Rehabilitation of Kurit Historical Dam
Abstract
In this paper, the importance of storing water in Tabas area, an old town near Kavir desert in Iran is explored and the achievements of its habitants more than 700 years ago in using the concept of arc dams for the first time and using the method of heightening the dam after sedimentation have been overviewed. In addition, the construction of a new concrete dam (completed a few years ago) to eventually solve the sedimentation problem and also to protect the old dam is discussed along with some explanation of the construction methodology.
Introduction
Iran is located in a seismic region and more than 70% of its area is highly prone to earthquakes. On December 26, 2003 Bam Citadel, the largest adobe structure of the world, and most of the city of Bam was destroyed by an earthquake with strength of 6.7 degree in Richter scale, leaving over 43000 life losses and over 60000 homeless (Figures 1 and 2).
Figure 1: Bam Citadel before earthquake
In an eastern part of Iran, near the famous desert of Kavir and in the neighborhood of Kerman province where the city of Bam is located, there is a historical and green city called “Tabas”. This city has been destroyed several times by strong earthquakes during its life (latest in 1978) and has been reconstructed again by its diligent natives. Figure 3 shows the location of this area.
Figure 2: Ruins of Bam Citadel after earthquake
Figure 3: The location of Tabas city
Water is so precious and scarce for agriculture in this area that the unit of its measuring is “cup” instead of cubic meter or liter. That’s why people have always put great effort to store water in wet seasons and use it in dry seasons. Construction of cisterns has been one of these efforts in this part of the country (Figure 4).
In eastern side of this area, among the mountains that
Figure 4: A destroyed cistern in Kurit (Tabas)
separate the dry desert from the north-eastern fertile and green area, there are few small river basins that have always been considered as important sources of water. One of them is the Mond river basin that brings about 8 million cubic meters of flood water per year to Kurit area -an important part of Tabas city- where the first arc masonry dam of the world was built about 700 years ago. This dam was the highest in the world for 550 years (Figure 5).
Seismic Conditions
Tabas is located in a part of Iran with a very high level of seismic risk. Some major faults that pass through this area sometimes cause heavy earthquakes with strength of approximately 8 degree in Richter scale. In 1978, an earthquake with the strength of 7.8 degree completely destroyed the city of Tabas and 25 villages around it, leaving more than 20000 casualties. Kurit village -where the reservoir water used to flow through- at the vicinity of the historical dam was also destroyed completely in this earthquake (Figure 6).It is remarkable that fortunately, the masonry body of Kurit dam survived these heavy earthquakes without incurring any damage.
Construction of Historical Dam
The Kurit dam was built in 14th century at the time of Il-Khanid dynasty. The main material of the dam body was stones with a special mortar called “Sarooj”. This mortar was anciently used for construction of important structures in Iran and was made of lime, ashes, sand and cleans water. The place of the dam was chosen in a narrow valley just after a wide area in the river bed where a good reservoir could be made. The basement of the dam and both side walls were in strong and consolidated limestone with no cracks in it.
Figure 5: The first arc dam of the world (Kurit Dam)
Figure 6: The ruins of Kurit village after 1978 earthquake
For construction of the dam, firstly, an arch ceiling on the river was built so that the water could pass underneath during the construction phase. This kind of diversion system was used in narrow rivers in that time. In Figure 7, another example of this technique is shown that was used in Abassi Dam around Tabas.
Figure 7: Diversion System of old Iranian Dams (Abassi Dam-Tabas)
After finishing the construction of the dam body above the arch ceiling, the lower part could be filled with proper masonry material in a dry season. This method may also be used in modern construction of dams in topographically favorable conditions as well.
Heightening and rehabilitation of the dam
Studying of the dam body shows that the dam has been heightened at least three times. These heightening have been done in the past when the dam crest had been damaged by friction and cavitations. Another important factor that was considered in heightening was sedimentation of the reservoir. Old people of the area remember their fathers telling they had to empty the silt sedimentation from the reservoir to have a higher capacity in summer. The final height of 63 m is obtained after occurrences of these heightening while the initial height was 52 m (Figure 8).
