Hydrological Sciences-Joumal-des
Sciences Hydrologiques, 47(2) April 2002
{73
Hydrological considerations for dam siting in arid
regions: a Saudi Arabian study
ZEKAI SEN
Hydraulics Division, Civil Engineering Faculty. Istanbul Technical University, Mas la k X0626,
Istanbul. Turkey
zsenfa'itu.edu.tr
KHALID AL-SUBA'I
Geology Department, Faculty of Sciences, Univcrsilv of Sana 'a. PO Box 13226. Yemen
Abstract In any dam siting study in arid regions, rainfall records, runoff measurements and their greatest magnitudes are very important. Unfortunately, the data are
scarce and, therefore, empirical approaches and charts obtained from similar regions
in other parts of the world are necessary for complete applications. The lack of
observed data presents the major problem tor runoff modelling in arid regions. These
regions have characteristically high rainfall intensity and consequent flash floods with
large amounts of sediments. Occurrence of rainfall is sporadic, both temporally and
spatially, which makes the interpretation of the rainfall-runoff relationship quite
difficult. Flood estimations play a significant role in dam siting from the point of view
of water availability. This paper presents the basic calculations of floods and sediment
amounts that are necessary in dam siting and construction in an arid area by
considering the southwestern part of the Kingdom of Saudi Arabia.
Key words dam siting; flood estimation; hydrological observations; data scarcity; arid regions;
Saudi Arabia
Considérations hydrologiques pour l'implantation de barrage en
régions arides: une étude en Arabie Saoudite
Résumé Dans toute étude d'implantation de barrage en régions arides, les
enregistrements de pluie et les mesures de débit sont très importantes, en particulier
pour les grandes valeurs. Malheureusement les données sont rares, et des approches
empiriques et des références obtenues dans des régions similaires du monde sont
nécessaires. Le manque de données observées est un problème majeur pour modéliser
l'écoulement en régions arides. Ces régions ont pour caractéristique de subir de fortes
intensités pluviométriques et d'importantes crues éclair particulièrement chargées en
sédiments. Les pluies sont sporadiques, à la fois dans le temps et dans l'espace, ce qui
rend difficile l'analyse de la relation pluie-débit. Les estimations de crue jouent un
rôle significatif dans l'implantation de barrage du point de vue de la disponibilité de
l'eau. Cet article présente les calculs de crues et de quantités de sédiments, qui sont
nécessaires pour l'implantation et la construction de barrage en zone aride, dans le
Sud-Ouest du royaume d'Arabie Saoudite.
Mots clefs implantation de barrage; estimation de crue; observations hydrologiques; rareté des
données; régions arides; Arabie Saoudite
INTRODUCTION
As stated by Pilgrim et al. (1988) and UNESCO (1979) classification, nearly half the
countries of the world face problems of aridity. There is therefore an obvious need for
improved understanding of the hydrology of arid and semiarid regions, and for the
development of appropriate techniques for modelling runoff. Even in those humid
regions where a large number of studies have been carried out, hydrological modelling
Open for discussion until I October 2002
Zekai §en & Khalid Al-Suba 'i
174
is at best of only moderate accuracy, and involves many assumptions, simplifications
and averaging over space and time. While some aspects of arid zone hydrology are
more amenable to simplified modelling, it is highly probable that greater errors and
uncertainty will continue to characterize results for arid zones. Recognition of these
problems is fundamental to a realistic approach to arid zone modelling, and to a
rational interpretation and application of the results obtained.
Study area
The Kingdom of Saudi Arabia extends over an area of some 2 250 000 km- in the
Northern Hemisphere between latitude 16°25'N and 32°15'N as shown in Fig. 1. It lies
within the arid region of the world and, therefore, the majority of its land is desert. The
total cultivated area is around 0.2-0.3% of the total land area and about one-third of it
is presently irrigated. Due to the scarcity of rainfall (except in the southwestern
region), the current irrigation is mostly supported by groundwater. A continuation of
the present trends of deep groundwater exploitation is likely to result in the depletion
of this resource in the near future. Conversely, the Tihamat Asir region of southwestern Saudi Arabia has relatively higher rainfall (the highest rainfall in the country)
resulting in 60% of the total surface water of the Kingdom, and it seems to offer a
better potential for sustained farming (MAW, 1986). Rainfall in southwestern Saudi
34*
3S*
3B*
40*
42*
4 4*
A"! Studied orm
Fig. 1 Location of the study area.
