prediction and countermeasures of saltwater intrusion in the qiantang estuary

www.seipub.org/awrp Advances in Water Resource and Protection (AWRP) Volume 2, 2014

62

Prediction and Countermeasures of Saltwater

Intrusion in the Qiantang Estuary Han Zengcui1, Shi Yingbiao*2, You Aiju3

Zhejiang Institute of Hydraulics and Estuary, Hangzhou 310020, China

[email protected]; *[email protected]; [email protected]

Abstract

This paper analyzed sixty years’ field data of chlorinity in

seven stations along the Qiantang estuary from 1953 to 2012.

In this period, the Qiantang estuary went through three

different stages: lower human activity intervention stage

before 1960, runoff regulation stage after the construction of

Xin’an reservoir in the upper stream from 1960, and

reclamation stage since 1980. With the social and economic

development, daily water demanded by our country,

industry, agriculture and environment were increased

rapidly. This paper analysed the temporal‐spatial chlorinity

variations in the three stages and their influencing factors.

Meanwhile, the variation of the chlorinity and number of

days over standard chlorinity also changed extensively year

to year and month to month. Based on the field data, 1D and

2D mobile riverbed numerical model had been developed by

the second author of this paper to study the saltwater

intrusion in Qiantang estuary. Optimal water discharge

mode of the Xin’an reservoir which took the tidal cycle into

consideration has been studied by the developed model, and

“large discharge in spring tide, small discharge in neap tide”

mode was proposed. The discharge mode has experienced

nearly 40 years of improvement and practice. Besides

applying the above discharge mode, in order to ensure the

security of water usage, other measures such as water

storage by channel networks, water intake moved to upper

stream, reverse regulation reservoir construction by the

estuary, negotiation mechanism among relevant authorities

were carried out. Thus, the win‐win result was achieved

between water resources saving and water supply security.

Keywords

Salt Water Intrusion; Mobile Riverbed Mathematical Model;

Predictive and Countermeasure Practice; the Qiantang Estuary

Introduction

The estuary region is densely populated and

economically developed. Domestic, industrial,

agricultural and environmental water demanded by

the estuarine plain are taken from the estuary. Due to

uneven temporal and spatial distribution of runoff,

and swift variation of strong and weak tides (marked

by the magnitude of the tide range), the river water

chlorinity often exceeded the standard value (0.25gL‐1

for domestic use; 0.35gL‐1 for agricultural and

environmental use), resulting in water supply cutoff

from time to time. It is one of the most important tasks

for social stability by taking a series of corresponding

measures to guarantee the water supply security. In

addition, the chlorinity of mixed saltwater and fresh

water in the estuarine reach is not only a trace material

for fluid motion, but also an important parameter for

the sediment, pollutants (such as CODMn) monitoring.

In this regard, the investigation of saltwater intrusion

has a practical and scientific significance.

In 1950s, seven regular shore chlorinity observation

stations (as seen in Fig. 1) were set up along Qiantang

estuary, taking samples at high water slack and low

water slack everyday to obtain the daily maximum

and minimum chlorinity, which show the variation

law in the natural condition with lower human activity

intervention.

FIG. 1 LAYOUT OF QIANTANG ESTUARY

In year 1960, a multi‐year regulating reservoir named

Xin’anjiang was built in the upstream of estuary with

major functions of electricity generation, electricity

front adjustment and emergency backup as well as

flood control and irrigation. The reservoir greatly

increased the low flow discharge in dry season and

decreased the chlorinity and exceeding standard time

of the estuarine reach. With the social and economic

Advances in Water Resource and Protection (AWRP) Volume 2, 2014 www.seipub.org/awrp

63

development along the Qiantang estuary, needed

water and diverted water increased so rapidly that the

river chlorinity was prone to exceed standards in

Hangzhou river reach from 1972 to the present.

Since 1968, regulating and narrowing project lasting

for over 40 years has been carried out to regulate the

shifting estuary. The estuary has undergone the regime

of the straight and curved shape as well as wet and dry

year. All the natural changes and human activities

influence the chlorinity characteristics in the Qiantang

estuary. The field data reflect the modes of saltwater

intrusion for these different periods, and also lay a

solid foundation for studies on the chlorinity variation

and forecast method.

Fresh water and saltwater mixed homogeneously

along the Qiantang estuary at every cross‐section. In

1950s, the mixed fresh water and tide prism segment

were used to simulate the longitudinal chlorinity

distribution (Ketchman 1951). With the hypothesis of

the tide motion as sinusoid curved and the diffusion

coefficient being proportional to the product of

characteristic velocity and length, an analysis solution

was further obtained by vertical integrating salt

continuity equation (e.g. Aron and Stommel 1951;

Charless 1976). Using the data of monthly averaged

chlorinity of the seven stations, this analysis of

solution of the saltwater intrusion was verified (Mao

1964). The formula for the flushing number which was

proportional to fresh water discharge and inversely

proportional to tidal range, river volume was given

(Han 1986). However, all the above solutions were

suitable for steady flow, far different from the real

unsteady state where fresh water discharge and tidal

rang change day by day. Since 1970s, 1D, 2D and 3D

unsteady mathematical models for saltwater intrusion

have been rapidly developed (e.g., Stiger 1976;

Harleman 1974; Abraham 1975; McDowell and

O’Connor 1977; Savenije 1986, 1992; Nguyen 2008; Luo

and Cheng 2005; Wang and Lu 2008; Ma and Liu et. al

2006). However, most models were fixed‐bed models

which were not suitable for the Qiantang estuary with

intensely changing riverbed. This paper introduced

numerical solution of moveable bed model, the

prediction and countermeasures for saltwater

intrusion in the Qiantang Estuary.

The Qiantang Estuary Saltwater Intrusion Feature

Basic Properties of the Qiantang Estuary

The basin area of the Qiantang estuary is 55,558km2

with the estuarine length is 282km (Han 2003). The

estuary is divided into three reaches: runoff reach

(78km), tidal currents and runoff effected reach (120km)

and tide dominated reach (84km). The 30km section

from Wenyan to Qibao is densely dotted with water

intakes and seriously affected by saltwater intrusion.

