SALTWATER INTRUSION ON THE MAIN RIVERS UNDER THE IMPACT OF CLIMATE CHANGE

Associate Professor. Nguyen Thi Bay

AGENDA

1. INTRODUCTION1.1. Introduce saltwater intrusion1.2. Impacts of saltwater intrusion1.3. Studies of saltwater intrusion

2. MODEL TO CALCULATE SALINITY IN THE MAINS RIVERS 3. CASE STUDY IN DONG NAI SYSTEM

INTRODUCTION

Wind

Sea

SaltwaterBrackishwaterFreshwater

River

River mouth

Introduce saltwater intrusion

• Agricultural production: Salinity affects production in crops, pastures and trees by interfering with nitrogen uptake, reducing growth and stopping plant reproduction.

• Water quality: The most significant off-site impact of dryland salinity is the salinization of previously fresh rivers. This affects the quality of water for drinking and irrigation—with serious economic, social and environmental consequences for both rural and urban communities.

• Ecological health of streams: Salt interacts with in-stream biota (animals and plants), changing the ecological health of streams and estuaries. The greatest threat to biodiversity is from the loss of habitat—both on land and in water.

• Terrestrial biodiversity: Much of the natural vegetation of salt-affected areas has been destroyed or damaged. This has caused major changes to the landscape and biodiversity including the destruction of remaining natural habitat in many agricultural areas and the fragmentation of many wildlife corridors.

• Irrigation: All irrigation water contains some salts, which may remain on the soil surface or on leaves of plants after evaporation. Therefore, any irrigation system has the potential to deliver an increased amount of salt to the soil.

Impacts of saltwater intrusion

• Some mathematical methods and water modelling software have been strongly developed and have become commercialized with high technology and contribution by famous scientists, allowing fast and economically approaches of finding the optimal solution for water engineering.

• The domestic software was easy to update, train, transfer, adapt and reflect more correctly the actual conditions in Vietnam as evidenced by the practical application of VRSAP-SAL, KOD, MIKE models, etc. For example, in scientific researches, several tasks can be named, such as planning/ managing the water resources and disaster prevention, environmental protection.

• To improve a modern model product, it is necessary to study and apply the exact standard equations and strong algorithmic solution to obtain accurate numerical results, as well as to improve computational time and also the capability to simulate a large modelling network. Many scientists in Vietnam built mathematical models to calculate saltwater intrusion such as: Nguyen Tat Dac, Le Song Giang…

Studies of saltwater intrusion

SOLUTION

FINITE DIFFERENCE

FINITE ELEMENT

FINITE VOLUME

Studies of saltwater intrusion

CALCULATING MODEL

• Main modules • Rainfall-runoff

• NAM, UHM

• Hydrodynamics• governing equations for different flow types

• Advection-dispersion and cohesive sediment • 1D mass balance equation

• Water quality• AD coupled for BOD, DO, nitrification etc

• Non cohesive sediment transport• transport material and morphology

Mike 11 model

Mike 11 model

qxQ

thB

02

2

RAC

QQg

xhgA

AQ

xtQ

Saint – Venant equation system:

Continuity equation

Momentum equation

Q - discharge, m3 s-1

A - flow area, m2

q - lateral flow, m2s-1

h - depth above datum, m C - Chezy resistance coefficient, m1/2s-1

R - hydraulic radius, m - momentum distribution coefficient

• Saint - Venant equations system• explicit methods• implicit methods

Time step j+1

Time step j

Time step j-1

Cross section i Cross section i+1 Cross section i-1

Space

Time

Reach

Solution

• Equations are transformed to a set of implicit finite difference equations over a computational grid

• alternating Q - and H points, where Q and H are computed at each time step• numerical scheme - 6 point Abbott-Ionescu scheme

Time step n+1/2

Time step n

Time Time step n+1

i i+1 i-1

Space

h1h3

h5

h7

2

4

6

Q

Q

Q

Center point

Solution

• Boundary conditions• external boundary conditions - upstream and downstream;• internal “boundary conditions” - hydraulic structures ( here Saint Venant

equation are not applicable) • Initial condition

• time t=0

Solution

Choice of boundary conditions

• Typical upstream boundary conditions• constant discharge from a reservoir• a discharge hydrograph of a specific event

