Luận văn Thạc sĩ Kỹ thuật: Assessment of salinity intrusion in the Red River Delta Vietnam

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..

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

LIST OF TABLES Vii

LIST OF FIGURES Vili

I INTRODUCTION 1

1.1 Problem Identification 1

1.2 Study Area Introduction 1

1.2.1 Geographical Condition 1 1.2.2 Hydrological Condition 3

1.2.3 Hydraulic Constructions in Study Area 4

4

5

1 Hoabinh Hydropower Plant

2 Son la Hydropower Plant: On-Going construction

1.2.4 Tidal Regime and Salinity Intrusion 6 1.25 Existing Land Use 6

2.1.3 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

3.1.1 Stratified Estuary 20 3.1.2 Partially Mixed Estuary 20

Chapter Title Page

3.1.3 Well Mixed Estuary jy 20 3.2 Numerical Computation 21

3.2.1 Finite Difference Tidal Hydraulics Equations 22

3.3 Model characteristics 23

Red-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.

1.2 Study Area Introduction

1.2.1 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.

Fig 1.1 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.1.1 and Fig.1.2 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.

104° E 105°R 106° E 107

Figure 1.2 River Network in Red River Delta

-3-LOCATION OF STUDY AREA

SCHEMA OF RIVER SYSTEM

Da Thao Lo Cau Thuong

River River River River River Lucnam River

Hoabinh Reservoir

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

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 8.26km*) 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.

1.2.3 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.

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.

2 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 (Source: Proceedings of the Workshop on Methodologies for EIA of Development Projects, Hanoi, July, 1999).

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

Fig 1.5 Location of Sonla and Hoabinh Reservoirs

1.2.4 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 30.0 ppt 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.

1.2.5 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 1.4 and Table 1.2.

1 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 1.3 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.

Table 1.2 Existing Land Use in 1998 (Unit: 1000 ha)

Total Area Agricultural Land Forestry Land Aquaculture Land

1,266.3 671.8 80.9 48.7

Table 1.3 Agricultural Crop Land (Unit: 1000 ha)

Annual Crop Land

Perennial Crop Land

Total Rice Land

620.9 576.4 10.1

33

Map of Landuse in Red River Delta

Landuse.shp

(8 Agriculture

GN Aquaculture I) Hydrological Infrastructure N

HN Water

Human influence Military Area

GE Natural Forest

>) Other Forest Ww E

(9) Other Unused Area

> Rural populated area

ME Unused mountains and hills Š

EM Urban populated area

Fig 1.6 Map of Land Use in Red River Delta

2 In Coastal Zone Area

Almost coastal zone area in Red River Delta no has agricultural land and has

traditionally depended on fishing and salt production Production of catching fish is getting decreased Life of many stakeholders in the area is below poverty line.

Coastal zone has 3 different sorts of water, including fresh water, brackish water

and brine.

Brine surface: set for the exploitation of sea products Some main sea products are

bream, Chinese herring, Khoai fish, grey mullet, Vuoc fish (perch), Van shrimps, Bop shrimps, and pawns At present, the seafood catching activities are natural and being carried out on small-scale A majority of aquatic products are used in processing traditional

lines such as fish sauce, shrimp paste and seafood.

Area of brackish water surface: Being mainly available in the Red, Thaibinh and

Traly river mouths thanks to an abundant source of short-lived creature, algas and aquatic botany as natural food used in process of breeding aquatic products Thaibinh province has about 20,705ha (Tienhai district has 9,949ha and Thaithuy district 10,756ha), of which

34

15,839ha is able to breed brackish water products (Tienhai 7,179ha and Thaithuy 8,660ha),including 10,386ha of tide-water region and 5453ha of low productivity me,transplantation salt-land likely being used for breeding brackish water sea-products Atpresent, about 3,629hais tapped for breeding shrimps, crab, arca, mussel and gracilaia,

Fresh water region: The total area of aquatic products breeding is about 9,256ha, ofwhich 6,020ha has been exploited for breeding Besides, more than 3,000ha of lowproductivity hollowed

In brief, the estuaries of Red River Delta offer good conditions for aquaculture asfollows:

= Water available for aquaculture development i large, estimated at over 1,000 ha,

- Natural food sources are abundant, natural seed stock, particularly sheimp, isdiverse in species composition

- High tide level assists in the supply and drainage of water a

natural food and seed from the sea and tothe sanitation of the rearin

so the reception ofponds

= The mangroves in costal zone help protect aquaculture ponds and contribute to thesupply of aquaculture seed (crabs, shrimp and certain species of fish) and feed (molluscs,trash fish, small mangrove erabs ete.)

the early 1980s, the aquaculture farming for export in Red River Delta hasbeen encouraged and promoted by the government Furthermore, a high economic returnleads tothe widespread practice of this lucrative activity

