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ISH0306 - Consultancy for the Development of Guidelines for Hydropower Environmental Impact Mitigation and Risk Management in the Lower Mekong Mainstream and Tributaries
Mekong River Commission
Office of the Secretariat in Vientiane 184 Fa Ngoum Road, Ban Sithane Neua, P.O. Box 6101, Vientiane, Lao PDR Tel: (856-21) 263 263 Fax: (856-21) 263 264
Office of the Secretariat in Phnom Penh 576 National Road, no. 2, Chok Angre Krom, P.O. Box 623, Phnom Penh, Cambodia Tel: (855-23) 425 353 Fax: (855-23)425 363
[email protected] www.mrcmekong.org
An Introduction to how Hydro- and Morphodynamic Models can help Test Effectiveness of Mitigation Guidelines and its Measures
Kees Sloff and Jenny Pronker
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What will you learn?
• Introduction to model types and characteristics for
hydrology, hydrodynamics, sediment and geomorphology
(morphodynamics) for the Mekong
• Setting-up a model: data requirements
• Modelling mitigation in the Mekong (ISH0306)
• Lessons learned, blessings and cursus, and further usage
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Modelling
• What and why modelling?
Guidelines and Recommendations for
Mitigation of Hydrological and Flow Impacts
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The Mekong pathways
Evaporation/rain
Rainfall
to runoff
of flow
and
sediment
River runoff
hydrograph modified
Hydrograph
Sand hunger
Salinity
intrusion
cascade
Tributary run off
Evaporation/rain Storage,
sedimentation
Tributary run off
Storage,
sedimentation
Tonle
Sap
Lancang
Dams
Presence and operation of reservoirs modify flows and sediment transport
All elements interact, and mitigation requires considering the entire system
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Quantification needed
• How does the system work (how much flow can we expect, where does sediment come from, etc.)?
• How big is the risk and damage due to HPPs? • Calculate the indicators, and test them (e.g., safety regulations,
trans-boundary agreements) • Calculate effectiveness of the mitigation measures (also w.r.t.
costs and benefits) • How do different schemes interact (cascade, cumulative impacts)
and how do they affect the different coupled processes (such as flow and sediment)?
• How does mitigation affect the turnover of the HPP (next)? • Etcetera …
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Models
Simple models: empirical, equilibrium states, spreadsheet
PROBLEM:
Physical models (laboratory scale models) Computational models (numerical models)
Advanced models: 1-dimensional models, catchment models
ANALOG: ANALOG:
Advanced models: high-resolution 2 and 3-dimensional models
ANALOG:
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Use of physical scale models
• Useful for local complex 3D flows (near the dam and turbines)
• Much work and time consuming
• Most problematic to do the right scaling:
- Use Geometrical scaling: ((keep the same)
- For flow use Froude scaling (ratio inertia/gravity):
- For flow use Reynold scaling (ratio inertia/viscous):
- For sediment use Shields scaling:
Pak Beng hydropower project - Overall Hydraulic Physical Model Investigation of Pak Beng HPP
Impossible to satisfy all scaling rules
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Use of computer models
• Use for various different applications from small-scale to large-scale (basin) models
• Physics represented with mathematical equations (obeying the laws of nature: conservation of mass, energy, momentum, etc.)
