcation ratio (Rb), drainage density (D) and

drainage frequency (F) (Table 2).

Fig. 5: An automatically delineated watershed boundary through the use of a digital

elevation file and the program TOPAZ and the streams shown were also automatically created by

WMS

Table 2: The drainage characteristics of terrain surfaces of the two selected basins

Wadi

name

A

Km2

BS

m/m AOFD

m

L km

SIN AVEL m

MFD km

MFS m/m

MSL m

CSS m/m

Rb D Km

-1

F Km

-2

Gasus 139 0.12 676 24.06 1.16 265 29.99 0.02 27.96 0.01 4.33 3.47 5.43

Gawasis 36 0.07 935 10.59 1.08 181 12.7 0.03 11.4 0.02 3.98 3.82 6.93

A is basin area (km2), BS is basin slope, AOFD is average overland flow (m), L is basin length (km), SIN is sinuosity

factor, AVEL is mean basin elevation (m), MFD is mean flow distance (km), MFS is maximum flow slope, MSL is

maximum stream length (m), CSS is stream segment slope, Rb is bifurcation ratio, D is drainage density (km-1

) and F is

stream frequency (km-2

).

Hydrograph generation

The time of concentration (Tc) of each catchment is defined as the travel time in minutes for

a drop of water to travel from the hydraulically most distant point of the watershed to the gauging

point downstream (Chow et al. 1988). It is estimated using the United States Soil Conservation

Service (now called the Natural Resources Conservation Service) method with a proper adjustment

factor for each catchment (SCS 1986). The lag time, which is the time interval between the

midpoint of excess rainfall to the time of peak discharge of the flood hydrograph, is estimated to be

0.6 of Tc according to the SCS method. The SCS method is chosen in this analysis because data is

scarce. For each catchment a runoff curve number (CN) was selected, based on the SCS method

(SCS 1986), taking into consideration the catchment characteristics, which include antecedent

moisture conditions, type of soils, initial abstraction of rainfall, slope and length of the longest

channel, watershed boundaries, urbanization, and land cover (Ponce and Hawkins 1996). This

includes the area, weighted average CN value, the average slope and the lag time. WMS (1996)

software is used for hydrologic analysis. This system is based on the HEC-1 flood hydrograph

package (HEC-1 1985) developed by the U.S. Army Corps of Engineers/Hydrologic Engineering

Center. It is designed to simulate the surface runoff hydrograph resulting from a rainfall storm event

by representing the basin as an interconnected system of components. Each component models an

aspect of the rainfall-runoff process within a sub-basin. Components may include sub-basin runoff,

stream channel, and a reservoir. The characteristics of each component are described by a set of


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parameters representing its physical process (Bedient and Huber 1992). The following data are

necessary for the application of WMS;

-Geomorphological information of each sub-basin (Basin area, mean slope and length of overland

flow).

-Geological information which include impervious drainage areas and percentage of channel flow

directly connected to the impervious drainage area. The first represents the land upon which no

infiltration takes place and the second is the regions of the basin from which the water flows

directly to the watershed.

-Hydrological information (Time of concentration, infiltration capacity, calculated weighted curve

number and rainfall intensity).

Input Data for WMS program

- Watershed characteristics data

The watershed characteristics data were automatically created by WMS from DEM file

(drainage area, length of main trunk and the slope, Table 2). The impervious drainage area and

percentage of channel flow directly connected to the impervious drainage area were calculated from

the geological map and the field observations.

- Rainfall data

Three storms, with different return period (Table 3) are used as input data in WMS program.

The rainfall intensity is calculated according to the storm duration (about 5 hours). Three scenarios

for the rainfall runoff relationship were carried out according to the different rainfall depths.

Table 3: Selected rainfall depths of different storms as input data in WMS program (after Ismail,

and Morad, 2009)

- Loss data The loss rates were calculated using the SCS (1986) method. The SCS combines infiltration

losses with initial abstraction and estimates rainfall excess by the following relationships;

Ia = 0.2 S…………………………………………………………………………………..………………..4

A dimensionless curve number CN is defined such as 0<CN>100

Where, Q is accumulated runoff depth or rainfall excess (mm), P is accumulated rainfall depth

(mm), Ia is initial abstraction (mm), S is potential abstraction of the soil (mm) and CN is the curve

number. The curve number (CN) is determined according to the type of soil.

Time of Concentration (Tc)

Return Period

(years)

Rainfall Depth

(mm)

Rainfall intensity

(mm/hr)

20 28.5 5.7

6.67 19.5 3.9

3.33 12 2.4

5...............................................................................................................25425400

CN

S

6..................................................................................................................