4 6*
49*
50'
52*
54"
56°
S 8*
SO*
Hydrological considerations for dam siting in arid regions
175
Arabia is likely in almost any month of the year. In general, rainfall maxima in the
region are in winter and spring. The winter rain is the greatest in terms of the number
of rainy days, but the largest amounts of rain fall in spring. The summer rainfall is very
noticeable in the region, since it rains almost every afternoon (more often during the
second half of July and during August) due to local convection.
Each seasonal rain is associated with different weather features. The winter rain is
caused by the disturbances from the Mediterranean Sea, the Sudan trough, and the
westerly waves in the upper atmosphere. The summer rain is caused by the northward
advance of the southwesterly monsoon and the Inter-tropical Convergence Zone. The
low-level Red Sea convergence is responsible for the abnormal amounts of rain during
spring, especially when it is supported by an upper trough at 700 mb. It also enhances
the Sudan trough and extends across the Red Sea (Al-Qurashi, 1981). The Asir
Mountains play a very important role in the rainfall occurrences during all seasons,
especially in summer. Higher elevations do not necessarily receive more rain: some
lower locations are more rainy due to the fact that they are on the windward side of the
mountains. The source of moisture is the Indian Ocean, the Arabian Sea, and the Red
Sea. The Mediterranean Sea also contributes some moisture during winter and spring.
The region is characterized by a semiarid climate with high intensity rainfall and
flash floods leading to large amounts of eroded sediments. Some of the wadis in the
central part of the region have runoff amounts in excess of 100 Mm J year"1. These
floods usually wash away Aqums (temporary small earth embankments), sometimes
uproot plants , carry away the valuable alluvial soil, and, in many cases, lead to loss of
livestock and destruction of houses, roads and other utilities (Maclaren International
Ltd, 1979; El-Khatib, 1980). It is, therefore, essential that flood control measures are
made available, not only for agricultural improvements, but also for further
infrastructure developments.
Although the evaluations by El Khatib (1980), Noory (1983), and Muller (1984)
for the estimation of surface water resources and the adopted criteria for arability and
irrigability are not identical, all of them suggest that the optimum development
necessitates the implementation of major projects, such as the construction of flood
retention dams, spatebreaks, or a series of diversion weirs in the wadis in order to
regulate the surface flows.
Tihamat Asir region consists of the southern part of the Red Sea coastal plain,
known as Tihamat and the western slope of the adjacent Asir highlands. This region
constitutes about 2% of the land area of Saudi Arabia and has 40% of the total runoff
of the country. The longest wadi of western Saudi Arabia Wadi Baysh, is located in the
central part of this region, which is intersected by several other westward flowing steep
and narrow wadis originating in the Asir mountains and discharging their waters into
the Red Sea (see Fig. 2).
After Al-Suba'i (1991), the Tihamat Asir region receives enough rain to support
vegetation and grow crops, but sometimes the rain causes destructive floods leading to
high sediment yield. The wadis in the central part of Tihamat Asir region (e.g. Baysh,
Gara, Shadan, Sabya and their tributaries, as well as Wadi Damad) are the most
cultivable and populated areas of the Kingdom. Therefore, the consultants gave these
wadis the top priority for development (El-Khatib, 1980; Noory, 1983). In the same
area, the management of surface dam waters and groundwater recharge have been
studied by Al-Muttair et al. (1989, 1994).
176
Zekai §en & Khalid Al-Suba 'i
Fig. 2 Tihamat Asir wadis.
The main puipose of this study is to consider the importance of surface water
utilization for agricultural development and to investigate protection in the region
against flood hazards. It is aimed at identifying and evaluating the factors that might
affect dam siting in the catchments and, hence, the water resources planning of the
proposed reservoirs. For hydrological investigations, this paper considers only the
catchments in the central part of Tihamat Asir region, extending over the Red Sea
littoral zone between latitudes 16°52'N and 18°05'N and stretching eastward from the
coast at longitude 42°35'E to the summit of the Red Sea escarpment at longitude
43°30'E as shown in Fig. 1. It thus embraces mainly the western slopes of the central
Hydrological considerations for dam siting in arid regions
177
part of the Asir highlands with an area of about 15 000 km~ extending to the Sa'dah
province of Yemen.
