3rd Mekong Forum on Water, Food & Energy 2013. Presentation from Session 9: Managing the impacts of dams across cascades.

1. IMPACTS OF LANDUSE CHANGE ON SEDIMENT
TRANSPORT IN THE YALI RESERVOIR CATCHMENT
Tarek Ketelsen, Luke Taylor, Mai Ky Vinh, Truong Hong
CHALLENGE PROGRAM ON WATER & FOOD IN THE MEKONG:
Optimizing the management of hydropower cascades at the catchment level (MK3)

2. Land management options for
reservoir benefits
1. Soil conservation
2. Enhancing reservoir life
UPSTREAM
RESERVOIR
Watershed
communities/
water user
sectors
DOWNSTREAM
Water use options of reservoir for catchment
benefits
1. Irrigation potential
2. Flood control
3. Fish passage requirements
4. E-flow
2

3. Yali
Reservoir, Viet
nam
Trung Nghia (TN)
sub -catchment
Kon Tum (KT)
sub -catchment
• What is the sediment
inflow to Yali?
• How do historic
sediment loads vary
with changes in
flow/rainfall and land
use?
• What are the
implications for
reservoir life?
3

4. Catchment suspended
sediment yield (SSY)
Proportion of catchment erosion which ends up
in the waterways
Global: (Vanoni, 1975)
10 – 20,000 t/km²/yr
Mekong: (Kummu, 2010)
18 – 489 t/km²/yr
3S Basin (Kummu, 2010)
220 – 240 t/km²/yr
Yali (KT & TN observation data 1990 – 2009)
79 – 1,051 t/km²/yr
4

5. Large variability in Yali catchment SSY
2009: Ketsana
1996: Wukong
5

6. Long term trends in SSY
Annual
increment in
SSY in 2005-09
is ~ double the
90-97 rate
6

7. Drivers of trends in SSY
A. Changes in
rainfall/flow regime
B. Changes in land use
Source: IWMI, 2002
7

8. A. Long term trends in Hydrology
High variability but no
clear increasing trend
8

9. B. Trends in land
use
9

10. Estimating Yali
sediment inflow
Trung Nghia (TN)
sub -catchment
1. PECC1 official design
eqn.
–
Kon Tum (KT)
sub -catchment
Based on long time series of
discharge at KT
2. PECC1 empirical
catchment SSY
relationships
–
–
based on long time series
discharge data (1960s-1990s) at
KT and TN
Scaled based on ungauged area
3. Observed sediment
loads KT, TN (1990-2009)
–
Scaled based on ungauged area
10

11. Estimated Yali sediment inflow
Suspended
sediment
yield
(Mt/yr)
SL Yali
(Catchmnent ) Ave
coarse
sediment
yield
(Mt/yr)
Suspend
SL (tot)
ed Load Bed load Yali
(MCM) (MCM) (MCM)
2.258
0.452
1.829
0.29
2.12
6.695
1.339
5.426
0.86
6.29
0.712
0.142
0.577
0.09
0.67
SL Yali (Empirical)
SL Yali (Design
Calc.)
2.030
0.406
1.645
0.26
1.91
2.139
0.428
1.733
0.27
2.01
SL Yali (basin SSY)
2.423
0.485
1.964
0.31
2.28
SL Yali
(Catchmnent ) Max
SL Yali
(Catchmnent ) Min
Note:
(1) Bed load was estimated based on PECC1 estimate:
Bed load = 0.2* suspended load
11

12. Yali Reservoir trapping
efficiency
• Brune’s method (1953) based on residence
time of reservoir
• Applied previously in the Mekong
• Sesan TE: 0.851 – 0.95 (Kummu, 2010)
• Yali: 0.83
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13. Yali annual sediment deposition
dynamics
SL(res)90-97 = 1.38 mcm
SL(res) = 1.67 mcm
SL(in) = 2 mcm
SL(res)05-09 = 3.13 mcm
SL(out) = 0.33
mcm
13

14. Next (final!) steps
1. Explore sensitivity of Yali sediment inflow to land use change
using the Revised Universal Soil Loss Equation
–
–
Maximum catchment erosion (aggressive deforestation scenario)
Minimised catchment erosion (aggressive afforestation scenario)
2. Estimate changes in reservoir half life based on change
upland management
14

15. Thank you for your attention

16. Sediment delivery ratio
•
•
Sediment transport cycle more
complex and slower than water
cycle
Signification proportion stored
in catchment:
–
–
–
–
•
Hillslopes
Gullies
Microtopographic features
Human structures (check dams,
reservoirs)
SSY = <10% of sediment
production in large catchments
(>100km²) (Annandale, 2003)
No.
I. Kon Tum city
II. Dak Gley
district
III. Tu Mo Rong
district
IV. Sa Thay
district
V. Ngoc Hoi
district
VI. Kon Plong
district
VII. Kon Ray
VIII. Dak Ha
district
VIII. Dak To
district
TOTAL
Reservoir
name
Total
Total
Main crop
under
irrigation
Rice
Rice
Total
Number of
Reservoirs
Reservoir
3
volume (m )
Total Irrigated
area (ha)
10
5
10,554,000
1,188,000
1759
198
Rice
1
750,000
125
Total
Rice
8
2,496,000
416
Total
Rice
6
3,138,000
523
Total
Rice
1
90,000
15
Total
Total
Rice
Rice & Coffee
4
19
648,000
19,875,000
108
7236
Total
Rice
15
3,048,000
508
69
% of long term mean annual inflow to Yali
41,787,000
~0.5%
16