protection of rabigh city agaist flash floods kingdom of saudi arabia

PROTECTION OF RABIGH CITY AGAIST FLASH FLOODS
KINGDOM OF SAUDI ARABIA
Saleh S. Almiman
Saudi Arabia
1. Introduction
Wadi Rabigh is located in the zone between 22o 40/, 23o 30/ North latitudes and 39o 00/, 40o 00/ East Latitudes
in Makkah region in west part of the Kingdom of Saudi Arabia. The total catchment area of wadi Rabigh is
about 4800 km2. Two important factors affecting the climatic conditions of Rabigh Basin are the topography
and the Red Sea nearness. The values of the meteorological factors describing the climatic features of the
basin have been determined using the nearby meteorological stations. The annual mean of rainfall and
temperature over the Basin are 98 mm and 27oC, respectively. Rainfall observations reveal the fact that the
driest months are in June and July. The annual mean of the evaporation is about 3248 mm and the
temperatures over the basin don't drop below zero. Relative humidity varies during a day and sometimes
reaches to 100%.
LEGEND
Rabigh Dam
Figure (1) Location map of Rabigh basin
For the purpose of protecting Rabigh city, a dam site (Fig. 1) on the main stream of wadi Rabigh, with an
upper catchment area of about 3450 km2, has been proposed. The dam's reservoir capacity was determined
based on the 100-yr flood and 50-yr transported sediment. Due to the reservoir operation, water will be
released downstream the dam within the recharge capacity. The released water will increase the underground
water table which, consequently, will increase the pumped water for the domestic users and for the irrigation.
So, detailed geological, hydrological, and engineering studies for the dam's design have been carried out.
Various alternatives for selecting the most feasible dam's site and type have been then presented and
compared. The catchment area of Rabigh Dam is shown in Figure (2).
Figure(2) Catchment area of Rabigh dam
2. Geology And Geophysics
In wadi Rabigh, basement granite and gneiss constitute both the reservoir area and dam site. This formation
of Precambrian age is watertight and stable. Geological conditions and engineering considerations have led to
the conclusion that the axis is suitable for the design of any types of dams.
Geophysical receptivity survey was carried out along the dam axis and the interpretation of the results
revealed that the maximum thickness of the alluvium is 16m. However, that needed to be correlated through
core drillings.
The required amount of aggregate material was available in the near vicinity of the dam axis in substantial
amounts. The indices used in the description of rock masses were given in accordance with British Standard
Institution BS 5930 Site Investigation.
3. Water Potentiality
3.1 Rainfall
There are no meteorological stations within the catchment of Rabigh, whereas, there are six nearby stations
have been used. Rainfall data shows that the rainy season is from October to May. The wettest months are in
January (≈17 mm) and in April (≈14 mm) while the driest months are in June (≈0.5 mm) and in July (≈0.1
mm). The seasonal rainfall percentage values for the study area are 32% in spring, 1% in summer, 23% in
autumn, and 44% in winter.
Average point rainfalls for the catchment area of Rabigh have been estimated using Thiessen method
utilizing the rainfall values with same durations and return periods for the stations' data located nearby the
catchment. The effective rainfall, which is a part of the total rainfall depths, has been determined using the
runoff/rainfall curves derived by U.S Soil Conservation Services and Curve Number (88). Table-1 shows the
average arial point rainfall and the effective rainfall depth within various return periods over the catchment
area in which the actual rainfall duration is twelve hours. The probable maximum areal and effective rainfall
values over the catchment upstream Rabigh dam's site have been estimated as 145.2 and 130 mm,
respectively.
Table (1) Rainfall characteristics over Rabigh basin
Return periods (years)
2
5
10
25
50
100
Average rainfall (mm)
Effective rainfall (mm)
19.4
33.4
42.7
55.0
63.2
71.9
3.3
11.5
18.2
27.9
34.7
42.5
3.2 Floods
The water potentiality values of Rabigh basin have been evaluated from the monthly stream flow data of J403 at wadi Rabigh runoff station. For the period 1969-1985, the monthly flow data at is available. This
station is located at a catchment area of about 4500 km2 while the proposed dam's location is located at a
catchment area of about 3456 km2. Accordingly, the monthly flow data at the station has been transferred to
the dam's location using the empirical formula Q = CA n
due to the
areal difference of 1044 km2, where Q is the mean monthly flow values (m3), C is the coefficient, A is the
catchment area (km2), and n is exponential. As a result, the annual flow at Rabigh dam could be estimated as
83.3 million m3. Floods that occur within the catchment area upstream Rabigh dam are due to rainfall during
winter and spring seasons. For the purpose of dam's design, the maximum flow volumes occurrence and the
maximum probable flood hydrographs at various return periods have been estimated at the dam's site.
