Green light for reservoir impounding at Karkheh

With a surface area of 1,648,000km2, Iran is a vast country with a varied climate and both arid and wet regions. Annual rainfall varies from 25mm in the Loot desert to about 2000mm in the Caspian Sea region.

The mean annual rainfall in Iran is 250-260mm, which is less than one- third of the global mean annual rainfall (860mm). About 72% of the country’s total rainfall is lost through evaporation leaving about 120x109m3 of available water, making it among one of the world’s dry regions. As such, Iran is determined to make use of its available water resources. Appreciating the role that water plays in national development, various programmes have boosted efforts in recent years to construct dams, manage wastewater and launch scientific projects for exploiting existing water resources. The total utilised water supply for the country is 82x109m3, of which 76x109m3 is used in agriculture, 5x109m3 for domestic and sanitary use, and 1x109m3 for industry.

In Iran the management of surface and ground water resources, implementation of water supply projects, and water supply management for agriculture and domestic use are carried out by regional water authorities. These are governmental organisations affiliated to the Ministry of Energy.

Massive projects of national interest, such as the Karkheh dam project, are implemented under the responsibility of the Iran Water and Power Resources Development Company (IWPC), formed in 1989. Other huge hydro power projects including Karun IV (1000MW), Karun III (3000MW), Masjed-e- Soleiman (2000MW - see IWP&DC, July 1999, pp18-22) and Upper Gotvand (2000MW) are also being constructed under the management of IWPC. All design work for the Karkheh dam project has been carried out by the Iranian consulting company mahab-ghodss. lahmeyer International of Germany has been retained to check, review and approve all the study and design works on behalf of the owner (IWPC). The engineering services of both consulting companies will continue until the impounding of the reservoir is completed.

The project

The Karkheh dam project, located in the province of Khuzestan some 20km west of the city of Andimeshk, is considered to be the largest public works project in Iran and one of the largest dam projects in the world presently under construction. The project is named after the third biggest river in Iran, the Karkheh river, with an average annual flow of some 188m3/sec at the dam site (Pay-e-Pol hydrometric station). The climate within the 43,000km2 catchment area varies greatly with temperatures of minimum below -25°C, and maximum 50°C, over the year. The mean annual precipitation within the catchment area varies within 300-800mm, half of which usually falls in winter. The Karkheh river is well known for its high flood potential mainly caused by storms and snowmelt in the Zagross mountains. The largest flood recorded during the past 46 years had a peak flow of about 5200m3/sec. The simulated probable maximum flood (PMF) gives a peak discharge of 23,700m3/sec at the dam site with a flood volume of about 8.1x109m3.

The Karkheh dam project has been designed as a multi-purpose project providing irrigation water for some 320,000ha of fertile land in the lower Karkheh basin, generating electric energy of about 950GWh/year, and controlling the river flow during flood seasons. The project’s most significant benefits will be related to the increased production of agricultural goods such as cereals, rice, sugar cane, soya, vegetables and fruits. The foreign currency savings made by producing the goods in Iran instead of importing them will be in the region of US$380M/year. The generation of electric energy is a secondary benefit, resulting in annual earnings of about US$40M.

Some 55% of the electrical energy produced will be firm energy, representing the most reliable energy source for the Iranian grid. Since the power plant is designed to generate peak power, a re-regulating dam will be constructed some 10km downstream of the Karkheh dam and will incorporate an 8MW power plant. The Karkheh power plant will be connected to the Iranian transmission system by two 400kV transmission lines. Implementation of the Karkheh dam will provide flood protection to 800km2 of agricultural land in the flood plains, which is estimated to save about US$17M annually.

Impounding of the huge reservoir, with a total storage capacity of some 7.6x109m3, began in March 2000 and is scheduled to finish in about 22 months. During the summer of 2000 irrigation water will be released for agricultural purposes.

In July 2000 the embankment dam and the concrete works of the spillway were 86% complete and 72% complete, respectively.

Contracts and project cost

In line with the general policy of the Islamic Government of Iran to support local industries and to boost the Iranian economy, all construction, supply and erection contracts for the Karkheh dam project were awarded to Iranian companies. During the spring of 2000 some 5300 labourers were employed at the project site and a further 1500 in various factories. Subcontracts were awarded to foreign companies for the supply of heavy equipment such as turbines and generators, and special civil construction works.

In 1992 the first works started with the construction of the river diversion works, which had a contract value of US$28.6M. In April 1994 the main contract was awarded to the Iranian construction company, Sepasad, for the construction of the earthfill dam and the spillway at a cost of US$247.7M. Then 11 months later, in March 1995, the contracts for the hydromechanical equipment for the spillway and power tunnels were awarded to Sadid and Neyr Perse for a combined contract value of some US$51.2M.