1

1

K

i

K

i

Ai

AiCNi

CN

3.................................................................................................................)8.0(

)( 2

SP

IaPQ


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Time of concentration is calculated for each basin by using the following equation

(Subramanya, 1984),

Where, Tc is time of concentration (min), L is Flow length (m) and S is average slope of the

main water course (m/m) which corresponds to MFD in Table 2.

Output of WMS program

The output of WMS program includes runoff amount and watershed hydrograph. The

hydrographic parameters of the studied ungauged drainage basins include time parameters (the time

of flood concentration Tc, lag time Tl , duration of unit hydrograph D, the time base Tb and the

time to peak Tp, peak flow Qp and flow velocity V. These parameters were estimated by

application of WMS software according to the following relationships (SCS, 1986);

Tl = 0.6 Tc ……………………………………....………………...……..……....……8

D = 0.133 Tc , …... ……………………………………… …………...,..…….….....9

Tb = 2.67 ( 1.2 Tc + 0.5 D )…………………….………..….………………,............…..10

Tp = D/2 + 0.6 Tc ……………….….…….………….………………...…......….11

Qp = 7.4074 x i x A / Tp ………………..………………………...…….……,….….…..12

V = 0.2278 Lb / Tc …………………(Jain et al. 1997)………..……………,,,…....…13

Where, Tc ,Tl, D, Tb, Tp, in minutes, Qp in m3/sec, A is the watershed area in km

2 and V is the

flow velocity (m/sec).

In the other hand, the groundwater management is discussed based on the geophysical study. It

deals with the subsurface conditions that clarify the geometry and configuration of the water

bearing formations. So, geoelectric method was applied using Vertical Electrical Sounding (VES)

of Schlumberger array (Kunetz, 1966). The parametric electrical resistivity measurements for the

different formations are carried out either in laboratory or in the field (11 VES across the main

channel of the two studied wadis). Their values were used during the preparation of the initial

models to interpret the subsurface changes under each VES station. A detailed quantitative

interpretation for the field curves was carried out by using two computer programs (Zohdy, 1989

and Vander Velpen B.P.A, 1988).

RESULTS AND DISCUSSION It is obvious from the isohyets map (Fig. 4) that the annual rainfall depth in the study area

increases in the NW-SE direction. As a general, Wadi Gasus receives less rainfall amount than

Wadi Gawasis. Moreover, Wadi Gasus chatchment receives average annual rainfall depth of 56 mm

while Wadi Gawasis chatchment receives 59 mm based on the TRRM records during 1998-2010. In

addition, Table 1 shows that the relation between the maximum rainfall depth in one day and the

return period is inversely proportional. It is clear from the table and the curve (Fig. 4). The

maximum rainfall in one day depth records during the period 1980-98 ranges from 0.1 and 28.5 mm

with the mean 7.6. The statistical analysis of these records show that the maximum rainfall in one

day with a probability level of 30 percent of exceedence is 12 mm (Table 1). In addition, on average

a maximum rainfall in one day of 28.5 mm or more can only be expected in 1 year out of 20. The

relation between the maximum rainfall in one day P max and recurrence period T is given by the best

fit relation (r2 =0.88);

Pmax = 9.949 ln (T) – 1.554, (r2 = 0.979)…………………………………………..14

This means that Pmax is expected to be 21.3 and 25.3 mm every recurrence period of

magnitude 10 and 15 years respectively. This must be taken into consideration in water

management of surface runoff.

Generally speaking, the drainage characteristics of terrain surfaces of the two basins show

that the stream order of the two basins related to 5th

order (Fig. 5). In addition, Table 2 shows that

7......................................).........1940....(........................................0194.0

385.0

77.0

KirpichS

LTc


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Wadi Gasus chatchment has great tendency to receive flash floods with peak runoff more than that

of Wadi Gawasis catchment. The drainage area (A), the basin slope (BS), the sinuosity factor (SIN),

mean basin elevation (AVEL), mean flow distance (MFD), and the bifurcation ratio (Rb) of Wadi

Gasus assure this finding. Otherwise, the short length of average overland flow (AOFD) of Wadi

Gasus reflects quick surface runoff entering the stream. Since the steeper the stream gradient, the

shorter the runoff residence time is, and the higher the potential for flooding (Gad and Abdel-latif

2003), accordingly Wadi Gasus has high potentiality for flooding. Moreover, Wadi Gawasis with

greater stream segment slope (CSS), maximum flow slope (MFS), drainage density (D) and

drainage frequency (F) than those of Wadi Gasus considers highly productive of surface runoff

(Gad 2010).

In the other side, the lack of accurate rainfall-runoff data is a major constraint on model

calibration and validation. The main uncertain parameter that affects the results of the simulation

process applying WMS model is the CN value (Ponce and Hawkins 1996). Selection of this value

depends on judgment about the watershed’s physical characteristics as well as its antecedent

moisture conditions. According to the geological assessment of the catchment areas (Fig. 2), two

main soil categories can be clearly identified. A curve number (CN) was assigned to each category.

