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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

mrcs@mrcmekong.org 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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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

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-03

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-03

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-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

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-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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-03

-30

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-03

-30

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-03

-30

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-03

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-03

-30

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-03

-30

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-03

-30

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-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

15-1-2018

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)