A CLIMATE RESILIENT MEKONG PROJECT
TECHNICAL MEMORANDUM ON PRELIMINARY ASSESSMENT OF
SEDIMENT MANAGEMENT AT MEKONG RIVER AND ITS TRIBUTARIES:
CASE FOR BUON TUA SRAH AND BUON KUOP DAM IN VIETNAM
Submitted by: A Climate Resilient Mekong Project
Major Initial Funder:
Project Coordinator:
Prepared by: Golder and Associates
44 Union Boulevard, Suite 300, Lakewood, Colorado, USA 80228
T: +1 (303) 980-0540 ; F: +1 (303) 985-2080
PRELIMINARY ASSESSMENT OF SEDIMENT MANAGEMENT AT MEKONG RIVER AND ITS TRIBUTARIES
1.0
INTRODUCTION
This technical memorandum presents the outcome of a preliminary study of sediment management for
some of the proposed and existing dams along the Mekong River and its tributaries. The objective of the
study is to identify practical methods for passing as much sediment as possible through cascades of
dams. This study evaluates the sediment management plans for Buon Tua Srah, Buon Kuop, Sre Pok 3 and
Sre Pok 4 dams are existing dams in Vietnam.
Sediment management techniques that are considered include drawdown flushing
(Atkins, 1996),
sluicing, density current venting (Morris and Fan, 1998), dredging, dry excavation, hydrosuction sediment
removal system (HSRS) (Hotchkiss and Huang, 1995), the inline sediment collector device, and
bypassing. The Reservoir Conservation (RESCON) model (Palmieri et. al., 2003) has been applied to
assess the feasibility of some of the methods (drawdown flushing, dredging, HSRS, and excavation).
Feasibility of other methods is assessed by hand calculation and using engineering judgment based on the
reservoir type and size and other available data (i.e., sediment and flow data).
2.0
MEKONG RIVER
Mekong River Sediment Management
2
2.1
Sre Pok River, Vietnam
An existing series of hydropower dams along Sre Pok River in Vietnam consists of the Buon Tua Srah
Dam, located at the most upstream reach of Sre Pok River, with the Buon Kuop, Dray Hlinh1/2, Sre Pok 3
and Sre Pok 4 Dams downstream.
2.1.1 Buon Tua Srah Dam
The catchment area upstream of Buon Tua Srah Dam is relatively small (2,930 km 2) compared to the
catchment area of all other downstream projects (areas > 8,000 km 2).
The Buon Tua Srah reservoir is
large compared to its inflow; with a capacity inflow ratio of 0.25. Its estimated trapping efficiency exceeds
90%. Therefore, it is anticipated that majority of the sediment entering the Buon Tua Srah reservoir is
trapped. Buon Tua Srah Reservoir statistics are given in Table 21.
Table 21 Reservoir Information of proposed Buon Tua Srah Dam
Item
Reservoir capacity
Bottom width of the dam
Top level of the reservoir
Minimum bed level
Available head
Reservoir length
Mean inflow
Mean annual sediment inflow
Units
Value
tons/year
787.0
1,630
487.5
441.8
45.7
45,700
99.5
530,700
By using RESCON, flushing feasibility is evaluated as follows:
Table 22 Feasibility of Buon Kuop Reservoir Flushing
Flushing Flow
LTCR
SBR
Result
Average Flow (217 m3/s)
1.5 times of average flow
2 times of average flow
0.08
14.6
15.1
15.5
Criterion
>1.0
For all assumed flushing flow magnitudes flushing is not deemed technically feasible (LTCR is much
smaller than 0.35). The best way to manage sediment at Buon Tua Srah Dam could be by using the
inline sediment collector system, which may be installed across the river within the delta of deposited
sediment, in the upstream reach of the reservoir.
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Mekong River Sediment Management
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2.1.2 Buon Kuop Dam
The Buon Tau Srah, Buon Kuop, Dray Hlinh 1/2, Sre Pok 3 and Sre Pok 4 Dams are located in the
upstream reach of the Sre Pok River in Vietnam. The salient features of the cascade of dams are
presented in Table 27, including their trap efficiencies, the long term capacity ratios (LTCR) and sediment
balance ratios (SBR). The latter information was determined using the RESCON method.
