ASSESSMENT OF SALINITY INTRUSION IN THE RED RIVER DELTA VIETNAM Le Thi Thu Hien A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering Examination Committee: Dr. Roberto Clemente (Chairman) Dr. Sutat Weesakul (Co-chairman) Prof. Ashim Das Gupta Dr.
Mukand S Babel Nationality: Vietnamese Previous degree: Bachelor of Engineering in Water Resources Engineering Water Resources University Hanoi, Vietnam Fellowship Donor: The Government of Denmark Asian Institute of Technology School of Civil Engineering Thailand May 2005 TABLE OF CONTENTS Chapter Title Page TITLE PAGE i ABSTRACT ii ACKNOWLEDGEMENTS 1H TABLE OF CONTENTS iv LIST OF TABLES Vii LIST OF FIGURES Vili I INTRODUCTION 1 1.2 Study Area Introduction 1 1. Hydraulic Constructions in Study Area 4 1. Hoabinh Hydropower Plant 4 2. Son la Hydropower Plant: On-Going construction 5 1.4 Tidal Regime and Salinity Intrusion 6 1.25 Existing Land Use 6 1.
In Coastal zone Area 8 13 Objectives of Study 10 1.4 Scope of Study 10 II LITERATURE REVIEW 11 2.1 Theoretical Study of Dispersion Coefficient and Salinity 11 Intrusion 2. Salinity Intrusion Study in Vietnam 17 2.2 Salinity Control Requirement for Irrigation and Aquaculture 18 IH THEORETICAL CONSIDERATIONS 19 3.1 Characteristics of Estuary 19 3.2 Partially Mixed Estuary 20 Chapter Title Page 3. Well Mixed Estuary jy 20 3.1 Finite Difference Tidal Hydraulics Equations 22 3.2 Finite Difference Satt Balance Equations 22 3.3 Model characteristics 23 Chapter I INTRODUCTION 1.1 Problem Identification Intrusion of salt-water in dry season is a well-known phenomenon in the Red- Thaibinh estuaries. In the rainy season from June to November the discharge of freshwater from upstream is high, the saltwater is pushed to the sea and the problem of salinity intrusion is not present.
But in the dry season from December to May of the next year the discharge of freshwater from upstream is small and the salinity intrusion problem becomes serious. In some branches of the Red river system, the distance of salinity intrusion may be up to 40 km. As the increasing of the freshwater intake for irrigation, the salinity intrusion is causes a lot of problems for irrigation, aquaculture and other economic activities due to lack of freshwater. The knowledge of the characteristics of salinity intrusion therefore is very necessary for solving the problems and utilizing the river.
Saltwater intrusion in Red river delta has been studied for several years ago. Many institutions involve research works for controlling and predicting Red-river salinity intrusion by various methods with mathematical models VRSAP (Water Resources Planning Institute, Hanoi Water Resources University), TL1, TL2 (Institute of Mechanics), the hydrological and meteorological models (Hydrological and Meteorological Services), the multivariable relational model (Institute for water resources research). However there is a lack of efficient numerical hydrodynamic models that consider effect of Hoabinh reservoir as well as calculation and prediction salinity intrusion in Red river delta. Sonla Hydropower Plant is going to built in upstream of Da River to reduce flood damage and improve irrigation in the Red River Delta.
Increasing inflow for irrigation in dry season can cause a change of salinity concentration for planning aquaculture area in Red River Delta’s coastal zone. How to supply sufficient freshwater for paddy crops while controlling salinity concentration for aquaculture area? This is an important issue to assess the entire effect of Sonla Hydropower Plant to downstream area. Moreover, global climate changes in some recent years have deep effect to hydrology condition of Red river delta. “Global Warning” could cause sea level rise 0.5 to 1 meter by the current century due to the “Greenhouse Effect’.