Annual runoff of the Lucibu hydrological station (tidal

limit) is 950m3/s, however, the runoff distribution is

uneven yearly and monthly: the monthly average

maximum discharge is 5170m3/s in the wet season

(April to June), while the minimum average low

discharge is only 140m3/s in dry season (November ~

February). Substantial variation in inter‐annual,

monthly runoff causes the large range of the river bed

siltation and erosion, after feedback, the high tidal

level, low tidal level, tidal range variations influence

river bed and chlorinity with different degrees.

The averaged and maximum tidal range along the

Qiantang estuary is shown in Table 1.

TABLE 1 AVERAGED AND MAXIMUM TIDAL RANGE ALONG

THE QIANTANG ESTUARY

Location average tidal range/m Max. tidal range/m

Ganpu 5.64 9.00

Yanguan 3.28 7.26

Cangqian 1.53 5.27

Qibao 0.79 4.28

Zhakou 0.56 3.72

Wenyan 0.47 3.17

The maximum and averaged tidal range of Ganpu

station is 9m and 5.70m respectively. For Qibao station,

which is the most sensitive to saltwater intrusion

among water intakes of Hangzhou city, its annual

average tidal range is only 0.79m, but the maximum

value is 4.28m. The reason for the substantial yearly

and monthly variation of tidal range is that the

riverbed of the Qiantang estuary consists of silt bed

material prone to erosion and deposition. The wet/dry

runoff and the tidal strength lead to bending or

straight river bed, changing stream length and the

changes of tidal ranges, and their interactions further

cause the variation of river bed. From 1968 to 2012,

about 80km length of the estuary was narrowed and

fixed, which limited the shifting mainstream, but the

variation range of riverbed erosion/deposition, tidal

range and chlorinity were decreased in the wet/dry

years.

Analysis of Main Influence Factors of Salt Water

Intrusion

Through 40 years of observation on chlorinity,

underwater topography and tidal level, the law of


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saltwater intrusion is mainly determined by following

factors.

1) The Magnitude of Runoff Discharge in the

Dry Season

Runoff discharge reflects the upper boundary

condition of freshwater value. As tide has a

semi‐monthly cycle and a semi‐diurnal cycle, the

chlorinity varies accordingly. The average runoff

discharge in certain period (especially in spring tide

period) is the most critical factor to the chlorinity,

and the value of chlorinity also relates to runoff

discharge in the pasted 30 days, 60 days and 90

days. The magnitude for the different frequencies

of discharge at Fuchunjiang power station is seen in

Table 2.

TABLE 2 THE MAGNITUDE FOR THE DIFFERENT FREQUENCIES OF

DISCHARGE

G.F. condition 15d 30d 60d 90d

95% before/ m3.s‐1 20 40 70 80

after/ m3.s‐1 130 140 180 190

90% before/ m3.s‐1 30 40 70 80

after/ m3.s‐ 138 140 180 200

70% before/ m3.s‐1 48 70 100 160

after/ m3.s‐1 185 260 300 360

50% before m3.s‐1 60 100 170 226

after m3.s‐1 284 366 420 460

The saltwater intrusion at the intake of waterworks

generally occurs when the discharge is less than

300m3/s. However, the phenomenon would not last

24 hours due to the tide variation, and there would

be 8‐16 hours while the chlorinity is lower than the

critical value and applicable for diversion. This is

very favorable for urban water supply system with

river networks or adjustable reservoir. Table 2

shows the contrast of the dry season discharge

between before and after the construction of

Xin’anjiang reservoir, the monthly averaged

discharge increases by 100‐250m3/s, however, it is

still frequently less than 300m3/s.

2) The Magnitude of Tidal Range

Magnitude of tidal range represents tidal dynamics

of the lower boundary condition. Taking the

Yanguan (①) and Ganpu (②) stations as the

representation of middle and downstream

respectively, Table 3 gives their monthly average

tide range before and after regulation, in conditions

of curved channels (in dry year) and straight

channels (in wet year). As the tidal range is in

proportion to the tidal volume, it can also represent

the strength of the tide.

From Table 3, it discloses that for the same month,

the difference of tidal range is very small in wet/dry

year or before/after regulation at Ganpu station.

However, at Yanquan station, the difference of the

tide range for the same month is 2‐6 times

difference in wet and dry years before regulation,

and this value has decreased 1.5‐3 times after the

river bed regulation. It suggests that the regulation

has controlled the shifting of the mainstream

substantially and those changes are favorable for

decreasing saltwater intrusion.

TABLE 3 MONTHLY AVERAGE TIDE RANGE BEFORE AND AFTER

REGULATION

Jan. Mar. May Jul. Sep. Nov.

①

beforewet 4.51 5.06 4.40 4.26 5.07 4.65

dry 0.70 0.92 0.67 1.25 2.04 1.36

afterwet 3.92 3.52 3.53 3.48 3.91 3.79

dry 1.07 1.60 2.40 2.20 2.02 2.06

②

beforewet 4.76 5.18 5.39 5.50 5.56 5.16

dry 5.02 5.41 5.56 5.82 5.81 5.50

afterwet 5.63 5.70 6.01 6.07 6.06 5.84

dry 5.43 5.64 5.70 5.84 6.07 5.72

3) The Magnitude of the River Bed Volume

The river bed condition is the inner boundary

condition for runoff/tide dynamics. The Qiantang

estuary is an estuary of strong shifting and widely

varying erosion and deposition. Besides the

monthly changes caused by astronomical tide,

runoff is another critical factor. Therefore, in wet

year, the river bed is eroded and the tide range

increases; in the dry year, the river bed is deposited

and the tide range decreases. Data of Yanquan in

Table 3 show that the difference of tide range

between wet and dry years is due to the feedback

by the magnitude of the river bed.

Table 4 compared tidal range of spring, middle and

neap tide in August and January in year 2010 and

2008 at Qibao station.

