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Advancement of the Pivot Point of Underwater Delta: A Study of Tarbela
Reservoir, Haripur, Pakistan
1* 2 1 3
Nausheen Mazhar , Ali Iqtadar Mirza , Zayanah Sohail Butt , Muhammad Nawaz ,
4
Muhammad Ameer Nawaz Akram
1Department of Geography, Lahore College for Women University, Lahore, Pakistan
2Department of Geography, Government College University, Lahore, Pakistan
3Department of Geography, National University of Singapore
4State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing
(LIESMARS), Wuhan University, China
*Corresponding author email: nausheen.mazhar@lcwu.edu.pk
Submitted date: 06/09/2020 Accepted date: 12/05/2021 Published online: 30/11/2021
Abstract
Tarbela dam, with the main functions of hydropower (3,750 MW installed capacity) and irrigation, is
being threatened by the menace of sedimentation. The underwater delta is advancing at a rapid pace towards
the main embankment dam, being at 5.25 miles from Main Embankment Dam (MED) in 2012. This study is
aimed at monitoring Tarbela reservoir's underwater delta pivot point advancement and its forecast mapping.
Secondary data regarding pivot point elevation and its distance from the MED was obtained from the
reservoir's library. Base map of delta direction was obtained from the surveying department at Tarbela dam.
The data was processed in Arc GIS and Microsoft Excel to perform meaningful analysis. The study concludes
that the average sediments deposited in the reservoir for the period of 1980-2012 were 0.1 MAF. Delta pivot
point was at a distance of 5.45 miles from the MED in 2012 and according to our study, it will be only at a
distance of 0.45 miles from MED in 2032. The In order to enhance the efficiency and life of Tarbela dam, this
study suggests that immediate sediment management, either active or passive methods e.g. dredging, hydro
suction and flushing must be adopted.
Keywords: Underwater delta, Pivot point, Main Embankment Dam, sedimentation, Tarbela dam, sediment
management, dredging, hydro suction.
1. Introduction
River basin erosion naturally leads to the silting
up of reservoirs (Vincenzo et al., 2017).
Building a dam usually alters the balance of the
sediment inflow and outflow. Rehman et al.,
(2018), state that annually on a global scale,
reservoirs loose annual storage of about 1%,
and this loss varies between 0.1 to 2.3%.
Chinese rivers carry 80 to 90% of the annual
sediment load accompanied by 50-60% of its
annual runoff, during its flood season which
leads to a loss of 66% of its reservoir capacity
(Wang and Hu, 2009).
Sedimentation leads to loss of storage
capacity which in turn causes a reduction in the
efficiency of the dam to generate power, control
the flood intensity, supply of water, navigation,
envi r onmen t al b e nefit s and recr e a tion
(Vincenzo et al., 2017; Hosseinjanzadeh,
Hosseini, Kaveh, and Mousavi, 2015; Wang
and Hu, 2009). It can also lead to abrasion of the
hydraulic machinery and blocking off the
intake tunnels. Navigation and ecology of the
region can get badly affected due to sediment
accumulation in the delta region (Wang and Hu,
2009). Various sediment-related problems take
birth upstream and downstream of dams, thus
the detailed study about this phenomenon and
the movement of its resultant underwater delta
is extremely significant.
Tarbela the only main reservoir on Indus,
the largest river of Pakistan, is the world's
largest earth and rock filled dam. The dam has a
height of 145m above the bed level (Khan and
Tingsanchali, 2009). At the time of closure in
1974, it had a total capacity of 14.3 billion cubic
mete r s and this capac i ty, d u e to high
sedimentation rate, drastically reduced by
17.4% in 1992 (Lowe and Fox, 1995). It has
five main tunnels, amongst them tunnel 1, 2 and
3 are equipped with power houses, with a
generation capacity of 3470 MW. Rest of the
two tunnels are kept reserved for irrigation
Journal of Himalayan Earth Sciences Volume 54, No. 2, 2021 pp. 40-46
40
41
flows and low-level flushing (Khan and
Tingsanchali, 2009).
