This is a follow-up on the previous article on the subject. Not that the policy makers will lose their sleep after the publication of this article or shake themselves out of their somnolence, but one feels impelled nonetheless to flag the importance of the issue. Tarbela may not be as glamorous a project as the components of ‘Vision 2025’ but it certainly is not as grotesque as the raising of Mangla.

It happens to be our last hope of survival. And it is important to operate the reservoir in a sustainable manner by flushing out its sediment inflows on a regular basis. At the design stage of Tarbela the facilities required for flushing out the sediment were incorporated in the project. John B. Drisko, who was a key member of the planning and design team of Tippetts-Abbett-McCarthy-Stratton (TAMS), the consultants for Tarbela, wrote in his 1962 paper on sedimentation of Tarbela, as follows:

“It appears that after sediment deposits had advanced to a point where they came within the influence of the currents flowing to the outlets or spillways, especially with reservoir level lowered as it would be every year, then substantial quantities of sediment would be carried along and discharged. The finer silts would be the first material to be carried away and, as the storage space decreased and the flow velocities through the remaining reservoir became greater, coarser material including sand would be carried through, or even eroded from previous deposits.”

The above concept formed the basis of TAMS recommendation. It involved adoption of the reservoir operating rule for flushing the sediment by releasing water through Tunnels 3 and 4, after lowering the level of water in the reservoir to the minimum operating level of 1300 ft every year from May 20 to June 20. When this recommendation was disregarded in the late eighties, things changed and the sediment started accumulating in the reservoir rapidly, causing the following problems:

* A rapid loss of the storage capacity.

* Increased risk of blocking the outlets, particularly in the event of an earthquake.

* Increased risk to the life and property down stream of the reservoir, especially in case of any physical damage to the dam and its spillways due to an earthquake, as a result of several billion tons of sediment trapped in the reservoir. Tarbela was not designed to withstand the maximum creditable earthquake that can occur at the site of the dam.

In order to minimize the rapid loss of the storage capacity, Wapda re-commissioned the services of TAMS in 1997, who reiterated their advice to Wapda that regular flushing of the sediments from the reservoir was the only viable option for sustaining the storage capacity. They recommended, for this purpose, construction of four new tunnels, an under-water dyke, and provision of deep-water dredgers for preventing the blockage of the new tunnels. The proposal would have entailed an investment of $600 million and would ensure a perpetual storage capacity of 6 million-acre feet (MAF).

However, an eminent American engineer named Andrzej Jezierski, through a letter of August 1999 addressed to the minister for water and power advised him that sediment can be flushed out very effectively, without the construction of new tunnels and an underwater dyke proposed by TAMS. Jezierski had spent 6 years at Tarbela between 1983 and 1992 as a project manager of MAIN/NESPAK, who were the consultants for Tarbela Hydropower Third Extension Project. Earlier, he had also worked for TAMS.

He advised that Tunnels 3 and 4 should be used for flushing out the sediments, as stipulated in the original reservoir operating rules. His advice was based on his extensive experience and knowledge of Tarbela dam. He went on to say that in order to ensure adequate supply of irrigation water and to extend the life of the reservoir, Tarbela should be operated as irrigation project; that is according to the original rule curve. This would also provide substantially the same power benefits as at present.

No measures for flushing out the sediment as advised either by TAMS or Jezierski, have been adopted. A project by the name ‘Tarbela Watershed Management’ was taken up in three phases. Phase 1 was started in 1983 , which lasted for 10 years. It cost Rs690 million. Phase II required Rs762 million. Not surprisingly, given the propensity of the government not to do any thing right, phase I resulted in almost three-fold increase of sediment inflow into the reservoir from 1981 to 1993. Phase II was started in 1993 and ended in June 2002. The project envisaged afforestation of an area of 100,000 acres for private land and terracing of 5,000 acres of denuded land. However, in pursuance of the TAMS report, Wapda did bring a proposal for concept clearance and a feasibility study for the following, subject to clearance by Indus River System Authority:

a) Feasibility study for introduction of multilevel intakes for tunnels 1 to 4.

b) Full feasibility study for phases 2 and 3 involving underwater dyke and a bypass. The proposal was cleared but no funds were earmarked. The Economic Affairs Division was asked to arrange funds. Nothing happened thereafter. It appears that sustainability of the project is a low priority with the government.

It should be realized that Tarbela is our most important and strategic national resource whose sustainable and efficient operation is of paramount interest. It is the source of over 45 percent of our canal water supplies. A policy of inaction seems to have characterized the attitude of the policy makers. Government should realize that delays are endemic to all public sector projects.

