01/01/2014
When it comes to passing sediment
downstream, proper dam design can make a significant difference. The authors
present an alternative design for Sambor Dam in Cambodia that could create a
sustainable reservoir with more than 70% of its capacity for hundreds of years.
By
George W. Annandale, Prakash Kaini and Jennifer Patterson
Although
dams and reservoirs are needed to provide water supply, energy production,
flood control and navigation, global reservoir storage capacity is diminishing
because of sedimentation. The average rate of global reservoir sedimentation is
estimated to be 1%.1,2 Over the past 30 years, global reservoir
storage volume has decreased because fewer dams have been built and reservoirs
are gradually being filled in by sedimentation.2 Increased water
storage capacity is needed to meet growing water demand, both through
implementation of sediment management plans for existing reservoirs and by
adding more reservoirs.
Another
issue related to reservoir sedimentation is the negative environmental impact
to the downstream river ecosystem. Reservoirs behind dams alter the natural
process of sediment transport in a river by trapping sediment.1 This
adversely affects fish migration, causes river degradation downstream of the
dam, and negatively impacts aquatic habitat and biodiversity by disconnecting
the river system and the nutrient flow. There is a need to balance development
aspirations and environmental needs. Sediment management should be an integral
part of the reservoir system for the sustainable use of the resource and to
safeguard the river environment.
This
article presents the outcome of an assessment of sediment management options
for Sambor Dam, which is being planned for construction on the Mekong River in
Cambodia. This study was undertaken as part of the Natural Heritage Institute's
work, with the Cambodian government, to examine options to minimize the
negative consequences of dams in the Mekong Basin. The design described in this
article is proposed as an alternative to the current design.
Sambor
Dam is the furthest downstream among all existing and proposed dams in the
Lower Mekong Basin, and the site is located near Sambor village. The proposed
dam is 56 m high and 18 km long, with a rated head of 16.5 m. It will create a
reservoir with 4,259 million m3 of storage. The trapping efficiency
(i.e., the ratio of trapped sediment to incoming sediment) of the reservoir is
estimated at more than 50% using the Brune method.3 The proposed
installed capacity of the powerhouse is 2,600 MW, with an estimated 11,740 GWh
of annual energy production.4,5,6
The
objective of this study is to formulate a design that will improve sediment
passage through the proposed facility to maintain sediment flows in the river.
Sediment is vital to the health of the downstream fisheries in the Tonle Sap,
mainstream of the Mekong River, and Mekong Delta. Tonle Sap is the largest
freshwater lake in Southeast Asia and is ecologically critical to the local
inhabitants. Tonle Sap is unique for two reasons: its flow changes direction twice
a year, and the portion that forms the lake expands and shrinks dramatically
with the seasons, up to five times.
Sediment
passage is also an important consideration with regard to the longevity of the
hydropower facility. This is an issue of great importance to the Kingdom of
Cambodia to ensure continued and effective use of the facility once the
concession period, which is typically 30 to 50 years, is over. The Reservoir
Conservation (RESCON) model,7 which was developed by the World Bank
in 2003, was used to evaluate the feasibility of drawdown flushing in this
study.
Sediment management
There
are various sediment management techniques for reservoirs: drawdown flushing,
bypassing, dredging, density current venting, and hydro-suction removal.
However, based on the large size of the proposed Sambor Reservoir (both its
footprint area of more than 600 km2 and its volume) and its
unfavorable geometry (i.e., its width of several kilometers and low gradient),
none of these are considered feasible.
However,
as illustrated in this article, passing sediment becomes viable if the design
is modified. The two techniques facilitating sediment passage in the modified
design are drawdown flushing and bypassing. They are jointly applied.
— Drawdown
flushing: Drawdown flushing involves emptying a reservoir and then allowing
the river to erode the deposited sediment and transport it downstream.8
A reservoir is brought to the original river-like condition by releasing flows
through bottom outlets or large radial gates if a dam is low enough. Although
drawdown flushing has been successfully accomplished at dams as high as 70 m to
80 m, it is deemed desirable to reduce Sambor Dam to lower heights, as shown
later in the article.