Table 23 Salient Features of Existing Hydropower Projects along Sre Pok River, Vietnam
Dam
Buon Tua
Srah
Buon
Kuop
Dray
Hlinh 1/2
Sre Pok 3
Sre Pok 4
Catchment area, km2
2,930
7,980
8,880
9,410
9,523
Mean annual flow, m3/s
99.5
217.0
241.0
251.0
253.0
Dam height, m
83.0
34.0
7.0
52.4
25.0
Dam length, m
1,041.1
1,828.0
242.0
1,510.0
410.0
Full supply level, masl
487.5
412.0
302.0
272.0
206.0
Minimum operating level, masl
467.5
409.0
299.0
267.0
202.5
Tail water level, masl
430.5
304.2
280.8
207.5
185.0
Bottom elev. of dam, masl
440.0
390.0
290.0
241.0
188.0
Vol. of reservoir, mill m3
787.0
36.5
2.9
223.0
27.0
Reservoir length, m
35,700
2,800
2,600
13,200
5,000
Bottom width of reservoir, m
1,630.0
1,380.0
-
1,510.0
410.0
Design head, m
47.0
95.0
18.5
62.5
20.0
Hydropower design flow (m3/s)
204.9
316.0
101.0
396.6
468.9
86
280
16
220
70
Energy production, GWh
358.4
1,459
94
1,002
299.0
Date of Commissioning
2009
2009
2007
2009
2009
530,700
1,464,500
2,253,010
2,728,900
714,995
Trap efficiency (%)
95%
22%
0.0%
75%
11%
Flushing discharge (m3/s)
99.5
217.0
241.0
251.0
253.0
LTCR
0.08
0.14
-
0.23
0.46
SBR
14.6
10.6
-
4.39
60.8
Installed capacity, MW
Net sediment inflow, tons/yr
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Mekong River Sediment Management
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The information in Table 23 indicates that Buon Trau Srah (trap efficiency = 95%) and Sre Pok 3 (trap
efficiency = 75%) Dams will capture the majority of sediment discharging into them, while Buon Kuop
(trap efficiency = 22%), Dray Hlinh 1/2 (trap efficiency ~ 0%) and Sre Pok 4 (trap efficiency = 11%) Dams
will likely allow most of the inflowing sediment to pass through. Based on cost considerations it is deemed
undesirable to pass sediment through Buon Trau Srah Dam. At 13.2km the reservoir of Sre Pok 3 Dam,
the second largest in the cascade, is much shorter than that of Buon Trau Srah (35.7km). Although the
SBR for Sre Pok 3 Dam is large, the long-term capacity ratio (LTCR) is only 23% (less than the criterion
of 35%). This means that implementation of drawdown flushing at Sre Pok 3 Reservoir is undesirable.
However, it might be possible to cost-effectively dredge deposited sediment from Sre Pok 3 Reservoir for
discharge downstream. The reason for this is that the Sre Pok 3 Reservoir contains a deep canyon-like
feature, which embraces the original river course. The embankment dam is located at the end of this
canyon (Figure 14). The confining nature of the canyon will force the majority of the sediment flowing into
the reservoir to deposit within it. This configuration may effectively limit dredging to the extents of the
canyon-like feature.
The opinion is expressed that the most cost-effective way to pass sediment through the Sre Pok River
cascade of dams will likely consist of dredging deposited sediment from Sre Pok 3 Reservoir and,
preferably, discharging it downstream of Sre Pok 4 Dam. This approach may result in a significant portion of
the sediment flowing in the Sre Pok River, downstream of Buon Trau Srah Dam, to be discharged
downstream of Sre Pok 4 Dam for delivery to Cambodia.