The first category consists of Miocene limestone with a relatively thin Quaternary soil cover and

sparse vegetation. For this category, the hydrological soil group ‘‘C’’ was chosen (Gawasis basin).

Thus, for mountain brush mixture with poor conditions the curve number (CN) was estimated to be

75. The second category consists of Cretaceous sandstone and basement rocks with no soil cover.

The hydrological soil group ‘‘D’’ was chosen, which indicates a hard surface with a very slow

infiltration rate (Gasus basin). The corresponding curve number was estimated to be 91. Antecedent

moisture condition (AMC) is another parameter to affect the choice of the CN value. Based on the

rainfall records and the distribution of the rainfall patterns, the average antecedent moisture

condition AMC-II was adopted for the first and third storms. For the second storm, a dry antecedent

moisture condition (AMC-I) was selected. The weighted CN value for the catchments upstream was

estimated to be 78 for the AMC-II, and 60 for AMC-I (Chow et al. 1988). The results of the model

(Table 4 and Fig. 6 and 7) show the following;

Fig. 6: The Simulated Hydrograph of the Wadi Gasus catchment applying HEC-1 model for 1st

storm event (left graph), 2nd

storm event (middle graph) and 3rd

storm event (right graph)


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Fig. 7: The Simulated Hydrograph of the Wadi Gawasis catchment applying HEC-1 model for 1st

storm event (left graph), 2nd

storm event (middle graph) and 3rd

storm event (right graph)

Table 4: Output parameters of the generated hydrograph applying WMS model in the two

catchment

Wadi name L (km) S (m/m) Tc (hour) Peak Time to Peak

(hour)

Volume

m3

Hydrograph parameters based on rainfall storm of 28.5 mm and return period 20 years

Wadi Gasus 29.99 0.01 1.567 288.65 1hrs. 50 min 1446646

Wadi Gawasis 12.7 0.02 0.619 33.39 1 hrs. 30 min 154845

Hydrograph parameters based on rainfall storm of 19.5 mm and return period 6.67 years

Wadi Gasus 29.99 0.01 1.567 7.16 2 hrs. 0 min 34623

Wadi Gawasis 12.7 0.02 0.619 4.4 1 hrs. 0 min 9597.6

Hydrograph parameters based on rainfall storm of 12 mm and return period 3.33 years

Wadi Gasus 29.99 0.01 1.567 0.21 2 hrs. 10 min 973.8

Wadi Gawasis 12.7 0.02 0.619 0.22 1 hrs. 30 min 971

The maximum flow rate resulted from daily rainfall storm of 28.5 mm which recurs every 20

years, ranges from 288.65 m3/s (Wadi Gasus) to 33.39 m

3/s (Wadi Gawasis) while it ranges

between 7.16 m3/s (Wadi Gasus) and 4.4 m

3/s (Wadi Gawasis) based on the hydrograph analysis

generated from daily rainfall storm of 19.5 mm. The normal flow rate which recurs every one year

out of four ranges between 0.21 and 0.22 for the two catchments respectively. In this meaning, the

runoff volume resulted from daily rainfall storm of 28.5 mm reaches 1.44 x 106 m

3 (Wadi Gasus)

and 0.15 x 106 m

3 (Wadi Gawasis). This huge runoff volume of Wadi Gasus, could be harvested by

construction of a storage dam in the outlet of the Wadi (Fig. 8). This storage dam should be

designed at the maximum storage volume (1.44 x 106 m

3). The best location of this dam is in the

main trunk of the Wadi at its outlet. In addition, to recharge the groundwater aquifer, the proper

location of the detention dams is the contact between the fracture basement outcrops and the

Cretaceous sandstone outcrops in Gasus basin and in the contact between the fractured Miocene

limestone outcrops and the Pliocene deposits outcrops in Gawasis basin. The old non-operational

phosphate mines there can used as cisterns for collecting normal runoff under the daily rainfall

storm of 12 mm after directing the runoff through network of small canals. The water harvesting for

these two basins assumes maximizing the utility from 1.5 million cubic meters. This can achieve by

construction of small irrigation communities (2 Feddan/family x 50 family x 3000 m3/Fed x 5 years


________________________________________________________________________________________________

= 1.5 x 106 m

3). During this period of 5 years the storage from the runoff of the second flood (12

mm of maximum daily rainfall) will recur and so on.

The geoelctric study along the main channels of the studied wadis revealed that the study

area is affected by step faults parallel to the Red Sea ؤoast. The upstream portion of these wadis

(Fig. 9) composed of three geoelectrical layers that correlated to Wadi fill, fractured basement and

fresh basement. While the middle part is composed of wadi fill and sedimentary layers (Wadi fill

sediments, variegated shale of Quseir Formation and Nubia sandstone) that cover the fractured

basement. At the downstream portion the fractured basement was not reached due to the effect of

the normal faults with downthrown side towards the Red Sea coast and due to sea water intrusion.