CLIMATE VARIABILITY
Some detailed regional studies on climate variability include work by Italconsult
(1973), Al-Qurashi (1981) and Al-Jerash (1985). Hydrological evaluations were an
essential part in most of the development project studies. In addition, regional studies,
including the entire Tihamat Asir basin, were undertaken by Al-Jerash (1983, 1985,
1988), Sen (1983) and Nouh (1987a, 1988). Similar studies have been presented for
some catchments in Iraq by Abdulla & Al-Badranih (2000).
Nouh (1987a) stated that the cyclonic climate system over southwestern Saudi
Arabia during the winter results in lower rainfall than that produced by the monsoon
system during the summer. The climatological investigations in the study area are
based on data from five meteorological stations in addition to 27 cumulative rainfall
measuring and 17 recording raingauges located in and around the study area. Table 1
summarizes the details of these stations. The observation records do not all date back
to the same points in time, neither were complete records available because of gaps in
the recorded time series. In hydrological modelling, the quality of the data very often
has a greater effect on the accuracy of the results than the quality of the model used.
The paucity of available data constitutes probably the greatest problem in arid and
semiarid zone modelling. The annual variations in the rainfall amounts result from the
effects of the cyclonic and monsoon climate systems, which contribute a maximum
annual rainfall of 650 mm throughout the region; the geographical variation results
from the strong orographic effect which includes a maximum of about 400 mm along
the escarpment ridge of the Asir Mountains. The orographic effect drops steeply in the
western direction and mildly in the eastern direction (Nouh, 1987b).
Some characteristic climatological values of the coastal plain, foothills and mountain areas (the main physiographic units in the region) are shown in Table 2. Generally,
it can be said that the climate of the study area is predominantly semiarid and is
characterized by moderate to high variations in diurnal and seasonal air temperatures,
high incident radiation, high wind speed (often resulting in heavy dust storms), and
sporadic rainfall. The vegetation is very sparse in the area. Thus, it has only very small
effects on the climate.
From the variations in air temperature given in Table 2, it can be seen that
temperatures in the foothills area are not distinctly different from those in the coastal
plain, and the range of data is small in both areas. In the mountain ranges, however, the
mean values are about 14°C. Mean monthly maximum, minimum and mean relative
humidity in the different areas are also presented in Table 2. The mean monthly
relative humidity (Table 2) lies within a very narrow range of about 52-67%. In
general, the range is small (44-52%) during summer and rather large (28-42%) during
winter. Table 2 also shows the mean annual maximum and mean annual Class-A
evaporation. The maximum evaporation is during the summer period (April-October);
it is about 60-70% of the annual evaporation.
The wind speed in the study region is generally high. The mean annual maximum
and mean annual wind speed are also given in Table 2. The range of the mean values is
Zekai Çen & Khalid Al-Suba ï
178
Table 1 Inventory of the hydrometeorological stations in and around the central Tihamat Asir catchments.
Malaki
Sabya
Abu-Arish
Darb
Aradah
Baysh
Damad
Jabal Fayda
SA001
SA002
SA101
SA 102
SA104
SA 106
SA107
SA110
Locationi :
Lat.
17°03'
17°10'
16°58'
17°42'
17°03'
17°22'
17°07'
17°16'
Jabal Sala
Jabal Fayda
Suq Al Ahad
ltwad No 3
Fatiyah
Gooz
Ilarub
Qadab
Wadi damad
Jadiyah
Kubah
Qui!
Suq Ayban
M'Khashel
Rayth
Jizan
llwad No 2
Itvvad No 1
Main ltwad
Wadi Baysh
Aradah
Jabal Salah
Ilarub
Kubah
Qui!
