SHOULD TARBELA BE WITH WAPDA?

The answer is no. Tarbela was constructed as part of Indus Basin Treaty to provide storage of 9.4 MAF. Tarbela makes a major contribution to the national economy. It provides nearly 30% of all the irrigation water available in dry season. 2100 MW of hydropower was to be generated as a by-product. By the year 1992, the generating capacity was raised to 3428 MW, with the 3^rd extension comprising four more units of 432 MW capacity each

In the original operating rule, stipulated by the designers, the’ minimum operating level’ (MOL) of the reservoir level was mandated to be lowered to 396 meters (1300 feet) for a month between May 20 through June 20 every year, before being raised to 492 meters (1550 feet). This rule was designed to ensure optimum condition for effectively flushing out the sediments through tunnels 3 and 4. As long as this rule was diligently followed, the delta formed by the sediments moved slowly towards the Dam at an average rate of one mile per annum.

In November 1985 (MOL) of the dam was raised to 408 meters (1339 feet). This was done to overcome violent vibrations to 4 power units (No.5 to 8). This was also done in the hope of starting construction of Kalabagh to make up for loss of water resulting from the raising of its MOL. With this decision, WAPDA almost converted Tarbela from irrigation to a power project. . From 1990 to mid 1996 WAPDA maximized the generation of electricity and as a result of good rains and more than normal water flows into the rivers, demand for irrigation was relatively less than normal. The effects of this decision were not felt. During this period MOL was gradually raised to 440 m which slowed downstream movement of the sediments and trapped about a billion tons in the live storage zone of the reservoir.

In 1997, dry cycle commenced and increased the demand for water. WAPDA lowered the MOL to 402 meters (below the revised operating level of 408 meters) but this had an unexpected effect of choking the cooling water system of four units on tunnel 3. It was therefore decided to raise the MOL to an unprecedented level of 412 meters (1350 feet), thus withholding water from agriculture.

In the year 2000 the MOL was lowered to 403 meters (1322 feet) and the Chief Engineer of Tarbela confidently reported:

“Tarbela units 11-14 are capable to run at low reservoir level if the modified procedure is followed. The turbine parts are designed to withstand the prevailing situation of sedimentation for next 5 to 10 years, whereafter the repair facilities would be available in Pakistan, to cope with the abrasion/erosion of hydraulic surface”.

The original operating rule was abandoned because of the following two apprehensions:

1)                           Removal of very large quantity of sediments from the reservoir through the tunnels would erode the turbines and impose huge expenditure on subsequent maintenance and repair of the power units, and

2)                           If the sediments were allowed to accumulate in the dead storage near the dam the possible forward slumping and movement of the sediment delta might block the intakes.

The policy of abandoning the original rule proved short sighted and only postponed the problem of erosion facing the turbines at a huge cost of losing storage capacity. It did not diminish the risk of blockage of the intakes but increased the possibility of slumping or liquefaction of the delta as a result of increased sediments. The primary objective of providing irrigation water was sacrificed to the secondary objective of generating power. This policy shift entails huge costs. Several billion tons of sediments have been trapped in the reservoir reducing its live storage capacity.

A panel of experts reported in 1995 that there was a close and direct relationship between the MOL and the rate of advancement of the sediment delta towards the Dam. Lower the MOL, greater the advancement of delta. The movement of the delta increases the volume of sediments in the dead storage zone but decreases it in the live storage zone. On the other hand, if the delta moves closer to the intake, it causes erosion of the power plant. These two conflicting phenomena, one of reducing the live storage and the other of providing cheap electricity present a paradox.

In 1997 M/s Tippetts-Abbett-McCarthy-Stretton International Corp. (TAMS) recommended that if additional remedial measures were not taken for management of sediments, the delta will cross the limit line as early as 2006, preventing the operation of tunnels 3 and 4 and few years later tunnels 1 and 2. They recommended following measures for sustainable operation of Tarbela with a reduced live storage capacity of 6 MAF (64% of the original capacity):

1)                           Raise the minimum operating level by 1.2 meters every year, for not more than 15 days in a year, because the sediment profile being within 3.2 k.m. of the limit line,  it poses a threat of overwhelming, the intakes of tunnels No.3 and 4.

2)                           Construct an under water dike by the year 2008, to protect the intakes of the right bank tunnels. The crest of the dike should be at 421 meters level.

3)                           Construct a bypass by the year 2015 for flushing out the sediments. The bypass should comprise four new tunnels to be built between the service and auxiliary spillways.