The salinity of the groundwater increases with depth. The presence of Quseir shale, that covers

Nubia sandstone, restricts the groundwater recharge, so it records high resistivity (over 350

Ohm.m).

Gawasis Basin

Gasus Basin

RED

SEA

33 50 34 00

2635

2630

Legend

Storage dam

Detention dam

Water divide0 2 4

km

Fig. 8: The proposed sites for the storage and detention dams in Wadi Gasus and Gawasis

The Wadi fill sediments are composed of gravels, sands and clays. The thickness of this

aquifer ranges from 20 to 60 m in Wadi Gasus and from 20 to 30 m in Wadi Gawasis. The average

resistivity of this layer ranges from 920 Ohm.m. to 83 Ohm.m. except at the down stream which

records 4 Ohm.m. due to sea water intrusion. The salinity ranges between 1649 and 13524 mg/l. The

high resistivity values may be attributed to the presence of fresh water with good potentiality.

The Quseir shales with some sand intercalations records a resistivity ranges from 15 Ohm.m.

to 5 Ohm.m. where their values decrease from the upstream portion to the downstream portion as a

result of decrease in sand ratio and sea water intrusion. Its thickness ranges from 25 m in the middle

part of Wadi Gasus to 57 m in the downstream portion of the Wadi. The absence of Quseir shales in

the upper part of the main channel of Wadi Gasus refers to good groundwater potentiality of this

area. Wdi Gawasis shows the same conditions with less thickness.

Nubia sandstone rests directly over the fractured basement and is composed of sandstone

with some shale intercalation. The resistivity of this layer ranges from 350 Ohm.m. to 700 Ohm.m. The difference in the resistivity may be attributed to shale intercalations. The Turonian-Santonian

Nubia sandstone confined aquifer with different groundwater salinities considers moderate

potentiality aquifer (Aggour 1997). The thickness of this aquifer ranges from 45 m in the middle

part of Wadi Gasus to more than 70 m in the downstream part of the Wadi while it ranges from 40 to

55 m in Wadi Gawasis. The recharge of this aquifer from the fractured basement through fault zones

is more pronounced and reflects good potentiality for this aquifer. The groundwater flow direction is

in SW direction.

The Lower Maestrichtian carbonate confined aquifer is defined in the mines of phosphate

ores in the study basins. Its water salinity varies greatly between 1810 and 13935 mg/l (Aggour,

1997). The Precambrian Basement complex aquifer represent the main aquifer in the study area. Its

groundwater discharges naturally through springs (Abdel Baki , 1996). The groundwater salinity of


________________________________________________________________________________________________

0 2 km

H scale

wadi fill

Quseir shale

Nubia sandstone

Fractured basement

basement

1

2

3

4

5

6

7

NW

SE

0

50

100

150

180

m

1

2

3

4

NW

SE

0 2 km

H scale

wadi fill

Quseir shale

Nubia sandstone

Fractured basement

basement

20

50

100

140

m

granitic rocks are less than 1000 mg/l while the salinity of metavolcanics groundwater vary between

334 and 14424 mg/l (Aggour, 1997).

Fig. 9: Geoelectric section of wadi Gasus (upper graph) and Wadi Gawasis (lower graph)

CONCLUSION AND RECOMMENDATIONS

Based on the above flood analysis and discussion, the following conclusions and recommendations

can be stated.

1. The storage of actual rainfall and flood measurements is a major constraint on hydrograph

generation and its calibration and reliability. Unfortunately, the available records are very limited.

The uncertainty related to the data has to be borne in mind when considering the reliability of the

simulation results.

2. To support future studies, an adequate rainfall-runoff monitoring system covering the watershed

of the two basins should be installed. This monitoring system should be able to record short-

duration rainfall-runoff events.

3. Flood-recording stations should be installed and rehabilitated at the three proposed points in the

watershed of both Gasus and Gawasis basins near the proposed sites of dam storage.

4. Afforestation, combined with terracing and check dams in the watershed upstream for two basins

will reduce the flood peakflow. This alternative is the recommended one instead the proposed dam

storage.

5. Afforestation, combined with dam storage of 1.5 million m3 produced about 60% of the annual

water requirements of new small irrigation communities (2 Feddan/family x 50 family).

6. Terracing with check dams measure would be more suitable and less expensive than dam storage,

and more desirable from the socio-economic, environmental, and risk perspectives. However, a

further detailed evaluation of these two options must consider all relevant parameters, including

social, economical, environmental, risk, etc.

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