Another fiction that enjoys easy acceptability amongst the water experts who oppose Kalabagh Dam (KBD) and are not engineers by training, is based on a far-fetched conspiracy theory, outlined as follows: “The de-silting of Tarbela reservoir is possible at very low cost but it is being allowed to silt-up to create justification for the construction of KBD.”
All rivers contain sediments: a river, in effect, can be considered a body of flowing sediments as much as one of flowing water. When a river is stilled behind a dam, the sediments it contains sink to the bottom of the reservoir. As the sediments accumulate in the reservoir, the dam gradually loses its capacity to store water. This happens in all storages, although the rate at which this happens varies widely. Generally, life of a dam is calculated in terms of the number of years in which it would be silted up to 80% of its storage capacity. In Pakistan the flows in river Indus above Tarbela are mostly from snow and glacier melt. The steep mountains have little vegetation cover. The fast moving water from the glaciers crush rocks and carry a lot of debris along.
The World Bank approved funding for Tarbela Dam in 1965 and its construction started in 1968. Irrigation Engineers by that time used to build under-sluices in the barrages in front of the Canal Head works in order to reduce entry of silt into the canals. In case of large dams, there was no proven technology available to tackle the siltation issues. Moreover, at that time countries building large dams like USA were less prone to siltation issue on account of blessed topography having vegetation in their respective catchment areas. Accordingly the science of watershed management and methodology to flush the silt was still in its incipient stage and there was no practice of keeping a provision in the design for flushing of mega reservoirs.
If we go into the history of sediment management, the Pakistani experience of Warsak Dam was horrible. Warsak, a multipurpose Dam on River Kabul was the first initiative of its kind after independence in Pakistan. The Governments of Pakistan and Canada jointly executed this first undertaking. The project took 5 years for completion and its operations started in 1960. There was no provision of silt exclusion in the design of Warsak dam, which resulted in major silting up of the reservoir during first five years of operation of the dam and by 1974, its storage was completely filled up. Since then it is being used as run of the river electricity generation project.
The project planners of Tarbela Dam were mindful of the siltation issue and stressed on the well-known option of keeping the silt away by watershed management. Accordingly Tarbela Watershed Management Project was started in 1971. However, it was only after completion of the dam in 1977 and start of its operation, that awareness about other methods to reduce siltation came to knowledge of engineers.
At that time, the only successful example in the world from where WAPDA engineers could learn and employ related techniques in regard to siltation problem was Sanmenxia dam in China. However, in China sediments are comparatively soft. The Yellow river has large flows and remodeled Sanmenxia can flush the muddied water carrying sediments when flows start rising and store the relatively cleaner water in later days to fill the reservoir. In case of Pakistan, morphology of sediments is markedly different than that of China.
For controlling the sedimentation, WAPDA in collaboration with renowned foreign consultants and institutions has been carrying out various studies to tackle this issue. The process of sedimentation in the Tarbela reservoir varies in summer and winter. When flows are high in summer, maximum sediment enters the reservoir and is deposited at the head of the sediment delta. When the reservoir levels are low in winter, incoming flows pass over exposed delta and rework (erode) to bring it forward. With the increase of trapped sediment, the front of delta advances towards the dam and outlets. The delta has been moving towards the dam on an average one kilometer per annum and presently its pivot point is at six kilometers from the dam. Despite more than six decades of research, sedimentation is still probably the most serious technical problem faced by the dam experts.
Until now, following five studies have been carried out on Tarbela sediment management:
1. Second Periodic Inspection Tarbela Dam 1992
2. World Bank Mission 1995
3. TAMS & HR Wallingford 1997-98
4. Fifth Periodic Inspection Tarbela Dam 2007 &
5. Sediment Management Study by Mott McDonald & HR Wallingford 2013
However, the most quoted study by the writers who appear to be influenced by the conspiracy theory is the TAMS Report of 1998 and the “Action Plan” suggested to install a sediment sluicing system at Tarbela Dam, so as to recover its lost capacity and extend its life by preserving its live storage. The TAMS study recommended the following actions:
a) Build an underwater dyke to protect the intakes of the tunnels;
b) Build four new tunnels to flush out sediments; &
c) Use dredgers to keep the intakes of four tunnels clear of sediments.
The measures proposed were not found practical on account of technical and economic consideration. Moreover, such action plan had never been applied anywhere in the world on mega reservoirs and WAPDA could not afford to take risk to construct four additional tunnels in the main abutments of the Dam when it already had 5 tunnels, 3 for electricity generation and 2 for irrigation. After careful evaluation of the suggested rectification measures, it was decided that Pakistan could not afford to be a guinea pig to implement untested solution in regard to our most important water storage. However, TAMS report was not shelved or abandoned but it was considered appropriate to have its recommendation examined by other experts. This was a useful decision because our apprehensions were later substantiated by the reports and studies which were conducted by other experts in years 2007 and 2013.
According to the recent study conducted in 2013 by Mott McDonald, the dam receives 500,000 tons of silt every day. It means that it is annually deposited with the sludge of 200 million tonnes. To quote their engineer, “There are two options in the world to tackle this gigantic problem which include the dredging and other is the flushing out of the silt during the flood days by opening the low level tunnels on the right bank. The said two options seemed not workable, as the dredging is an uphill task as there is no site available on either side of banks of the dams where the dredged silt would be piled. Moreover, this is an extremely costly affair. To evacuate sediment entering into reservoir each day we need one thousand trucks of 20 ton capacity per hour. Likewise, flushing out of the silt by constructing four additional tunnels is also a risky job on account of two reasons. One reason is that the turbines would have to be closed down for at least one month meaning an end to electricity generation from the dam for one month. The second reason is more risky as the silt would be flushed out from the dam resulting in the piling of the silt downstream to choke Ghazi-Barotha pond. This would be more dangerous.”
The recommendations of TAMS report have not been implemented because of these concerns and WAPDA is currently operating the reservoir in such a way that the rate of delta advancement towards the intakes may be reduced. Raised intakes of Tunnels T3, T4 and T5 are in various stages of planning and construction to safeguard the electricity generation. Meanwhile if Diamer Basha and Dasu Projects are built upstream of Tarbela, it would help trap the sediments and increase the life of Tarbela by about 45 years.
I have explained this in great detail because some knowledgeable writers from Sindh including a down to earth and a talented person, Mr. G. N. Mughal, in their discussion with me quoted the TAMS report and wanted to know why WAPDA was not implementing their recommendations.