A package for the electro-mechanical equipment (400MW) for the power house was awarded to Farab for a price of US$185.1M in mid 1996. The civil works for the power house are being completed by SAB for some US$32M. The Rah-Sahel construction company was selected for the construction of the Dasht-e-Abbas conveyance tunnel at a contract value of US$43.9M.

It is scheduled that all civil works for the project will be finalised by mid 2001 and the power plant will start commercial operation by the end of the same year. The total project costs will then reach US$622M which, for a project of this size, is very low.

The entire project was financed mainly by the Ministry of Energy with a small component of ECA finance for the electro-mechanical installations.

The river diversion scheme required before the construction of the embankment dam comprises four reinforced concrete culverts, each 10.5m high, 5m wide and 790m long, along with a 60m high upstream cofferdam (subsequently integrated into the main dam) and a 20m high downstream cofferdam. The concrete culverts were constructed in an open pit requiring excavation of 1.5M m3 of soil. The concrete culverts had to be designed for the full earth pressure from the 127m high dam and had a total concrete volume of 223,000m3 with an average reinforcement content of about 60kg/m3 concrete.

During the river diversion phase, the waterflow through the culvert system was controlled by bulkhead and fixed-wheel gates (10.5m high and 4.6m wide) which were installed and operated in an intake tower with four 30m high shafts. A 110m long and 35m wide stilling basin is arranged at the outlet of the culverts to avoid erosion and damage during flood periods.

The entire river diversion scheme was designed to discharge safely a flood with a return period of 100 years, with peak flow of 7080m3/sec. Under this flood condition the combined discharge capacity of the four culverts would reach 3680m3/sec when impounded (to an elevation of about 163m asl).

Several alternative river diversion schemes were reviewed during the feasibility study phase in addition to the selected culvert concept. These were mainly: •An open channel with one diversion tunnel.

•Diversion tunnels only.

•A combination of culverts and tunnels.

Because of poor geological conditions, in particular the low compressive strength and moduli of deformation of the surrounding rock mass, the temporary rock support measures needed for tunnels and slopes were extensive. Therefore from a safety, economics and construction point of view the culvert concept was finally found to be superior.

Prior to impounding, three culverts were converted into bottom outlets to ensure that the downstream water demand can be maintained. At elevation 220m asl (normal reservoir water level) the total discharge capacity of the three bottom outlets will be 360m3/sec. The fourth culvert was concrete plugged and serves as access and ventilation for the three bottom outlets.

The 127m high earthfill dam, constructed from sandy gravel\ conglomerate fill with a central clay core, was designed with upstream and downstream slopes of 1V: 2.25 H and 1 V:1.8 H, respectively. The total length of the 12m wide crest of the dam is 3030m, and the dam contains 32M m3 of earth material, making the Karkheh dam one of the biggest embankment dams under construction worldwide. During the months of December 1999 and January 2000 the maximum daily and monthly peak placing rates reached 43,500m3/day and 904,000m3/month, respectively.

Because of its weak foundation conditions, the dam is supported with a wide upstream stability berm (at elevation 140m asl) and two wide downstream stability berms (at elevation 170m asl and 135m asl). The upstream slope of the dam is protected with a 2m thick layer of limestone riprap which is quarried 70km from the dam site. For cost reasons the limestone riprap was replaced with soil-cement at lower levels (below elevation 192m asl).

The central clay core features upstream and downstream slopes of 1V:0.25 H and consists of a mixture of 60% clay and 40% sand and gravel. The clay core was constructed in lifts of 13-15cm compacted by means of sheep foot rollers (four passes) and smooth rollers (two passes). The core material has a moisture content of 12-15%. The clay core is protected on the upstream and downstream sides with sand/gravel filters of varying thickness (4-7m) depending upon the height of the dam section.

Again because of the heterogeneous foundation condition caused by pervious alluvium and weakly cemented conglomerate interbedded with impervious mudstone layers, a positive cut-off wall was constructed to minimise seepage under the dam foundation. About 5% of the conglomerate has no fine grain matrix (open gravel conglomerate) and consequently has a high permeability of about 100 Lugeons.

Seepage control by means of cement grouting was envisaged during the initial design phase. However grouting tests indicated that the construction of a reliable grout curtain by means of conventional grouting practices was questionable. Eventually a 2930m long plastic concrete cut-off wall (modulus of elasticity 2000-5000MPa) with a maximum depth of 80m was selected. The total area of this cut-off wall reaches about 142,000m2, being one of the largest ever constructed worldwide. The thickness of the cut-off wall varies between 0.8-1m for different sections of the dam depending upon the height of the dam. The cut-off wall extends 2-8m into the overlying clay core to keep the seepage gradient within allowable limits and to reduce tension stresses in the adjacent core and in the upper part of the wall. Downstream of the cut-off wall a 1300m long drainage and inspection gallery (3.2m high and 2.6m wide) was constructed with a total concrete volume of 46,000m3.