The mega reservoir Tarbela is being
threatened by the menace of sedimentation
since 90% of its inflow comes from snow and
glacial melt. River Indus, in an average year, is
capable of bringing along 200 million tons of
sediments and deposit it in Tarbela reservoir
(WAPDA, 1996). Since 1974, Tarbela reservoir
has lost some 20% of its gross capacity and
almost 40% of its original dead storage (Tate
and Farquharson, 2000). The reservoir has an
89% trapping efficiency (Ali and Boer, 2007).
Another dam of the world with such high trap
efficiency is Burdekin Falls Dam, Australia.
According to the calculations of Lewis et al.,
2013, the measured trap efficiency of the
reservoir Burdekin Falls Dam, Australia ranges
between 50-85%.
A huge underwater delta has been created
in Tar b ela r e servo i r due to the he avy
sedimentation, whose pivot point was 10 km
from the d am t oe i n 200 9 (Kh a n an d
Tingsanchali, 2009). Tarbela Dam Project
2009, recorded a 917m advance of delta per
year that led to 80% of the live storage to
become unavailable (Tate and Farquharson,
2000). Khwaja and Sanchez (2008) postulated
that the delta will reach the main dam around
2030. H.R Wallingford's (2011) study also
in d i c a t es t h a t a t th e cu rr en t ra te o f
sedimentation the reservoir's dead and live
storages will be largely filled up by 2030, while
the intake tunnels will be blocked much before.
Average inflow of sediment at Tarbela is 0.55
MST per day, while 0.11 MAF storage of the
reservoir is lost per year. While at Mangla, the
second largest reservoir of Pakistan, the
average inflow of sediment is 0.164 MST per
day with a storage loss of 0.034 MAF per year
(Haq and Abbas, 2007). According to Rahman
(2005), the gross capacity of the reservoir has
been reduced from 11.620 MAF to 8.782 MAF
in the year 2004 i.e. (24.42%) and the usable
capacity from 9.679 to 7.973 MAF i.e.
(17.63%).
The total inflow of sediments is estimated
to be 208 million tons per year, which is
composed of 59% of fine sand, 34% silt, and
7% clay. Lowe and Fox (1995), state that
according to an estimated 99% of the sediment
that comes into Tarbela reservoir gets trapped
and settles mainly in the shape of a delta
deposit, which continues its advancement
towards the Main Embankment Dam. This
delta “top-set beds have a slope of about s=
0.0006 to 0.0008”. According to Mazhar et al.,
2021, am o ng the fa c to rs affec t i ng the
sedimentation of Tarbela reservoir, temperature
and rainfall in the catchment area of Indus river
and inflow coming into Tarbela reservoir, most
significant one is inflow.
Agha, (2011) indicates that as the quantity
of sediments increases, the front of the
underwater delta advances towards the main
embankment dam. The underwater delta is
approaching the main dam at a fast rate and may
cause the problem of clogging the intakes that
feed the t u rbine s . Fu r t hermo r e, t h e
downstream facing slope of the delta may cause
landslides (Lowe and Fox, 1982). Larger
landslides can be caused due to earthquakes
(TAMS, 1998).
Ahmed and Sanchez (2011) postulated
that the major threats to Tarbela Dam Project
(TDP) due to rapid sedimentation were storage
loss, clogging of power intakes, abrasion of
equipment and concrete surfaces by sand and
degradation downstream. The study of the bed
level of the dams is essential for the assessment
of the risk of clogging of the intakes. The bed
levels of Tarbela continue to rise each year,
showing an increasing tendency in the last 5
years, Roca (2012). The aim of the present
study is to monitor the changes in the shifting of
the pivot point of the underwater delta in
Tarbela reservoir from 1981-2012 and forecast
its advancement till 2032.
