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Alternative Technologies for Freshwater Augmentation
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The average rainfall in the Zikt area is about 160 mm/year, and the evaporation
rate is about 3500 mm/year. These
regions face an acute water shortage, and are located in an arid zone with no
perennial streams. Groundwater is the
only available water resource, and the only source of recharge water is
rainfall. The rainfall pattern is an
important factor, noting that most of the rain falls during the winter
season. The rainfall system is one of
the effective factors in situations where most rainfall takes place in
winter. These rainfalls are usually
intense and only last a few hours, thereby causing flash floods.
The geology of the lower part of the
catchment and the dam site is shown in Figure 59.
The entire catchment lies in the Pre-Permian to Upper Cretaceous
allochthonous beds that constitute the larteral Ophulite layers.
The strata consist of ultrabasic peridotites
and serpentinites that protrude in the coastal plain in the form of small
hills. There are numerous faults from
the north or northwest towards the south or southeast (part of the wadi is
thought to be a result of a fault). The
wadi is deeply incised in the mountain range and is narrower at the dam site
(to a width of about 180 m) bounded between a both sides of a very steep
gully. The wadi then turns abruptly
from the north to the east, to the north, and back again to the east, before
entering the coastal plain. The
abutments at the dam site are composed of massive rocks in the steep
mountainsides, whereas the wadi bed material are mostly gravel with a variety
The main aquifer in Wadi Zikt consists of
coarse sand and gravel along the wadi and extends to the coast.
Groundwater is also found locally in the
serpentinite and the Iperidotites bedrock, by virtue of permeability formed
through fractures and weathering effects on the bedrock.
The average water table in the wadi is
about 20 m before the land surface. It
fluctuates seasonally according to the storage volume of the dam.
Groundwater quality in the Wadi Zikt, and
downstream near Dadnah, is acceptable. The salinity values range between 500-2,000 ppm,
but increases near Zubairah by the coastal strip to reach about 10,000 ppm in some wells.
Thus, the dam plays an important role in
controlling seawater intrusion by providing extra recharge to the groundwater.
Dam Structure and the Storage Lake
The dam was designed and constructed to a
height of 21 m, and a length at the crest of 234 m.
It was constructed in the form of zones, using fine material with
low permeability, to ensure that the time required for the water to reach the
dam core from the upstream edge was relatively short.
Because the water will need a much longer period of time to
percolate through the core, it essentially becomes impervious.
Zones surrounding the dam core are designed as
filter zones and protect the piping of the core material through the outer
layers of the dam. The upstream surface
of the dam is covered with boulders and rocks to resist the erosive effect of
the lakeís water (Figure 60).
Figure 59. Geology of the Zikt catchment and dam sites
The Zikt Dam closes the main Zikt Wadi
course, retaining about 3 million m3 of floodwater.
The excess water from the reservoir is
released over a main spillway excavated in the rocks at a distance of 500 m
east of the Dam. Water is released
downstream of the Wadi through the control outlets.
The spillway is a channel cut through the rock
of the damís right bank. The spillway
channel is provided with a low concrete sill (approximately 150 m length)
capable of discharging the computed probable maximum flood (PMF) of 1,677 m3/s,
equivalent to more than 11 m3/s per meter of spillway length.
The spillway releases the water downstream
of the dam through the rock channel.
Figure 60. Typical dam cross-section
Three gabion bunds are constructed approximately 2.6 km
downstream of the main dam area, in order to contribute to groundwater recharge
from an additional catchment area of 11 km2 between the main dam and
these bunds. The gabion bunds are constructed to lengths of 330, 162 and 50 m,
and heights of 5.3,2.6 and 1.9 m, respectively.
The main purpose of the reservoir is to recharge groundwater
through infiltration into the ground and toward the area downstream of the
wadi. The efficiency of infiltration
may be gradually decreased, however, due to settlement of fine material at the
reservoir bottom. This requires adopting a routine flushing technique
for the sediment. The Zikt Dam incorporates a special tower
equipped with a manually-operated gate and an outlet pipe, for the purpose of
flushing the sediment. The release is
carried out through a 800 mm diameter pipe with a discharge capacity of about
Water Table Changes
Three piezometric wells have been since 1992 to monitor
groundwater levels downstream of the Zikt Dam.
A fourth well was constructed in 1998.
Monitoring wells no. 1 and 2 are located about 500 and 14 m downstream
the dam, respectively, whereas well no. 3 is about 3,500 m downstream of the
dam near agricultural farms with large groundwater consumption.
