Study on Influencing Factors of Flushing Time of Puyang River in the

Upper Tributary of Qiantang River

Hongqing Zhang

1, 2,*

, Yilong Lou

and Zhen Wang

Zhejiang Institute of Hydraulics & Estuary, Hangzhou 310020, China

Zhejiang Institute of Marine Planning and Design, Hangzhou 310020, China

Quzhou water resources and soil and water conservation management center, Quzhou 324000, China

College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, China

Keywords: Tidal river, Flushing time, Influencing factors, Hydrodynamic model

Abstract: Aiming at the prevention and control of the water environment of the Puyang River in the upper reaches of

the Qiantang River, two-dimensional hydrodynamic model was established to study the influence factors of

flushing time of Puyang River in the upper tributary of Qiantang River. The goal is to provide a new way to

solve the problem of river water quality. The research results show that under the action of different

influencing factors, the flushing time of the Xijiang and Dongjiang has a gradual increase trend from upstream

to downstream. However, due to the supporting effect of the Fuchun River flow on the Puyang River outlet,

the flushing time of area below Meichi increased first and then decreased from the upstream to the downstream.

At the same time, the influence of discharge of Fuchun River on the flushing time roughly extends to the

middle and lower reaches of the Xijiang and Dongjiang River. The discharge of Fengqiao River has little

effect on the flushing time in the Xijiang River and area below Meichi, and the influence of the discharge of

Fengqiao River on the flushing time of the Dongjiang River Basin is roughly 18km~25km away from the

entrance of the Dongjiang River. The influence of the discharge of tidal intensity of Qiantang River on the

flushing time of the area below Meichi is within 5km of downstream exit. The initial water level and the

discharge of the Puyang River are the main factors affecting the flushing time of the river basin, and the

discharges of the Fuchun River and the Fengqiao River also have a significant effect on the flushing time.The

research results can provide technical support for the prevention and control of the water environment of tidal

rivers.

1 INTRODUCTION

The Puyang River is a first-level tributary of the upper

reaches of the Qiantang River. The quality of the

water environment in the basin not only affects the

production and life of the people along the river, but

also affects the drinking water safety of millions of

people in the lower reaches of the Qiantang River.

With the rapid economic development, there are more

and more industrial enterprises around the Puyang

River, and industrial wastewater and domestic

sewage are directly discharged into the river. At the

same time, the Puyang River is a tidal channel, and its

water environment is affected by both upstream

runoff and the tidal effect of the Qiantang River,

resulting in complex water quality issues (Wu et al.,

2003). Although the Puyang River Basin has

continuously increased its pollution control efforts in

recent years, there are still water quality problems.

Since the end of the last century, the process of

material transport has become an important issue in

the water environment system. When the polluted

water body is exchanged with a clean water body, the

pollutant is diluted, its concentration is reduced, and

the water quality of the water body is improved.

Therefore, many scholars express the exchange

capacity of water bodies inside and outside the system

through the transmission time scale (Takeoka, 1984;

Monsen et al, 2002; Sandery & Kämp, 2007). Among

them, flushing time is a kind of transmission time

scale. Dilute time can explore the law of material

transport between the system and the external

environment and the characteristics of the material

exchange within the system itself (Takeoka, 1984;

Monsen et al, 2002). In recent years, scholars have

Zhang, H., Lou, Y. and Wang, Z.

Study on Inﬂuencing Factors of Flushing Time of Puyang River in the Upper Tributary of Qiantang River.

In Proceedings of the 7th International Conference on Water Resource and Environment (WRE 2021), pages 419-425

ISBN: 978-989-758-560-9; ISSN: 1755-1315

419

carried out a series of studies on the water

environment using flushing time (Ding et al., 2003;

Wang et al., 2004; Wan, 2009; Zhu, 2011) including

using the flushing time to study the migration of

pollutants in the estuary and assess the quality of the

water environment. Therefore, carrying out relevant

research on the flushing time of tidal rivers can

provide guidance for the harmonious development of

water conservancy projects and the water ecological

environment of the river basin, the establishment of a

long-term mechanism for river water environment

improvement, and the management of sewage

discharge along the river.