2. Materials and methods
2.1. Study site
Tarbela reservoir in district Haripur,
Pakistan, is chosen as the study site of the
present study. The region is one of the wettest
parts of Pakistan, with maximum rainfall being
received in early spring and in the summer
season. This study area receives less intense
rainfall events, but the highest number of
rainfall days (Hussain and Lee, 2016). The
42
Fig. 1. Study area map, Tarbela reservoir, district Haripur, Pakistan
region around the Tarbela reservoir experiences
significant decadal precipitation variability of -
17.45 to -13.63mm (Mazhar et al., 2016). The
study area lies in the region experiencing a 0.4-
degree centi grade risi ng trend of mean
temperature, which might lead to increased
snowmelt and the resultant rise in sediments
being carried to the reservoir downstream.
(Mazhar et al., 2015). The monitoring of the
pivot point of the underwater delta has been
performed through mapping of the underwater
delta advancement which has been a neglected
part of literature till present. Fig. 1 presents the
study area map.
2.2. Delta pivot point advancement map
Delta pivot point advancement map was
prepared using delta direction map obtained
from Tarbela Dam library as a base map. The
data of elevation of pivot points and their
distance from Main Embankment Dam was
obtained from official reports in Tarbela Dam
Library and was entered in the attribute Tab. of
Arc GIS, and yearly pivot points were plotted
on the flow line, while their elevation was
shown using proportional symbols in Arc GIS.
2.3. Forecast of delta pivot point advancement
The average advancement of the pivot
point for the 32 years was calculated to be 8.16
mil e s. T h e p e r-ye a r d elta piv ot p oint
advancement was calculated by dividing the
average advancement of 32 years by the total
number of years, the result was 0.25 miles
advancement per year. Later the per year
advancement rate was multiplied by 5 to get
five-year advancement Fig. i.e. 1.25 miles
advancement per five years. Lastly, 5, 10, 15
and 20-year advancement was calculated by
simply subtracting 1.25 from 2012 Fig., then
subtracting 1.25 from the 2017's forecasted Fig.
and so on. The calculations are shown in Table
1.
43
3. Results and discussion
3.1. Main reservoir sedimentation
The mean sedimentation coming in the
reservoir from 1980 to 2012 was 0.100 MAF.
From Fig. 2 it is obvious that 2012 was the year
that experienced abnormally high sediment
inflow. 1984, 1994, and 2010 presented a
considerably higher amounts of sediments
being deposited in the reservoir, i.e., 0.159,
0.199 and 0.197 MAF respectively.
3.2 Trap efficiency
Table 2 presents trap efficiency for the
years 1980-1996, which can be supported with
Fig. 3, where drastic fluctuation in 1997 and
1998 was due to very low inflow. The trapping
efficiency went through slight fluctuations
from 1999-2004.
3.3. Delta Pivot Point Advancement
The pivot point of the delta of Tarbela
reservoir was at a distance of 11.11 miles away
from MED in 1981 and was only 5.45 miles
away from MED in 2012. Its elevation has also
varied a lot over the years, being 1296 ft. high in
1981 and 1382 ft. in 2012.
The map shown in Fig. 5 shows the location of
the delta pivot point of the Tarbela reservoir for
the period 1981-2012. The volume of the
proportional symbols in pink color shows the
elevation of the pivot point in feet, the higher
the elevation of a point the thicker the deposit is
from the bed. The pivot point was at a distance
of 11.11 miles from the MED in 1981, it
advanced towards the MED and was only at a
distance of 8.56 miles from the MED in 1996.
Fig. 4 presents a clear declining trend of the
distance from the main embankment dam line.
On the contrary, the obvious ascending trend of
the delta pivot point is also identifiable.
However, these both lines coincide with the
years 2000-2003 when the distance from MED
decreased sharply from 8.93 in 1999 to 6.59 in
2002. In the same years, delta pivot point
elevation decreased from 1373 ft. in 1999 to
1330 ft. in 2002.