SA111
SA! 12
SA118
SA123
SA 124
SA 125
SA 126
SA127
SA 129
SA113
SA135
SA 136
SA 140
SA 143
SA 145
SA 148
SA201
SA202
SA203
SA204
SA205
SA206
SA207
SA208
SA209
17°03'
17° 12'
16°43'
18°14'
17°34'
17°08'
17°27'
17°16'
17°10'
16°48'
16°48'
16°41'
17° 19'
16°54'
17°37'
16°55'
18°02'
17°57'
17°46'
17°34'
17°02'
17°03'
17°27'
16°48'
16°41'
Suq Ayban
M'Khashel
Abha
Serat Abida
Amir
I-laraja
Temniyah
Al Mala
Garha
Zahrah Al-Jamb
Thailand
SA210
SA211
A00I
A004
A103
A104
AI2I
A213
A303
N103
N203
17°19'
16°54'
18°12'
18° 10'
18°06'
I7°56'
18°02'
18° 10'
18°06'
17°40'
17°40'
Location name
Number
Long.
42 57'
42 37'
42 50'
42 14'
43 05'
42 32'
42 47'
43°08'
Elevation Year of
Equip- Complete years
(m a.s.l.) installation ment
190
40
69
65
223
70
70
860
1967
1965
1953
1966
1953
1966
1966
1957
C
C
N
N
N
N
N
N
43°07'
43°03'
42°58'
42°31'
42°37'
42°27'
42=53'
42°45'
42°54'
42-59'
43° 14'
43°08'
43°()2'
43°08'
42°84'
42°32'
42°29'
42°28'
42°20'
42°36'
43°05'
43°07'
42°53'
43°14'
43°08'
900
800
40
900
540
540
110
150
80
240
120
305
340
600
4
820
460
150
200
223
900
540
240
120
1957
1953
1965
1970
1967
1967
1967
1967
1967
1967
1967
1967
1970
1971
1972
1984
1986
1967
1967
1969
1971
1971
1971
1971
1971
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
R
R
R
R
R
R
R
R
R
43°02'
43°08'
42°29'
43°06'
42°47'
43°22'
42°45'
42°50'
42°53'
43°38'
43°37'
1000
340
2200
2400
2100
2350
2300
2030
2060
2020
2000
1971
1971
1982
1982
1965
1965
1965
1967
1966
1964
1966
R
R
C
C
N
N
N
R
N
N
R
1968-1986
1966-1986
1954-1962, 1964- 1969
1980-1986
1966-1968, 1970- 1984
1966-1968, 1970- 1986
1967-1986
1957-1958, 1960- 1972,
1974-1986
1958-1966, 1970- 1986
1954, 1960
1966-1970
1971-1972, 1975
1969-1971
1968-1986
1968,1970
1968, 1970- 1971, 1973-1975
1971-1986
1968,1971- 1986
1968-1973, 1980, 1982-1986
1968, 1970- 1986
1971-1986
1972-1976, 1978, 1980-1986
1974-1975, 1981--1986
1984-1986
1970
1970
1971-1972, 1979--1980
1970-1986
1978-1984
1978-1980, 1982--1986
1972-1973, 1978--1979,
1981-1986
1975, 1979--1982, 1984
1979-1984, 1986
1982-1986
1982-1984
1965-1984
1965-1984
1965-1984
1967-1984
1966-1984
1964-1984
1966-1984
C: Complete met. station; N: Nonrecording raingauge; R: Recording raingauge.
relatively small except for two values, which represent the extremes of the coastal
plain (Jizan) and the mountain range (Serat Abida). Summer months are characterized
by increased wind velocity in the afternoon when dust storms may appear.
Hydrological considerations for dam siting in arid regions
179
Table 2 Climatological values of the main physiographic units in the study area.
Physiographic
unit
Station
name
Rainfall
(mm)
Max. Mill. Mean
Temperature
(°C)
Max. Min. Mean
Coastal plain
Jizan
Sabva
171
179
35
37
26
25
30
31
Max. Min.
40
86
87
38
Mean
67
63
Foothills area
Malaki
Mountain
range
Abha
Serai
Abida
Relative humidity
(%)
livaporation
(mm)
Max.
Mean
Wind speed
(km h
Max. Mean
4290
3812
45
11
27
40
92
103
11
7
442
97
286
37
25
31
83
37
60
3932
3556
10
7
558
282
216
63
444
189
22
24
13
11
17
18
84
74
31
29
58
52
3060
3275
2443
3004
9
39
6
13
RAINFALL PATTERN
In general, greater spatial variations of rainfall occur in the arid zone than in more
humid regions, although the magnitude of these variations differs in different regions.
