THE Karkheh river originates from the middle and southwest zones of the Zagros Mountain range, located on the west and northwest regions of Iran. After crossing a distance of about 900km from north to west, the river reaches Huralazim Lagoon, on the border of Iran and Iraq.
Karkheh is the third largest river (considering water yield) in Iran, after Karun and Dez. The watershed of Karkheh consists of an area of 43,000km2, which covers Hamedan, Kermanshah, Kurdestan, Ilam, Lorestan, and Khuzestan provinces.
The main headwaters constituting Karkheh river are Seymareh, Kashkan, Qarresu, Gamasyab, and Cherdavel rivers. Probable floods and resulting damages are characteristic of the Karkheh river.
In 1956, US-based company Development and Resources Corporation began studies on water and soil resources development of the Karkheh river watershed, for the irrigation of north-west lands in Khuzestan plain.
Successively, some Iranian and international companies carried out studies concerning dam construction on this river. In 1966, a UK company, Sir Alexander Gibes, conducted reconnaissance studies of the irrigation development of Kurdestan and Kermanshah provinces – upstream of the Pay-e-Pol hydrometric station. This was followed in 1969 by complementary studies by Iran-based Ab-o Kahk and Electro Consult of Italy.
Feasibility studies of Karkheh reservoir dam were also performed by Iran Development and Resources Consulting Engineers, and in 1980, the company proposed the construction of a dam and reservoir 54km upstream of Pay-e-pol – named Karkheh Zero. This project was designed for the storage and regulation of water for irrigation of 340,000ha of lands of downstream plains. It was also designed for power generation amounting to 934GWh per year, and to aid in flood prevention
After the Islamic Revolution, further studies were carried out by the ministry of Jehad-e Sazandegi. The final studies of phase one of the Karkheh zero reservoir dam were completed by Mahab Ghods Consulting Engineers in 1989.
During the years 1989-90, Mahab Ghods tried to locate axes more suitable than Karkheh zero, which required 7.6km length and a volume of 72.85km3. It studied the axes of A, B, C, D1, and D2, in collaboration with a Chinese-based company called Caitec. Through preliminary studies, axes A and C were selected to compare with Karkheh zero axis. After conducting geological studies and drilling exploratory boreholes, primary designs for both axes A and C were developed.
Finally, axis C, which is located 21km northwest of Andimeshk, and 12km upstream of Naderi Bridge was selected because it offered the most suitable geological conditions. It also offered the most desirable conditions for the execution of the diversion system.
Diversion system
Karkheh dam diversion system consists of a concrete culvert with four waterways, intake structures, stilling basin, pre-cofferdam, cofferdam and downstream dyke.
In order to prevent any damage during the flood seasons, the upstream and downstream of the dam were closed during construction of the dam body, by means of a cofferdam and downstream dyke.
This meant the area between the two points became dewatered, allowing construction activities at the main structure to continue safely. Since the cofferdam of the Karkheh project is designed to be a part of the main dam, a pre-cofferdam was built, to allow the cofferdam to be constructed with the suitable technical specifications in a dry condition.
The crest level of the pre-cofferdam, at 122.5m asl for a dry period, is designed to meet a 20-year flood event, and have the maximum discharge of 860m3/sec. The design level of the cofferdam of the Karkheh diversion system is 163m asl for a 100-year flood.
The 32m long, 30.6m wide intake structure of the diversion system, includes: four openings; intake tower; slots for the gates; and closure gates.
In order to close the water course through the waterways, gates of different types, have been installed. These include:
• At waterways I and II, two bulk-head gates, the weight of each amounts to 32 tons.
• At waterway III, a fixed wheel gate, weighing 62 tons.
• At waterway IV, a roller gate, weighing of 82 tons.
For the lifting and installation of parts of the culvert gates, a cantilever gantry crane, with the capacity of 120 tons, was used. This crane, which had been installed 160m asl on the crest of the intake tower, was disassembled after impounding and permanent closure of the gates.
On the upper part of the intake tower, there are orifices and windows, which are used as inlets for the water flowing into the bottom outlet.
In order to dissipate the energy of the water, passing through the culvert, a 128.5m long, 35m wide stilling basin was constructed.
Spillway
Having considered parameters such as the dam height, reservoir volume, type of the dam, and its importance in irrigation and hydroelectric power supply, and the location of the dam, the spillway of Karkheh dam has been designed based on the reliable discharge of probable maximum flood.
The inlet structure of the 110m high spillway includes the ogee, the middle and the abutment piers, radial and stoplog gates, access bridge on the dam crest, the bridge for the gantry crane, and hydromechanical equipment for the possible repairs of the gates. The ogee crest of the spillway is placed at the level of 209m asl.
The slope of the upstream face of the ogee is 1:1 and the width of each of the middle piers of the spillway is 4m.
Six sets of radial gates are employed to control the flow of water through the spillway, each 18m high and 15m wide, weighing 170 tons. This is one of the largest types of radial gates, manufactured in Iran. Each gate is operated by two servomotors, each with a capacity of 26 tons.
In order to repair the radial gates, four 15m wide stoplogs are used. For the purposes of shifting around or replacement of stoplogs, a gantry crane of 35 tons with the rail length of 130m, is used.
Seepage control
In order to control seepage within the dam foundation, a plastic concrete cut-off-wall was utilised.
This main cut-off, which is on the same axis as the dam body, is constructed under the foundation, in the centre of the clay core.
Since the power house building is located on the left abutment, and having considered the effect of impounding and seepage through conglomerate layers, another cut-off wall was constructed under the east side of the power house building. To connect this cut-off to the main cut-off wall, a grout curtain has been executed through the access gallery.
Grouting
After impounding in 1999, the water level of the reservoir was increased. In order to investigate and study the effect of the impounding on different structures of the dam, a panel consisting of international and Iranian experts was held at the Karkheh dam site in July 2000.
This panel closely investigated and discussed the amount of seepage into the dam foundation. It was observed that the amount of seepage under the abutments was higher than the middle parts of the foundation. The reason for this was considered to be the existence of permeable layers, deep under the foundation, where the cut-off wall could not be constructed.
Recommendations were made for the execution of a grout curtain, on the left bank, around the power house building, the drainage gallery and on the right bank on inspection gallery, under the dam body.
Concrete gallery
A concrete gallery was designed and constructed a short distance from the downstream cut-off Wall for the following purposes:
• Access to the foundation, under the clay core.
• Access to the cut-off wall.
• Monitoring the cut-off wall.
• To complete repairs, if needed, during the dam operation.
Other applications of this gallery could also include:
• Monitoring, and obtaining data concerning piezometric pressure of the water in different layers, under the dam foundation.
• Possible increase of the depth, and improvement of the seepage flowing under the dam foundation.
• Monitoring, and controlling the settlement of the dam foundation.
• Possible drainage, and decreasing the water pressure of each geological layers under the dam foundation.
Three individual tunnels have the duty of transferring water to the Karkheh power house. At a distance of 61m from each tunnel inlet, there is a vertical shaft, 80m high, which is used for the installation and complete closure of water flow, during any necessary repairs of the tunnel.
To access these shafts, a 60m long bridge is linked to the dam crest. The intake structure of the tunnels has six openings, and there is a 6.5m wide, 20m high trash rack at each opening inlet.
The Karkheh dam structure was completed in February 2001. Unit one of the plant, which will feature three Francis turbines with vertical axis, was commissioned in September 2002 and units two and three will be commissioned in June 2003.
