School: LGS 55 Main
Topic: Reservoir Sedimentation
District: Haripur, KPK
Haripur District, located in the province of Khyber Pakhtunkhwa is the proud holder of Tarbela Dam, the
largest earth filled dam in the world. Built in 1976 over the Indus River, the dam has a reservoir with a
catchment area of 169,600 square kilometers. The main purpose of the project is to regulate the flows
of the river for irrigation use and to generate hydroelectric power by conserving snow melt monsoon
flows of the Indus River. One major issue that authorities face is the accumulation of sedimentation in
the reservoir that has increased the dead storage level of the reservoir to 1380 feet from 1378 feet. This
issue has posed serious threats to the dam life, by limiting it to 2030 and by reducing its total power
generation capacity. A recent government report suggests, that the filling up of reservoir will also
deprive 1.3 acre of irrigation land from water. In order to tackle this problem, proactive actions must be
taken by WAPDA. We believe the problem of reservoir sedimentation can be best tackled by a mixture
of both preventive and curative measures .An effective preventive measure that reduces the amount of
silt entering the reservoir is a proper watershed program, and as for a curative measure hydraulic
flushing can be used to reduce the level of silt accumulated in the dam.
One root cause of sedimentation is the inadequate watershed
environment of the northern areas that has caused soil erosion of soft
rocks such as shale, slate and mudstone that make their way into the
reservoir. Over the years, the rate of soil erosion has increased due to
deforestation, cultivation of crops on steep slopes and unchecked
grazing. It is predicted with such watershed environment Tarbela will
be completely filled with silt in the next thirty years. This can be solved
by initiating a watershed management program in Indus watershed
areas such as Kaghan Valley, Duar watershed and Skardu. The main
aim of this program will be to identify priority land and soil-related
issues in the watershed and use cost effective measures to prevent
FIG:1a
soil erosion. These measure include strip farming (Figure 1(a)),
afforestation, contour bunds that reduce the flow of water and micro
catchment areas for tree plantation that reduces soil erosion and
retain soil moisture (Figure 1 b) This program can be put into action by
authorities such as the Forest Department of KPK, WAPDA and
international organizations devoted to promote afforestation like
WWF. This solution will gradually reduce the amount of silt entering
FIG:1b
the Tarbela reservoir and ensure the sustainability of mountainous
areas. The total cost of this program that targets all Indus wetland is
$25000. This program can be even more successful if the locals living in the wetlands are integrated in
the system.
Secondly, a curative solution is also required to reduce the present accumulation of sedimentation in
the reservoir .For this purpose we propose the process of hydraulic flushing. Through this process the
accumulated sediments at the base of the reservoir are washed off .This process can be used in Tarbela
during the winters when the water flow is minimal. The low volume of water collected in the reservoir
can be used to flush out previously deposited silt. While the process is being conducted the lower sluices
of the dam are to be opened so that the flowing water can take fine and coarse sediment particles with
it (FIGURE 2). In order to prevent an increase flood risk downstream the flushing must begin at the
raising part of the flood wave. The WAPDA authority will have to effectively manage sluices as they can
be blocked by debris and sediments. Moreover, the process must only begin before the flood season in
order to prevent water wastage. This process is cost effective as compared to dredging and pumping of
sediments and will efficiently combat
reservoir sedimentation at Tarbela.
This will improve the storage capacity
as well as increase the power
generation capacity of the reservoir.
In addition to this, the collected silt
from the hydraulic process can be
used as a fertilizer for the nearby
agricultural grounds.
FIGURE 2