Also, the raingauge density is much lower. This leads to the question of whether the
normal approach of use of a model with lumped inputs is adequate or not, and the
likelihood of large sampling errors resulting from sparsely gauged data being unrepresentative of average rainfalls over a drainage basin. A factor that partially counteracts
these problems is that many arid drainage basins for which modelling is required are
small, but in basins less than 10 km" it is not uncommon for a flood event to occur at
the outlet and be due to an intensive partial-area storm that is not adequately represented by the gauged readings (Pilgrim et al., 1988).
According to Al-Yamani & Sen (1993), there are four distinctive seasons of
rainfall occurrences within the Kingdom of Saudi Arabia, due to the movement of
various air masses. However, all four do not prevail in one location. For instance, the
central area, known as Rub-Al Khali (Empty Quarter), does not receive rainfall except
during a few winter days.
The general rainfall pattern may be observed from a hydrological map of the area:
the seasonal isohyets, based on daily data, are roughly parallel to the coastline with
annual rainfall of less than 100 mm in the coastal areas, increasing eastward to a
maximum of 600 mm above the first range of the Asir Mountains. Behind this first
range, the rainfall pattern shows decreasing values. Orographic rainfall is more regular
in the highlands than in the coastal plain and foothills, where convective rainfall
occurrences are more common. The variation of annual rainfall in the southeast-centre
of the study region is significant. Over a distance of only about 15 km, the annual
rainfall varies from 300 to 600 mm. Some characteristic rainfall values from the main
area are presented in Table 2. In the coastal areas, the rainy period is confined to the
summer and winter, whilst to the east, the rainy period is proportional with elevation;
in the highlands the rain extends almost throughout the year.
The depth-area curves for the catchments of the wadis in central Tihamat Asir
region are shown in Fig. 3. These curves can be used in estimating the runoff volume
at each proposed dam site (see below).
One of the important factors within the context of the erosion phenomenon is the
hourly intensity of precipitation. It has been found that the best representative
expressions for rainfall erosivity are those which include the value of the maximum
rainfall intensity of 30-min duration (/30). Therefore, the distribution of this value has
been statistically evaluated for the study area and represented in Table 3. The analysis
Zekai $en & Khalid Al-Suba ï
180
Fig. 3 Depth-area curves for mean annual precipitation of central Tihamat Asir
region.
Table 3 Distribution of 30-min rainfall intensity in central Tihamat Asir catchments and surrounding
areas.
Physiographic Station
region
Foothills and SA001
coastal plain SA002
region
SA203
SA204
SA205
SA207
SA208
SA209
SA210
SA211
All
Mountainous
region
Both regions
A001
A207
A213
SA201
SA202
SA206
N203
All
Percentiles:
95% 90% 85%
25.2
19.4
24.4
28.0
21.6
25.0
23.2
22.2
21.6
22.6
23.5
13.0
10.0
11.0
19.0
19.6
11.5
12.2
11.8
20.6
19.6
16.4
20.4
21.3
16.2
21.6
19.8
20.0
17.3
17.0
18.6
9.4
6.4
7.5
13.0
9.4
11.4
6.6
8.0
15.0
80%
75%
70%
65%
60%
55%
50%
40%
30%
15.4
13.0
15.8
17.6
13.9
17.2
17.1
16.4
13.0
13.3
15.4
12.4
10.3
14.8
14.0
11.9
14.4
15.2
14.2
11.1
11.4
12.7
10.0
9.0
12.4
12.0
9.9
12.1
13.4
13.1
9.9
10.6
11.0
8.3
8.5
10.0
10.8
8.4
10.6
11.4
11.7
9.0
9.4
9.6
6.1
7.2
8.3
9.6
6.3
8.4
9.0
9.8
6.6
7.4
7.6
5.2
6.8
7.8
8.5
5.5
7.7
8.4
9.3
5.7
6.2
6.7
4.6
6.0
6.8
7.6
4.9
7.0
7.2
8.7
5.1
5.7
6.0
3.3
4.9
5.3
6.2
2.9
5.9
5.8
6.8
3.8
4.6
4.6
7.0
4.4
5.6
12.0
7.7
6.6
4.8
5.8
11.4
5.3
3.5
4.4
10.1
6.5
5.0
4.0
4.4
9.5
4.2
2.6
3.8
9.3
4.8
3.6
3.2
3.5
8.0
3.2
2.0
2.6
7.6
3.7
3.4
2.8
2.6
6.8
7.1
8.0
8.8
10.2
7.4
9.3
10.0
10.9
7.4
8.2
8.6
2.4
1.6
2.2
6.4
3.1
