vnmc mekong dam study: modelling

Greater Mekong Forum on Water, Food and Energy:

Phnom Penh, Cambodia,

21 October 2015

Study of the Impacts of Mainstream Hydropower on the Mekong Delta

Modelling

Study of the Impacts of Mainstream Hydropower on the Mekong River

Outline• Model Configuration

• Modelling of flows

– No operation

– Peak-hour operation

– Dam Break

• Saline Intrusion

• Sediment Transport

– Trapping of silt

– Flood plain sedimentation

• Water Quality

• Coastal sediments

• Summary


Model Configuration

SWAT/MIKE Basin provide inflow of water,

sediment, nutrients to mainstream Mekong

River.

MIKE11 modelling mainstream Mekong River

and Delta flows, water levels, sediment

transport, morphology and salinity intrusion.

MIKE21C modelling reservoirs, operation,

hydraulics, sediment and morphology.

MIKE21 modelling coastal hydraulics.

EcoLab based on MIKE11 modelling nutrient

transport.

Model Configuration

MIKE11

MIKE 21

MIKE11

MIKE21C

SWAT/MIKE

Basin


MIKE11 model setup - Delta

Delta Model setup

� Upstream model: Data for set-up of

MIKE 11 from Chiang Sean to Kratie

provided by MRCS.

� Downstream (delta) model: SIWRP, high

resolution and well proven,

supplemented with new bathymetric

information;

� Cambodia floodplain model: From JICA

WUP study by DHI. Well calibrated and

used extensively to study the flow in

the Mekong River and Tonle Sap Lake.

Detail updates from SIWRP setup.


Storage characteristics of the planned 11 reservoirs

compared with the Mean Annual Flow

Project

Full Supply Level

Lowest Supply Level

Active Volume

Inactive Volume

Mean Annual Flow

Mean Active

Storage Time

Operation

m m Mil. m3 Mil. m3 Mil. m3/y Days

Pakbeng 345.0 340’0 442.4 1.295.0 96,500 1.67 Daily

Luangprabang 310.0 300.0 734.0 1.311.7 100,000 2.63 Daily

Xayabuly 275.0 270.0 738.1 678.3 124,800 2.16 Daily

Paklay 240.0 235.0 383,5 967.8 130,700 1.07 Daily

Sanakham 215.0 210.0 206.1 61.7 133,800 0.56 Daily

Pakchom 192.0 190.0 440.9 656.0 141,600 1.14 Daily

Ban Kum 115.0 110.0 651,5 1.458.5 294,600 0.81 Daily

Latsua 97.5 90.0 550.0 1.000.0 294,600 0.68 Daily

Don Sahong 75.1 71.0 115.0 476.0 325,100 0.13 Daily

Stung Treng 52.0 51.0 150.8 518.0 405,800 0.14 Daily

Sambor 40.0 38.0 1,450.0 4,001.1 432,500 0.98 Daily


Longitudinal water surface profile.

MIKE11 Mainstream – With Reservoirs


Mainstream M11: Discharge Kratie - Without - With

Dams 1985-2008 - Full Supply Level Operation

(Max Power Production over 24 hours)


Dam Operation: Daily operation of power production

for 8 hours during the daytime and 16 hours filling

until next morning



Mainstream M11: Water Level at Kratie - Without

and With Dams operated 24 hours 2007 Baseline



Indicator: Areas of changes in water level

between Baseline and Scenario 1 - 2007

The expected impacts from

the 11 Mainstream Dams on

water levels in the

Vietnamese part of the

Delta are relatively small, on

average less than 2 cm.


Dam Break – Major focus on Sambor.

Reservoir Information

Dam Type Earth Rock Dam

Dam length 18000m

Dam height 56m

Full Supply level 40m

Checked Flood level 40.6m

Dead Level 38m

Crest Level 45m

Design Flood Discharge – PMF (1000 Yrs.) 80400 m3/s

Checked Flood Discharge (10000 Yrs.) 88300 m3/s

Total Storage 5206 mcm


Dam Break – Peak Discharges - Sambor.

Scenario Peak Discharge (m3/s)

Overtoppi

ng - 2000

flood

Overtoppi

ng -

Climate

change

Piping –

2000

flood

Piping –

Climate

change

S1 BFT=1hr,

BW=167m

127107 139788 - -

S2 BFT=10hr,

BW=406m

- - 113569 121240

S3 BFT=13hr,

BW=473m

- - 110058 117813

S4 BFT=13hr,

BW=676m

189879 195798 - -

S5 BFT=18hr,

BW=861m

180616 186927 - -

Highest

calculated

discharge from

Dam Break at

Sambor is

195,798 m3/s


Dam Break – Flooding in Vietnamese Delta.

The Dam Break flood peak at

Sambor is about 8 m. At

Phnom Penh around 0.6 m.

In the Delta below 0.4 m.


Salinity – Baseline CalibrationSalinity baseline model

calibrated against

measurements in the Delta.

y = 0.9636x

R² = 0.9512

0

5

10

15

20

25

30

35

0 10 20 30 40

Simula

ted

Observed

Salinity 2008

Series1

Linear (Series1)


Scenario 1 – Changes in Salinity Intrusion

Changes in salinity

levels (g/l) are

relatively small in

the Delta, around

or below 1 g/l for

2007 with daily

dam operation.


