23ème Congrès de la Commission Internationale des Grands Barrages, Brasilia, 2009
Topic: Multiple Benefits
TARBELA DAM: A NUMERICAL MODEL FOR SEDIMENT MANAGEMENT IN THE RESERVOIR
Khawaja Bilal AHMED*, Martin SANCHEZ*
* Université de Nantes, Faculté des Sciences et des Techniques,
UMR 6112 CNRS – Laboratoire de Planétologie et Géodynamique,
2 rue de la Houssinière, BP 92208, 44322 Nantes, France
Bilal.Khawaja@univ-nantes.fr
Abstract
The study of the morphology of rivers is one of the most complex problems related to fluid
dynamics. This is because the river flow changes over time and space. Therefore, knowledge based on
experience and good understanding of physical phenomenon involved in sediment transport, are two
crucial aspects.
The sediment transport is an important process that occurs in the rivers, estuaries and coastal
regions. In many situations, this process and the morphological changes that result may have an
adverse impact on the hydraulic structures and the environment. Thus, the reservoirs of dams,
navigation canals and ports could be rendered unusable by the accumulation of sediment. The
maintenance of these structures in operational state may require dredging, which may represent a
significant cost for the authorities. Furthermore, the structural integrity of the structures could be
compromised by excessive washouts of the bed around them.
Issues related to the sediment transport are highlighted through the presentation of the problems
posed by the rapid filling of Tarbela dam on the Indus River in Pakistan. The inactive pool behind the
dam generates favourable conditions for particle settling, such that important storage capacity is lost.
Sediment transported in suspension by Indus is deposited in the reservoir at an annual rate of about 200
million tons, corresponding to about nearly 98 % of the sediment inflow. The composition of the deposits
varies between sand and fine silt or clay fractions. The location of the delta imposes the danger of
clogging of the five low-level tunnels of the dam.
The sediment delta induced by the dam is explained by deposition of suspended sediment
resulting from low current velocities in the reservoir. Since the capacity of the river flow was found
negligible, it was concluded that the sediment transport in suspension is governed by the rate of erosion
so the Meyer-Peter and other formulas could not be used to estimate the annual sediment transport. A
simulation of average monthly variations in Tarbela reservoir has been realised. Calculation of inflows
and outflows permits to evaluate the water level variations. Inflow is linked with snowmelt and rainfall
upstream from the reservoir. Outflow depends on evacuations by spillways and tunnels for electricity and
irrigation. In this study, five potential remedial measures are considered. The interest and viability of
these measures are evaluated on the basis of the possible sediment evacuation and economical cost.
Methods to try to lessen the problems are usually based on reduction of the inflow of sediment,
the manipulation and control of the reservoir processes and the mechanical removal of deposits. A
numerical model is being realized taking in account different transversal sections of the reservoir up to
100 kilometres, annual hydrological cycle, monthly reservoir storage levels, rate of erosion, rate of
deposition, granulometric distributions and settling velocity to define and explain the morphological
changes and deposition of the sediment in the reservoir over different time periods. Software MIKE 21
will be used to carry out a numerical simulation of different scenario of flushing out of the sediment to
ensure a continuity of the normal operation of this dam. Simulations describe hydraulic conditions,
sediment transport and depth evolutions in time. The initial conditions are defined by initial bottom levels
and bed sediment characteristics (cohesionless deposited sediment but also some rigid cohesive
material). During the operations of sediment evacuation, the hydraulic regime in the reservoir would
resemble to a fluvial regime with important velocities all along.