2.4
2.4
2.0
5.6
1.6
1.2
1.6
4.9
2.9
2.1
2.2
1.6
4.8
1.2
1.0
1.4
4.2
2.3
1.0
1.8
1.2
4.0
0.6
0.8
1.0
3.7
1.8
0.9
1.4
0.8
3.2
0.3
0.4
0.6
3.2
1.0
0.5
1.2
0.4
2.4
3.5
4.4
4.8
2.4
4.2
4.6
5.4
2.6
3.6
3.2
0.2
0.2
0.2
2.7
0.6
0.4
0.8
0.2
2.0
1.0
Hydrological considerations for darn siting in arid regions
181
of the available rainfall intensity data from 17 stations in and around the study area
(see Table 3) shows the existence of two climatic zones as follows:
- the western part characterized by strong pluviométrie intensities and represented
by stations in the coastal plain and foothill areas; and
- the eastern part characterized by a moderate pluviométrie intensity and represented
by stations located in the mountain range.
To represent the rainfall erosivity index, the maximum 30-min rainfall intensity at
the 75 percentile in this table is multiplied by its depth. This yields 60.5 mm mm h'1
(11 x 5.5) for coastal and foothill areas and 6.1 mm mm If1 for mountainous areas. The
latter value will be used only for the Wadi Baysh catchment, because nearly 80% of
this catchment is located in the mountainous area.
RUNOFF FEATURES
There are no perennial surface flows in the study area and the annual runoff volume is
usually concentrated in the form of flash floods of short duration and sizable
magnitude which occur during the rainy seasons. Rainfall is immediately converted
into runoff causing flash floods for the following reasons:
- In the upper parts of the catchments, there is hardly any soil to store water by
moistening. The slopes of the mountains normally are steep and the rocks are
generally impervious. Therefore, infiltration loss and retention by filling the
depressions are negligible.
In the foothill parts of the catchments, the high intensity rain seals the surface of
the bare soil very quickly, and consequently, only a shallow depth of soil moisture
may be achieved before ponding occurs and surface runoff is initiated.
The wadis of the central Tihamat Asir region originate in the Asir mountains along
the Red Sea escarpment. The catchment areas of the larger wadis namely, Baysh,
Damad and Jizan, are situated in the mountainous region. These wadis often have a
base flow in the upper range throughout the year. The smaller wadis (e.g. Akas,
Tafshah, Qura and Wu'al) originate on the western escarpments and foothills of the
Asir Mountains. These wadis remain dry most of the year. The water reaches the plains
and then the Red Sea only during floods.
Out of the twelve wadis in the study area, only Baysh, Damad and Jizan have
runoff stations. The station characteristics are shown in Table 4, which also includes
Wadi Khulab station located near the southern edge of the study area.
Table 4 Characteristics of runoff evaluation stations.
Sequence
Name
Station
no.
Location:
Wadi
Bavsh
Wadi
Damas
Wadi
Jizan
Wadi
Khulab
SA415
17°34'N
42°37'f
200
SA417
17°09'N
42°53'E
SA418
17°03'N
SA421
16°43'N
Altitude
Period of
(m a.m.s.l.) record
Years of
complete
record
Basin
areao
(knr)
Equipment
1959--1986
17
4652
130
1958--1986
17
1108
Runoff stage
recorder
Runoff stage
recorder
42°57'H
178
1953- -1986
16
1430
43°01'E
99
1969--1986
11
900
Runoff stage
recorder
Runoff stage
recorder
Zekai §en & Khalid Al-Snba 'i
182
FLOOD EVALUATION
Problems in the analysis of flood records from arid areas include those of gauging and
measurements, low or zero annual maxima in a number of years, and the suitability or
otherwise, of the mean annual flood as the appropriate scaling factor for the dimensionless curves (Farquharson et al., 1992). Mean runoff values form the basis for any
irrigation and flood protection measures. In addition, annual flood and flood frequency
data are needed for dam siting as well as for dimensioning of the hydraulic structures
for flood control and irrigation purposes. In the following, these quantities will be
evaluated for the wadis on which the dams are proposed.