• Typical downstream boundary conditions• constant water level• time series of water level (tidal cycle)• a reliable rating curve (only to be used with downstream boundaries)

Avoiding Errors

• Hydraulic jump cannot be modelled, but upstream and downstream conditions can

• Stability conditions• topographic resolution must be sufficiently fine• time step

• should be fine enough to provide accurate representation of a wave• if structure are used smaller time step is required• use Courant condition to determine time step

• or velocity condition 𝑣 𝛥𝑡𝛥𝑥 ≤1   to  2

𝐶𝑟=𝛥𝑡 (𝑣+√ h𝑔 )

𝛥𝑥

 Advection-dispersion equation

qCAKCxCAD

xxQC

tAC

2

Where:

C : Concentration (Kg/m3 or g/l)

D : Dispersion coefficient (m2/s)

A : Cross section area (m2)

K : Linear decay coefficient (1/s)

C2 : the source/sink Concentration (kg/m3 or g/l)

q : lateral flow (m3/m/s)

CALCULATION MESH

Upstream boundary

Downstream boundary

Hydraulic boundary in Mike 11 model decide accuracy of simulating result

USING MIKE 11 MODEL TO CALCULATE SALILITY IN THE MAINS RIVERS OF

DONG NAI SYSTEM

Boundaries and calculation meshHydraulic regime of SGR is effected by

regulation of three upstream reservoirs as: Tri An reservoir on Dong Nai River (DNR) (Vinh Cuu Dist., Dong Nai Prov.), Dau Tieng reservoir on Sai Gon River (Tay Ninh Prov.) and Phuoc Hoa reservoir on Be River (Phu Giao Dist., Binh Duong Prov.).

Consequently, study area will be extended from below the three reservoirs to Dinh Ba, Long Tau, Thi Vai and Soai Rap Estuaries as Figure.

A mesh applied to Mike 11 (measurement and inheritance) includes: 79 large and small branches, 674 cross sections, 68 distributaries and tributary points. Maximum distance dx on river is 500 – 1000m and 100 – 200 m for minimum dx. To decrease simulation time, the distance is smaller for the small branches and longer for large branches.

Verifying model

Verifying model

Validating model

Validating model

Manning coefficientRiver Manning

coefficient River Manning coefficient

Dong Nai 0,035 Thi Vai 0,022Sai Gon 0,033 Soai Rap 0,022Nha Be 0,032 Dinh Ba 0,020

Long Tau 0,026 Vam Co Tay 0,028Dong Tranh 0,021 Vam Co Dong 0,028Dong Mon 0,020 Vam Sat 0,020

Buong 0,030 Go Gia 0,020Be 0,033 Ben Luc 0,031

Phu Xuan 0,021 Rach Chiec 0,033

Verifying  Advection-dispersion

Validating  Advection-dispersion

Dispersion parameterRiver Dispersion coefficient (m2/s) River Dispersion coefficient (m2/s)

Dong Nai 25 Thi Vai 16

Sai Gon 25 Soai Rap 23

Nha Be 23 Dinh Ba 23

Long Tau 22 Vam Co Tay 22

Dong Tranh 18 Vam Co Dong 24

Dong Mon 6 Vam Sat 16

Buong 8 Go Gia 9

Be 25 Ben Luc 13

Phu Xuan 12 Rach Chiec 6

Salinity of saline boundary (SB) No. Value Goal Color

1 < 0,25‰SB 1(0,25‰)

Drinking water supply usage (normal treatment)

2 0,25‰ - 0,5‰SB 2 (0,5‰)

Drinking water supply usage (normal treatment)Conservation of aquatic plants and other purposes

3 0,5‰ - 1‰SB 3 (1‰)

Irrigation usage or other purposes with equivalent requirement of water quality

4 1‰ - 2‰SB 4 (2‰)

- Brackish aquacultures- Reduce salt-sensitive crop yields

5 2‰ - 4‰SB 5 (4‰)

- Brackish aquacultures- Reduce crops yields

6 4‰ - 8‰SB 6 (8‰)

-Suitable for some kinds of brackish aquacultures- Reduce salt-tolerant crop yields

7 8‰ - 18‰SB 7 (18‰)

- Suitable for some kinds of brackish aquacultures- No irrigation

8 > 18‰ Salination, unusable.