A rise in sea level enables salt water penetrates upstream and inland, and would threaten human uses of water particularly during droughts. To bring a reasonable operation for both electric production and saltwater prevention is urgent duty of Hoabinh reservoir in the future, finding a numerical model for simulation and prediction salinity intrusion in Red river delta for future is also very important.2 Study Area Introduction 1.1 Geographical Condition Red - Thai Binh River System is the second largest river system in Vietnam, after Mekong River. It originates from Nguy Son Mountain in Yunnan province of China. Sông Lúc Nam sônghơn sy, Sông Bir Vọng ` ` ou =vat Trai bus \\ _ Song Fig.1 Red River System in Vietnam Territory The whole basin areas occupy 169,020km” of which 86,720km” (representing 51%) are located in Vietnam’s territory as shown in Fig.
It is a population density area with high economic potential. The North Delta and Midland Region cover 14,590km” with a population of 18.56 millions in the year 2000. Asa large river basin with a complex topography including mountains and hills (covering 90% of the area), delta and coastal areas, the Red - Thai Binh river basin, hosting a diverse and more and more developed socio-economy, makes a significant contribution to the national economy.2 River Network in Red River Delta -3- LOCATION OF STUDY AREA CHINA ‘LF OF TONKIN 0 50 100 = kilometres Fig.2 Hydrological Condition The Red River Delta is in reality the delta of two river systems: the Red River System and Thai binh River System. The Red River System consists of 3 major river branches namely the Da, Lo and the Thao Rivers.
The Thaibinh River System is also comprised of 3 river branches, which are the Cau River, Thuong and the Luc Nam River as shown in Figure 1. The two river systems are connected through the Duong and Luoc rivers forming the Red and Thaibinh River Basin. SCHEMA OF RIVER SYSTEM Da Thao Lo Cau Thuong River River River River River Lucnam River Hoabinh Reservoir Viettri Sontay East Fig.4 Schema of River Network System 29 Table 1.1 Catchment’s Area and Distributed Flow of Red River Delta’s Branches Catchment’s | Area Percentage in Distributed Flow Catchments Area Red River Delta to Red and Thaibinh river (km?) (%) (%) Da _ _ 27585.3 Water resource of Red River is plentiful. Annual average volume at Sontay station is 114km? corresponding with 3643m°/s of discharge.
Inflow in Thaibinh River is less low due to upstream rivers of Thaibinh River (Cau, Thuong, Lucnam) have annual inflow very small. Total water volume of Thaibinh river at Phalai is 8.26km” (equal to 7.2% ones of Red river at Sontay station) with annual discharge is 318m’/s. Apart from inflow from Cau, Thuong and Luc Nam River, one numerous inflow is passed from Red River at downstream of Phalai through Duong River. This flow is nearly triple are compared with Thaibinh’s.
(25km* compare with 8. In addition, Thaibinh River also gets supplementary volume from Red river through Luoc River with total volume is 13 km? per year before flowing to the sea. In the dry season, water level in Red River fall down very low; in somewhere freshwater altitude of river is less than altitude of field’s surface inside the dyke. However, water resources of Red River keep in plentiful state so the lowest monthly average inflow at Sontay is 691m’/s.3 Hydraulic Constructions in Study Area 1.
Hoabinh Hydropower Plant Hoabinh Hydropower Plant was built in 1980 in the northern mountainous province of Hoabinh with assistance from the former Soviet Union. Major objectives e Flood prevent for whole Red River Delta. e Electricity generation e Water supply for irrigation to whole of downstream Red River Delta in dry season. Some characteristics of Hoabinh reservoir Surface of the reservoir F=200 km2 Length L=230 km Average width B=1 km Average depth H=50 m Volume V=9.5 billion m3 Capacity P=1,920 MW Average annual production of electricity E=8 billion KWh 30 Hoabinh Hydropower Plant has been completely constructed in 1979 with 8 electricity generation units.