• Software tools require training and experience
• Easy to run scenarios, vary the settings, rerun simulations
Averaging in space: replace the varying conditions
on the smaller scale with ‘empirical relations’: this
usually reduces the accuracy of the models, but
increases the aplicability
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multiconsult.no Dimensions
Full 3D DNS (CFD):
Variations in time (unsteady)
Spatial varying in 3
dimensions
Details turbulence
Reynolds average 3D:
Variations in time (unsteady)
Spatial varying in 3
dimensions
Averaged turbulence models
(LES, eddy viscosity, k-
epsilon)
Delft3D
2DV (width average):
Variations in time (unsteady)
Spatial varying in 2
dimensions
2DH (depth average):
Variations in time (unsteady)
Spatial varying in 2
dimensions
3D processes parametrized
1D cross-section average:
Variations in time (unsteady)
Spatial varying in 1 dimension
0D basin model
(catchment models, water-
balance models)
multiconsult.no Hydrology models versus hydrodynamic models
• Used to calculate rainfall to runoff
• Usually catchment models and
simple flow routing (0-D)
• Time steps in the order of weeks to
months (output monthly average
discharge, etc)
• Does not include a detailed
physical description of fluids
• Used to calculate physics of flow
processes (velocities, water
levels,…)
• 1-D to 3-D models
• Time steps in the order of minutes to
hours
• Also includes modules for sediment,
geomorphology and water quality
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Model elements for assessing main-stream dams
Evaporation/rain
Rainfall
to runoff
River runoff
hydrograph
modified
hydrograph
Salinity
intrusion
cascade
Tributary run off
Evaporation/rain Storage
Tributary run off
Storage
Tonle
Sap
Lancang
Dams
2D/3D
2D/3D
1D
0D
0D
HP
model
HP
model
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Environmental impacts and mitigation
• What to model?
http://ariverstail.com
Hydrology, sediment yield
Reservoir sedimentation, flushing,…
Erosion, fish habitat change, water levels,…
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Build on existing DSF for LMB, plus specials
Additional or alternative components
Flood Modeller Free (250 nodes)
WUP-FIN (Tonle Sap)
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Modelling
• Setting-up models for the Mekong and its tributaries
Guidelines and Recommendations for
Mitigation of Hydrological and Flow Impacts
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Setting-up a model
• Step 1: numerical grid
1-dimensional: follows thalweg 2(3)-dimensional: cover wet area
Grid size: hundreds of meters to kilometers (approx. > river width)
Grid size: meters to several tens of meters (approx. > river depth)
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Setting-up a model
• Step 2: topography (bed levels)
1-dimensional: follows thalweg 2(3)-dimensional: cover wet area
Cross-sections prescribed (e.g perpendicular to axis)
Bed level in each grid cel
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Setting-up a model
Step 3: conditions
• Initial conditions
- Flow situation at time = 0 (start condition)
- Bed sediment composition (grain size distribution)
• Boundary conditions
- Water discharge series at inflow(s)
- Water levels at outflow(s)
- Sediment load at inflows
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Setting-up a model
Step 4: closure • 1: Bed roughness - Chezy C or Manning’s n - Bed forms - Vegetation
• 2: Sediment transport - Transport formula - Advection/diffusion
• 3: etcetera
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Baseline data used for the new sediment/flow model (Delft3D)
Mekong Delft3D model
Hydrology conditions: • Discharges for BDP2030
development (Chinese dams and tributary dams)
• Follows from hydraulic run Dec.2015 version DSF
Topography (same baseline as used for DSF): • Hydrographic atlas
(1998) (soundings navigation program)
• SRTM data (2000)
Bed composition: • MRC – DSMP
(discharge sediment monitoring pro.) results
Sediment inflow for calibration: • Historic MRC – DSMP
(discharge sediment monitoring pro.) results
Sediment inflow for reservoir runs: • MRC – DSMP predicted
sediment loads for Chiang Saen
• Tributary loads (70% trap efficiency for future dams)
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Setting-up a model
Step 5: calibration and verification
• Use historic data to calibrate the model
- Water levels and flow velocities at key-locations (for a range of discharges)
- Bed-level changes observed in time, sediment loads
- Sufficiently long periods
• Use another data set to do the verification simulation (with the settings obtained during calibration)
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Setting-up a model
Step 6: Runs
• Take care of output (make sure indicators are quantified)
• Coupling of models for hydrodynamics and hydropower
• Think about scenarios and simulation period
• Etc.