‘There are many districts in coastal zone convert of salt fields, intertidal areas and

‘mangrove forests into aquaculture ponds with highly profitable, at least inthe short term,

Fig L7 Distiets along the Coast Having Aquaculture Production

Basing on different conditions about topography area, tidal regime, salinityconcentration and so on, the sort of aquaculture species and pond size in each regional area

is diferent

‘Table 1-4 Aquaculture Productions in Districts along The Coast (Souree data: 2002)

3s

Location Sortof Aree

Province District —

Latitude Longitude Species (ha)

ShrimpNghiahung 19°56-20°00N | 106°07-106"12E Crab 1040

Venus ClamsNam dinh

Haihau | 20°00-20°1SN | I0612-10622E | Shrimp 2000

ShrimpGiaottuy | 20710-20°20N | 1062210637E | 2957

Tiebai — 20182027N | 10627-10637E_ Shrimp 2000

‘Thai bình, Shrimp

Thaithuy | 20°24-20°37N | 106°24-106°37 2500

Mud Crab)Hai phong Tienlang 20°30-20°SSN | 106°28-106740E | Shrimp 10001.3 Objectives of Study

This study is an attempt to describe the effect of Hoabinh Hydropower Plant andSonla Hydropower Plant (going-on construction) to salinity intrusion in Red River Delta

nd the changes of the flow characteristics of the lower Red River Delta in time and space

at present and future condition

In order to archive the above requirements, the mathematical model of MIKEII isused to evaluate the characteristics of the freshwater flow and salinity intrusion based! onthe recent observed data, The result will be estimated under the conditions of sea level risedue to the Greenhouse Effect

The objectives ofthis study are as follows:

‘To estimate the longitudinal dispersion coefficients at different braches inthe Redriver delta at present

To assess the effect of Hoabinh reservoir t0 salinity intrusion in condition with or

‘without the reservoir

To assess the future-effect of Sonla reservoir to salinity intrusion

‘To forecast characteristics of flow and salinity intrusion in the Future

Chapter 11LITERATURE REVIEW3⁄1 Theoretical Study of Dispersion Coeff

2.11 Mathematical Formulas

nt and Sali Intrusion

For many years, a number of systematic attempts have been made with more or lesssuccess to correlate the intrusion of saline water with tidal characteristics on the basic ofactual observations of salinity condition in the estuaries

TAYLOR (1935) developed the turbulence theory and used the statistical approach,

40 formulate the dispersion coefficients forthe case of two-dimensional motion as follow:

wr

Dh persion coefcens in x andy dtections

cst he velo Macaton nan deen

Ra, incauo-artlation of elon andy dieons

‘A requirement is that the velocities be measured according to the Lagrangianstandpoint, However, actual data for velocity are normally obtained by measurementstaken at fixed points, that is, they are expressed in the Eulerian point of view Therefore,the TAYLOR theorem cannot be applicable to the data available in most cases, A

‘wansformation between the Eulerian and Lagrangian description of velocities was made byHAN and PASQUILL (1957), WADA et al, (1975); they suggested that the dispersioncoefficient can be expressed as follows

D, = Bu, 23)

D,=Bv eT, 4)where

wp, Vp: the Eulerian velocities fluctuation in x and y direction

a dimensionless parameter depending upon the scale of turbulence.

‘Ty : the Eulerian time scale,

KETCHUM (1951) has presented an approach to the steady state salinity intrusionproblem based on dividing an estuary into segments whose lengths are equal tothe averageexcursion of a particle of water during the flood tide, Complete mixing is assumed within

neat at high ride, and exchange coefficients are based on this assumption, As aresult of the complete mixing assumption this method is limited to steady-state studies ofestuaries where the well mixed condition is approached Estuaries of this type arecharacterized by very large rations of tidal prism to freshwater discharge and are a ratherlimited class as compared to the partially mixed estuary

ARON and STOMMEL (1951) have proposed a mixing-length theory of tidal

‘mixing as a means of treating the time average (over tidal cycle) salinity distribution in &rectangular estuary The one-dimensional conservation-of-salt equation was employed with

a convective term forthe river flow and horizontal eddy diffusivity The latter is assumed

to be equal to the product of the maximum tidal velocity at the estuary entrance, the tidalexcursion length, and a constant of proportionality By integrating the conservator

Fa

‘equations, 4 family of salinity distribution curves is obtained in terms of the distance along

the estuary divided by the total length of salinity intrusion, The results are primarily usefulasa classification of estuaries by means of a “flushing number” oblained by a best fit offield salinity measurements with one of the family of curves

They used the steady state model to study the problem of salinity intrusion equation

D=100au, en

in which:

a: radius ofthe pipe

vu: the shear velocity

‘This result was probably the best known as well as the simplest of all equationsdescribing turbulent dispersion,