Chashma and Ghazi-Barotha hydropower projects, costing $300 million and $2.2 billion respectively, show that we would require 20 years to complete any major dam. Obviously, if Tarbela is left to be filled with sediments at the current rate, it will be impossible over a long period of time to fight the looming water and food shortages. We would become a wasteland with all-consequent social disruption and starvation.

It is still possible to salvage the situation by minimizing the loss of the remaining storage capacity of Tarbela. Following measures are necessary:

1. As stipulated in the original operating rule, minimum water level of 1300 ft should be maintained from May 20 to June 20 every year, irrespective of demand for irrigation or power. This step will ultimately create a condition of equilibrium between the sediment inflows and outflows, and will also help in establishing a stable delta.

2. Tunnels 3 and 4 should be used for flushing out the sediments by operating the power plants on Tunnel 3 on the base load so that maximum quantity of water is released through it, carrying the maximum amount of sediment. Assuming an average discharge of 1,000 cubic meter per second, 2500 parts per million (ppm) sediments concentration and 250 days of operation per year, Tunnel 3 would flush over 72 million tons of sediments annually.

3. “Tarbela Units 11 to 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 5-10 years, whereafter the repair facilities would be available in Pakistan, to cope with the abrasion/erosion of hydraulic surfaces.”

4. The feasibility of reconnecting Tunnel 3 to its abandoned outlet control structure should also be investigated so that it can be used for lowering the reservoir in an emergency. This step would provide additional security for rapid lowering of the reservoir if the dam is endangered.

5. Tunnel 4 should be converted to power without disconnecting its outlet works from the reservoir. Tunnel 4 should then be used for base load. There are a number of factors, which influence the movement of the sediments in the reservoir, making it rather difficult to predict flushing capacity of the tunnels.

On April 24, 1997 the discharge of Tunnel 4 contained 580,000-ppm sediments, and if one assumed a discharge of 1000 cubic meter per second at that time, the tunnel flushed an unbelievable 2.8 million-ton sediments per hour, without suffering any damage. This rate of flushing is of course not sustainable but it does show that the existing tunnels are capable of flushing the bulk of the annual sediment inflows, and eventually create equilibrium.

6. Tunnels 1 and 2 should be operated only if the water demand is more than the Tunnels 3 and 4 could handle. When the reservoir level falls below 1139 ft the Generating Units No. 5 to 8 should be shut-down to avoid any damage due to excessive vibrations. Any shortfall in the water demand, above the combined capacity of Tunnels 1 to 4 should then be met by opening Tunnel 5. If the shortfall still persists the outlet control gates of Tunnel 4 should be opened.

7. In the long run, say after 5 to 10 years, repair facilities would be required to cope with the erosion of hydraulic surfaces due to silt-laden water. Such facilities should be planned in the light of experience gained at Warsak. It may be pointed out that the turbine runners and wicket gates of Generating Units 11 to 14 are made of stainless steel castings, which has proved to have properties of very good resistance to abrasion. This material has good welding properties also and therefore, easy to repair its erosion by welding. The use of similar stainless steel at Warsak was found reasonably durable, although sediments of the Kabul River are far more abrasive than the sediments of the Indus River.

8. To avoid the risk of blockage of intakes either due to liquefaction during an earthquake or slumping of the sediment delta due to other causes, deep-water dredgers of reasonable capacity should be arranged to clear the blockage as quickly as possible. With collaboration between the local industry and the foreign manufacturers, very large barrage mounted dredging systems, operating on the available hydropower can be fabricated and assembled at site. Very large dredgers are being built worldwide. The Suez Canal Authority purchased a dredger of 24,000 cubic meter per hour dredging capacity in 1984.

The dredgers are necessary to guard against the prolonged blockage of the intakes due to the liquefaction or the slumping of the sediment delta, which provision should be made immediately, even if for some reason GOP decides against the recommended flushing of the sediments.

The measures 1 to 4 above should ensure flushing of sediment on a sustainable basis and ultimately create equilibrium between the sediment entering and leaving the reservoir. Thus the remaining storage capacity of Tarbela would become a perpetual storage. The suggested remedies may perhaps not be the best. It is suggested that a team of engineers, experienced in the design and the operation of large water resources and hydropower projects handling the sediments, should examine the above remedies and carefully deliberate on the likely impact of the above measures. They might as well come up with better and more cost effective remedies. But what is important to emphasize here is that inaction is not an option. Our public servants have specialized in the art of humming and hawing so that they don’t have to accept responsibility for decisions, which might eventually come to haunt them.