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Case Study 4: Wadi Zikt Dam for Artificial Recharge

In the absence of perennial streams and because of low rainfall in the United Arab Emirates (UAE), groundwater is the main source of water for agriculture and domestic use in rural areas. Excessive withdrawal of groundwater, however, has resulted in a rapid fall in the water table, and increased soil and water salinity.

To increase groundwater recharge, the UAE authorities constructed about 40 dams and recharge structures, with a total water storage capacity of about 100 million m³. These dams were built to utilize floodwaters caused by the rains. These structures are very useful and effective in increasing groundwater recharge, and also in improving its quality and storage capacity. The water saved and recharged is used for drinking and agriculture purposes.

Dams are built in different shapes, depending on their use of storage, detention, diversion, etc. In this project, emphasis was placed on detention dams constructed for safety against floods and for reducing their negative impacts. These dams are categorized into two main types. The first type is where the water is temporarily stored and ultimately discharged through outlets designed for that purpose. The water release is carried out at rates that do not exceed the discharge capacity of the Wadi course. In the second type, the water is retained for the longest possible period and left to infiltrate down into permeable ground layers. Both types are used efficiently for groundwater recharge.

Technology Description

• Climate

The most important climate elements to consider in constructing dams and recharge structures are rainfall and evaporation. It is important to select the dam sites in areas where there is sufficient rainfall to produce runoffs in the Wadis. Evaporation also is an important factor to be considered in designing the different types of dams, and in determining the operational criteria.

• Morphology

The morphology and topography in the area where a dam is to be constructed also is an important factor in selecting the site for the dam. The reservoir and the recharge area also should be in a suitable geomorphologic unit. Suitable conditions are not only important for the effectiveness of the dams, but also to reduce the construction costs. Most dams are built in the mountainous regions having natural Wadi courses. The downstream areas are usually gravel plains, which serve as good recharge areas. The sandy desert plains or coastal plains are not geomorphologicaly suitable for constructing recharge structures.

• Geology

The suitability of the various types of rocks and soils as foundations for the dams constitutes a geologic factor that must be taken into consideration. The foundation geology at a dam site often dictates the type of dam suitable for the site. The geologic aspects also represent an important consideration for the conditions of the dam abutments. Suitable geological conditions at the site also are important with regard to the availability of construction materials. If such materials are locally available, the total project costs can be significantly reduced.

• Hydrology

Study of the hydrologic conditions are very important for safe and economic design of dams and spillways. Hydrology plays an important role in the selection of the type of dam and appurtenant structures. Water flow characteristics and flood volume produced by rainwater can significantly affect construction costs. Hydrologic studies examine the capacity and type of dams, dam safety conditions and spillway capacity.

In arid regions, and in the absence of perennial streams, the dams can be constructed at Wadis when there are sufficient flows resulting from rainfall The annual precipitation yield of the Wadi must be carefully computed in designing the dams.

• Hydrogeology

In construction of groundwater recharge dams, the hydrogeology of the area is an imporatnt factor in selecting the site for the dam and the type of dam to be constructed. The underlying layers must be permeable to facilitate water infiltration, and the thickness of the alluvial beds must be sufficient to take all the water. Sandy gravel layers are considered best for groundwater recharge processes. The geometry of aquifers should be studied in designing the dam recharge capacity. The recharge structure design, recharge improvement and well excavation are carried out on the basis of the aquifer characteristics. Water losses in calcite karstic layers, or retaining water for beneficial uses, are the most difficult and expensive problems.

• Social Conditions

There are no limitations or required social conditions for the application of this technology. It is important, however, to respect the existing water investment and water use rights downstream of he Wadi.

• Economics Conditions

For recharge dams that also provide flood control, selection of the best dam site must balance the project costs and benefits. Because water is a very precious resource in the UAE, evaluation of the economic feasibility of water projects is less rigorous. Moreover, because flood control also is an important aspect of the project in most cases, the cost:benefit ratio relation may exceed the justifiable expenditure.

In all cases, safety must come first. Nevertheless, it is always the designer’s intention to select the development project as inexpensively as possible.