This paper takes Puyang River as the research

object and establishes a two-dimensional

hydrodynamic model to study the influence factors of

flushing time of Puyang River in the upper tributary

of Qiantang River, such as discharge of Puyang River

, discharge of Fuchun River , discharge of Fengqiao

River , tidal intensity of Qiantang River , and initial

water level . The research results provide guidance for

the prevention and control of water environment in

tidal rivers.

2 METHODS

2.1 Model Establishment

In this paper, the two-dimensional hydrodynamic

model of Puyang River Basin is constructed by using

mike21hydrodynamic module developed by DHI.

Maps of study area and model grid are shown in

Figure 1. The research objects in the Zhuji section of

the Puyang River are divided into Xijiang River,

Dongjiang River and area below Meichi. Among

them, from Wansha Brige to Caojiangkou to Meichi

is called Dongjiang River. And Fengqiao River is the

main tributary of Puyang River, which meet at

Caojiangkou.

The entrance boundary of the mathematical model

is located inWansha Bridge of Puyang River and

Luojia Bridge of Fengqiao River. The exit boundary

islocated in Sanjiangkou. Among them,Puyang Jiang-

WanSha Bridge and Fengqiaojiang-Luojia Bridge are

the flow import boundary, and the measured flow data

of Zhuji Station and Fengqiao Station were adopted

respectively. Sanjiangkou is the water level

boundary, and the exit water level is calculated based

on the one-dimensional model of the Qiantang River

by coupling the flow of the Fuchun River and the tidal

intensity of the Qiantang River.

Figure 1: Map of study area and model grid.

2.2 Model Verification

The flood level verification was carried out on the

model using the measured data of each boundary

during the flood period from June 13 to June 21,

2011. During the model debugging process, the

Manning coefficient has a certain influence on the

model. Default value (0.020 m1/3/s.), the relative

error interval between the calculated water level and

the measured water level is -7.2%-10.2%. The final

debug value of the manning coefficient in the model

is 0.035 m1/3/s, and the verification results of Linpu,

Meichi and Zhuji along the Puyang River are shown

in Figures 3-5. It can be seen from the Figures 2-4

that: (1) The relative error interval between the

calculated water level and the measured water level is

-1.2%-3.2%, the highest flood level error is controlled

within 4cm; (2) The calculated and measured flood

level process changes are basically the same. The

model’s rate parameter is basically reasonable and

can be used for the analysis and study of the

distribution of the flushing time of Puyang River.

Figure 2: Comparison of measured and calculated data of

Linpu station.

WRE 2021 - The International Conference on Water Resource and Environment

420

Figure 3: Comparison of measured and calculated data of

Meichi station.

Figure 4:

Comparison of measured and calculated data of

Zhuji station.

2.3 Calculation Method of Flushing

Time in the Model

Based on two-dimensional hydrodynamic model, the

tracer is selected as the substance in the Puyang

River, and the initial value of the tracer mass

concentration is set to 1 kg/m

, and the inlet and outlet

boundary water bodies are assumed to be clear water,

that is, the mass concentration of the tracer is 0. After

setting the boundary conditions in the hydrodynamic

model, the conservative tracer in the Puyang River

gradually diffuses under the action of hydrodynamics,

and the change function of regional material quality

with time is obtained, see formula (1) for details.

Then, substituting formula (1) into the flushing time

definition expression, get the formula for calculating

the water dilution time, see formula (2) for details.

This section must be in one column.

12 2 1 3

() ()

1+ ( )

Mt CtV

ee Ct

MCV

ββ

ββ β β

−

⋅

−⋅

-t

（

） +=

(1)

12 12

βββ

ββ

−−

=−

(2)

In the formula: M

(t) and C

(t) are the function of

the tracer mass and mass concentration in the Puyang

River, respectively; β

and β

are the exponential

terms in the tracer concentration curve according to

the double exponential law; V is the volume of water

at any point in the Puyang River; M' is the initial

amount of dissolved substances.

This article takes 16 equidistant calculation points

along the central line of the main channel in the

Xijiang River, the Dongjiang River and area below

Meichi. According to the aforementioned calculation

method, calculate the flushing time of each

calculation point, and then discuss the distribution

characteristics of the flushing time of the main

channel in each region under the different factors.