It is economically costly and physically difficult to gauge all streams in a region.
Consequently, some methods have been followed to estimate runoff coefficients and
volume for the catchments of ungauged streams. The runoff coefficient, CR, is the
fraction of precipitation which immediately appears as surface runoff. It is given by the
ratio of the annual discharge volume of the stream to the annual volume of precipitation of its catchment.
Calculation of the mean values of CR for the catchments of the four gauged
streams in and around the study area is shown in Table 5. The CR value is found to
vary from 0.048 to 0.078. Figure 4 shows a plot of logCn against log^ (catchment
area). This suggests a straight line relationship of the form:
CR=A-^
(1)
Table 5 Annual surface runoff and catchment runoff coefficient of gauged wadis in Tihamat Asir
region.
Year
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
Total (10 6 nr)
Average (106m3)
Catchment area
(km2)
Annual precip.
(10" m 3 )
Runoff coeff.
Wadi Baysh
22.15
85.50
134.09
45.83
87.18
76.89
45.38
36.64
35.09
40.09
37.99
95.76
121.16
150.91
66.08
110.14
102.24
1293
76
4652
Wadi Damad
3A85
22.35
24.30
11.03
32.80
82.06
59.06
52.37
55.96
24.11
17.95
51.60
66.85
42.53
7.90
62.64
38.42
687
40
1108
Wadi Jizan
39.93
18.70
41.46
83.92
63.92
57.60
67.64
98.44
58.38
43.26
94.69
78.90
38.39
26.50
60.11
40.03
911
57
1430
Wadi Khulab
1586
570
755
405
0.048
0.071
0.076
0.078
55.67
17.91
32.90
38.33
35.14
43.90
46.66
28.07
13.16
23.58
12.42
347
32
900
Hydrological considerations for dam siting in arid regions
183
o Baysh
» Jizon
•
Damod
o Khulob
Fig. 4 Runoff coefficients as a function of area for the gauged wadis in Tihamat Asir
region.
and this is assumed to be valid for the range of catchment areas considered in this
study. Hence, equation ( 1 ) has been used to estimate CR for the catchments of the
ungauged wadis from their catchment areas as shown in Table 6. On the other hand, it
is possible, for given catchment areas, to estimate annual precipitation amounts from
the depth-area lines in Fig. 3, and these are shown in the third column of Table 6.
Additionally, annual precipitation volumes, VA, for the entire catchment of each of
these wadis were calculated by the multiplication of the second and third columns and
the results are entered in the fourth column of the same table. The final annual runoff
volume, VR, is given in the last column of Table 6 as the product of annual precipitation volume and the corresponding calculated mean runoff coefficients. In a similar
manner, and by using only the catchment upstream areas of the proposed dam sites,
mean annual runoff volume at dam sites can be estimated.
Table 6 Runoff coefficients and mean annual runoff volumes of wadis in the Tihamat Asir region.
Wadi
name
Baysh
Ikas
Qura
Shahdan
Was'a
Sabya
Damad
Jizan
Catchment
area, A
(km")
4652
68
309
213
324
675
1108
1430
Mean annual
precipitation, P
Precipitation
volume, VA
Mean runoff coefficient,
CR
Runoff volume, VR
(10° m J )
(mm)
(lO-'rrO
1591
25
131
109
145
306
571
755
Measured
Calculated
Measured
Calc.
0.05
0.07
0.07
0.05
0.22
0.13
0.15
0.13
0.10
0.08
0.07
76
40
57
76
6
17
16
18
29
46
58
342
370
423
510
446
453
515
528
Note: Calculations are done according to the following equations (the volume results are taken as round
million cubic metres): VA = AP; VR = VACR.
Zekai §en & Khalid Al-Suba 'i
184
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CATCHMENT ABEAS I K m ' l
Fig. 5 Calculated and measured peak discharges of wadis in Tihamat Asir region and
Yemen.