The sediment pass-through may therefore consist of about 80% of the sediment flowing into Buon Kuop
Dam to naturally pass through to Dray Hlinh 1/2. The very low trap efficiency of Dray Hlinh 1/2 will lead to
almost all the sediment flowing into it to pass through, downstream to Sre Pok 3 Dam. About 75%
(possibly more) of the sediment flowing into Sre Pok 3 Dam will deposit within it. To pass sediment
through Sre Pok 3 Dam it will be necessary to dredge a significant portion of the deposited sediment for
discharge downstream. It is desirable to discharge that sediment downstream of Sre Pok 4 Dam. The
reason for this is that the incremental catchment between Sre Pok 3 and 4 Dams are fairly small, resulting
in the incremental natural flow being small as well. It may therefore be difficult to successfully execute
drawdown flushing in Sre Pok 4 Dam if the material dredged from Sre Pok 3 Dam is discharged into that
reservoir.
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Mekong River Sediment Management
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Figure 14. Sre Pok 3 Dam and Reservoir, indicating location of deep channel.
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Mekong River Sediment Management
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3.0
CONCLUSION
In the upstream reach of Sre Pok River in Vietnam, there exist five hydropower dams in series— Buon
Tua Srah, Buon Kuop, Dray Hlinh1/2, Sre Pok 3, and Sre Pok4. None of these dams have low-level outlet
and therefore majority of the sediment is trapped in these reservoirs. By using RESCON, sediment
management option(s) for each case is studied. Buon Tua Srah is located at the most upstream reach.
The catchment area contributing to this reservoir is relatively small. It is expected that most of the
sediment entering to this reservoir is trapped. Dredging is a feasible sediment management option for this
reservoir. In addition, inline sediment collector system may be installed at the upstream reach of the
reservoir. The second dam in series after Buon Tua Srah is Buon Kuop. The reservoir length of Buon
Kuop is less than 3 km and HSRS is the most feasible option for sediment management at this reservoir.
Mekong River Sediment Management
Golder Associates Inc.
25
Sept 25, 2012
113-82352
It is logical to sluice sediment through Dray Hlinh 1/2 dam, which is a low head power plant. Next in series
after Dray Hlinh 1/2 is Sre Pok 3, which is comparatively a large reservoir. It is 13 km long and there is no
sediment passing facility from the dam at present. RESCON model shows dredging as the most effective
sediment management option. Another existing dam is Sre Pok 4, which is located immediately
downstream of Sre Pok 3. The sediment management at Sre Pok 4 depends on the strategies applied at
Sre Pok 3. If sediment is dredged from Sre Pok 3, then the dredged sediment may be added to the
system downstream of Sre Pok 4. In such a case, there may be a minimal sediment management needed at
Sre Pok 4. The best alternative at Sre Pok 4 is also dredging due to lack of low level outlet at the dam. There
are few other projects under consideration along Sre Pok River, such as Duo Xuyen upstream of Buon Tua
Srah, Lower Sre Pok 4 and Lower Sre Pok 3, both downstream of Sre Pok 4 and in Cambodia. This study
has not covered sediment management plans for these three projects.
4.0
REFERENCES
Brune, G.M., 1953. Trap Efficiency of Reservoirs. Trans. of American Geophysical Union 34(3), 407-418.
Morris, G.L., and Fan, J. 1998. Reservoir Sedimentation Handbook, McGraw-Hill Book Co., New York.
Hotchkiss, R. H., and Xi, H. 1995. Hydrosuction Sediment Removal System (HSRS): Principals and Field
Test. J. of Hydr. Res. Pp 479-489.
Atkinsin, E. 1996. The Feasibility of Flushing Sediment from Reservoirs, TDR Project R5839, Rep. OD
137. HR Wallingford.
Palmeiri, A., Shah F., Annandale, G. W., and Dinar, A. 2003. Reservoir Conservation, Volume I. The
RESCON Approach. World Bank.