run
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Example use of model for sediment mitigation: Pushing the ‘sediment balance’ buttons
Entrainment and deposition
Inflow of sediment
Outflow of sediment
transport
High at begin of flood season
High shear stress: net entrainment Low shear stress: net deposition
Releasing through the main outlets
High velocity and turbulence: much transport Mostly sand, silt and clay
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River- en reservoir modelling Mekong ISH0306
Reservoir reach
Cross-section
1D model
3D model
Flow + sediment
Flow + sediment
• Main channel indicators for flow and sediment
• Reservoirs (first basic in 1D, correct with 3D outcomes)
• Use 1D models as backbone for the more detailed reservoir models (input/output of discharges, water levels, sediment)
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Pool level from hydropower model z(t)
Tributary Qtrib(t), Qtrib,s(t)
Outflows Qout(t)
Qout(t) and Qs (t)
Upper model boundary
Lower model boundary
Delft3D DSF
HEC-ResSim
HEC-ResSim
Example ISH0306: mitigation of risks/impacts in cascade
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0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0.0E+00 2.0E+04 4.0E+04 6.0E+04 8.0E+04 1.0E+05 1.2E+05 1.4E+05
Dis
char
ge (
m3/s
)
Hydrodynamic time (minutes)
Discharge at Chaing Saen
Example ISH0306: Simulating long term dynamics (cascade)
Pak Beng Luang
Prabang Xayaburi Pak Lay
Sanakham Vientiane
Reservoir sedimentation,
sand bars
Bed composition, grain size
(>)7 year Chiang Saen and tributary inflow
Paksane (or Pakse)
Sediment transfer
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Hydrology HP design & operation
Simulation without mitigation
Simulation with mitigation
Assessment without mitigation
Propose mitigation measures
modelling
sediment, fish,...
Power & value loss
Re
stri
ctio
ns
Ch
ange
s in
riv
er-
be
d d
ynam
ics
Op
timise
d o
pe
ration
w
ith m
itigation
O
ptim
ised
op
eratio
n
with
ou
t mitigatio
n
Modelling set up & tools
Input DSF (BDP)
hydropower modelling & Detailed reservoir models & Downstream models Optimise for
power generation with/without mitigation
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Modelling
• Example ISH0306 case study and other examples
Guidelines and Recommendations for
Mitigation of Hydrological and Flow Impacts
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Setting-up the 3D model: physics
• Xayaburi reservoir (bridge Luang Prabang to Xayaburi): Lateral sand bars
Long (days)
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Setting-up the 3D model: physics
• Luang Prabang reservoir (downstream Pak Beng): turbulence at rock outcrops
Long (days)
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Schematisation Delft3D models ISH0306
Numerical grid
• Bed level, velocities, water levels, etc. are computed in each grid cell Highest
resolution in main channel
Chiang Saen to Pakse: 6 sub models
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Mekong Delft3D model ISH0306
• The model is based on the same data as used for the DSF models
Min elevation ca 230 m
Min elevation ca 280 m
Xayaburi dam
Luang Prabang dam
Operation level ca 275 m (so upper reach is still free flowing)
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Rocky patches
Sandy beaches
Mekong River at Xayaburi
“Inflow from China”
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• Eddies form sand bars
Eddy
Delft3D
Sand bars in bed-rock channel
rock
beach
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Just downstream of Luang Prabang
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Hydraulic roughness (calibration)
Bed rock n=0.045
Alluvial n=0.025
Chiang Sean to upstr. Vientiane Upstream Vientiane to Pakse
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Sediment transport
Class Grain size Model
Gravel 4 mm Engelund and Hansen