TABLE 4 THE SPRING, MIDDLE, NEAP TIDE AT QIBAO STATION

Time spring middle neap

①~④ ①~④ ①~④

August in 2010 3.08~3.28 1.8~2.44 0.04~0.82

January 1.55~1.81 1.01~1.18 0.01~0.32

August in 2008 1.33~1.48 0.86~0.94 0.09~0.18

January 0.23~0.30 0.12~0.15 0.05~0.08

It shows the average semi‐month tidal range in

spring tide is 8 times larger than in the neap tide for

the two years, and it was caused by the different

river bed. In winter, the river bed is deposited and

the tidal ranges are smaller, while in August the

river bed is eroded and leads to a larger tidal range.


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Therefore, water chlorinity in the river reaches of

Hangzhou seldom exceeds the standard. All neap

tide in August has a small tide range, there are

rarely more than 10 days successively exceeding

standard (with exception in successive dry months).

4) The Position and Scale of Water Intakes

Water intakes provide water for the city dweller,

agriculture and environment. The water intakes at

Nanxingqiao, Zhakou, Baitaling and Shanhusha are

the most important for urban water supply,

confining to 8km scope (seen in Fig. 2).

FIG. 2 WATER INTAKES DISTRIBUTION IN QIANTANG ESTUARY

The quantity of water withdrawal is rapidly

increasing with the development of society and

economics. In 1970s, the water demanded by

Hangzhou city was only 0.15 million ton per day.

At that time, it could be regulated by the river

networks in cities when the river water chlorinity

exceeded the critical value. Later on, the needed

water increased to 0.3‐1.2 million ton per day, and

saltwater intrusion occured occasionally, thus the

plain reservoir Shanhusha reservoir was

constructed to store fresh water, meanwhile,

dispatch on salt water intrusion was initiated by

Xin’anjiang reservoir to solve the problem.

In 2013, the water diversion of Hangzhou city

reached to 25m3/s (planned maximum permission

diversion discharge is 35m3/s) with high guarantee

rate requirement, while agricultural and

environmental water consumption also increased to

100‐150m3/s with relatively low guarantee rate

requirement. Due to staggering in time, the

simultaneous discharge would not be so great. In

addition, the distance of water intakes is about 20

km. The maximum successively and exceeding

standard times were different, so the

counter‐measures were also different.

The Temporal and Spatial Variation of the Chlorinity

In the estuary such as Qiantang estuary, the differences

of chlorinity at any two arbitrary points in the same

cross‐section are within ± 30%. According to the

classification method (Simmons 1963), The Qiantang

estuary belongs to the strong mixing type, at least at

85% of the time. The convection diffusion equations

with 1D unsteady model can describe it. From this

point of view, studying on saltwater intrusion problem

for this estuary is relatively simpler than others.

The chlorinity intakes are influenced by the runoff and

tidal range. In the full tidal process, the highest

cholornity value arises at the high tidal slack, and the

minimum value is in the ebb tidal slack. At some water

intakes, chlorinity in flood tide exceeds the standard

but not in the ebb tide, which provides a favorable

condition for water diversion and regulation. Unlike

the estuary of the Yangtze River and Pearl River, where

the successive exceeding standard time is as long as

20‐50 days (Shen 2003; Bao 2009), the Qiantang

estuary’s continuous over‐standard time is only 0.5‐5

days. Therefore, this paper stipulates that only in the

statistics of measured chlorinity: when the maximum

and minimum chlorinity are both greater than 0.25g/L,

the over‐standard time is recorded as 1 day; when the

maximum chlorinity exceeds the standard but the

minimum does not, the exceeding standard time is

recorded as 0.5 day.

As water intakes are in the upper stream of the estuary,

the spring tide and dry flow period (from July 1st to

November 15th) is prone to saltwater intrusion and

severe sediment deposition. At this time, the tide range

of stations upper stream of Yanguan increases rapidly

due to astronomical tide and sediment deposition.

Therefore, movable riverbed model is required to

simulate the saltwater intrusion, coupling the flow

equation, bed deformation equation and chlorine

conservation equations (Han 1987, 2012). In addition,

the empirical relationships of observed tidal data are

calibrated in different months. This is one of

complexities and difficulties for simulation of the

saltwater intrusion in the Qiantang estuary that differs

from the Yangtze and Pearl estuary.

In Table 5, ①represents the period before the

construction of Xin’anjiang reservoir (in 1955‐1960).

②represents the period after the completion of

Xin’anjiang reservoir but before the regulation (in

1972‐1975). ③represents the period after the regulation


66

(in 1976‐2010). The monthly maximum, yearly average

chlorinity for each station are listed in Table 5.

Fig. 3 and Fig. 4 show that in the dry season the

chlorinity of each station changes in a monthly and

semi‐monthly, and the longitudinal distribution,

reflecting the influence of runoff discharge and tidal

range on the saltwater intrusion and over‐standard

days at the water intake.

TABLE 5 MONTHLY MAXIMUM, YEARLY AVERAGE CHLORINITY

FOR EACH STATION

Station monthly maximum Yearly average

① ② ③ ① ② ③

Wenyan 0.2 0.1 0.1 0.0 0.0 0.0

Shanhusha 0.5 0.1 0.3 0.1 0.0 0.0

Zhakou 1.5 0.1 0.2 0.3 0.0 0.0

Qibao 2.6 0.6 0.9 0.8 0.2 0.3

Cangqian 4.8 0.9 1.7 2.0 0.5 0.6

Yanguan 7.3 3.9 1.6 2.7 2.2 1.2

Ganpu 9.3 8.2 6.7 6.2 5.6 5.5

FIG. 3 EFFECT OF DISCHARGE AND TIDAL RANGE ON

CHLORINITY

Zhakou station is used to reflect the domestic intake

conditions as shown in Table 6.