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Attachment 1: Sediment Management Options for Lower Sre Pok
Dam
Name
Technique
Flushing
Flushing
(alternative)
Lower Sre
Pok
Phase I Start
(no. of years
after
commissioning)
19
Phase II
start
(no. of
years
after
commiss
ioning)
Empty
30
30
HSRS
Technically not feasible
Dredging
Immediate
Excavation
Sluicing
Sediment
Collector
71
141
Power
Reduction
(%)
Duration (days)
Phase I
Frequency
(yr)
Phase II
Frequency
(yr)
Phase I
Sediment
Removed (m3)
Phase II
Sediment
Removed (m3)
Remove
Refill
15
30
100
19
11
15,598,192
9,255,432
‐
365
‐
0
1
1
841,403
841,403
‐
730
‐
100
71
70
58,898,203
58,898,203
May not feasible flushing is the feasible
May not be appropriate because system is too large for this
technique
Attachment 2: Sediment Management Options for Buon Kuop Reservoir
Dam
Name
Buon
Kuop
Technique
Phase I Start
(no. of years
after
commissioning)
18
Phase II
start (no.
of years
after
commissi
oning)
25
Power
Reduction
(%)
Duration (days)
Empty
Remove
Refill
15
3
15
100
15
2
15
100
(yr)
Phase I
Sediment
Removed
(m3)
Phase II
Sediment
Removed
(m3)
7
2,722,150
2,013,656
Phase I
Frequency
Phase II
Frequency
(yr)
18
Flushing
Flushing
(alternative)
immediate
HSRS
immediate
1
‐
365
‐
0
1
1
261,456
261,456
Dredging
immediate
1
‐
365
‐
0
1
1
335,160
335,160
Excavation
45
89
‐
180
‐
100
37
37
12,400,915
12,400,915
Sluicing
Sediment
Collector
immediate
1
‐
365
‐
0
1
1
205,030
205,030
1,464,500
Attachment 3: Sediment Management Options for Buon Tua Srah Reservoir
Dam
Name
Buon
Tua Srah
Technique
Phase I Start
(no. of years
after
commissioning)
Phase II start
(no. of years
after
commissioning)
Flushing
Flushing
(alternative)
Technically not feasible
HSRS
Technically not feasible
Dredging
Excavation
Sluicing
Sediment
Collector
Power
Reduction
(%)
Duration (days)
Empty
Remove
Refill
Phase I
Frequency
(yr)
Phase II
Frequency
(yr)
Phase I
Sediment
Removed
(m3)
Phase II
Sediment
Removed
(m3)
Technically not feasible
immediate
1
‐
365
‐
0
1
1
414,382
414,382
299
338
‐
730
‐
100
299
38
663,099,644
678,846,177
1
‐
365
‐
0
1
1
74,298
74,298
Technically not feasible
immediate
Attachment 4: Sediment Management Options for Sre Pok 3 Reservoir
Dam
Name
Technique
Flushing
Flushing
(alternative)
HSRS
Sre Pok 3
Dredging
Excavation
Sluicing
Sediment
Collector
Phase I Start
(no. of years
after
commissioning)
Phase II
start (no.
of years
after
commissi
oning)
9
14
Power
Reduction
(%)
Duration (days)
Empty
Remove
Refill
15
3
15
100
15
3
15
100
(yr)
Phase I
Sediment
Removed
(m3)
Phase II
Sediment
Removed
(m3)
9
5
12,318,725
7,089,153
Phase I
Frequency
Phase II
Frequency
(yr)
Technically not feasible
immediate
1
‐
‐
‐
0
1
1
1,417,831
1,417,831
299
338
‐
730
‐
100
299
38
60,966,717
60,966,717
Technically not feasible
Technically not feasible due to large river size
Attachment 5: Sediment Management Options for Sre Pok 4 Reservoir
Dam
Name
Technique
Flushing
Flushing
(alternative)
HSRS
Sre Pok 4
Dredging
Excavation
Sluicing
Sediment
Collector
Phase I Start
(no. of years
after
commissioning)
Phase II
start (no.
of years
after
commissi
oning)
68
81
Power
Reduction
(%)
Duration (days)
Empty
Remove
Refill
15
3
15
100
15
3
15
100
(yr)
Phase I
Sediment
Removed
(m3)
Phase II
Sediment
Removed
(m3)
68
13
4,954,190
1,295,426
Phase I
Frequency
Phase II
Frequency
(yr)
Technically not feasible
immediate
1
‐
‐
‐
0
1
1
99,648
99,648
88
176
‐
730
‐
100
88
88
8,769,039
8,769,039
Technically not feasible