The maximum flow volume data observed at Rabigh runoff station have been utilized to derive the flow
volumes at different return periods up to 100-year at the dam site. The maximum 1-, 3-, 5-, and 7-day flow
volume series of the available station with course data of the year 1969-1984 have been analyzed. Different
extreme distribution functions such as Normal, Log-Normal, Gumbel, and Gama-2 Parameter have been
applied to the data series to obtain the probable maximum flow volumes as shown in Figure (3). Results
showed that the most critical flood duration is the 7-day with various recurrences estimated for wadi Rabigh.
FREQUENCY
DIMENSIONLESS VARIABLE
Figure (3) Frequency analysis of the annual maximum flow volumes
Due to the difference in catchments' areas upstream the flow station and the dam site, flow volumes with
various recurrence intervals have been estimated for the station site by frequency analysis and transferred to
the dam's site using the empirical formula V = CA n where V is the volume (million m3), C is the coefficient, A
is the catchment area (km2), and n is exponential. As a result, Figure (4) shows the flood hydrographs for
various return periods at the dam's site using the superposition of the runoff hydrograph with 2-hr time lag.
The estimated probable maximum peak discharge value is about 9543 m3/sec, which could be used in the
dam engineering studies.
3.3 Sediment Analysis
Suspended sediment measurements carried out in 1972 and 1973 at 14 locations in the Arabian Shield-South
comprises a total of 148 measurements. These data have been used in the estimation of the suspended
sediment yield transport of wadi Rabigh. The sediment rating curve for each station have been obtained by
plotting the available data at suspended sediment and discharge in order to establish a relationship of
discharge (m3/sec) and sediment yield (ton/day). The represented equation of the sediment-rating curve could
be estimated by applying the least square technique to the following polynomial equation:
Log Q S = a + b ( Log Q W ) + c (Log Q W ) 2 + d (Log Q W ) 3 + .....
(1)
where; QS is the sediment yield (ton/day), QW is the water discharge (m3/sec) has been used in the
computation of the rating curve equation following the logarithmic transformation of the discharge and
sediment quantities. The constants in the above equation have been estimated by the least squares technique.
The polynomial equation with the minimum value of standard error has been adopted for that solution.
Results showed that the bed load has been accepted as 20% and the annual average sediment yield
(summation of the suspended and bed load) at the dam site has been calculated as 408 tons/year/km2,
assuming the Bulk density is equal to 1.35 tons/m3. As a result, the annual average sediment yield at the dam
site has been estimated as 300 m3/year/km2. By considering the total area upstream the dam site, the probable
annual average sediment volume is expected to be 1.1 million m3/year which leads to 52 million m3/year for
the 50-year probable maximum floods.
Figure (4) Flood hydrographs with various recurrence intervals
4. Selection of Dam type
Three different alternatives of the dam's type were analyzed. The first type was a concrete gravity type and
second type was a rockfill type with asphaltic concrete core while the third type was a concrete faced rockfill
type dam. The appurtenants of each alternative were defined and the financial comparisons between the
different items were also analyzed, as listed in Table-2. Considering the cost of the three alternatives, it was
found out that the concrete gravity type is the most economic one. This is due to the fact that the concrete
gravity type has the advantage of having the spillway placed over it, whereas for the fill type dams, big
amounts excavations of rocks are required to place the spillway. General layout of the selected dam and
appurtenant structures are given in Figure (4).
Table (2) Financial comparisons between different dams' alternatives, (million SR)
No.
1
2
3
4
5
Items
Dam body and
cofferdam
Diversion channel
Tunnels
Tunnel inlets,
outlets, and intakes
Spillway
Total Cost (106 SR)
Alternative (I)
Dams' Types
Alternative (II)
Alternative (III)
Concrete Gravity
Asphaltic concrete core
Concrete faced rockfill
161.6
89
59.1
10
NI
NI
NI
30.9
40.8
NI
31
31
NI
84.3
84.3
171.6
235.2
215.2
NI: Not Include
On the other hand, the geological conditions prevailing and economic considerations have revealed that the
dam site is suitable for the design of concrete gravity dam. The aggregate material required for the
construction of the dam and appurtenant structures can be provided in abundant quantity from the alluvium
deposits. Particularly at the downstream, in the near vicinity, aggregate material is available in huge quantity.