2.4 Boundary Condition

The simulation conditions of this paper are based on

the daily average flow of Zhuji Hydrological Station,

the daily average discharge flow of Fuchun River, the

daily average flow of Fengqiao River Hydrological

Station, the daily average tidal range of Wenyan

Station, and the daily average water depth of Zhuji

Hydrological Station from 1991 to 2012. The range is

determined, namely discharge of Puyang River

condition, discharge of Fuchun River condition,

discharge of Fengqiao River condition, tidal intensity

of Qiantang River condition and initial water level

conditions. Among them, the simulation of each

factor includes 5 groups, and the other factors in each

group are unchanged, which are shown in Table 1

below.

3 RESULTS AND ANALYSIS

3.1 The Influence of Discharge of

Puyang River on the Flushing Time

The distribution of the flushing time under different

discharge of the Puyang River conditions are shown

Figure 5. It can be seen that the flushing time of the

Xijiang and Dongjiang has a gradual increase trend

from upstream to downstream. However, due to the

supporting effect of the Fuchun River flow on the

Puyang River outlet, the flushing time of area below

Meichi increased first and then decreased from the

upstream to the downstream. From the perspective of

the distribution curve spacing of the flushing time

under the different discharge of Puyang River

conditions, when the discharge of Puyang River

increases from 150 m

/s to 300 m

/s, The impact of

the Puyang River flow on the flushing time of the

Xijiang River, the middle and lower reaches of the

Dongjiang River and area below Meichi is more

significant than when the discharge of Puyang River

increases from 300m

/s to 750m

/s.In general, the

flushing time of the Xijiang River, Dongjiang River

and area below Meichi decreases with the increase of

discharge of Puyang River .

Study on Inﬂuencing Factors of Flushing Time of Puyang River in the Upper Tributary of Qiantang River

421

(1) Xijiang River

(2) Dongjiang River

(3) Area below Meichi

Figure 5: The distribution of the flushing time under

different discharge of the Puyang River conditions.

3.2 The Influence of Discharge of

Fuchun River on the Flushing Time

The distribution of the flushing time under different

discharge of the Fuchun River conditions are shown

Figure 6. It can be seen that the variation law of the

flushing time along the three regions is the same as

the aforementioned discharge of Puyang River factor.

In general, the flushing time of the Xijiang River,

Dongjiang River and area below Meichi increases

with the increase of discharge of Fuchun River.

Table 1: The simulation conditions.

Impact factor

Condition

No.

Discharge of

Puyang

River(m

/s)

Initial water

level (m)

Discharge of

Fuchun

River(m

/s)

Discharge of

Fengqiao

River(m

/s)

Tidal

range

(%)

Discharge of

Puyang

River

CN1 150 9.3 1600 15 50

CN2 300 9.3 1600 15 50

CN3 450 9.3 1600 15 50

CN4 600 9.3 1600 15 50

CN5 750 9.3 1600 15 50

Discharge of

Fuchun

River

CN6 300 8.8 1600 20 50

CN7 300 8.8 3200 20 50

CN8 300 8.8 4800 20 50

CN9 300 8.8 6400 20 50

CN10 300 8.8 8000 20 50

Discharge of

Fengqiao

River

CN11 150 7.35 1600 10 50

CN12 150 7.35 1600 20 50

CN13 150 7.35 1600 30 50

CN14 150 7.35 1600 40 50

CN15 150 7.35 1600 50 50

Qiantang

River tidal

intensity

CN16 300 8.8 4050 20 10

CN17 300 8.8 4050 20 30

CN18 300 8.8 4050 20 50

CN19 300 8.8 4050 20 70

CN20 300 8.8 4050 20 90

Initial water

level

CN21 300 7.3 1600 20 50

CN22 300 8.8 1600 20 50

CN23 300 10 1600 20 50

CN24 300 10.5 1600 20 50

CN25 300 11 1600 20 50

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422

From the perspective of the distribution curve

spacing of the flushing time under the different

discharge of Fuchun River conditions, the influence

of discharge of Fuchun River on the flushing time

roughly extends to the middle and lower reaches of

the Xijiang and Dongjiang River. At the same time,

the discharge of Fuchun River has a smaller impact

on the flushing time of the Puyang River than the

discharge of Puyang River.