Drawdown
flushing is usually executed during a low-flow period and preferably just
before a high-flow period, to allow the reservoir to be refilled with water
once drawdown flushing has been completed.
Criteria
exist to assess the potential for successful drawdown flushing.9 In
general, drawdown flushing is successful in narrow, steep reservoirs where the
river can migrate across the entire valley floor, thereby removing sediment
from all portions of the reservoir. To implement drawdown flushing, it is
necessary to have available low-level outlets that are large enough to drain
the reservoir (i.e., large enough to pass the flushing design flow, which can
range between one and two times the mean annual river flow) and to freely
discharge re-entrained sediment downstream.
Flushing
feasibility is determined based on two criteria: sediment balance ratio (SBR)
and long-term capacity ratio (LTCR). SBR quantifies the ability to re-entrain
and transport previously deposited sediment during drawdown flushing. SBR is
the ratio between the amount of sediment that may be removed by the water
flowing through the reservoir during drawdown flushing and the amount of
sediment that has deposited in the reservoir between flushing events. LTCR is
defined in Figure 1 on page 21 and in the following equation:
Equation
1
LTCR =
Area B / Area A + Area B
The
major criteria used in the RESCON model to assess the potential success for
sediment removal by using drawdown flushing are SBR greater than 1 and LTCR
greater than 0.35.
— Bypassing:
Two principal bypass techniques exist; one uses bypass tunnels and the other
uses river modification or existing river channels. In the case of Sambor Dam,
the use of bypass tunnels is not desirable because the local topography does
not allow for this arrangement. Tunnels require mountainous terrain, which is
absent in the vicinity of the dam. However, the width of the Mekong River at
the location of Sambor Dam extends over several kilometers, consisting of a
number of braided channels. Using some of these channels to facilitate
bypassing is possible, as demonstrated by the proposed alternative design.
Figure 2 shows
a bypass scheme designed for Nagle Dam in South Africa. The dam is located at
the downstream end of a large river meander, and its reservoir is located in
the meander bend. The river contains a high sediment load, but the reservoir
has been successfully operated since 1950 with the bypass arrangement.
Proposing an alternative design
As an
alternative to the proposed Sambor Dam design, this study revealed that a
smaller reservoir (see Figure 2) with a bypass diversion would provide valuable
benefits. Table 1 on page 21 presents the main features of the proposed and
alternative dams. The alternative design would have reduced power output, but
the reduction in construction cost arising from lowering the dam height by 5 m
and decreasing the length to 3 km (vs. 18 km for the original design) will be
significant. Additionally, the associated 80% reduction in inundated area and,
therefore, reduced resettlement requirement (a few thousand vs. about 20,000
people) has great value.
The goal
of the alternative design is to create bypass flows that exceed twice the
average flow (i.e., > 27,600 m3/sec), which corresponds to monsoonal
flows. Flows exceed 27,600 m3/sec at this site about 13% of the time
in any given year. About 28% of the total sediment load in the river will be
bypassed through such an arrangement, never entering the reservoir. This means
that 72% of the total sediment load — equaling 58,398,405 tons per year — will
pass into the reservoir, of which some portion will pass downstream. It is
estimated that use of a bypass channel alone will result in about 67.6% of the
total sediment load in the river being passed downstream of the alternative
Sambor Dam. When combining drawdown flushing with bypass, an additional amount
of sediment will be discharged downstream, equaling about 90% of the sediment
deposited in the reservoir. This will result in a sustainable reservoir that,
in principal, will indefinitely retain more than 70% of its original capacity
The
alternative design presented in this article results in a significant
improvement when compared to the originally proposed design. For the original
case, the analysis results indicate it is not hydraulically possible to remove
the average amount of sediment deposited in the reservoir when using the
average annual flow for flushing. Even if the flushing flows are increased by
50% to 100%, it is still not possible to hydraulically remove deposited
sediment satisfactorily. The reason for this is that the channel that may erode
into the reservoir bed during such flushing will be much narrower than the
total width of the reservoir. Even though the sediment transport capacity of
the water used for flushing may be high enough to transport the average amount
of sediment deposited within a year, the narrow channel will make it
geomorphologically unfeasible to remove the majority of the deposited sediment
from such a wide reservoir.