The peizometric wells lie along the longitudinal section
(profile) of the wadi. The readings are
recorded monthly, and more frequently during the flood periods.
The phreatic changes of the water table
are directly related to the floodwater volume retained in the damís lake, and
to the distance of the monitoring wells from the dam (see Table 32).
The water level rise in the first well is
rapid, reflecting the effect of recharge concurrent with flood events.
The water rises in the farther downstream wells
after a longer time lapse. The pattern
of phreatic changes resulting from flood events reflects several
characteristics, including (1) a rapid rise in the water table near the dam,
(2) a decrease in the water table in the towards the downstream end of the
wadi, and (3) an increase in the response time (time interval between the flood
event and maximum rise in the groundwater table) towards the downstream end of
Groundwater Recharge through Zikt Dam
It is noted that 18 flood events have been experienced since
the construction of the dam (up to 1998).
The estimated total volume of stored water in the damís lake from these
flood events is about 14.3 million m3.
The largest flood event occurred on 23 July, 1995.
The estimated volume of this flood event was estimated as follows:
|∑ Water storage loss due to infiltration
||0.5 million m3
|∑ Stored water in the reservoir
||3.0 million m3
|∑ Water retained through the gabion bunds
downstream of the dam
|0.5 million m3
|∑ Estimated total flood volume
||4.0 million m3
To evaluate the role of the Wadi Zikt Dam
in the recharge process and its effects on the aquifer, the above-noted flood
event was utilized. The water level in
the dam reservoir reached 13.5 m and the corresponding storage volume reached 3
million m3. The outlet pipe
was closed, and the only water release from the reservoir was through
infiltration. The following observations were recorded:
- The water flows over the spillway crest for more than 5 hours, and about 0.5
million m3 of water were retained by the downstream bunds;
- As a result of the flood, a base flow was observed on the main wadi course near
well no. 1 (500 m downstream of the dam).
This base flow increased with time and covered larger distances
downstream of the dam, in the wadi course between wells no. 1 and 2;
- The base flow continued for about 3 months,
proving the effectiveness of the recharge process and its contribution to
aquifer recharge at the downstream regions of the wadi.
The water levels in the wells before and after the flood are highlighted
in Table 32.
Table 32. Water level changes in monitoring wells downstream of Zikt Dam
||Water levels before rain (m)
||Water levels after rain (m)
||Total rise in water table (m)
||Distance from dam (km)
|1. downstream of dam
||0.0 (up to 6.0)
|2. upstream of gabion bund
|3 -- downstream of gabion bund
|4 -- near Duadnah Unit
The effect of recharge is clearer and faster in the wells
near the dam (well nos. 1, 2 and 3, which are within 2 km of the dam).
Well no. 4 (5 km from the dam) showed showed
a delayed effect to the recharge. The
water levels recorded a rapid rise, reaching about 2.89 m at the beginning and
remaining steadily throughout the year.
Operation and Maintenance
The major factors affecting the performance of most dams are
the outlets and spillways, and continuous monitoring of their performance is
necessary. This includes external and
internal inspection of metal and concrete parts for any abnormal changes.
Recharge dams have to be operated to provide
maximum benefits. The outlets also must
be opened and closed intermittently to provide maximum groundwater
recharge. The depositing silt has to be
removed during the dry season. Safety
inspections also must be carried out whenever the dam has suffered any damage
from large floods.
Effectiveness of the Technology
Wadi Zikt Dam operates very effectively, and
contributes to the groundwater recharge.
The dam also provides good protection against damages caused by floods.
The Wadi Zikt Dam does not introduce any
negative impacts to the beneficiaries or the users at the downstream end of the
Wadi. Moreover, it results in water-quality improvements after flood events.
Sources of Information
Mohamed Saquer Al-Assam
Director, Water and Soil Department
Ministry of Agriculture and Fisheries
P.O. Box : 1509
Dubai, United Arab Emirates
Tel: 00 971-4-228161
Al-Assam, M.S. 1994. Dams in United Arab
Emirates and their role in recharge of the ground reservoir.
Al-Assam, M.S. 1999. Country Report,
Project for the Preparation of the Source Book for the Alternative Technologies
for Freshwater Augmentation in West Asia Region.
Internal Technical Report, ACSAD, Damascus, Syria.
Halker and Associates. 1993.
Final design report for Wadi Zikt Dam.
Mohamed Said Abdallah. 1995. Water sources in
United Arab Emirates. Technical report.