FURUMOTO and AWAYA (1955) proposed a numerical model to calculatesalinity intrusion in tidal estuaries by mean of transforming the independent variable x inthe advective-dispersion equation into the storage volume V, They oblained thelongitudinal distribution of the dispersion coefficient in the estuary based on the quisteady transformed dispersion coefficient equation with the aid of the observed S-Vrelationship and fresh water ilove

THOMAS (1958) applied TAYLOR’s concept to flow in an infinitely wide two

«dimensional open channel in which the flow is described by a power-law distribution He

‘obtained a complicated functional relationship between dispersion and Reynolds number

ELDER (1959) duplicated THOMAS's effort, for assuming a logarithmic velocityprofile, obtained a remarked simple result

D=5.93hu 28)

in which his the depth of flow

PRITCHARD (1959) presented a mathematical model representing the variatio

of salinity concentration from tidal eycle to tidal eyele:

38

where

De: time-averaged over a tidal-cyele of dispersion coefficient

Dy was obtained by integrating the steady state equation corresponding to Eq (2.9)

9, Xí ^ 2.10)

The integration (2.14) with respect tox yields:

-0,5=p/45 ean

By fitting data to Bq (2.15) D', could be obtained

IPPEN and HARLEMAN (1961) used the steady state model and analyzed theresults of salinity intrusion experiment in the tidal plume of the Waterways ExperimentStation (WES) to show that

D, the dispersion coefficient a station x at the low tide

D) : the dispersion coefficient at x=0 at low tide,

Xx: O atriver mouth

[B the distance seaward from x=0 to the point where S=So at low tide,

he parameter Do is found to be correlated with a stratification parameter Gi,where:

G _rateofenergydissipationperunitmassoffiuid _ (2.12)

J rateofporentialenergygainedperunitmassoffluidHARLEMAN and ABRAHAM (1966) re-analyzed the WES data and found thatthe stratification parameter G/J was related to another parameter called “estuary number”

ED They formulated the following correlations:

5, =oos("] oa G13

9955

2-8 =0306) aw

arwhee

tidal ism, dened as the volume of wate

Fp densinetric Froude numer

a G16

[eaz

Ve

ug: maximum tidal velocity,

1h: depth at the ocean velocity

‘Ap: change of density over the entire length of the estuary,

Q¢: fresh water discharge

39

Ts tidal period,

STIGTER and SIEMON (1967) used the unsteady state diffusion equation tò

‘study the salinity intrusion in a constant width representation of the Rotterdam Waterway

‘The unsteady state diffusion equation

2 (as) S(as)-2 (0.4) aun)

They appisd boundary condiions repeating fom tidal cyle to tidal cycle, thusreiting alo t cgeaing me tuying alaty iatabulon The dspesioncocsint Wasassume to be in form:

G18)

“The value of

‘condition for salinity

FISCHER (1966-1968) made an important step in the development of methods forpredicting longitudinal dispersion coefficient in natural stream based on Taylor's theory,

He presented two ways of predicting a dispersion coefficient for a natural stream: theMethod moment and the Routing method His methods required field measurement ofchannel geometry, concentration and cross-sectional distribution of velocity,

\ any instant of time was determined by using the ocean boundary

‘The method of moment is based on the equation:

2.19)

220)

where

the variance ofthe concentration distribution with respect to distance along the

the variance of the concentration distribution with respect to time, measured at

‘fixed point in the stream,

ithe mean velocity ofthe flo

7: the time of passage of centroid af concentration

Subscripts 1 and 2 reler to the two measuring stations

Inthe Routing method, a value of Dy is assumed The validity of D may be tested

by the beginning with a measured concentration curve at a particular time, applying thetheory (o predict a concentration curve at the same later time, at which one Was actuallymeasured The comparison between the observed data and routed results demonstrates thevalidity of the predict dispersion coefficient

BOICOURT (1969) wsed the approach of Prichard to study the salinity of Upper

‘Chesapeake Bay He obtained the dispersioncoefficient by integrating equation

os

Ss fa av

aq tt Mar

pi A=——2BELLA and SCREENLY (1972) relied on the assumption that

40

KT is a constant dus

measured data

‘isthe cross-section area,

‘They derived the longitudinal dispersion coefficient, which was assumes constantduring a time period:

the lime period of (2241) and could be computed from

"

Aidt

223)

where:

M total mass of salt

‘The value of Q, A, S are measured at station

THARCHER and HARLEMAN (1972) improved the model used by Stigter and

‘Siemons (1967) They extended the problem to transient boundary condition and proposed

4 formula in which the dispersion coefficient varied with time and space:

LL length of estuary from sea entrance to head of tide

So: salinity of sea water

‘Thatcher and Harleman found that the dimensionless param

‘well with the estuary number inthe following form

K/(U,L) correlatedK

the deviation of velocity from the ro

4; the traverse mixing coefficient

tional mean velocity

LIU (1977) also suggested a similar equation as Equation (2.28)

p,=p2, 229)

where:

a coefficient

Q: the discharge ofthe river

RR: the hydraulic radius

LIU deduced an expression to estimate the coefficient

230)

YVONGVISESSOMJAI, ARBHABHIRAMA and APICHATVULOP (1978)formulated a mathematical model to investigate the effect of upstream fresh waterdischarge and tidal conditions on the salinity concentration and intrusion length along the(Chao Phaya and the Mae Klong rivers The dispersion coefficient expression suggested by

‘Thatcher and Harleman was used in this mode inthe following form:

‘© KI is equal to 600 (mJs)(ppukm) and K2 is equal to 400 (m)/5)(ppUkm)

are appropriate for the Chao Phaya river

‘© KH is equal to 100(mŸ9)/(ppUkm); K2 is equal to 200(m1/9)/(ppUkm) forthe

‘Mae Klong river

PENPAS (1979) showed that D, boing @funetion ofthe product [28 x]

632)

PRANDLE (1981) analyzed the measured data from eight estuaries and shows thaithese data could be fitted reasonable well with each of three expressions the dispersioncoefficient

233) 34) 635)

21.2 Numerical Model

‘The following are some popular numerical models of salinity intrusion that are

‘mentioned in many references

stuary Model (FWQA)4) Hydrodynamic

4

FWQA is usually called as ORLOB following the name of Dr Geral T Orlob,

‘This model was to be used in actual eases, Both set of Saint-Vanant equations anddispersion equation are solve with a consideration of tidal effects The firstpplication of FWQA was Sacramento-San Joaquin, California,

b) SALFLOW of Delf Hydraulics

SALFLOW (1987) is production of cooperation between Hydraulics Institute of.Netherlands and Mekong Committee It is one of the newest achievements in

‘numerical salinity intrusion model

‘Test model in Netherlands achieved good results and doing apply in Mekdelta

In addition, there are modules of salinity intrusion in some hydrodynamicmodel in recent year as ISIS (English), MIKEI L (Danish) and HEC-RAS (US) buthave not applied in Vietnam

2.13 Salinity Intrusion Study in Vietnam

Salinity Intrusion in Mekong Delta Project (Southern of Vietnam) in 1980 underthe Mekong Committee assistance promoted the research of salinity intrusion in Vietnam,Within the framework ofthis project, some of saltwater and salinity intrusion models werefound by Mekong Committee and Institute of Water Resources Planning and Institute of

‘Mechanics These models are used in eesearch of Mekong delta planning, in estimate effect

of anti-salinity-intrusion constructions to enlarge erop area in the dry season as well as

prediction salinity intrusion, These models have important contribution to study of salinity

‘THUY (1985) studied the characteristics of tide inthe Red River estuary He foundthat the tidal properties vary greatly from the rainy season to dry season and the

‘predominant components of tidal waves are diurnal

‘THUY (1987) applied a numerical model to study the flow in the river systemduring Mood and dry season He found that in dry season, tidal waves could propagate

‘more than 100 km upstream along main branch of the river system,

PHÚC (1990) used ID numerical model with much success However in the

‘model, te effects of density differences were not considered The data used for calibration

‘were limited and the verification of the model was not possible Moreover, data were used

in the model such as datum of all station was not possible to bring to standard altitude;ross sectional areas of river system were not measured atthe same Yeurs Thus the results

‘were very limited,

NGO (1991) based on the recorded data of salinity concentration at stations along,estuaries of the Red River System has draven some primary remarks on the characteristics

of salinity intrusion there Details of salinity intrusion in each tributary ofthe iver network

‘were not investigated,

4B

UY (1992) applied a numerical model to determine the dispersion coefficient forthe prediction of salinity intrusion in the Mekong estuarine network He found thatdispersion coefficient varies in the same manner as those of salinity concentration.

CA (1996) based mainly on two previous publications by Vu (1990) and Vu et al(1991), Using many year recorded data of salinity concentration at stations along thecestuaries, monthly-average salinity concentration at each estuary is computed The salinityintrusion length in each estuary was also estimated Details of salinity concentrationdistributions along the estuaries were studied using a numerical model of the transport anddispersion of salinity He found that in the dry season, the salinity intrusion length is aslong as 20 km in the mai river and mie than 20 km for some tributaries Inthe main riverand tributaries with high freshwater discharge, the maximum salinity concentration is

‘observes in January while for the tributaries with low freshwater discharge, the maximumsalinity concentration is observed in March,

HUNG's study (1998) of saline intrusion in the Red river delta has been alsolimsted by data used for calibration of the model and the dispersion coefficients were notaccounted for the saline gradient along to estuaries also the verification was not carried out,