Coarse Sand 1 mm Engelund and Hansen
Medium Sand 0.35 mm Engelund and Hansen
Fine Sand 0.125 mm Engelund and Hansen
Silt&Clay (mud) > 0.06 mm Partheniades and Krone
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Sediment transport
Annual Sediment Load at Luang
Prabang Damsite
Km 2038
Input to Xayabouri
10.7 Mt/yr
Gr: <1%
C Sand:6%
M Sand 11%
F Sand: 63%
Silt+Clay: 20%
2.4 Mt/yr
Gr: <1%
C Sand:<1%
M Sand: <1%
F Sand: 2%
Silt+Clay: 98%
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1-J
an-3
0
1-J
un
-30
1-N
ov-
30
1-A
pr-
31
1-S
ep-3
1
1-F
eb-3
2
1-J
ul-
32
1-D
ec-3
2
1-M
ay-3
3
1-O
ct-3
3
1-M
ar-3
4
1-A
ug-
34
1-J
an-3
5
1-J
un
-35
1-N
ov-
35
1-A
pr-
36
1-S
ep-3
6
Sed
ime
nt
load
(m
3/s
)
Luang Prabang, computed reference
Silt and Clay
Fine Sand
Medium Sand
Coarse Sand
Gravel
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1-J
an-3
0
1-J
un
-30
1-N
ov-
30
1-A
pr-
31
1-S
ep-3
1
1-F
eb-3
2
1-J
ul-
32
1-D
ec-3
2
1-M
ay-3
3
1-O
ct-3
3
1-M
ar-3
4
1-A
ug-
34
1-J
an-3
5
1-J
un
-35
1-N
ov-
35
1-A
pr-
36
1-S
ep-3
6
Sed
ime
nt
load
(m
3/s
)
Luang Prabang, computed with dams
Silt and Clay
Fine Sand
Medium Sand
Coarse Sand
Gravel
Scenario 0.2 BDP 2030 Scenario 1.1 Run of River without dam with dam
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Dam Operation
• Gates opened/closed automatically to control pool level according to operation rule (HP model)
• Use of PID controller
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Mitigation measures (some examples)
aerator
Diversion channel
Fish passage
resettlement
Ramping (limit water level change)
Sediment flushing and sluicing
Artificial flood
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Sediment mitigation
• Progressive flushing and sluicing
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
17001750180018501900195020002050210021502200225023002350
Ele
vati
on
(m
+ M
SL)
Chainage (km)
Min water level LLW (MRC)
Minimum bed level
Mean bed level
Operating level
Luang Prabangkm 2010
Xayaburi damkm 1930
Pak Lay (1)km 1818
Sanakhamkm 1736
Pak Bengkm 2188
340 m - 335m
310 - 315 m
270- 275 m
240 - 245 m
215 m (low) - 220 m (full)
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Some results: bed level at Pak Beng reservoir
-1
-0.5
0
0.5
1
1.5
2
2.5
3
-20020406080100120140160
Be
d-l
eve
l dif
fere
nce
(m
)
Distance from dam (km)
Pak Beng reservoir(smoothened bed-level sedimentation/erosion, long term)
Without dam, 28 years
With dam, 10 years
With dam, 15 years
With dam, 20 years
With dam, 28 years
Dam
Ero
sio
nSe
dim
enta
tio
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Some results: flushing (dynamics)
215
220
225
230
235
240
245
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
400.0
450.0
500.0
01
-03
-30
02
-03
-30
03
-03
-30
04
-03
-30
04
-03
-30
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-03
-30
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-03
-30
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-03
-30
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-30
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-03
-30
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-03
-30
10
-03-
30
Wat
er
leve
l at
the
dam
(m
)
Sed
ime
nt
con
cen
trat
ion
(m
g/l
)
Date
Pak Lay reservoir, increased draw down flushingSediment outflow
Silt&Clay
Fine Sand
Medium Sand
Coarse Sand
Gravel
Water level dam
0.5 m/hr 0.2 m/hr
215
220
225
230
235
240
245
0.0
500.0
1,000.0
1,500.0
2,000.0
2,500.0
01
-03
-30
02
-03
-30
03
-03
-30
04
-03
-30
04
-03
-30
05
-03
-30
06
-03
-30
07
-03
-30
08
-03
-30
09
-03
-30
09
-03
-30
10
-03
-30
11
-03
-30
12
-03
-30
13
-03
-30
14
-03
-30
14
-03
-30
Wat
er
leve
l at
the
dam
(m
)
Sed
ime
nt
con
cen
trat
ion
(m
g/l
)
Date
Pak Lay reservoir, unconstrained draw down flushingSediment outflow
Silt&Clay
Fine Sand
Medium Sand
Coarse Sand
Gravel
Water level dam
Ramp down 0.5 m/hr Ramp down >2 m/hr
Full draw down (to 225 m) with 0.5
m/hr ramp down; 0.2 m/hr ramp up