TABLE 6 NUMBER OF DAYS EXCEEDING STANDARD

Mean annual Maximum annual Minimum annual

① ② ③ ① ② ③ ① ② ③

All day/d 55 0 2 85 0 43 26 0 1

Half day/d 85 6 13 145 12 127 40 0 5

Comparison shows that in period ③, both the whole

exceed day and half exceed day are greatly reduced

than that in period ①. This is the combined effects of

low‐flow increase by Xin’anjiang reservoir and the

maximum tidal range reduction after the river

constriction.

FIG. 4 THE LONGITUDINAL DISTRIBUTION OF CHLORINITY

The monthly average discharge, the monthly

maximum tidal range at Qibao, and the instantaneous

maximum chlorinity of the water intake station in 2003

are listed in Table 7. It is clear that the chlorinity at

water intakes are less than 0.25g/L when the runoff

discharge is over 318m3/s, inversely, when the runoff

discharge is in 318 ~ 170 m3/s, chlorinity at all water

intakes exceeds the standard. The smaller the

discharge is, the higher the chlorinity is. In September,

October and November in year 2003, the monthly

average runoff discharge was 318‐170m3/s, accordingly,

the exceed standard time was 103, 132, 109h at

Nanxingqiao intake and 12.5, 60, 28h at Shanhusha

intake respectively. However, the maximum successive

exceeding time of the above two stations was 69, 41h

respectively. This was a serious saltwater intrusion

incident in recent years. The incident is the result of the

zero discharge of the Xin’anjiang Reservoir, due to

maintenance of transformer substation during that time.

TABLE 7 MONTHLY DISCHARGE, TIDAL RANGE AND CHLORINITY OF 2003

Time Jan.‐Aug. Sep. Oct. Nov. Dec.

discharge/m3s‐1 547~1768 318 173 172 215

maximum tidal range/m 1.68~3.05 2.96 2.76 2.31 1.46

chlorinity of Nanxingqiao/gL‐1 0.01~0.1 1.96 2.1 2.1 0.75

chlorinity of Shanhusha/gL‐1 0.01~0.04 0.57 1.07 1.4 0.28

The Numerical Prediction Model of Saltwater Intrusion

The Qiantang Estuary is well‐mixed so that 1D and 2D

mathematical models can be used to describe the


67

macro law of motion of its water flow, sediment,

chlorinity and so on. 2D numerical model of salinity

can be used to describe river bends near the intake

with varying morphology. Ever since 1970s, there were

substantial literatures concerning 1D and 2D saltwater

intrusion, the majority of which, however, was

confined to the solution of fixed beds, rather than

moveable beds. As the Qiantang estuary is a

wandering estuary with strong tide, high sediment

concentration and intensely changing river bed, the

high silting intensity decrease flood tidal volume at the

middle and later periods during the dry season. This is

one of the special difficulties for the medium and long

durational prediction of the saltwater intrusion in the

Qiantang estuary. Therefore, concerning the predicted

formula, the conservation equations of water mass,

momentum and salt content should be coupled with

the sediment continuity equation and the riverbed

deformation equation of the moveable bed model.

1D Mobile Riverbed Salinity Mathematical Model

The sediment laden by the tide flow of the Qiantang

estuary mainly contains fine grain suspended load,

which also largely contributes to the easily eroded and

deposited feature of the estuary.

The governing equations of the 1‐D movable bed

mathematical model composed of shallow water

equations, sediment transport and riverbed

deformation equations, salinity convection diffusion

and state equations, written as follows (Shi 2004, Han

1986, 1987, 2012):

lqx

Q

t

A

(1)

022

2

x

AHg

ARC

QQg

x

ZgA

A

Q

xt

Q

z

(2)

)( *21 STSTBx

SEA

xx

QS

t

AS

(3)

)( *21

STSTt

Zos

(4)

)(1

x

CAD

xAx

Cu

t

C

(5)

100035.1 C (6)

where x is the longitudinal coordinate；t is the time；Q

is the discharge of the water；B is the river width; Z is

the tidal level;Ａ is the flow area; Cz is the Chezy

coefficient; R is the hydraulic radius; S is the

cross‐section averaged sediment concentration; S* is the

cross‐section averaged sediment transport capacity;T1

is the ratio of the near‐bed and section‐averaged

concentration of the suspended load; T2 is the ratio of

the near‐bed and section‐averaged sedimend transport

capacity of the suspended load; ω is the settling

velocity of sediment; Z0 is the cross‐section averaged

elevation of river bed; ql is the lateral discharge; E is the

longitudinal diffusion coefficient of sediments; γs is the

dry density of sediment; ρ is the density of water flow,

sediment concentration and chlorinity mixture; C is the

cross‐section averaged chlorinity；D is the longitudinal

dispersion coefficient of chlorinity. By dimensional

analysis and the measured data calibration, D is

suggested to determine as follows (Han 1981).

uHC

C

TQ

W

gH

uD if

10])·exp(1[00

2

(7)

In which, u2/gh is the Froude number; u, H is

cross‐section averaged flow velocity and water depth

respectively; g is the acceleration of gravity; Wf/Q0T is

the ratio of the flood volume to fresh water volume in

a tidal cycle; Wf is the flood volume; Q0T is the fresh

water volume in a tidal cycle; Ci/C0 is the ratio of the

local cross‐section averaged chlorinity to that of fresh

water.

The boundary conditions and initial condition should

be given to solve Equations (1) ~ (5). The processes of

the fresh water discharge, sediment concentration and

chrolinity are given as follows for the upper boundary

condition at the Fuchunjiang Hydroelectric station.

Q(0,t)=Q0(t), when Q(0,t)>0, S(0,t)=S0(t), C(0,t)=C0(t)

The processes of the tidal level, sediment concentration

and chrolinity are given as follows for the lower

boundary conditions at Ganpu cross‐section.

Z(l,t)=Z0(t), when Q(l,t)<0, S(l,t)=S1(t), C(l,t)=C1(t)

The initial conditions: Z(x, 0) =Z*(x), S(x, 0) =S*(x), C(x,

0) =C*(x).

In the paper, the verification of the saltwater flow and

salinity were carried out using the salinity

mathematical model developed by the authors in 1980

and 1997.

