Disastrous Sedimentation

Sedimentation and silting are a regular feature of irrigation structures, weirs and dams.

One of the most important contribution of Civil Engineering in this field has probably been the use of the energy of flowing water to maintain the balance of the system.

The science has been extensively researched, but is considered to be still in its empirical stage.

In this respect, Prof.G.G Stokes has contributed a scientific approach and has developed the famous "Stokes Law" to form the basic principle of sedimentation process. It says ;

Where "Vs" is the Settling Velocity of the Particle, "Ss" is the Specific Gravity of the Particle

"g" is the Gravitational Pull and "d" is the Diameter of the Particle.

The "Stokes Law" suggests a relation between the Settling Velocity of Particles to their Diameter, while in suspension. And it states that, for a given specific gravity of particles and under a constant gravitational pull, the settling velocity will be directly proportional to the diameter of the particles. Which implies that, the courser particles will have a higher settling velocity than those of the smaller sizes.

The application of "Stokes Law" is found in the design of sedimentation tanks and in the study of sedimentation process in large dams.

As a confirmation of Stokes Law, it has generally been found that the courser particles, in suspension, gets deposited earlier than the finer ones because of a higher settling velocity.

This phenomenon has been further confirmed by G.L Assawa in his excellent book; Irrigation Engineering (Published by Wiley Eastern Ltd., 1993, ) as "The courser sediment deposits farther away from the dam, while the finer sediment is deposited closer to the dam".

The applicability of this theory has been further confirmed in the case of Tarbela Dam, where a huge sediment mound has been developed, over the past twenty years, at a certain distance from the dam body.

The application of this law to our KBD case suggests that, the courser particles, in suspension of KBD waters, will have a higher settling velocity than those of the finer ones, and will hence deposit earlier than the finer particles. This suggests that the courser particles will start depositing at the farthest end of the reservoir, followed by next higher size and ultimately the finest in more close proximity to the dam body.

Therefore, within the present design parameters of KBD, it is feared that the sedimentation process will initiate near Nowshera City (the farthest end of the reservoir) and progress downwards, thereby putting the thickly populated towns to high flood risks from the very outset.

In addition, a major impact of the sedimentation process is Aggradation, which causes the bed level, and correspondingly flood levels, to rise to unforeseeable heights, causing heavy losses.

In this respect, G.L Assawa in his book; Irrigation Engineering (Wiley Eastern Ltd. Dehli, 1993), states:

"Where the sediment load in a River is in excess of the sediment transporting capacity, the excess sediment gets deposited in the river bed and the sediment load entering a given reach is greater than the sediment load leaving the reach during the same time. This causes a rise in the bed level (and hence the flood level) and decrease the bed slope with time. This phenomenon is known as Aggradation"

The high sediment load carried by River Indus has an important bearing on the design and operating rules of the KBD. This becomes more important, because a number of towns, upstream of Attock, are sensitive to the flood levels, feared to be aggravated with the increased sedimentation overtime.

In the case of Kalabagh Dam, the hydraulic drop will be far greater than that of Tarbela, or even Mangla, which will consequently accelerate the sedimentation process.

The siltation process at the reservoir of KBD is feared to be further accelerated due to the diminished velocity from water diversion into the Ghazi Barotha channel, and the reduction in hundred Monsoon days in River Indus to fifty days.

On top of all this, the silt delta in Tarbela reservoir is travelling at a rate of ½ mile per year, and is feared to ultimately get deposited in the KBD reservoir, thus reducing the life span of the dam.