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

174 Zekai §en & Khalid Al-Suba 'i

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* 4 6* 49* 5 0 ' 52* 54" 56° S 8* SO*

A"! Studied orm

Fig. 1 Location of the study area.

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 MmJ 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

178 Zekai Çen & Khalid Al-Suba ï

Table 1 Inventory of the hydrometeorological stations in and around the central Tihamat Asir catchments.

Location name

Malaki Sabya Abu-Arish Darb Aradah Baysh Damad Jabal Fayda

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!

Suq Ayban M'Khashel Abha Serat Abida Amir I-laraja Temniyah Al Mala Garha Zahrah Al-Jamb Thailand

Number

SA001 SA002 SA101 SA 102 SA104 SA 106 SA107 SA110

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

SA210 SA211 A00I A004 A103 A104 AI2I A213 A303 N103 N203

Location Lat.

17°03' 17°10' 16°58' 17°42' 17°03' 17°22' 17°07' 17°16'

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'

17°19' 16°54' 18°12' 18° 10' 18°06' I7°56' 18°02' 18° 10' 18°06' 17°40' 17°40'

i :

Long.

42 57' 42 37' 42 50' 42 14' 43 05' 42 32' 42 47' 43°08'

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'

43°02' 43°08' 42°29' 43°06' 42°47' 43°22' 42°45' 42°50' 42°53' 43°38' 43°37'

Elevation (m a.s.l.)

190 40 69 65

223 70 70

860

900 800 40

900

540 540 110 150 80

240 120 305 340 600

4 820 460 150 200 223 900 540 240 120

1000 340

2200 2400 2100 2350 2300 2030 2060 2020 2000

Year of installation

1967 1965 1953 1966 1953 1966 1966 1957

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

1971 1971 1982 1982 1965 1965 1965 1967 1966 1964 1966

Equip­ment

C C N N N N N N

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

R R C C N N N R N N R

Complete years

1968-1986 1966-1986 1954-1962, 1980-1986 1966-1968, 1966-1968, 1967-1986 1957-1958, 1974-1986 1958-1966, 1954, 1960 1966-1970 1971-1972, 1969-1971 1968-1986 1968,1970 1968, 1970-1971-1986 1968,1971-1968-1973, 1968, 1970-1971-1986 1972-1976, 1974-1975, 1984-1986 1970 1970 1971-1972, 1970-1986

1978-1984 1978-1980, 1972-1973, 1981-1986 1975, 1979-1979-1984, 1982-1986 1982-1984 1965-1984 1965-1984 1965-1984 1967-1984 1966-1984 1964-1984 1966-1984

1964-

1970-1970-

1960-

1970-

1975

1971,

1986 1980, 1986

1978, 1981-

1979-

1982-1978-

-1982, 1986

1969

1984 1986

1972,

1986

1973-1975

1982-1986

1980-1986 -1986

-1980

-1986 -1979,

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

Coastal plain

Foothills area

Mountain range

Station name

Jizan Sabva

Malaki

Abha Serai Abida

Rainfall (mm) Max. Mill.

171 27 179 40

442 97

558 216 282 63

Mean

92 103

286

444 189

Temperature (°C) Max. Min. Mean

35 37

37

22 24

26 25

25

13 11

30 31

31

17 18

Relative humidity (%) Max. Min. Mean

86 87

83

84 74

40 38

37

31 29

67 63

60

58 52

livaporation (mm) Max. Mean

4290 3812

3932 3556

3060 2443 3275 3004

Wind (km h Max.