1) Verification for Short Duration

For fixed bed mathematical model of short duration

salinity simulation, combination of the equations

(1), (2) and (5) were adopted and sets of results had

been provided in many literatures (Han 1981, 1987,

2012). Fig. 5 is the verification results of the

chlorinity in the cross‐section of Nanxingqiao and


68

Zhakou.

FIG. 5 THE CONTRAST OF THE CHLORINITY BETWEEN

CALCULATION AND MEASUREMENT

Table 8 shows the simulated results of the

maximum chlorinity and duration of over‐standard

hours (C>0.25g/L) in the water intakes of Hangzhou

city. The result proves that the chlorinity in the

main intakes, such as Zhakou and Shanhusha, is

lower than 0.25g/L for quite a few days within one

month, so that the water can be taken between

intervals. Itʹs much better than the Pearl River and

Yangtze River estuaries where duration of

exceeding standard is more than 15‐55 days, which

makes water intake impossible.

TABLE 8 THE CHLORINITY VERIFICATION CALCULATION RESULT IN THE

SHORT DURATION

Items

Max chlorinity

gL‐1

Exceeding standard

hours

① ② ③ ① ② ③

Measured

in Oct. 1995

25 2.82 1.18 0.79 20 <12 6

26 4.00 1.07 1.38 24 10 9

27 4.20 1.78 1.94 24 <12 12

28 3.60 1.22 1.40 24 24 10

29 3.30 0.63 1.40 21 <12 8

Calculated

25 3.03 1.93 0.64 22 12 6

26 3.22 2.11 1.43 24 8.6 8

27 3.24 2.66 1.78 24 8.6 12

28 3.63 1.83 1.38 24 23 11

29 2.90 1.63 1.12 20 7 9

(Note: ①, ②, ③ refers to Nanxingqiao, Zhakou and Shanhusha

station, respectively)

2) Verification for Long Duration

Generally, the chlorinity simulation for short

duration is satisfied by means of the fixed bed

model. However, the moveable bed mathematical

model of the above equation (1)~(5) should be

adopted to forecast the chlorinity for relatively long

term simulation from July to December. This is

because on the one hand, the astronomical tide is

changing; on the other hand, the bed volume from

Zhakou to Yanguan of the Qiantang River differs

0.05 billion m3 due to the change of runoff, and

with the same bed volume for different month, the

tidal range at Qibao also differs from 0.4~0.8m (see

Fig. 6).

FIG. 6 MONTHLY MAXIMUM TIDAL RANGE UNDER

DIFFERENT RIVER VOLUME

Fig. 7 compares the measured chlorinity at Qibao to

its simulated data using the fixed and moveable

bed models, which obviously proves the difference

between moveable and fixed bed. Agreement

between the measured and the calculated result

using the moveable bed model is fairly good,

especially during the low chlorinity period. The

calculated result using the fixed bed model deviates

far from the measurement, which has a great

impact on the calculation of the exceeding standard

duration and may lead to overestimated

conclusion.

FIG. 7 CONTRAST OF CHLORINITY SIMULATED BY MOVEABLE

AND FIXED BED NUMERICAL MODEL

The Establishment and Verification of 2D Model

Although the Qiantang Estuary is a well‐mixed

estuary, there is a main channel and shoal on certain

cross section, and the water depth and flow velocity

also differ, resulting in the difference of chlorinity in

the plane. Therefore, 2D salinity transport numerical

model should be established for the research.

Considering the impact of water density change on the

water pressure after the saltwater intrusion

longitudinally and vertically, the two‐dimension


69

shallow water equations of the conservation form, and

the control equation of chlorinity to convective

diffusion are written as follows (Shi 2012):

Sy

G

x

F

t

U

(8)

Where U= (h, hu, hv, hc) T is the vector of flow and

chlorinity variables. The flux terms F, G, and source

term S are defined as:

2 212

2 t

t

x

hu

uhu gh h

xF u v

huv hy x

chuc E h

x

; 2 21

22

t

t

y

hv

u vhuv h

y xG v

hv gh hy

chvc E h

x

;

2 2 20

2

2 2 20

2

cos( )2

sin( )2

s

sz

sz

s s

Q A

z u u v gh cgh fhv Q u A

x xC hS

z v u v gh cgh fhu Q v A

y yC h

Q C A

where h is the water depth; u and v are depth‐averaged

flow velocities in the x, y direction, respectively；c is

the chlorinity of the water; g is the acceleration of

gravity; z0 is the bed elevation, Cz is the Chezy

coefficient; f is the Coriolis coefficient; νt is the flow

turbulent viscosity coefficient, decided by

Smagorinsky empirical relation (Smagorinsky 1963)；ρ

is the saltwater density, ρ=ρ0+αc ；ρ0 is the fresh water

density; Ex and Ey are turbulent diffusion coefficient of

chlorinity in x, y directions, respectively; Qs and Cs are

the discharge and chlorinity in water intake or outlet,

respectively ; A is the area of control cell.

The turbulence viscosity coefficient (νt ) is determined

by the following Smagorinsky empirical relations, and

Ex and Ey are determined by Elder formulation in the

model.

2 221

2T e

u v u vA

x y y x

(9)

Where, α is a constant coefficient with 0.1‐0.25; Ae is an

element area.

The governing equations are solved with the finite

volume method, and an unstructured grid of triangles

is used as the control cell in order to accurately fit the

irregular river boundaries. The core of the finite

volume method is the flux calculation of the control

body at cell faces. Many schemes have been used, such

as TVD (Harten 1980), MacCormack (MacCormack

1972), BGK (LAI 2008) and KFVS (Pan 2006). In this

thesis, the fluxes are computed by Roe’s method (Roe

P L. 1981). The diffusion fluxes are calculated by the

central difference scheme of the unstructured grid. In

order to reach an advanced and stable calculation

numerical scheme, the variable slopes are limited so as

to adjust and control the dissipative and dispersive

effects of figures. Similar to MUSCL, the first order

accuracy is improved to the second order. Confined by

the length of this article, the detailed deduction of

numerical scheme is seen in the literature (Shi 2012).

