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When differences in space are to be computed, the question is if the values in time step j or time step j+1 should be used. If the time step j is used an explicit solution is given. If the values at time j+1 are used, an implicit solution is given. An implicit solution is more stable than an explicit one and longer time step can be used. An explicit solution is simpler to program.

- 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
- 6. • 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
- 7. SOLUTION FINITE DIFFERENCE FINITE ELEMENT FINITE VOLUME Studies of saltwater intrusion
- 8. CALCULATING MODEL
- 9. • 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
- 10. 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
- 11. • 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
- 12. • 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
- 13. • 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
- 14. 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)
- 15. 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
- 16. 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)
- 17. CALCULATION MESH Upstream boundary Downstream boundary Hydraulic boundary in Mike 11 model decide accuracy of simulating result
- 18. USING MIKE 11 MODEL TO CALCULATE SALILITY IN THE MAINS RIVERS OF DONG NAI SYSTEM
- 19. 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.
- 20. Verifying model
- 21. Verifying model
- 22. Validating model
- 23. Validating model
- 24. 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
- 25. Verifying Advection- dispersion
- 26. Validating Advection- dispersion
- 27. 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
- 28. 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.
- 29. 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
- 30. Saline boundary 1 and 2
- 31. Sceanrio 2013 SB 1: at the HAB upstream : 0,5 km SB 2: at the HAB downstream : 9km Hoa An Bridge Cat Lai station
- 32. Hoa An Bridge Cat Lai station Sceanrio 2020 SB 1: at the HAB upstream : 5,5 km SB 2: at the HAB
- 33. 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
- 34. Saline boundary 3
- 35. Scenario 2013 : SB3: at the downstream of Hoa An bridge :19km Hoa An Bridge Cat Lai station
- 36. Hoa An Bridge Cat Lai station Scenario 2020 : SB3: at the downstream of Hoa An bridge :8km
- 37. Hoa An Bridge Cat Lai station Scenario 2030 : SB3: at the downstream of Hoa An bridge: 5km
- 38. Saline boundary 4 and 5
- 39. 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
- 40. Hoa An Bridge Cat Lai station Sceanrio 2020 SB4: at the HAB downstream: 24km SB5: at the HAB downstream: 33,5km
- 41. Hoa An Bridge Cat Lai station Sceanrio 2030 SB4: at the HAB downstream: 21km SB5: at the HAB downstream: 32,5km
- 42. Saline boundary 6 and 7
- 43. 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)
- 44. 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)
- 45. 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)
- 46. 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.
- 47. 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
- 48. 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
- 49. 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
- 50. Year: 2013 Hoa An Bridge Cat Lai station SB1 SB2 SB3 SB4 SB5 SB6 SB7
- 51. Year: 2020 Hoa An Bridge Cat Lai station SB1 SB2 SB3 SB4 SB5 SB6 SB7
- 52. Year: 2030 Hoa An Bridge Cat Lai station SB1 SB2 SB3 SB4 SB5 SB6 SB7
- 53. Long Tau Groins (prevent salinization)

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