INFORMATION REGARDING PROPOSED
SEDIMENT MANAGEMENT STUDY OF TARBELA RESERVOIR

1.     Introduction

Tarbela Dam Project is the capstone of the Indus Basin Plan envisaged as replacement of water previously fed by eastern rivers. The project is the backstay of Pakistan’s economy and facilitates supply of irrigation water and production of power for industrial, agricultural and domestic uses. Civil works of Tarbela Dam started in 1968 by the international contractors M/S TJV under design and supervision of M/S TAMS, were completed by 1974.

Main Features / Components of the Project

The project consists of 9,000 feet (2,743 meters) long, 470 feet (143 meters) high (above the river bed) earth and rock-fill embankment across the entire width of the river with two spillways cutting through the left bank and discharging into a side valley. Main spillway has a discharge capacity of 650,000 cusecs (18,406 cumecs), and auxiliary spillway 8,50,000 cusecs (24,070 cumecs). Two auxiliary embankment dams 2340 ft. (713 m) & 960 ft. (293 m) long respectively close the low level ridges in the left bank valley. A group of 4 tunnels, each about half a mile long through the right abutment rock, have been constructed for irrigation releases and power generation. Irrigation tunnel 5 situated on the left bank, for which NESPAK were the Project Consultants, was put into operation in April 1976.

The reservoir is 60 miles (97 km) long, 100 square miles (260 square kilometers) in area and had initial gross storage capacity of 11.62 MAF (14.34 BCM) with a live storage capacity of 9.68 MAF (11.95 BCM). Due to 0.11 MAF per year silt carried by inflowing water, the gross and live storage capacity of reservoir as per Hydrographic Survey 2007 has reduced to 8.067 MAF and 6.841 MAF (9.955 BCM & 8.442 BCM) respectively. The total catchments area above Tarbela is spread over 65,500 square miles (169,600 sq. km), which largely brings in snowmelt supplies in addition to some monsoon rains. Average annual inflow at Tarbela is 64 MAF.

The power station on the right bank near the toe of main dam houses fourteen power units on Tunnels 1, 2 and 3. Ten (10) units each with generating capacity of 175 MW were installed on tunnel 1 and 2 by 1985. Four (4) units each with an installed generating capacity of 432 MW on tunnel – 3 have been commissioned in June, 1994. The total power potential of the project has thus been enhanced from originally planned 2,100 MW to 3,478 MW.

Statistical Sketch of Structures 

Reservoir
Length                                  60 Miles
Depth                                   450 Ft
Surface area of Reservoir      100 Sq Miles
(at conservation level)
Usable Capacity                    9.68 MAF
Gross Capacity El.1550 Ft      11.62 MAF
Annual Sediment Load :0.11 MAF(201MST)           
Deposition till 2007               3.553 MAF
Catchment Area                   65,500 Sq Miles

Auxiliary Dam No.1

Length at Crest El 1565 Ft 2340 Ft
Max. Height                       345 Ft

Auxiliary Dam No.2

Length at Crest El 1565 Ft       960 Ft
Max. Height                           220 Ft

Main Embankment Dam
Length at Crest El 1565 Ft                   9000 Ft
Max. Height                                        470  Ft

Service Spillway
Gates       : 7  : (50 Ft wide x 61 Ft high)
Discharge Capacity     :  6,50,000 Cs

Tunnels at Right Bank
Number                                   4
Length                                     2400-2700 Ft
Diameter u/s of gates shaft       45 Ft
Diameter d/s of gates shaft                
Tunnels            1,2 & 3              43.5 Ft
Tunnel              4                      36 Ft

Auxiliary Spillway
Gates   : 9         :(50 Ft wide x 61 Ft High)
Discharge Capacity    :  8,50,000 Cs
Power Station
Installed capacity (Unit 1-10)175MWeach  
                    (Unit 11-14)   432 MW each

Tunnel at Left Bank
Length                                     3,675 Ft
Diameter u/s of gates shaft        45 Ft
Diameter d/s of gates shaft        36 Ft   

 

 

 2.         Project Location

Tarbela Dam is situated on the Indus River at a distance of about 40 Air Miles (about 100 K.M. by road) North West of Islamabad and about 50 KM upstream of Attock.

3.         Project Objectives

The purpose of this study is to analyze and evaluate the recommendations made by 5th Periodic Inspection of Tarbela Dam on Sediment Management of Tarbela Reservoir for maximizing its economic return and prolong the useful life of the dam. The recommendations inter-alia propose WAPDA to commission a study for sediment transport dynamics investigation in the Indus River downstream of Tarbela Dam to assist in decision making regarding the feasibility of sediment evacuation from Tarbela Reservoir and other reservoirs, either existing or planned.