AN NIEN, NGUYEN (1999) has summarized studies relating to saline intrusion inVietnam and has pointed out that at estuaries the salinity is in the range of 22-28ppt Thesaline inưusion length inthe Red river delta is not so long The distributaries connected tothe open sea is at acute angle thus bands affected by salinity are narrow with the width of12m,

‘The above studies are the first studies in some rivers without consideration ofwhole river system

23 Salinity Control Requirement for Irrigation and Aquaculture

Control of salinity concentration is primary importance in development ofaquaculture in coastal zone as well as water intake to ierigate for crop fields in the dry

According to Water Quality Standards (TCVNS943-1995) and Quality Criteria ofWater for Aquatic Life (2§TCNI71-2001); (28TCNI91-2004), salinity concentration isrequired for water intake into paddy fields and aquaculture ponds as followings:

= Gate of weirs under the dykes can be opened to intake for rice seeds fields while

salinity concentration is 1g/l With growing-paddy, maximum salinity concentration isallowed in 4g/1

- The procedure for intensive culture of Tiger shrimp assign that salinity

concentration of shrimp ponds as well as for nursery of shrimp from postlarvae 15 topost-larvae 45 is from 10 to 30 (past per thousand) (the best range: 1Sppt-25pp0)

= River water ean be used for men and livestock with salinity concentration is

‘The characteristics of flow and salinity intrusion in the rivers are governed by threepredominant factors, which control the magnitude and direction of current at different{depth and at different distances from the estuary month, The three factors are

4) The effect of tide in generating the tidal current and turbulence;

4i) The effect of upstream discharge of freshwater in producing a net seaward

transport; an

iit) The effect of gravitational forces due to density differences between the

‘upstream freshwater and downstream seawater o the sediment

The rise and fall of the tide at the mouth and the associated exchange of water

‘masses through the entrance result in the temporary storage of large amounts of sea water

in the estuary during high tide and the drainage of this water seaward dung low tide Thetotal volume so exchanged is known as the tidal prism, which, for a given estuary, isvariable only with tidal amplitude

OF significance in relation to this tidal prism is the continual inflow of fresh waterfrom upland sources which results in a volume of fresh water equal to discharge ratetotaled over the tidal period While this discharge rate may vary slowly with time, the ratio

of fresh-water volume to the tial prism has proved of value in the general classification of

‘The type of an estuary depends on the ratio of volume of seaward freshwater Ilow

in a tidal cycle and volume of the tidal prism which govern the mixing of the estuarine

‘water, The volume of seaward freshwater How in a tidal eyele is

‘A: cross-sectional area at the estuarine mouth,

Up: seaward freshwater flow velocity

‘a= “4: mean tidal velocity in a half tidal eyele

uu: maximum tidal velocity

45

are in the seaward direction, At the salt wedge, the upper fresh water flow is seaward butthe lower sea water is reversed in direction.

3.1.2 Partially Mixed Estuary

‘When the ration of volume of seavard fresh wate flow in a tidal cycle and volume of thetidal prism, Eq, 3.3, is in the range of 0.2 to 05, the estuary type is partially The tidalvelocity is strong enough to produce sufficient turbulence to induce mixing horizontally

‘and vertically between fresh and salt water, therefore, there exists no distinet interface offresh and salt water However there exists gradient of salinity in the vertical direction due

to the partial mixing

Well Mixed Estuary

‘When the seaward fresh water flow is small, during the dry season, with respect totidal low, Q, /P<0.1, the fresh and salt water are well mixed due to strong tidal action.wer this condition, there exists only a litle gradient of salinity in the vertical directionand salinities decrease progressively from the sea water atthe mouth of the estuary

In these four estuaries, stratification is strongly season-dependent resulting in partlystratified conditions or well-mixed inthe dry season when the dominant stratifying is tidalStaining, tidal advection, and bed-generated turbulent mixing The rainy season ischaracterized by stratified conditions when estuarine circulation and advection ofratification by tidal currents and river flow are the main stratifying mechanisms,

In the dry season, almost of estuaries in Red river delta is partially mixing with theration between Volume of freshwater ina tidal cycle and tidal prism as Follows

‘Table 3.1 Classification of Mixing Type of Main Braches of Red River Delta

No | River Gly | Uotins)| QP | Mixing type

1 [Bata T011 [0a | 079 | Parilly mixed

(Source of data: Center of Estuary and Coastal Engineering)

‘Therefore, applying one-dimension model to evaluate the salinity intrusion in rivers

‘of Red river delta is appropriate

‘A: c1oss section aa [m"}

1; Manning's roughness coefficient

S salinity concentration at time t's"

us ida velocity [m's"]

Q discharge [m's")

De: dispersion coefficient,

4 lan flow {ms}

Si itera salinity concentration

Both the tidal dynamics and the salt balance equations are solved using implicitFinite difference method The domain, the x1 plane, is discretized into rectangular grids ofsize Ác by Ar Value of water level (H), discharge (Q), and salinity (S) at all grid pointsare to be determined