This mathematical model was developed by the

second author of the paper in 2006.

Fig. 1 shows the simulation area and the hydrological

validation points. The upper boundary of the model is

located in the Fuchunjiang Hydroelectric station and

the lower boundary is in the Ganpu section of the

Qiantang Estuary. The computational area is 790km2

which composed of 11710 triangles with minimum

side length of 100m, and the time step is 4s. The

measured and calculated results of the flow velocity,

the chlorinity and tidal process among the 12

chlorinity monitoring sites and 13 tide stations from

Oct. 25 to 30 in 2007 have been compared. Fig. 8 shows

the contrast between the calculated flow velocities and

the measured flow velocities of Qibao (701#) and

Ganpu (710#); Fig. 9 shows the comparison between

the chlorinity of Qibao and Ganpu (710#).

FIG. 8 THE VALIDATION CURVE OF WATER FLOW PROCESS,

OCT. 2007

It can be seen from the figure that the strong tidal

movement in the Qiantang estuary and the serious


70

saltwater intrusion result in the chlorinity in Qibao,

reaching as high as above 0.7g/L. Based on the model.

The calculated figures of the vertical average flow

velocity and the chlorinity agree well with the

measured figures, which clearly reflect the saltwater

intrusion of Qiantang estuary affected by the strong

tide.

FIG. 9 THE VALIDATION CURVE OF CHLORINITY PROCESS,

OCT. 2007

Optimized Water Release Method Based on Tidal

Range Scale

During the dry season of spring tides, the water release

of the upper Xin’anjiang Reservoir should be increased

in order to prevent the saltwater intrusion. Since the

tidal stage change periodically within 15 days, water

release method decides the optimum distribution of

water resources to guarantee the lowest chlorinity and

the longest qualified intake hours. Therefore, three

modes of water release are investigated: the first mode

is to release water evenly within 15 days; the second

one is to increase 30% of water release during the

spring tide and decrease 30% during the neap tide; and

the third method is to release 70% of the water during

the later period of the spring tide. The calculated results

are listed in Table 9. Obviously, the second mode is the

best: it has the lowest chlorinity and shortest days in a

month while the chlorinity exceeds the limits.

TABLE 9 A COMPARISON OF THE WATER RELEASE MODES

Modes of Releasing Water No. 1 No. 2 No. 3

Max. chlorinity/gL‐1

Qibao 5.83 3.37 7.40

Zhakou 2.61 1.44 4.80

Shanhusha 1.10 0.07 2.70

Days of Exceeding the Limit 6.6 2.1 6.2

In order to verify the actual effect of the prediction,

two verification periods without water release

prediction and with water release prediction are

chosen. The two periods should have similar tidal

range and water release volume in 15 days. The former

period is from Jul. 23 to Aug. 1, 1972 without optional

water release, and the latter period is from Oct. 1 to

Oct. 15, 2006 with the released water completely under

prediction (cf. Table 10).

TABLE 10 A COMPARISON OF CHLORINITY IN ZHAKOU WITH/WITHOUT

PREDICTION

time (1972) 7/25 26 27 28 29 30 31 8/1

Without

prediction

Water

release/m3s‐1344 244 183 147 192 2.51 257 412

Tidal range in

Qibao/m 0.88 1.21 1.53 1.65 1.44 1.27 1.18 0.93

Max

Chlorinity/gL‐10.10 0.30 1.34 3.88 4.14 2.61 1.48 1.20

Min

Chlorinity/gL‐10.02 0.06 0.80 1.45 2.70 1.48 0.80 0.50

Exceeding

standard/d

Half day: 2d, All Day: 6d, water

supply was seriously affected

time (2006) 10/4 5 6 7 8 9 10 11

With

prediction

Water

release/m3s‐1298 382 357 492 372 364 300 321

Tidal range in

Qibao/m 0.4 0.7 1.2 1.46 1.44 1.52 1.39 1.1

Max

Chlorine/gL‐10.02 0.02 0.10 1.03 0.98 0.70 0.33 0.12

Min

Chlorine/gL‐10.01 0.01 0.01 0.12 0.15 0.10 0.01 0.01

Exceeding

standard/d

Half day: 4d, All Day: 0, water

supply was secured

From Table 10, the former is 10% larger in water flow

and 8% smaller in tidal range under similar conditions.

The result shows that the former released discharges

completely according to the power requirements

without consideration needs of saltwater intrusion. It

turns out that the actual discharge is smaller in spring

tide, the chrlonity have been larger. Comparatively,

under predictions, more water is released in the spring

tide, the max chlorinity decreases from 4.14 g/L to 1.03

g/L, only one fourth of the former. Whatʹs more, the

former has 6 days with over‐limit chlorinity, in sharp

contrast to the latter. It proves sufficiently that it

functions well to release water according to the tidal

range and it has contributed greatly to improve the

water supply security of Hangzhou City.

The Application of the Prediction Model

Anti‐saltwater Intrusion Control and Application

Confirmation

The common measures for preventing saltwater

intrusion are city river regulation, building regulation

reservoir on river bank, moving the intake to the

upstream, increasing discharge of large reservoirs, etc.


71

They are practiced in the Qiantang estuary. Before the

operation of Xin’anjiang reservoir, daily water supply

was so low that household water storage, well water

and channel net storage could be adopted when the

chlorinity of water exceeded the standard. When the

river water quality improved, the water supply and

drainage water storage could be resumed again. When

the Xin’anjiang reservoir was built, the low‐water

discharge increased from 50~100m3/s to 250~300m3/s,

and greatly lighten saltwater intrusion.

