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United Nations Environment Programme
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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augmentation
in West Asia>


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 of properties.

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.

Spillway

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

Gabion Bunds

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.

Emptying System

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 2.8 m3/sec.

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 the wadi.

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
Well no. Water levels before rain (m) Water levels after rain (m) Total rise in water table (m) Distance from dam (km)
1. downstream of dam 12.30 0.0 (up to 6.0) 12.30 0.5
2. upstream of gabion bund 14.10 2.02 12.08 1.5
3 -- downstream of gabion bund 9.97 1.28 8.69 2.0
4 -- near Duadnah Unit 23.57 20.68 2.89 5.0

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.

Cultural Acceptance

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

Contacts

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
Fax: 00971-4-232781

References

Al-Assam, M.S. 1994. Dams in United Arab Emirates and their role in recharge of the ground reservoir. Technical report.

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.

 

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