Table 2 on
page 24 presents the flushing feasibility based on the RESCON analysis for the
alternative and originally proposed designs. The low values of LTCR for the
originally proposed design (0.08 to 0.12) are evidence of this conclusion.
Interpreted differently, over the long term it will only be possible to retain
about 8% to 12% of the original reservoir volume. The rest of the space will be
consumed by deposited sediment.
Figure 4 on
page 24 relates the anticipated change in reservoir volume due to sedimentation
to the number of years after project commissioning. Stable conditions will
likely be reached in about 100 to 150 years, when the reservoir at the proposed
Sambor Dam is expected to reach a state of geomorphic equilibrium. On the same graph,
it is indicated that implementation of reservoir sedimentation management
approaches (particularly drawdown flushing and bypass) in the alternative
design will likely lead to a state of geomorphic equilibrium within about 50
years. This means that significantly more sediment will be released downstream
from the alternative design over both the short and long term. The length of
the short term is defined as the duration of the concession period. The long
term is essentially an indefinite period but practically may be set at, say,
300 years.
Conclusion
In the
authors' opinion, consideration of sediment management conditions for the
proposed Sambor Dam indicates that short-term operational problems may be less of
a concern than short- and long-term environmental and social concerns. The
anticipated disinterest of the concessionaire in sediment management due to the
fact that sedimentation will not significantly impact operations during the
concession period is not the determining factor as to whether the dam and
reservoir should be designed and operated to facilitate sediment management and
passage, the authors believe. More important, the authors feel, is the need to
preserve the long-term natural resource values provided by the Mekong River,
Mekong Delta and Tonle Sap, thereby creating intergenerational equity and
ensuring the livelihood and well-being of tens of millions of people.
Results
of the study indicate it is possible to develop a dam design that increases the
amount of sediment that can be passed through the Sambor Reservoir. But the
alternative design comes at a price, resulting in an installed hydropower
capacity of only about 65% of the originally proposed capacity. At the same
time, the alternatively proposed dam and ancillary facilities will be much
smaller than the proposed dam, thereby resulting in lower construction cost.
The
alternative design is still economical. The fact that the amount of sediment
passed downstream over the long term can be increased about 14-fold when
implementing the alternative design is worth considering. In addition to
improving sediment passage, additional benefits include better conditions for
fish migration and spawning and lesser need for resettlement.
Notes:
1. Morris, G.L.,
and J. Fan, Reservoir Sedimentation Handbook, McGraw-Hill Book Co., New
York, 1998.
2. Annandale,
George W., "Going Full Circle," International Water Power and Dam
Construction, May 2011.
3. Brune, G.M.,
"Trap Efficiency of Reservoirs," Transactions of American
Geophysical Union, Volume 34, No. 3, 1953, pages 407-418.
4.
"Feasibility Study Report of Sambor Hydropower Station, Class A
Engineering Design," China Southern Power Grid Co. Ltd., Guangzhou, China,
2008.
5. "MRC
Sea for Hydropower on the Mekong River Mainstream," Volume I- Inception
Report. International Center for Environmental Management, Sydney, Australia,
2009.
6. Meynell,
Peter-John, Mainstream Hydropower Dams in Cambodia and their Environmental
Impacts (Report for IFREDI), 2011.
7. Palmeiri,
A., F. Shah, George W. Annandale, and A. Dinar, Reservoir Conservation,
Volume I, The RESCON Approach, World Bank, Washington, D.C., USA, 2003.
8. Atkinson,
E., The Feasibility of Flushing Sediment from Reservoirs, TDR Project
R5839, Rep. OD 137, HR Wallingford, Oxfordshire, UK, 1996.
9. White, W.R.,
Flushing of Sediments from Reservoirs, Thematic Review IV5, World Commission on
Dams, 2000.
George Annandale, D.Ing., PE, is principal and program leader, Prakash
Kaini, PhD, PE, is project hydraulic engineer, and Jennifer Patterson, PH, is
senior fluvial geomorphologist with Golder Associates.
All sites are working..This is great thing..A huge thanks from my side…:)
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