Firstly, the water level and discharge along the river are determined from the tidaldynamics Secondly, using these water level and discharge values the salt balance equation

is solved to give the salinity along the river

‘The general outline of the MIKE 11 is used to compute the tidal hydraulies andsalinity intrusion in the Red river deta is as follows:

+ Use the finite difference formulae with Abboth and loneseu 6-point implicitscheme

+ Use the interpolate formulae to determine the intermediate grid points

3.2.1 Finite Difference Tidal Hydraulics Equations

Continuity equation — h centered

= Momentum equation ~Q centered

OF +0)

a a a2)

4

32.2 Finite Difference Salt Balance Equations

‘Tobe simple, lateral flow is clinsinated An arbitrary six point scheme constructed

& and term of salt balance equation

a osFig 36 show how the weighting coefficients are assigned The weighting coefficientsbby means of weighting factors and applicable to

—

gt tm=l

HN tsi)valsp!+s/)+ gi esi]

nan an: G9

Fig 32 Arbitrarily Weighted Six-point Compatational Molecule

33 Model characteristics

On unsteady salinity intrusion ina tidal estuary is solved usingice equation while the tidal dynamics are solved using a system of one-dimensional,

‘unsteady continuity and momentum equations based on Manning's roughness coefficient

Both tidal dynamics and salt balance equations are solved using finite difference

‘methods for implicit schemes The tidal dynamics are calibrated by varying Manning'sroughness coefficient until the model produces the water level and discharge relationships

‘observed in prototype

Assumptions

To develop and calibrate the hydrodynamic and salinity intrusion model, the

‘mathematical model has considered the following basic assumptions

1 The model is one-dimensional, and that is all quantities vary along thelongitudinal axis of the estuary

2 Vertically well-mixed salinity condition exists,

3 The seasonal lateral ouvinflow from groundwater is negligible

dimension salt

MIKE II is ä professional engineering software package for the simulation oflows, water quality and sediment transport in estuaries, rivers, iigation systems, channels,and other water bodies MIKEI is based on an integrated modular structure with a varity

‘of basic modules and add-on modules, each simulating certain phenomena in river systems.MIKE 11 includes basic modules forthe following

Rainfall - RunoffHydrodynamicsAdvection-<ispersion and cohesive sedimentsWater quality

[Non-cohesive sediment transport

‘The modules used for this study consists of hydrodynamic (HD) and dispersion (AD) modules The application the MIKE 11 model in this study takes place in

adveetion-two steps:

‘© Computation of the river flows and water level by MIKE 11-HD

‘© Computation of the river salinity concentration using MIKEII-AD

Chapter 1VDATA COLI ECTION

“The collected data involves geometry data of cross

and salinity data at stations in the Red-Thaibin River System in 199:

ections, hydrology, hydraulic

2002 and 2003.4a Geometry

‘Two sets of geometry data in 1993 and in 2000 along main branches of whole RedRiver Delta are collected The accuracy of both of topographical data is very high andreliable In this study, geometry data of cross-section in 2000 is used,

49

Hydro-topographic measurement was carried out in 2000 for 634 cross-sections

‘They cover river reaches of 1,086 km The distances between cross-sections are about Tkm

to 5km,

beEtbtggee

é

Fig 4.1 Spatial Distribution of Available Cross-Section in Red-Thaibinh River System

42 Hydrologiealand Salinity Dat

ata of water level, discharge and salinity concentration in the dry season mainlyfrom December to April of the next year at the stations along the main branches of the Redriver and Thaibinh River in 1993, 2002, 2003 are collected Table 4.1, 42, 4.3 shows thelocation of stations, the measured components of each set of data, respectively Stationriver networks of each year above are displayed in Figure 4.2, 4.3,4.4

Measured data in 1993 for Red river basin are very good for hydrodynamicsimulation, However, the measurement of salinity concentration was focus on Bulat estuarywhich is the main branch of Red river basin, Salinity concentration along the otherbranches as Day, Ninhco, Traly was not measured or measured at only one station.Therefore, these data are not enough to calibrate and verify for model

Measured data in 2003 is the newest data, However, the measured campaign waslimited in Red river basin from only Hanoi station to the sea Hence the change of

"upstream inflow of Red River Delta cannot study

‘Measured data in 2002 in all of Red River delta is the most adequate to requirement

of this study The data are both homogenous and long enough in order to be used in

‘numerical computation of salinity intrusion For this reason, hydrological and salinity datafrom Sth to 20th March, 2002 is used in this study