Sediment Transport

• Bed material load 10-20 %

• Wash load 80-90 %

Bed material load is the portion of the

sediment that contains material derived from

the river bed. Bed material load typically

consists of all of the bed load, and the

proportion of the suspended load that is

represented in the bed sediments. The

importance of bed material load is that its

composition is that of the bed, and the

material in transport can therefore be actively

interchanged with the bed. For this reason,

bed material load exerts a control on river

channel morphology.

Wash load (in contrast) is the material that is

not present in the river bed. It is carried in

suspension without interaction with the river

bed and thus has negligible influence on the

river channel morphology.


Sediment Transport – Baseline


Sediment Transport – Baseline

In consistency with the Baseline Mainstream

Hydrological Model covering the period up to

and including 2008, the hydropower

development up to 2008 is included in the

Baseline. A total of 30 hydropower schemes

in tributaries to the Lower Mekong

Mainstream are included in the Baseline


Baseline: Sediment Concentration

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

Se

dim

en

t co

nce

ntr

ati

on

(g

/l)

Simulated

C Rating Curve

Assumptions:• Rating curve based on DSMP data from 2011

• Simulated concetration = Clays + Silt + Fine Sand

Kratie:


Sediment Transport – Scenarios 1

Pak BengLuang Prabang

Xayabuly Paklay

SanaKham

SangThong_Pakchong

Bankum

LatSua StungTrengSambor

0

10

20

30

40

50

60

70

80

,0 200000,0 400000,0 600000,0 800000,0 1000000,0 1200000,0 1400000,0 1600000,0 1800000,0 2000000,0

Mil

l To

ns/

ye

ar

Scenario 1 - Cumulative

Deposition Silt

Scenario 1 - Transport Silt

Baseline Transport Silt

Baseline Transpoirt Clay


Sediment Modelling

Flood Plain Sedimentation (Silt and Clay downstream Kratie):


Model Approach & Parameters

Shields Curve

• Two fractions - silt and clay

• Silt constitutes 60% of TSS at Kratie

and clay 20%

• Silt constitutes 45 % of TSS at Tan

Chau and clay 55%

• Floodplain sedimentation

information and model parameter

from articles: N.V.Manh et.al 2013,

2014, 2015

• Results presented for baseline year

(2007)


Sediment Transport – Sediments measured and

simulated 2009 and 2011 Chau Doc – Tan Chau

cenarios With Dams

With all 11 planned reservoirs in operation (Scenario GA.1) the annual load to the

Delta reduces by approximately 86%. In Scenario GA.3 the reduction is about 15%


Silt Concentration

Scenario Total silt and clay transport (million tons/ per year)

2007 2008

KratieTân Châu + Châu

ĐốcKratie

Tân Châu + Châu

Đốc

Baseline 52.63 32.06 68.81 42.31

Scenario 1 (11

dams)22.41 13.88 24.98 15.30


Tonle Sap - Clay Deposition – Baseline & Scenario 1_2007-08

Baseline

Scenario 1


Nutrients - Baseline

Conceptual

outline of nutrient

model – Example

Phosphorus.

The Nitrogen

model is similar,

however with 2 in-

organic state

variables, nitrogen

and ammonium/

ammonia


Nutrients – Baseline – Model calibration

The phosphorus

and nitrogen

models have been

calibrated against

the sediment and

nutrient survey

obtained for 2014

as part of the

MDS Additional

studies providing

unique insight into

the sediment and

nutrient relations

in the Mekong.


Scenario 1 – Changes in Annual N-deposition

Deposition of

sediments and

their contents of

nutrients.

gN/m2/year


Nutrient Indicator: Transport to the Delta

Annual sediment deposition distribution in the

Vietnamese part of the Mekong Delta. Scenarios

GA.1, GA.3, GA.4 and GA.8.b

GA.3

GA.4 GA.8

Nutrient (Attached to Silt & Clay)

(1000 tons/ per year - 2007)

Kratie Tân Châu + Châu Đốc

Phosphorus (P)

Baseline 19.1 11.6

Scenario 1 (11 dams) 10.1 6.3

Nitrogen (N)

Baseline 48.7 29.6

Scenario 1 (11 dams) 21.2 12.9

Potassium (K)

Baseline 290.0 176.1

Scenario 1 (11 dams) 123.4 75.0


2D/1D Coastal models overview


Sediment plume: Annual average (silt)

Baseline

Scenario 1

Change


Summary• Model Configuration

• Modelling of flows

– No operation

– Peak-hour operation

– Dam Break

• Saline Intrusion

• Sediment Transport

– Trapping of silt

– Flood plain sedimentation

• Water Quality

• Coastal sediments

• Summary

• Flows: Relative modest impact

except immediately d/s dams and in

case of dam break

• Saline intrusion: Some impact

depending on operation

• Sediment Transport: Significant

impact in particular for silt (lower

concentration and reduced flood

plain sedimentation

• Water Quality: Significant reduction

of nutrients

• Coastal sediments: Reduced

sediment concentration mainly in

the vicinity of estuary areas.

vnmc mekong dam study: modelling

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