This presentation is part of the ProSPER.Net Young Researchers' School 2017 ‘Water Security for Sustainable Development in a Changing Climate’.

1. SALTWATER INTRUSION ON THE MAIN RIVERS
UNDER THE IMPACT OF CLIMATE CHANGE
Associate Professor. Nguyen Thi Bay

2. AGENDA
1. INTRODUCTION
1.1. Introduce saltwater intrusion
1.2. Impacts of saltwater intrusion
1.3. Studies of saltwater intrusion
2. MODEL TO CALCULATE SALINITY IN THE MAINS RIVERS
3. CASE STUDY IN DONG NAI SYSTEM

3. INTRODUCTION

4. Wind
Sea
SaltwaterBrackishwaterFreshwater
River
River mouth
Introduce saltwater intrusion

5. • 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

7. • 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

8. SOLUTION
FINITE DIFFERENCE
FINITE ELEMENT
FINITE VOLUME
Studies of saltwater intrusion

9. CALCULATING MODEL

10. • 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

11. Mike 11 model
q
x
Q
t
h
B 





02
2














RAC
QQ
g
x
h
gA
A
Q
xt
Q

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

12. • 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+1Cross section i-1
Space
Time
Reach
Solution

13. • 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+1i-1
Space
h
1
h3
h5
h
7
2
4
6
Q
Q
Q
Center
point
Solution

14. • 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

15. 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)

16. 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
𝑣𝛥𝑡
𝛥𝑥
≤ 1 to 2
𝐶𝑟 =
𝛥𝑡 𝑣 + 𝑔ℎ
𝛥𝑥

17. Advection-dispersion equation
qCAKC
x
C
AD
xx
QC
t
AC
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)

18. CALCULATION MESH
Upstream boundary
Downstream boundary
Hydraulic boundary in Mike 11 model decide accuracy of
simulating result

19. USING MIKE 11 MODEL TO CALCULATE
SALILITY IN THE MAINS RIVERS OF
DONG NAI SYSTEM

20. Boundaries and calculation mesh
Hydraulic 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.

21. Verifying model

22. Verifying model

23. Validating model

24. Validating model

25. Manning coefficient
River
Manning
coefficient
River
Manning
coefficient
Dong Nai 0,035 Thi Vai 0,022
Sai Gon 0,033 Soai Rap 0,022
Nha Be 0,032 Dinh Ba 0,020
Long Tau 0,026 Vam Co Tay 0,028
Dong Tranh 0,021 Vam Co Dong 0,028
Dong Mon 0,020 Vam Sat 0,020
Buong 0,030 Go Gia 0,020
Be 0,033 Ben Luc 0,031
Phu Xuan 0,021 Rach Chiec 0,033

26. Verifying Advection-
dispersion

27. Validating Advection-
dispersion

28. Dispersion parameter
River 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

29. 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.

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

31. Saline boundary 1 and 2

32. Sceanrio 2013
SB 1: at the HAB upstream : 0,5 km
SB 2: at the HAB downstream : 9km
Hoa An Bridge
Cat Lai station

33. Hoa An Bridge
Cat Lai station
Sceanrio 2020
SB 1: at the HAB upstream : 5,5 km
SB 2: at the HAB

34. 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

35. Saline boundary 3

36. Scenario 2013 :
SB3: at the downstream of
Hoa An bridge :19km
Hoa An Bridge
Cat Lai station

37. Hoa An Bridge
Cat Lai station
Scenario 2020 :
SB3: at the downstream of
Hoa An bridge :8km

38. Hoa An Bridge
Cat Lai station
Scenario 2030 :
SB3: at the downstream of
Hoa An bridge: 5km

39. Saline boundary 4 and 5

40. 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

41. Hoa An Bridge
Cat Lai station
Sceanrio 2020
SB4: at the HAB downstream: 24km
SB5: at the HAB downstream:
33,5km

42. Hoa An Bridge
Cat Lai station
Sceanrio 2030
SB4: at the HAB downstream: 21km
SB5: at the HAB downstream:
32,5km

43. Saline boundary 6 and 7

44. Hoa An Bridge
Cat Lai station
Sceanrio 2013
SB6: At CLS downstream: 39,5km
length: 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)

45. 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)

46. 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)

47. Zone II: Long Tau river
Maximum salinity on Long Tau fluctuates from 14,8-
28‰
Zone III: Dong Tranh, Go Gia and Thi Vai through Dong Nai
province
Maximum 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.

48. 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

49. 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

50. 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

51. Year: 2013
Hoa An Bridge
Cat Lai station
SB1
SB2
SB3
SB4
SB5
SB6
SB7

52. Year: 2020
Hoa An Bridge
Cat Lai station
SB1
SB2
SB3
SB4
SB5
SB6
SB7

53. Year: 2030
Hoa An Bridge
Cat Lai station
SB1
SB2
SB3
SB4
SB5
SB6
SB7

54. Long Tau
Groins (prevent
salinization)