4)                           Procure dredgers having an annual dredging capacity of 0.5 mcm should be for dredging the intake channel to carry the silt laden water to the new tunnels.

The estimated cost of TAMS proposal was US$ 663 million. This proposal is cheaper than the contentious and palpably jinxed undertaking of Mangla Dam at a cost of $ 1 billion. The action plan proposed by TAMS consist of three phases:

Phase I

Comprises modification of the reservoir operation rule so as to provide security of the intakes against clogging up to the year 2008 while at the same time maximizing reduction in live storage. In the proposed operation rule, the minimum reservoir level would be increased by four feet every year and the period of annual draw down limited to 15 days.

Phase II

Comprises construction of under water dike to protect the intakes of tunnels from sediments.

Phase III

Comprises flushing the annual inflows of sediments in the reservoir with a low level  high capacity bypass to be situated on the left abutment.

The objective is to provide 6 MAF live storage on a sustainable basis which is not possible if the priorities are stood on their head by sacrificing storage to power generation.

As an alternative to the above, but only to ensure the security of the intakes against inundation for as long as possible MOL has to be raised by six feet each year. Those way tunnels 3 and 4 are likely to be free of the risk of inundation up to the year 2025 and tunnels 1 and 2 up to the year 2040. Then onwards some powerhouses will have to be shut down or replaced by thermal plants, if excess capacity available with the IPPs is not sufficient to meet the demand. Under this option spillways have to be modified to resist the abrasion from high sediment loads. This proposal does not fulfill any of the objectives of the action plan.

A third alternative is to go for dike only option which comprises construction of an under water dike in two stages to protect tunnels 1 to 4 against inundation. In the long run this would effectively convert Tarbela into run of river hydropower project with only a small live storage, and obviously merits not a thought.

TAMS proposal itself suffers from the following drawbacks:

a)                            The construction of an under water dike may not, as claimed, prevent sediments from entering the right bank tunnels. A similar dike at Warsak failed.

b)                           Huge quantity of water, 22% of the annual flows will be needed under TAMS proposal to flush the sediments out, and maintain live storage level.

c)                            Disposal of the huge quantity of sediments along the left bank of the barrage of Ghazi Barotha Hydropower Project (GBHP) will adversely affect the GBHP; and

d)                           Geological conditions are not favourable for construction of a bypass.

The policy of inaction, which defines the attitude of l the Government entails huge national cost and does not portend well for the future of this country. We cannot afford to lose our storage capacity having lost 2.4 MAF already. With the prospects of Kalabagh or Basha having receded in to history, and the raising of Mangla most likely to be lost in intense controversy, the most feasible and cost effective option is to do something about Tarbela. The following is proposed:

1.      Treat Tarbela as an irrigation project and assign only a secondary role to power generation. In any case there is enough unutilized power generation capacity within the country, which could easily be utilized at a little higher cost than the hydel power.

2.      Institute the original operating rule of MOL of 396 meter (1300 feet) between May 20 and June 20 every year to reduce annual loss of live storage.

3.       Tunnels No.3 and 4 should be effectively used for flushing out the sediments. These tunnels have an annual discharge capacity of more than 26,000 million cubic meters, which is more than sufficient to flush out the annual sediment inflow (the discharge assumed by TAMS for their proposed bypass was 18,000 million cubic meter).

4.      Tunnel No.4 should be converted to power so that instead of spilling the water through it, hydropower should also be generated.

5.      To prevent the possibility of liquefaction during an earthquake or slumping of the delta, deep-water suction dredging system of reasonable capacity should be arranged to clear the blockage as quickly as possible. The required barge mounted dredging systems can be fabricated locally in collaboration with foreign manufacturers.

The attractive feature of the Tarbela project, according to Kirmani report (July 1964), is the possibility ‘for successive development of off channel storages in the Haro and Swan river basins aggregating to some 40 MAF.’

The Tarbela dam was proposed to be constructed in two stages. In the first stage 400 feet high earth and rock filled with a capacity of 8.4 MAF was to be constructed and in the second the height was to be raised by 50 feet to increase the gross capacity to 11.1 MAF. The maximum pool level of the reservoir in the ultimate stage will be at elevation 1550 which is slated to feed the off channel reservoirs on the Haro and Swan rivers. This interesting aspect of Tarbela appears to have been abandoned.

Lastly, Tarbela owes its mismanagement to its control of WAPDA. It should have been independent of WAPDA.A separate organization should be set up or in the alternative its control may be handed over to an existing organization, IRSA (Indus River System Authority). The head of the organization should be an expert in water with a power engineer as a subordinate so that correct priorities are maintained.