‘Measured campaign was carried out in the same time at all rivers from upstream toGulf of Tonkin Salinity was observed at every hour during high tide and every bihourlyduring low tide, At every station, salinty is measured at three depths (surface, middle andbottom) Fig 49 illustrates the salinity concentration at three depths at Balat(km0) station

sọ

Salinity concentration tends to increase from surface to bottom of water depth Water

levels are measured at

happens when water level is also the maximum

‘Table 4.1 Collected Data in 2003 for Numerical Model(Time of measurement: Feb/1/2003 to March/27/2003)

ety hour Fig 4.10 presents measured water level and salinityconcentration at Balat(km0) station in the Red River The maximum salinity concentra

Fig 42 Locations of Gauging-Stations in Measured Campaign by The Year 1993

Table 4.2 Collected Data in 2002 for Numerical Model

N ‘Measured

No] Rher EP | ot si Coxe | °0EAEMGI Postion | Conponens

[es Tangtie[ [aiue | H | Q S

1 [Da 40103 | 47 Tos Bini Tf toss | ãmp ff

2 thao | 25658 | 3 | Yen B: 2 | is [21a |»

3 la -44500 | 15 Và Quang 3 [tosis [aise |e |=

4 [ Booag —T H0 | 4L Thuong Cat + 8s [aro f= f=

‘ho Tan we | 10606 | 2000 |*[ }+

Tien Hoang | Mới | mm | +] | +

s | Day 126300 | 69 | Doe Bo 16 | tosas | 3015 | | +

inh Bình 1s | 10558 | 2016 | *

Phu Ly 18 | 1055 | ano | + | +Phu Le ar} 6e | anos |[*[ +

6 | Nhho | 44000 | 36 | Bian ta m0 | roar | 3 |*| |

‘Trac Phương w | tosis | 2019 | «|

7 |NamDnh | 28800 | 9 | Nam Dink 13 | I8 | 30% | * | =

BaLat (kad) | tosas | amis | |

Bala | 6M | 309 | +] | = Con Nam ww | 16638 | 2018 | *

8 Hong | 237307 151 | Neo Dong 9 | 10626 | a7 fe] | +

Vu Thun s | mm | 2020 | | + HaNoi 7 | 10551 | aor | + | « Son Ti 6 | 163 | 2109 |+| + Din Co 34 [863g | mất [*[ T*

9 |Taty 360 | 3% TT Bin 33 | 10620 *

Quyet Chien 3 | 10612 +l

1 [Tae | 69050 | 33 | Trew Duong s | 10607 a

1í [How 18000 | T9 Quan Khai 2910638 =

Dong Xuyen 2 | 10638 :Ƒ

Cat Pháo han a7 | 10620 +] is

12 | thai inh | 92100 | an | Ch zy ee "nh

Pha Lat 2s | 10617 ae

TF [Ga 2900 | 5 _[Ba Na Trang Trang) 34 [106.27 :

14 [Mai 1500 | 2 Tien Tien 3 | 10631 a

Xenh Khe(Quang Pius) 36 [106.2 =

lan 35600 | 20 trang Trang 35 | 10629 *|rị*

16 | Mi 10 | 3

17 [tai Va | 2700-13

18 | Lach Tray 42100 | 28 | Kien An 3P | I6 | má [+ |=

30 | Kinh Thay) 37200 | 25 Ben Binh 30] T63 | 2198 | [= =

21 | Han T900 | 5

Ximang Hoang Tach 31 rosa | nas ff

22| DaBae 18800 | 12 | Don Son 33 | tosas | 2100 | +] |

Do Nghị 33 | 10646 | oss [+ | +

Tay 3000 [+

| Rink Mow 3000 | 16 | An Pha 3% ee | ma pep

‘en Pha Binh án | tosai | ms ef

34 | cam 20700 | 13 CaaCem 40 | 10637 | as2 | ©

Cao Kenh 39 | toss | mse | + | +

Toa CES

3

‘Table 4.3 Collected Data in 1996 for Numerical Model

(Time of measurement 12 to 11/23/1993 and 11 10 3/17/1996)

Geographical Postion | eared

No) River | Coie] Sion — a] Rais |g s

Longtede | TaSuớelim | 1 (MaaBnh C10518 | 2049 + [+

2 TRE CÔ 1017 | 2115 +Phe | 3 |WQam toss [aise

- LEGEND

Gauging Station

River SegmentUpstream Boundery

43 Based Maps

1 Based map of Red River - Thaibinh River in ArcView of sale 1:250000 with

database management system on topography, hydrology, transport system, communes andland use for Red-Thaibinh River are collected to use in application of this study

Fig 45 Based Maps in ARCVIEW of scale 1:250000

2 Based map of Vietnam in MAPINEO environment including several basic maplayers: topographic, geologic, administrative boundaries, basin boundaries, river andstream network, transportation network, hydraulic works systems, ete They are used fordetermination of gauging-stations are showed in Fig 4.6, 4.7, and 4 8