Fig. 2. Tarbela Main Reservoir Sedimentation in MAF, 1980-2012, Hydrographic Survey
Table. 1 Calculations of Delta Pivot Point advancement forecast.
44
Fig. 3. Tarbela Reservoir Trap Efficiency, 1980-2004.
Fig. 4. Tarbela Reservoir Pivot Point Advancement, 1981-2012.
Fig. 5. Delta Pivot Point Advancement towards
M.E.D, 1981-2012.
Fig. 6. Forecasted Delta Pivot Point Advancement
till 2032.
45
However, the pivot point retreated in 1997 and
was 8.93 miles away from MED. In the year
2000, the pivot point rapidly plunged forward
to remain only 6.50 miles away from the MED.
A major retreat was experienced by the pivot
point again in the year 2003 when the pivot
point was at a distance of 7.79 miles away from
MED. Fig. 4 proves that the delta pivot point
abnormally retreated because of very low
inflow in the reservoir in 2003. Later, from
2004 to 2011 the pivot point steadily advanced
towards MED and moved from 6.59 to 6.02
miles.
In 2012, h o w e v e r, the pivot po i n t
advanced further and was only 5.45 miles away
from the MED. Elevation of the pivot point was
only 1296 ft. in 1981 and rose to 1382 ft. in
2012. The elevation of pivot point dropped
drastically from 2000 to 2003, the same years
when the delta pivot point retreated away from
the MED. The most appropriate explanation
could be that the lower inflows lead to lower
sediments coming into the reservoir and thus
lesser elevation of delta pivot point. These were
the same years when UIB received lower
rainfall than normal. Fig. 4 explains the
relationship between pivot point advancement
and its elevation. Various efforts have been
made by experts to forecast the rate of delta
pivot point advancement. The findings of this
study can be supported by the results of a recent
study, where the greatest bed elevation of the
Tarbela reservoir was indicated to be between
range lines 7-12, which lies near the MED
(Mazhar et al., 2021). The pivot point
advancement map (Fig. 6) presents the drastic
picture of the intake tunnels of the reservoir
becoming unable to operate perhaps by 2032
when the pivot point would be only 0.45 miles
away from the MED.
4. Conclusion
The average sediments deposited in the
reservoir from 1980 to 2012 was 0.100 MAF.
Drastic fluctuation in the trap efficiency of the
reservoir in 1997 and 1998 was due to very low
inflow. Delta pivot point was at a distance of
5.45 miles from the MED in 2012 and
according to the forecast of this study, it will be
only at a distance of 0.45 miles from MED in
2032, with an advancement rate of 1.25
miles/year. The results of this study are
consistent with Wallingford's (2011) study that
co n clu ded th a t a t t he c ur r en t ra te o f
sedimentation the Tarbela reservoir's dead and
live storage capacity will largely be filled up by
2030. This can lead to catastrophic impacts on
the irrigation and energy sector of the country.
The study provides the policymakers with a
practical mapping method to continuously
monitor the advancement of the underwater
pivot point of delta in the reservoirs across the
globe so that timely measures can be taken to
avoid complete clogging of intake tunnels.
Acknowledgment
This research paper is an excerpt from the M.S
thesis of the first author. The author would like
to thank the continuous guidance and support of
Mr. Ghulam Ali Varaich, Sr. Officer Electronics
(R), Tarbela Dam, Pakistan.
Authors' Contribution
Nausheen Mazhar proposed the main
concept, collected the data from the field,
performed the mapping analysis and was
involved in write up. Ali Iqtadar Mirza finalized
the methodology and did the proof reading and
supervision of the paper writing process.
Zaynah Sohail Butt assisted in performing the
mapping analysis and conducting review.
Muhammad Nawaz did technical review before
submission and helped raise the standard of
paper before submitting to the journal.
Muhammad Ameer Nawaz Khan provided
assistance in the statistical analysis and helped
in addressing the reviews.
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