When the daily water supply of cities increased from

0.1 million tons to 0.3~1.2 million tons, the water

release discharge of Xin’anjiang reservoir has to be

increased, the intakes have to be moved to the

upstream, and the methods of anti‐adjustment and

anti‐saltwater reservoir must be implemented jointly

in order to guarantee the water supply security of

Hangzhou city. The effectiveness of these measures

largely depend on the accuracy of the prediction of the

river water chlorinity. During these years, the river

width, runoff, daily water supply, water supply

facilitate and other conditions are constantly

changing, as a result, the required water release

discharge of the Xin’anjiang has to be adjusted each

day accordingly. If the chlorinity of water doesnʹt

exceed the standard at all intakes, the pre‐discharge is

more than required; if the days when the chlorinity

exceeds the standard are too long, then the

pre‐discharge is less than required.

Therefore, itʹs quite difficult to predict the best fit

water release. The elaboration of the anti‐salt water

intrusion prediction and practice is as follows.

1) The Comparison of the Case According/not

According to Prediction

From September 19 to 25, 1994, the 7‐day and

15‐day real water discharge were in compliance

with the predicted values, as shown in Table 11.

Therefore, the continuous over‐limit chlorinity

duration at Nanxingqiao was less than 24 hours.

From the end of October to the beginning of

November, 2003, because of the transformer

maintenance in Xinʹanjiang Hydroelectric Station,

the actual discharge of 15 days was only half of the

predicted value so that continuous exceeding

standard duration of Nanxingqiao and Shanhusha

was 80h and 48h respectively.

2) The Practice Confirmation of the Long‐term

Prediction

Since 1978, the actual release discharge of the

Xinʹanjiang and the Fuchunjiang hydroelectric

station has been almost implemented according to

the prediction. Table 12 shows the relationship

between over‐limit chlorinity days of a typical

intake, Zhakou and the main factors (the minimum

runoff of 30d, and the maximum annual tidal range

in Qibao).

1) Small discharge is the main reason for the

chlorinity of water to exceed the standard all days

and half days. Before the Xinʹanjiang reservoir was

built (in 1960), the monthly average runoff was

148m3/s, the tidal range 1.12m, the maxium

chlorinity 5.0g/L, and the exceeding standard

duration was 62 days (all day: 45, half day: 17.2).

After the reservoir was built, the runoff discharge

increased to 443m3/s in the wet years with straight

channel (in 1988~1997), which directly contributed

to the improvement: the tidal range was 3.45m, the

maximum chlorinity was 1.23 g/L, and the

exceeding standard duration was declined to 20

days (all day, 0.66 day; half day, 19.3 days). When

the reservoir doesnʹt release water according to the

prediction, such as zero water discharge (1978), or

water release not with prediction (2003), itʹs quite

possible for the chlorinity of water to exceed

standard for so long as 103 days and 32 days, which

proves the essentiality of the prediction.

TABLE 11 A COMPARISON ACCORDING/NOT ACCORDING TO PREDICTION

Time Tidal range /m

Q

/m3.s‐1

Exceeding standard

duration/h

① ② ③ ④

According

to

prediction

Sep. 19 2,28 410 270 12 0

20 2,59 540 569 20 0

21 2,69 540 550 20 0

22 2.74 540 687 17 0

23 2.65 540 502 15 0

24 2.43 540 417 13 0

25 2.28 410 273 11 0

1994

7d ave. 490 456

15d ave. 410 430

Not according

to

prediction

Nov.

6 1.34 320 0 7.5 0

7 1.42 340 0 24.0 19.0

8 1.58 360 506 24.0 24.0

9 1.75 360 836 24.0 16.5

10 1.59 360 367 11.5 0

11 1.70 360 256 0 0

12 1.26 340 191 0 0

2003

7d ave. 348 308

15d ave. 300 164

(Note: ①, ② refers to Predicted and Facts, ③,④ refers to

Nanxingqiao and Shanhusha station, respectively)

2) The river conditions of the downstream also play

an important role on deciding the tidal range in

Qibao. From 1988 to 1997, the straight river


72

channels in the wet year period brought the tidal

range to 3.45m, which led to the serious saltwater

intrusion. As a result, the release discharge had to

be increased to 443 m3/s with 19.7 days of over‐limit

chlorinity per year. In comparison, from 1998~2009,

with improved river channels, the water release

was decreased to 120m3/s, the tidal range was 0.8m,

and the average exceeding standard days was only

2.6d, 6 times shorter than the former time.

TABLE 12 THE DAYS EXCEEDING THE STANDARD AND ITS FACTORS IN

ZHAKOU

Year

Monthly

minimum

discharge

/m3.s‐1

Tidal

range in

Qibao

/m

Max

chlorinity

/g.L‐1

Exceeding standard

days/d

Half

day

All

daytotal

1955‐1960 148 1.12 5.0 17.2 45.2 62.4

1972‐1987 350 2.07 0.98 6.2 3.9 10.2

excluding

1978 367 2.09 0.54 3.6 0.36 3.70

1988‐1999 443 3.45 1.23 19.7 0.66 20.4

2001‐2009 325 2.64 0.62 2.6 0.35 3.25

excluding

2003 2.1 0 2.2

3) Comparing data of winding river channels of

1972~1987 with that of 2008~2009, the latter one

belonged to the later period of river regulations

when the tidal range was 0.57m larger, and the

runoff was 7% smaller. The average exceeding

standard days reached 3.25 days, shorter than the

previous 10.2 days. Excluding the special situation

of 1978 and 2003, the two figures decreased to 3.70

days a year and 2.2 days per year, which proved

the improvement on saltwater intrusion brought by

the river regulations.

4) It is essential for coordinate relevant authorities

to carry out all the water resource optimum

measures. The large‐scale Xinʹanjiang reservoir

plays the role of electricity generation, electricity

front adjustment and emergency backup as well as

flood control, irrigation and water supply, with the

regulated storage capacity of 1.0 billion m3. In 1978,

the electricity sector declared that on the premise of

emergency backup, the water supply security of the

citizens in the lower reaches should be guaranteed.