Design Standards

The basic principle of design is to produce a functional structure at minimal cost. To achieve minimal costs, the dam is designed to make maximum use of the most economical materials available, including excavated materials for the dam and the foundations of its appurtenant structures. Design standards for different types of dams (storage dams, detention dams with and without spillways, earth-fill dams, rock-fill dams, concrete dams, gravity dams, concrete arch dams, etc.), vary according to the type of dam. Earth-fill dams are the most common type, principally because their construction allows use of locally-available materials and material resulting from excavation of the foundations. Further, the materials are readily available for use in dam restoration and reconstruction, in case of a total dam failure due to a flood exceedings its spillway capacity.

The most common type of earth-fill dams are zoned embankment dams. A zoned embankment dam is characterized by a central impervious core, covered with zones of more permeable materials. To ensure the safety of earth-fill dams, the following design principles must be taken into consideration:

• The embankment, foundation, abutments and reservoir edges must be stable and should not deform under different loading conditions;
• Seepage through the embankment, foundation, abutments and reservoir sides must be controlled to prevent excessive uplift pressure, an unbalanced situation in regard to the dam sections, and piping of particles through the dam body. The quantity of water lost via seepage must be kept within safe limits;
• The reservoir edges must be stable under all operating conditions;
• The embankment must be safe against overtopping, by constructing adequate spillways and outlets for water exceeding the dam capacity;
• The freeboard must be sufficient to prevent over-topping by waves;
• Sufficient camber to allow settlement of the foundation and embankment, but not included as part of the freeboard;
• The upstream slope must be protected against wave erosion, and the crest and downstream slope must be protected against wind and rain erosion.

Specification of Construction Materials

Materials for various types of dams that are usually available at, or near, the dam site include (1) soils for the dam body and sides, (2) rocks for protecting the dam body and sides (rip-rap), and (3) concrete aggregate (sand, gravel, crushed stones)

Information on Foundations

Zoned embankment dams are the most economical to construct, primarily because they allow the use of materials obtained from excavating the dam site. The zoning must, of course, be consistent with the requirements for dam stability.

Zoned embankment dams consist of a central impervious core surrounded by zones of significantly more permeable materials. The zoning scheme may divide the dam into various zones, based on the characteristics and gradation of the materials available for construction. The permeability of each zone should increase as one moves toward the outer slopes. Suitable drainage materials are used to cover and support the impervious core. Permeable materials are placed in the downstream sections to prevent the building up of pressure from percolating water, and to lower the water surface line during its passage through the dam. The slopes of earth-fill dams must be protected against destructive wave action. Rocks are usually used to protect these slopes.

Equipment Required for Construction

The construction of earth-fill dams does not require special equipment. Normal excavation machines and other construction equipment are typically needed for the job, including bulldozers, excavators, shovels (wheel loaders), compactors, rollers, heavy trucks, tippers, etc. Other equipment may be used for other types of dams (sheet walls, grouting and slurry trenches, etc.). Some especial excavation techniques may be used to excavate wells, and emptying and filling of the soil. It is preferable that all equipment and manpower required for construction are locally available. The local contractors also should be required to have all necessary resources to carry out construction according to the required specifications.

Case Study of Wadi Zikt Dam

Wadi Zikt is located at the eastern edge of the mountains and releases floodwater to the coastal plains and subsequently to the sea near Dahnah (in Gulf of Oman). The Wadi Zikt Dam is located at 6.5 km southwest of Duadnah town on the East Coast, at the point where the wadi leaves the mountains and enters the plains. The project includes construction of a 21 m high main dam, a separate spillway channel, a controlled outlet and three additional earthfill embankments (gabion bunds) downstream of the dam. The dam storage capacity is about 3 million m³. The main dam catchment area is about 68 km² (Figure 58) and the additional catchment area of the earthfill embankments is about 11 km².

The dam was constructed in 1991 to retain floodwater, for the purpose of groundwater recharge and to protect the areas downstream of the Wadi from flash floods.

Figure 58. Zikt catchment basin

 

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