The annual volume of the wadi flows in the study region varies widely. Figure 5
shows the calculated and measured instantaneous peak discharge values available for
wadis in southwest Saudi Arabia and Yemen. The mean annual flood volume, VT, and
discharge, QT, with recurrence intervals of 50, 100 and 200 years, are considered
significant for sediment yield evaluations, reservoir capacity determinations, and flood
control and irrigation purposes.
In order to determine the values of Vj and Qr for the wadis under consideration,
the region curve of Nouh (1988—Fig. 6) for prediction of flood frequency in southwestern Saudi Arabia has been used together with the mean annual discharge Q
calculated from the values of VA from Table 6. The results are displayed in Table 7.
Table 7 Expected mean annual flood discharge, Qr, and mean annual flood volume, VT (101 nr) for
different return periods for wadis in the central Tihamat Asir region.
Wadi
Baysh
Ikas
Qura
Shahdan
Was'a
Sabya
Damad
Jizan
Mean
annual
discharge,
Q
880
0.64
192
182
209
340
520
672
Return period (years):
25
50
Qr
3187
232
69
660
756
1230
1910
2434
VT
275
20
60
57
65
106
165
210
Qr
3724
271
81
771
884
1437
2232
2844
200
100
Vr
322
23
70
67
76
124
193
246
Qr
4948
360
1080
1024
1157
1909
1966
3779
Vr
428
31
93
89
102
165
256
327
Qr
5414
394
1182
1121
1286
2089
3247
4136
VT
468
34
102
97
111
181
280
357
Hydrological considerations for dam siting in arid regions
I
-
I
1
1
0
I
I
1
L
1
I
185
1
1
2
3
4
5
6
7
8
Reduced Variute. Y (T I
Fig. 6 Regional curve for precipitation of flood frequency in Tihamat Asir region
(after Nouh, 1988).
For the study region, the available records of the Ministry of Agriculture and Water
(MAW, 1986) showed that the magnitude of some individual daily flows exceeds the
total annual flow for some years. Sediment yield data available from Wadi Jizan
Reservoir indicate that extreme flood events are associated with high energies, that is
neglecting them leads to a great difference between the estimated and measured
sediment yield, before and after dam construction. The life-time of the reservoir was
estimated by the designer to be 91 years, yet, after only 12 years (1970-1982) of
operation, it was predicted to be 27 years. It is also observed that the upstream crosssections of wadis Barada, Khashabat and Mishref, which flow into this reservoir, showed
little or no change in 12 years and that 13, 25 and 51%, respectively, of the trapped
sediment in these wadis was deposited in only about 2 years (Al-Khafif, 1986).
However, it is interesting to note that the mean annual discharge values with recurrence
interval of 100 years, Q\oa, for the investigated wadis are very similar to the values of the
instantaneous peak discharge for the same wadis as observed from Fig. 5. Thus, the
values of £?ioo were used in sediment yield evaluation at the proposed dam sites.
CONCLUSIONS
The study area with the highest precipitation in Saudi Arabia shows characteristics
typical of a semiarid region in its mountainous parts, with mean annual precipitation
reaching up to 600 mm, and of an arid region in its hilly and coastal plain parts, with
186
Zekai §en & Khalid Al-Suha 'i
mean annual rainfall decreasing to 100 mm as the coast is approached. The rain might
fall in almost any month of the year and usually has high intensity which causes
destructive floods leading to abundant sediment yield. The wadis Baysh, Damad and
Sabya (including Harub and Qasi tributaries) have the highest discharge and they are the
most hazardous in terms of flood. Wadi Baysh provides the largest arable area, which
could be developed for any agricultural purposes. The runoff coefficient is related
empirically to the catchment area. The available hydrological, climatic and sediment
data of the region are inadequate. It is therefore, recommended to upgrade the existing
rainfall and runoff stations of the region to employ modern automatic recording stations
and maintain them continuously. Furthermore, it is recommended to install a new
network of rainfall gauges to cover ungauged catchments as well as the upstream parts
of the gauged ones. Streamgauges for ungauged wadis and sediment sampling stations
for representative wadis are also needed.
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Received 5 January 2001; accepted 16 July 200Ï