7

58

Fig 4.8 Map of Salinity Intrusion Observation Stations

Sty Concentration athe depth we: Sora Mle si to,

‘ah Son

Fig 49 Salinity Concentration at Three Depth of Water at Balat (km0) Station

9

Salinity Concentration and Water Elcvdlon ta Lak) Staton

Fig 4.10 Salinity Concentration and Water Elevation at Balat (km) Station

40

33.1 Hydrodynamic Module

‘The HD module contains an implicit, finite difference computation of unsteadyflow in rivers and estuaries, Within the standard HD module, advanced computationalformulations enable flow overa variety of structures to be simulated

Broad-crested weirsCulverts

Regulating structuresControl structuresDam-defined structures

‘Tabulated structuresThe hydrodynamic model takes into account the main rivers and eanals in the area,The connectivity of the river systems and influence of other rivers outside the model area isidentified from observed data The river slop and flow direction is computed in the model

by considering the cross sections The flood plain depression within the model area isdefined as flood cells or storage cells, which have been connected to the main river throughthe link channel, The runoffs from catchments of the region are directed to thehydrodynamic model either as lateral inflows distributed over a reach or as point inflows

‘The area to be attached at cach point or reach has been determines examining the detaileddrainage pattern, established with the help of vopo maps

332 Advection-Dispersion Module

the AD module is based on a one-dimensional equation of conservation of mass of

4 dissolved or suspended material, ic the dispersion equation The dispersion equation is solved numerically using an implicit finite difference scheme, whichhay negligible numerical dispersion An AD simulation is carried out on the basis of resullsfrom the hydrodynamic model

advection-The movement of salt in the model depends on flow velocities and the amount of

‘mixing, which oceurs with water of differing salinity Water level and discharge

‘boundaries of hydrodynamic model must be specified as either open or closed for salinity

Times series of salinity concentrations needs to be specified in open boundaries,

‘where salt can enter or leave the model In closed boundaries, itis assumed that no nettransport of salt occurs, Usvally downstream water level boundaries are open concentration

‘boundaries where as upstream discharge boundaries ae closed,

“The program for salinity intrusion in an estuaty is describe in Fig 34

41

‘CALCULATION OFWATER LEVEL AND

DISCHARGE RY HD.

‘CALCULATION OFSALINITY BY AD:

Fig 3.3 Flow Diagram of Program for Salinity 1

34 Statistical Criterion

For the evaluation of the performance of the mathematical model comparedpreferably against measurements, some statistical parameters are often used in order to

‘obtain some quantitative measure Tor the deviations

34.1 Efficiency Index or Coefficient of Efficiency

Efficiency index can be used for the measuring the efficiency or accuracy of therode

‘iis observed (measured) data at tie

¥ is mean value of observed data ữ Sx

‘Yi computed (predicted) data at time ï

N numberof datapoints

342 Standard Deviation (s)

where,

Y mean value of computed data ir

343 Root Mean Square Error (RMSE)

Root mean square error is used to compare the performance of two or more

‘models when used for the same data set

RMSE = 614)

4B

cov XY

R 615)

Ÿ@-ñ*ứ,~P)

mmcov XY G16)

48 - Sea Level Rise

34.1 General Introduction

A worldwide rise of sea level is among the predicted consequences of the

“Greenhouse Effect’ the global warming expected as a result of the accumulation in theEarth's atmosphere of carbon dioxide and other greenhouse gases generated by industrialand agricultural activities It has been suggested that increasing concentrations of these

‘gases will lead to a rise of between 1.5" and 4.5°C in mean atmospheric temperature duringthis century

‘Such an increase will cause expansion of the volume of near surface ocean water,and partial melting of snowfield, ice sheets and glacier, releasing water to augment the

‘oceans, thereby producing worldwide sea level rise (BATH and TITUS, 1984)

There have been various scenarios for the scale of this sea level rise, Analysescarried out by the Environmental Protection Agency in Washington led Hoffman to predictthat global sca level would rise a meter during the next 50 to 150 years, with the mostlikely scenario of an accelerating sea level rise attaining a meter a century

‘Sea level rise scenario based on the prediction from the Climatic Research Unit,fersity of East Anglia, that global sea water level will stand to 18 em higher by the year

“2030 and the prediction from the Environmental Protection Agency, Washington, that itwill rise Im by the year 2090 as shown in Figure 4.2.5 (BIRD, 1989)

AA global sea level rise of one meter would greatly, modify coastal environments,producing erosion and submergence, especially on low-lying sectors Such a rise willtenable the highest tides to reach levels of at last 1m above their present limits, allowing for

a possible increase in tide range as nearshore water deepen

Arising sea level will result in submergence and widening of the mouths of rivers

‘with increased penetration by salt water, which may invade underground aquifers A sealevel rise will also enlarge and deepen lagoons, causing erosion of fringing swamp areas,

44