45 11

10

9 39

speed

Mean

11 7

7

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

180 Zekai $en & Khalid Al-Suba ï

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 region

Foothills and coastal plain region

Mountainous region

Both regions

Station

SA001 SA002 SA203 SA204 SA205 SA207 SA208 SA209 SA210 SA211

All

A001 A207 A213 SA201 SA202 SA206 N203

All

Percentiles: 95%

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

90%

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

85%

15.4 13.0 15.8 17.6 13.9 17.2 17.1 16.4 13.0 13.3

15.4

7.0 4.4 5.6

12.0 7.7 6.6 4.8

5.8

11.4

80%

12.4 10.3 14.8 14.0 11.9 14.4 15.2 14.2 11.1 11.4

12.7

5.3 3.5 4.4

10.1 6.5 5.0 4.0

4.4

9.5

75%

10.0 9.0

12.4 12.0 9.9

12.1 13.4 13.1 9.9

10.6

11.0

4.2 2.6 3.8

9.3 4.8 3.6 3.2

3.5

8.0

70%

8.3 8.5

10.0 10.8 8.4

10.6 11.4 11.7 9.0 9.4

9.6

3.2 2.0 2.6 7.6 3.7 3.4 2.8

2.6

6.8

65%

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

60%

6.1 7.2 8.3 9.6 6.3 8.4 9.0 9.8 6.6 7.4

7.6

1.6 1.2 1.6 4.9 2.9 2.1 2.2

1.6

4.8

55%

5.2 6.8 7.8 8.5 5.5 7.7 8.4 9.3 5.7 6.2

6.7

1.2 1.0 1.4 4.2 2.3 1.0 1.8

1.2

4.0

50%

4.6 6.0 6.8 7.6 4.9 7.0 7.2 8.7 5.1 5.7

6.0

0.6 0.8 1.0 3.7 1.8 0.9 1.4

0.8

3.2

40%

3.3 4.9 5.3 6.2 2.9 5.9 5.8 6.8 3.8 4.6

4.6

0.3 0.4 0.6 3.2 1.0 0.5 1.2

0.4

2.0

30%

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

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 Location: no.

Altitude Period of (m a.m.s.l.) record

Years of Basin Equipment complete areao record (knr)

Wadi Bavsh Wadi Damas Wadi Jizan Wadi Khulab

SA415

SA417

SA418

SA421

17°34'N

17°09'N

17°03'N

16°43'N

42°37'f

42°53'E

42°57'H

43°01'E

200

130

178

99

1959-

1958-

1953-

1969-

-1986

-1986

-1986

-1986

17

17

16

11

4652

1108

1430

900

Runoff stage recorder

Runoff stage recorder

Runoff stage recorder

Runoff stage recorder

182 Zekai §en & Khalid Al-Snba 'i

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 dimen-sionless 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 Wadi Baysh Wadi Damad Wadi Jizan Wadi Khulab 1970 22.15 3A85 39.93 1971 85.50 22.35 18.70 1972 134.09 24.30 41.46 55.67 1973 45.83 11.03 - 17.91 1974 87.18 32.80 83.92 32.90 1975 76.89 82.06 63.92 38.33 1976 45.38 59.06 57.60 35.14 1977 36.64 52.37 67.64 43.90 1978 35.09 55.96 98.44 46.66 1979 40.09 24.11 58.38 28.07 1980 37.99 17.95 43.26 13.16 1981 95.76 51.60 94.69 1982 121.16 66.85 78.90 1983 150.91 42.53 38.39 23.58 1984 66.08 7.90 26.50 12.42 1985 110.14 62.64 60.11 1986 102.24 38.42 40.03 Total (106nr) 1293 687 911 347 Average (106m3) 76 40 57 32 Catchment area 4652 1108 1430 900 (km2) Annual precip. 1586 570 755 405 (10" m 3) Runoff coeff. 0.048 0.071 0.076 0.078

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

(mm)

342 370 423 510 446 453 515 528

Precipitation volume, VA

(lO-'rrO

1591 25

131 109 145 306 571 755

Mean runoff coefficient, CR

Measured

0.05 -----0.07 0.07

Calculated

0.05 0.22 0.13 0.15 0.13 0.10 0.08 0.07

Runoff volume, VR

(10° mJ)

Measured

76 -----40 57

Calc.

76 6

17 16 18 29 46 58

Note: Calculations are done according to the following equations (the volume results are taken as round million cubic metres): VA = AP; VR = VACR.

184 Zekai §en & Khalid Al-Suba 'i

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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 25

Qr 3187

232 69

660 756

1230 1910 2434

period (years): 50

VT

275 20 60 57 65

106 165 210

Qr 3724

271 81

771 884

1437 2232 2844

Vr

322 23 70 67 76

124 193 246

100

Qr

4948 360

1080 1024 1157 1909 1966 3779

Vr

428 31 93 89

102 165 256 327

200

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 185

I I 1 I I 1 L 1 I 1

- 1 0 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 cross-sections 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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