After that, the coordinating team of electricity,

water supply and research (where the writer of this

thesis works) authorities put forward the minimum

discharge mode of the Xinʹanjiang. Power Station

and Fuchunjiang Power Station, in order to prevent

the chlorinity of water from exceeding the standard,

based on the river topography at the end of the

rainy season of that year. It turned out that even

when the actual runoff was larger than predicted,

the exceeding standard days didnʹt occur; when the

runoff was 20% smaller than predicted, the

exceeding standard days would appear. Therefore,

three authorities of the team agreed on the

reliability of the prediction and took measures

accordingly for the next 30 years.

Research on Guarantee Rate of Water Supply of

Hangzhou City

As a most important intake of water supply of

Hangzhou city, Shanhusha located at the fringe of

saltwater intrusion. During the dry season with spring

tide, the chlorinity always exceeded the standard, and

the water quality also faceed the risk of regional water

pollution and sudden pollution, which led to the low

guarantee rate of the water intake. Consequently, the

Hangzhou Municipal Government advocated building

another emergency reservoir. Enhancing the capacities

of water supply and saltwater prevention, the

combination of two reservoirs aims to solve the

problems of normal and backup water supply.

Therefore, the research was carried out based on the

previous prediction model.

The boundary conditions of the model include the

released discharge (Q), tidal level process and

chlorinity of Ganpu section, the total water diverted

from the estuary (q).The boundary conditions satisfy

the guarantee rate of water supply of 95%. The

calculated results are shown in Table 13.

From Table 13, under the present conditions, the water

supply guarantee rate of Hangzhou City is about 80%.

In order to reach P=95%, during a period of 45d,

chlorinity of 32.3d are over‐limit in the Shanhusha

TABLE 13 THE CALCULATED RESULTS OF THE 95% GUARANTEE RATE FOE HANGZHOU WATER SUPPLY

Shanhusha Wenyan Changansha

① ② ③ ① ② ③ ① ② ③

Q=250m3/s

q=117m3/s

(P=95%)

Continuous exceeding /d 1.9 8.9 7.9 0.2 0.4 0.3 0 0.2 0

Total exceeding/d 7.5 12.2 12.6 2.4 6 4.7 0 1.3 0

Average Chlorinity/mgL‐1 413.8 850.6 700.2 119.1 282.2 197.5 36.1 88.1 55.9

Max chlorinity/mgL‐1 1778 2572 1833 742.7 1321 771.4 261.9 561.1 261.8

Total /d 32.3 13.1 1.3

(Note: ①, ②, ③ refers to the 15d of the first, second and third spring neap cycle.)


73

water intake, with a successive 8.9 days during the

second spring neap cycle, which is unable to satisfy the

water supply of Hangzhou City. As a result, the water

storage capacity of the reservoir should be 8.5*106 m3.

But in order to satisfy the second and the third spring

neap cycle, the water release discharge in Xinʹanjiang

reservoir should be increased to 250m3/s~300m3/s, with

the total runoff of 0.38~0.8 billion m3; or the water

intake will be moved to the upstream of Chang’ansha

with the successive over‐limit days being only 0.5 day.

Considering that the water utilization rate is only 10%

by means of increasing the release discharge of

Xin’anjiang reservoir, the scheme of diversion water

directly from the Xinʹanjiang reservoir is being

discussed.

Conclusions

This paper describes the conditions and practice about

the saltwater intrusion for Qiantang estuary, which

goes through two periods: natural condition and

human activities influence (reservoir building and

estuarine width narrowing). Those human activities

drastically have changed the characteristics of salt

water intrusion and met the need of social and

economic development.

(1) Runoff is the main factor which decides the

chlorinity in the intakes of Hangzhou City. Before the

construction of the Xin’anjiang reservoir, the average

exceeding standard days was 62.4 days per year. And

in semi‐month, the successive exceeding standard day

could be 8 days, making it possible to store water in

another 7 days by the city channel network during the

neap tide to backup for spring tide. After the

Xin’anjiang reservoir was built, the discharge of dry

seasons increased by 200m3/s, with the average

exceeding standard days declined to 2.2 days per year.

This is one advantage of the Qiantang estuary in

preventing the saltwater intrusion.

(2) As the Qiantang estuary is a strong mixed estuary

with drastic shifting, the difference of chlorinity

between arbitrary points in cross‐section is within

±30%. For the short period saltwater intrusion

prediction, 1D fixed bed model can be adopted. But for

long‐term prediction such as 1‐6 months, the variation

of river bed should be taken into consideration, when

the movable bed model must be used. At present, this

kind of reference about saltwater intrusion is rarely

been seen. The long‐term (half year) movable riverbed

model of this thesis has been successfully applied to

predict the saltwater intrusion in Qiantang Estuary for

more than thirty years.

(3) Based on the law of saltwater intrusion and the

characteristics of the recent river channels, the low

flow from Xinʹanjiang Reservoir could be increased,

with the principle of “spring tide more discharge, neap

tide less discharge” (discrepancy ±30%), which can

shorten the days of saltwater intrusion and improve

the availability of water resources. Practice showed

that the release discharge based on the prediction can

satisfy the minimum flow demand for water intakes

and guarantee the water supply for Hangzhou city,

and also save 20% water resources, which brings great

social and economic benefits.

(4) The countermeasures to deal with saltwater

intrusion for the Qiantang Estuary are storing water in

channel network, building regulation reservoir in river

bank, moving water intake to the upstream and

increasing release discharge from large reservoir, etc.

At present, the water supply guarantee rate of

Hangzhou is only 80%, less than the national standard

95%. Therefore, it is necessary to increase the release

discharge from Xin’anjing Reservoir to satisfy the

minimum low‐flow from Fuchunjing Hydropower

Station of over 300m3/s. .On the other hand, for the

need of backup water resources in emergencies, it is

necessary to build a new reservoir with a capacity of

8.5 million m3. Besides, it is also very important to

coordinate relevant authorities for the water resource

optimization.

ACKNOWLEDGMENTS

The research reported in this paper was supported by

the Ministry of Water Resources Nonprofit Research

Program of China (No. 201101056) and the Key Science

and Technology Innovation Team Building Project of

Zhejiang province, China (No.2010R50035).

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prediction and countermeasures of saltwater intrusion in the qiantang estuary

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