It has raised the discharge of flow of Da (Black river) and Red rivers in dry season up to 400-600 m/s. The flow regulation also facilitates to put saltwater into river mouth in dry season. Sonla Hydropower Plant: On-Going Construction Sonla Hydropower Project to be constructed on the Da River, it is far from Hoabinh Hydropower Plant nearly 250 km towards upstream and about 320 km of Hanoi. The proposed Sonla Dam would be the largest dam in Vietnam.
The Sonla Hydropower Station Project will be the largest of its kind in south East Asia. Sonla Hydropower together with Hoabinh Hydropower Plant will improve Vietnam's electricity fuel mix, reduce flood damage and improve irrigation in the Red River Delta. Sonla reservoir will hold a total of 25 billion m3 of water. Together with the Hoabinh reservoir, the water volume will total 36 billion mỶ.
With the Sonla reservoir, safety discharge to Hoabinh in the dry season is 759 m°/s, raising 115 m°/s if has only Hoabinh reservoir. Electricity of Vietnam (EVN) plans begins construction on Sonla Hydropower Plant late 2005. First turbine expected operable 2012, the entire of construction expected compliable in 2015. Major objectives e Energy production: 14.16 billion KWh/year e Regulation flood stream: very important for Hoabinh Dam and downstream areas, including Hanoi (ensuring water level in Hanoi during flood season not to exceed 13 m).
e Water supply: providing to the Red River Delta about 6 billion m3; during dry season will ensuring a sanitary run-off of 300-600m3/sec e Creating new opportunities for regional socio-economic development. Some characteristics of construction Normal water level: 265 m Dam height: 177 m Volume of reservoir: 25.4 billion m3 Surface of reservoir: 440 km2 Installment capacity: 3.600 MW 31 có iS ov YY? # 4% wh ` AK "=> \ & S; l 1a @S> b Hồ Sơn La # hi, Hồ Hoa Bình Fig.5 Location of Sonla and Hoabinh Reservoirs 1.4 Tidal Regime and Salinity Intrusion The mixing of fresh and marine waters also is accelerated by tidal action. The tidal regime in this area is irregularly diurnal, but is more regularly diurnal upstream. The maximum tidal range along the coast of the Delta is approximately 4 m.
The tidal transfer speed in the river mouth approaches 95-150 cm/sec. and the tidal influence extend 150-180 km from the river mouths (Source: Nguyen Ngoc Thuy, 1982). Due to low terrain and improved river mouths so much, seawater and salinity are easy to go Red River Delta in almost of annual. In Thaibinh River, low river bottom datum, large estuary and upstream inflow create a good condition for severe saltwater intrusion up far from the sea to Lucnam, Cau and Thuong River.
In the Red River, distance of saltwater intrusion was recorded at location which is 10 km far from Hanoi station above and 185 km far from the sea. Salinities increase from about 0.5 ppt in the rivers to 30. Fluctuation widely of salinity depends on the flow in the river and state of the tide. Salinity concentration l ppt can intrude about 30 — 40 km in average in the main branches with complicated characteristic.5 Existing Land Use Almost the entire delta has been reclaimed for agricultural land, aquaculture ponds, forestry and urban development.
Approximately 53% of the delta is agricultural land, 6.4% is forestry land and there are only some 3.8% of permanent lakes and ponds for aquaculture as shown in Fig. In general The principal land use throughout the delta is the annual cultivation of rice, in addition to the perennial crop as main fruit species. Rice occupies around 93 percent of the total annual crop area as shown in Table 1. Corn, sweet potato and cassava followed behind.
The whole region produces about three million tons of rice per year (an average yield of 2,835 kg/ha in 1995). 32 To facilitate rice production, some 1,080 km of embankments, 34,400 km of canals, 1,310 drains, 217 reservoirs and 1,300 pumping stations have been constructed. In spite of the low salinity of estuarine water, the production of table salt by traditional measures in estuarine waters has been developed. Each year the salt fields of this area have provided North Vietnam a table salt production of 20,000 — 30,000 tons.2 Existing Land Use in 1998 (Unit: 1000 ha) Total Area Agricultural Land Forestry Land Aquaculture Land 1,266.