Result analysis

Area of analysis:Zone I: Dong Nai, Nha Be, Dong Mon and Buong Rivers. Zone II: Long Tau RiverZone III: Dong Tranh, Go Gia and Thi Vai Rivers

Saline boundary 1 and 2

Sceanrio 2013

SB 1: at the HAB upstream : 0,5 km

SB 2: at the HAB downstream : 9km

Hoa An Bridge

Cat Lai station

Hoa An Bridge

Cat Lai station

Sceanrio 2020

SB 1: at the HAB upstream : 5,5 km

SB 2: at the HAB

Hoa An Bridge

Cat Lai station

Sceanrio 2030

SB 1: at the HAB upstream : 9,5 km

SB 2: at the HAB upstream : 2 km

Saline boundary 3

Scenario 2013 :

SB3: at the downstream of Hoa An bridge :19km

Hoa An Bridge

Cat Lai station

Hoa An Bridge

Cat Lai station

Scenario 2020 :

SB3: at the downstream of Hoa An bridge :8km

Hoa An Bridge

Cat Lai station

Scenario 2030 :

SB3: at the downstream of Hoa An bridge: 5km

Saline boundary 4 and 5

Hoa An Bridge

Cat Lai station

Sceanrio 2013

SB4: at the HAB downstream: 32km

SB5: at the HAB downstream: 36km

Hoa An Bridge

Cat Lai station

Hoa An Bridge

Cat Lai station

Sceanrio 2020

SB4: at the HAB downstream: 24km

SB5: at the HAB downstream: 33,5km

Hoa An Bridge

Cat Lai station

Sceanrio 2030

SB4: at the HAB downstream: 21km

SB5: at the HAB downstream: 32,5km

Saline boundary 6 and 7

Hoa An Bridge

Cat Lai station

Sceanrio 2013

SB6: At CLS downstream: 39,5kmlength: 3,5km

SB7: Consist of Nha Be (the whole length of 9km), a part of Dong Nai from confluence upstream 1km (a distance of 3,5km from Cat Lai)

Hoa An Bridge

Cat Lai station

Sceanrio 2020

SB6: At CLS downstream: 39km length: 5,5km

SB7: Consist of Nha Be river, a part of Dong Nai river from confluence upstream 1km (a distance of 3,5km from Cat Lai)

Hoa An Bridge

Cat Lai station

Sceanrio 2030

SB6: At CLS downstream: 37km length: 4,5km

SB7: include the whole Nha Be river, a part of Dong Nai river from confluence upstream 3,5km (a distance of 1km from Cat Lai)

Zone II: Long Tau riverMaximum salinity on Long Tau fluctuates from 14,8-28‰Zone III: Dong Tranh, Go Gia and Thi Vai through Dong Nai provinceMaximum salinity in this area fluctuates from 18,1-33‰, Particularly the river section through Dong Nai, the maximum salinity reaches 32,1‰. Therefore the whole zone is in 8th level of salinity.

Scenario 2013

SB7: ~ 5km from confluence of Long Tau – Nha Be – Soai Rap

SB8: from lower border of SB7 to Long Tau estuary , about 4 km long inside the study area

Hoa An Bridge

Cat Lai station

Hoa An Bridge

Cat Lai station

Scenario 2020

SB7: ~ 3km from confluence of Long Tau – Nha Be – Soai Rap

SB8: from lower border of SB7 to Long Tau estuary , about 6 km long inside the study area

Hoa An Bridge

Cat Lai station

Scenario 2030

SB7: ~ 1,5km from confluence of Long Tau – Nha Be – Soai Rap

SB8: from lower border of SB7 to Long Tau estuary , about 7,5 km long inside the study area

Year: 2013

Hoa An Bridge

Cat Lai station

SB1

SB2

SB3

SB4SB5

SB6

SB7

Year: 2020

Hoa An Bridge

Cat Lai station

SB1

SB2

SB3

SB4SB5

SB6

SB7

Year: 2030

Hoa An Bridge

Cat Lai station

SB1

SB2

SB3

SB4SB5

SB6

SB7

Long Tau

Soai Rap

Groins (prevent salinization)