Figure 8: The difference in age and type of materials shows that at least three heightening efforts have happened so far.
Construction of the new Kurit Dam
In the last 50 years, the amount of silt settled in the reservoir was so big that its removal was impractical. Gradually, some trees started to grow in the silt sediment behind the dam. This sedimentation caused water overtopping every year which in turn decreased the height of the dam as a result of friction. So, the government (Ministry of Energy of Iran) decided to build a new dam upstream of the old one. This new dam had several important roles. The first function was to collect 8 million cubic meters of water per year and store it for downstream farming. The second function was to protect the historical dam from friction and damage. Furthermore, the new dam protects the downstream land from flood damages and provides a nice area for recreation as well. The following picture (Figure 9) that has been taken before the construction of the new dam shows that the reservoir was almost fully filled with sediments and only a small fraction of its total 63- meter height was available for storage. The loss of approximately one meter of the dam height as a result of overtopping friction can also be observed.
Figure 9: The old dam reservoir
Basic Considerations
In design and construction of the new dam, some important factors were considered: All of the sediments could not be removed, so the new dam had to be high enough to make an acceptable reservoir size and protect the old dam. On the other hand, the outlet of spillway and the intake and irrigation system had to be designed so that the water flow could pass down the old dam without disturbing it. A geology fault on the right side near the old dam had to be considered in engineering design. Consideration of these factors led to an architectural design that is shown in Figure10
Figure 10: Layout of the new dam
Geotechnical studies
Geotechnical studies showed that the compacted silt and clay sediments in the reservoir from the depth of 20 meters to the rock bed was completely impermeable and grouting of the side walls and the limestone bed could make a proper watertight curtain (Figure 11).
Figure 11: Geotechnical Study
So, the engineer decided to remove only 20 meter depth of the sedimentation and put the new dam foundation on the compacted silt and clay material. After the sediment was removed, a reinforced concrete slab was constructed on the compacted silt and clay layers and fixed on the sides and then the dam body was placed on the slab. The contact grouting was injected between the slab and the basement sediment. Figure 12 shows the reinforced concrete slab on the basement and the dam body above. Figure 13shows the grout curtain of the new dam.
Figure 12: Reinforced concrete slab
Figure 13: Grout curtain of the new Kurit Dam
Other Works
Other executive works of new Kurit dam were done similar to conventional concrete dams. Pre cooling and post cooling of material and concrete, instrumentation, grouting of joints, access galleries and all other necessities were done according to technical specifications. Controlled blasting was carefully performed in excavations near the old dam. After completion, every visitor would testify that the Kurit area, with the new dam beside the old one is one of the most beautiful dam sites in Iran. Figures 14~18 show some executive activities of construction of the new dam and Figure 19shows the completed new dam next to the old one.
Figure 14: Controlled blasting of foundation
Figure 15: Shuttering
Figure 16: Grouting gallery
Figure 17: Post cooling
Figure 18: Chipping of the surface
Figure 19: Access road
Figure 20: At the first glance, it seems that the new dam is protecting the old one like a mother protecting her child; but in fact, the new dam is the youngster getting the shape, place and of course the name (Kurit) of the old mother and now proudly protecting it.
Conclusion
For a long time, the great efforts from the people of Tabas have saved the old historical Kurit Dam to the present. The principle motive of these efforts was the value of water in this area. However, the great challenge is still to come. The problem of water in the near future all over the world will make us contemplate and work hard on construction and rehabilitation of hydraulic structures and most importantly, dams. New facilities using modern methods and new studies will greatly contribute to accomplishing this goal whereas preserving old and historical structures will send the message of ever growing value of water from our ancestors to the future.
Thanks To
-Yazd Water Authority (Ministry of Energy of Iran) -Technical office of Ev Yol Construction Company -Ab-Pooy Engineering Consultants
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