A large selection of construction equipment was employed for the construction of the earthfill dam comprising: 358 tip trucks, 11 dumpers, 79 loaders, 74 bulldozers and 30 compactors (sheep foot and smooth).

Spillway The spillway is located on the right abutment and is a conventional chute spillway with a stilling basin as an energy dissipation structure. The spillway was designed to discharge the predicted PMF peak flow of 23,700m3/sec, resulting in a maximum outflow of about 18,900m3/sec.

The spillway has a 110m wide and 25m deep approach channel. The gated overflow weir is equipped with six radial gates, each 18m high and 15m wide with a radius of 22m; a 700m concrete chute of 110m constant width; and a 164m long stilling basin.

To achieve optimum hydraulic conditions, 25m high concrete guide walls are constructed on both sides of the approach channel upstream of the weir structure. The weir structure itself is subdivided with 4m thick and 59m long concrete piers into six 15m wide bays. The concrete piers also act as supports for the access bridge from where the stop logs can be placed for maintenance purposes.

The bottom of the chute is lined with 1-1.5m thick high velocity resistant reinforced concrete. The longitudinal and transverse joints in the bottom slab are sealed with waterstops to prevent high velocity flow penetrating into the foundation. A systematically arranged drainage system is provided underneath the slabs to eliminate potential uplift pressures. For maintenance and repair purposes the chute is divided into two equal sections by a partition wall extending down to the stilling basin. The side walls of the chute were designed to provide a freeboard of 0.6m for the 10,000-year flood and are 8-13.5m high. To prevent cavitation damage to the concrete caused by the high flow velocities (maximum about 42m/sec), three aerators were constructed along the chute.

The chute terminates in a 164m long stilling basin (at elevation 93m asl), which was designed for the 1000 years flood, giving a conjugated depth of about 31.5m. An open cut pit with a maximum depth of 61m had to be excavated to reach the foundation level of the stilling basin which was about 26m below the natural ground water level.

Dewatering this pit was done using surface pumping with an average capacity of 1000 litres/sec. The uplift stability of the stilling basin during spillway operation represented a challenging design task because of the deep foundation level and the high ground water level. Several counter measures were studied including pumping systems, tension piles, slab weight and drainage systems. Finally, on the basis of costs, construction time and reliability a 7m thick concrete slab with an extensive drainage system was selected to safeguard the stability of the stilling basin during operation. The concrete side walls of the stilling basin are of the gravity type with heights up to 34m. In constructing the spillway and its stilling basin, total volumes of earth of 1.6Mm3 and 3.6Mm3, respectively had to be excavated. Altogether 758,000m3 of concrete was placed.

For the production of this huge volume of concrete, batching plants with a total nominal capacity of about 335m3/hr were mobilised, achieving monthly production rates of up to 40,000m3. To control the heat of hydration, Type II portland cement, ice chips and cooled water were used. The ice chip production plant had a capacity of 8tons/day.

Power intake and waterways

The power intake is a conventional submerged six-barrel bellmouth structure located on the left abutment of the dam with two bellmouths feeding one power tunnel. Therefore each of the six bellmouth structures were designed for half of the rated turbine discharge of 163.8m3/sec and are equipped with fixed trashracks each with an area of 264m2. Overall the power intake is 60m long, 23m deep and 20m high, with a total concrete volume of 21,850m3.

Each power tunnel is provided with a gate shaft about 85m downstream of the power intake which houses 8.5m high and 4.8m wide fixed wheel gates. The three concrete-lined pressure tunnels of 436-456m length have an internal diameter of 7.2m which reduces to 5.3m along the high head tunnel section. With these diameters, flow velocities of 4m/sec to 7.4m/sec result at rated conditions.

The 70cm thick tunnel linings are steel reinforced with about 120kg of steel per m3 of concrete.

All three power tunnels are steel lined (ST-52), starting about 15m upstream of the dam axis. The steel liners are between 287m and 300m long, of varying thicknesses (20mm, 28mm and 30mm) depending on the internal water pressures.

The power house was designed and constructed as a conventional surface powerhouse 117.5m long, 53m wide and 55m high. It is located on the left abutment immediately downstream of the embankment dam and houses all electro-mechanical equipment for the three 133MW capacity generating units. Three butterfly valves are also installed within the power house.

The construction of the surface power house required 91,200m3 of concrete and excavation of about 3M m3 of soil/rock material consisting of weakly cemented conglomerates and mudstone. All excavated material was used for the construction of the earthfill dam. About half of the excavated construction pit, which had maximum slopes of 100m height, was located below the natural groundwater level. An extensive dewatering system by means of wells and pumps had therefore to be installed.