The geological description of the deposits indicate that these can be used as concrete aggregates after
processing.
(184.50)
(177.00)
(175.50)
(142.10)
(130.00)
(107.00)
(184.50)
(175.50)
(177.07)
(146.00)
(NATURAL GROUND)
ALLUVIAL FILL
ALLUVIAL FILL
ALLUVIIUM
(104.00)
Figure (4) Cross sections of the concrete gravity dam
5. Characteristics of Selected dam
Reservoir
Reservoir level during probable maximum flood
Reservoir level for dead storage
Volume of spillway at crest elevation
Volume of reservoir for flood control
Volume of reservoir for dead storage
Dam
Type
Crest elevation
Crest width
Crest length
Riverbed elevation
Elevation of foundation
Height above foundation
Total volume of concrete
Spillway
Spillway design flood
Peak probable maximum flood
Type
Location
Number of Bays
Width of each Bay
Spillway crest elevation
Maximum spillway discharge
Type of dissipater
Radius of deflector
Lowest elevation of dissipater
Height of guide walls
Outlet Structures
High level bottom outlets
Number of intakes
Number of penstocks
Elevation of intake structures
Dimensions of intake structures
Dimensions of intake stop logs
Diameter of penstocks
Number of valves for each penstocks
Dimensions of slide gates
Low level bottom outlets
Number of intakes
Number of penstocks
Elevation of intake structures
Dimensions of intake structures
Dimensions of intake stop logs
Diameter of penstocks
Number of valves for each penstocks
Dimensions of slide gates
: 183.25 m.a.s.l
: 161.50 m.a.s.l
: 220.35 MCM
: 140.0 MCM
: 80.35 MCM
: Concrete Gravity
: 184.50 m.a.s.l
:5m
: 381.00 m
: 125.00 m.a.s.l
: 104.00 m.a.s.l
: 80.50 m.a.s.l
: 555000 m3
: Probable max. flood
: 9543 m3/sec
: Uncontrolled overflow ogee
: On the dam body
: 12
: 14 m
: 175.50 m.a.s.l
: 7856 m3/sec
: Flip bucket
: 15 m
: 144.00 m.a.s.l
:7m
:2
:2
: 148.0 & 155.0 m.a.s.l.
: 3.00*2.1 m
: 1.60*1.85 m (clear)
: 1.60 m
: 2 slide gates
: 1.00* 1.00 m
:2
:2
: 130.0*135.0 m.a.s.l
: 5.30 * 4.5 m
: 3.2*3.7 m (clear)
: 3.20 m
: 2 slide gates
: 2.0*2.0 m
6. Conclusions
The objectives of study aimed to reduce the probable damage of the city of Rabigh and to manage the water
resources inside the basin. That can be done through the selection of the most potential sites for harvesting
flooded water. Possible sites on the wadi were investigated, taken into consideration the hydrological and
geological effects. Different alternative solutions for wadi Rabigh water management could be presented and
analyzed. The most feasible alternative suggestion was the concrete gravity dam, 35 km east of the Rabigh
town and spillway over the dam's body. The flood peak discharges and volumes up to 100-year return periods
had been calculated using the synthetic adopted for the dam site, taking into account the engineering
economy and inadequacy in the observation period of the runoff station. The runoff observed values at
Rabigh runoff gauging station showed that there were some years without any runoff on the wadi. However,
for the rest of the observation period, it seemed to be possible, according to the observed runoff values, to
release water downstream during at least 3-6 months per year within the recharged capacity. Groundwater in
the aquifer would be a reliable source of water for domestic purpose and it would be more than the
requirements of Rabigh town. As for ratability of the project, an increase in the domestic water supply by
1.67 million m3/year would give a benefit of 16.7 million SR, which would result in a benefit/cost ratio of 1.
It was obvious that more water can be used from the aquifer annually than the aforementioned value which
means that the project is even neglecting the benefits due to prevention of flood damages.
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