Duration 10 days
Full draw down (to 229 m) with no
constraints to ramping
Duration 14 days
Inflow
upstream
(∙103 ton)
Outflow dam
(∙103 ton)
Net (∙103
ton)
Inflow
upstream
(∙103 ton)
Outflow
dam (∙103
ton)
Net (∙103
ton)
Gravel 1.5 1.5 0.0 2.2 2.1 0.1
Coarse Sand 5.9 9.2 -3.2 8.6 9.2 -0.6
Medium Sand 17.5 28.4 -10.8 25.5 55.1 -29.6
Fine sand 53.4 132.1 -78.7 77.6 280.8 -203.3
Silt/Clay 351.4 456.8 -105.4 499.3 1,575.9 -1,076.6
Total 429.8 628.0 -198.2 613.2 1,923.2 -1,309.9
Qmax=9743 m3/s Qmax=18520 m3/s
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Some results: shear stress during flushing
During normal operation or after full draw down
During a rapid draw down
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Draw down flushing simulation
0.1 m/hr
0.2 m/hr
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Example Taiwan: turbidity current venting
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Power outlet converted to low level sediment sluice (2010)
Release clear water from surface spillways to dilute turbid outflow
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Shape of the Shihmen Reservoir
and (planned) sluices/tunnels
Dahan River
TDR monitoring
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2DV model (Delft3D Open Source)
50 layers Dx = 200 m
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Sediment concentration 2DV
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Conclusions from the Taiwan modelling work
• Sluicing tunnel has highest 𝑉𝑒𝑓𝑓
• Location is important
• Reservoir operations as well
• Delft3D (RANS) able to capture turbidity currents
in Reservoirs
• Good comparison with TDR (concentration
measurements)
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Modelling
• Final comments, lessons learned and take-home messages
Guidelines and Recommendations for
Mitigation of Hydrological and Flow Impacts
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Interpretation and uncertainty
• Principle of ‘least astonishment’
• Model is as good as its input: ‘garbage in = garbage out’
• Ockham’s raiser: simple theory is best
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Some lessons learned
• System approach needed (coupled basin model, 1D model and 2D/3D models) for the larger picture and interactions between processes and reservoirs
• Integrate detailed 2D/3D models with larger-scale 1D and basin models
• Accuracy of models:
- Limited by the data availability
- ‘Fit for purpose’ means that tools are adequate for making the right decisions
- Remaining uncertainty translates to adaptive approach and monitoring program
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Some more lessons learned
• Monitoring is an essential part of measuring success of the measures
• Mitigation can cause new problems
• Modelling is a powerful tool if sufficient data is available to calibrate the model
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Concluding remarks
• For reservoir sediment management it is necessary to consider the full river system (all pathways of sediment, pulses, etc.)
• Sediment management in a cascade will benefit from joint operation
• Flushing and sluicing is modeled best with a 2DH or 3D model (flushing channel, dynamics of draw down, etc.)
• Hard flushing can be damaging for the downstream: use full dynamic model
• Models can be used to simulate the impacts of sediment augmentation / nourishments, such as the placement of sediment dikes
• Allow time for analyzing the uncertainties in the model (sensitivity analyses, stochastic modeling)
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‘Take home’ message: use model as a tool
• Models are not an exact copy of reality, just an approximation
• Future conditions are uncertain, so they cannot be exact
• Do not forget their purpose
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Questions?
Nam Gnoung Dam (Theun-Hinboun)