The Sultanate of Oman through the Ministry of Regional Municipalities, Environment and Water Resources has been exploring the potential of the Wadi Dayqah Dam and Water Supply Scheme to Muscat and Quriyat since the early 1970s. Initially studies were undertaken to identify where along the Wadi Dayqah it might be possible to construct a major dam for water supply purposes. Wadi Dayqah was investigated as it is one of the few wadis in Oman which has a flow of water in its middle reaches throughout the whole year.
The initial studies identified a number of possible sites and the need for measurement of the natural flows in the Wadi to demonstrate how much water could be available for collection. Gauging stations were established at a number of locations along the Wadi and flows have been measured since 1978 near the village of Mazara, about a kilometre downstream of the site on which the Main Dam is now going to be constructed.
While Wadi Dayqah does flow throughout each and every year since records started, it is not the base flow which requires the dam to be constructed - it is the flood flows. The base flows are already used for local irrigation and domestic consumption adjacent to Mazara, and at Hay al Ghaf where the falaj is fed from an aquifer that is supported by the base flow. The main objective of the scheme is to catch the floodwaters that would otherwise flow to the sea and to use them, after suitable treatment, for the supply of potable water to Muscat and Quriyat.
Floods usually occur in Wadi Dayqah during the wetter spring months. They do not occur regularly and may actually occur in almost any month of the year. They occur immediately after heavy rainfall over any part of the inland areas that are drained through Tul at the head of the upper Dayqah gorge.
As much of the area is mountainous with extensive areas of bare rock, the rainfall runs off rapidly and there is little infiltration into the ground to be retained for future use. In addition, because the run off from the bare rock is rapid the scale of the flood flows is large and they have a short duration. As each flood event is very short there is also little benefit gained in the downstream reaches, and the bulk of the storm water flows into the sea, often causing considerable damage. It has been estimated that in the period of records since 1978, more than 720 million cubic metres (m3) of water have been lost in this way.
In addition, to the flood losses directly to the sea more than half of the normal surface flow passing Mazara disappears into the ground before Hayl al Ghaf. This water has been demonstrated to pass through the karstic limestone beneath the wadi bed gravels to emerge in freshwater springs below the sea off the coast to the south east of Daghmar. This again is of no benefit to the people of Oman. A dam above Mazara will retain this lost base flow, and some part of the flood flows depending on the reservoir capacity.
The site selected for the Main Dam to halt the flow in the wadi is located upstream of the village of Mazara, some 60 km (direct) to the south east of Muscat in Al Hajar Ash Sharqi. Here the average flow is 60 million cubic metres per year. The 100 million m of storage to be provided will support a yield of 35 million cubic metres per year, of which 10.3 million cubic metres is to replace existing irrigation supplies with the balance used for public water supply.
This site was selected as the most suitable of any of the possible sites. The site has to be capable of retaining a large quantity of water, so there has to be a suitable valley shape, and the natural bed and walls of the wadi must be of suitable rock to minimise the amount of work necessary to create a reasonably water tight reservoir. The site above Mazara can be closed by a dam some 75m high across the wadi, together with a second dam to close the gap through a low saddle in the wadi walls presently used for access to the small community of residents living upstream of the future dams. The dam size is the optimum for this site and sufficient, the foundation conditions are also better than at any other possible site.
The Wadi Dayqah scheme to supply water to Muscat and Quriyat will be implemented under two separate contracts. The pact signed yesterday is for the construction of the Wadi Dayqah Main and Saddle Dams. These create the reservoir and permit storage of a large proportion of the flood flows. A second contract will be awarded in due course to take the water from the reservoir, to treat the water to comply with the Omani standards for drinking water quality, and to transfer that water to the existing users in Hayl al Ghaf and Daghmar, to future users in the Quriyat Wilayat, and the remaining balance to add to the existing water supplies to the city of Muscat. This second contract will comprise two pumping stations, some 130km of pipelines, a water treatment works and treated water storage.
The Main Dam is 75 metres high and some 400 metres in overall crest length. It is constructed of roller compacted concrete which is placed and compacted like highway fill instead of being poured or pumped as conventional concrete. This dam is founded within the narrow limestone gorge through which the wadi flows.
The foundation rock here is sound and appropriate for this form of construction. No water will normally be released to the wadi downstream of this dam, but there are facilities to maintain a flow in each of the three falaj that presently pass through the site. For a future flood of sufficient magnitude that it overfills the reservoir, there is an ungated spillway extending over half of the dam length. The largest recorded flow in Wadi Dayqah in 1982, exceeded 5000 cubic metres per second, and there is local evidence of a flood in 1927 that may have reached 9000 cubic metres per second.
The dams are designed for the Probable Maximum Flood estimated from rainfall records as 18,398 cubic metres per second. This flood would result in 10.5-metre depth of flow over this spillway. For normal releases to supply there is an intake tower that provides means for taking water at 4.5-metre depth intervals.
This facility is provided as the water quality is usually better closer to the water surface. The Saddle Dam is nearly 48 metres high, with a crest length of some 360 metres. This dam closes the low point in the wadi walls through which the flow would otherwise escape. Here the foundation materials are of weaker sandstones and mudstones and hence construction will be of rockfill with a central clay core. This construction spreads the applied loads to the foundation across a large area. The crest elevation of this dam provides a further 0.6-metre of freeboard above the highest water level in the reservoir in the most severe flood event.
The construction of these dams will take some three years from starting on site on August 1, 2006. Closure of the diversion culverts through the Main Dam is programmed for January 2009 to enable capture of the possible floods in that spring. Once this closure is achieved it will then depend on the magnitude of the natural flow that occurs to determine when full supply to Muscat can commence. There must first be sufficient water stored in the reservoir to maintain the supply to existing irrigation users and the local supplies to Quriyat before commencing regular supplies to Muscat. However, should it be necessary, the transfer system could be operated to provide a quantity of water to Muscat for a short period as soon as completed.
© Oman Daily Observer 2006