(1)Xijiang River

(2) Dongjiang River

(3) Area below Meichi

Figure 6: The distribution of the flushing time under

different discharge of the Fuchun River conditions.

3.3 The Influence of Discharge of

Fengqiao River on the Flushing

Time

The distribution of the flushing time under different

discharge of the Fengqiao River conditions are shown

Figure 7. It can be seen that the variation law of the

flushing time along the three regions is the same as

the aforementioned discharge of Puyang River factor,

also. In general, the flushing time of the Xijiang

River, Dongjiang River and area below Meichi

increases with the increase of discharge of Fengqiao

River.

From the perspective of the distribution curve

spacing of the flushing time under the different

discharge of Fengqiao River conditions, the

distribution curves of the flushing time along the

Xijiang River and area below Meichi under different

conditions basically coincide, that is, the discharge of

Fengqiao River has little effect on the flushing time

in these two regions. And the influence of the

discharge of Fengqiao River on the flushing time of

the Dongjiang River Basin is roughly 18km~25km

away from the entrance of the Dongjiang River.

(1)Xijiang River

(2) Dongjiang River

(3) Area below Meichi

Figure 7:

The distribution of the flushing time under

different discharge of the Fengqiao River conditions.

Study on Inﬂuencing Factors of Flushing Time of Puyang River in the Upper Tributary of Qiantang River

423

3.4 The Influence of Discharge of Tidal

Intensity of Qiantang River on the

Flushing Time

The distribution of the flushing time under different

tidal intensity of Qiantang River conditions are shown

Figure 8. It can be seen that the variation law of the

flushing time along the three regions is the same as

the aforementioned discharge of Puyang River factor,

also. In general, the flushing time of the Xijiang

River, Dongjiang River and area below Meichi

increases with the increase of discharge of tidal

intensity of Qiantang River.

(1) Xijiang River

(2) Dongjiang River

(3) Area below Meichi

Figure 8:

The distribution of the flushing time under

different tidal intensity of Qiantang River conditions

From the perspective of the distribution curve

spacing of the flushing time under the different tidal

intensity of Qiantang River conditions, the

distribution curves of the flushing time along the

Xijiang and Dongjiang River under different

conditions basically coincide, that is, the tidal

intensity of Qiantang River has little effect on the

flushing time in these two regions. And the influence

of the discharge of tidal intensity of Qiantang River

on the flushing time of the area below Meichi is

within 5km of downstream exit.

(1) Xijiang River

(2) Dongjiang River

(3) Area below Meichi

Figure 9:

The distribution of the flushing time under

different initial water level conditions.

3.5 The Influence of Discharge of

Initial Water Level on the Flushing

Time

The distribution of the flushing time under different

initial water level conditions are shown Figure 9. It

can be seen that the variation law of the flushing time

along the three regions is the same as the

aforementioned discharge of Puyang River factor,

also. In general, the flushing time of the Xijiang

River, Dongjiang River and area below Meichi

increases with the increase of initial water level.

WRE 2021 - The International Conference on Water Resource and Environment

424

From the perspective of the distribution curve

spacing of the flushing time under the different initial

water level conditions, when the initial water level

increased from 7.3m to 8m, the impact of the flushing

time in the three regions was small; When the initial

water level increased from 8.8m to 10m, the impact

of the flushing time in the three regions increased

significantly. Among them, the Xijiang River and

Dongjiang River have a relatively large range of

changes, and area below Meichi is relatively small.

The reason is related to the topography of the cross-

section of the river bed in each region. The bottom

slopes of the Xijiang River and Dongjiang River are

steeper and the upper parts of the Xijiang River and

Dongjiang River are gentler, while the riverbed of

area below Meichi is wider, and the slope changes at

the bottom and upper part of both banks are smaller

than those of the Dongjiang River and Xijiang River.

When the initial water level increased from 10m to

10.5m and then increased to 11m, the flushing time

curves of the three regions also changed evenly.

4 CONCLUSIONS

This paper takes Puyang River as the research object

and establishes a two-dimensional hydrodynamic

model to study the influence factors of flushing time

of Puyang River in the upper tributary of Qiantang

River. The research results provide guidance for the

prevention and control of water environment in tidal

rivers，got the following conclusions:

(1) Under the action of different influencing

factors, the flushing time of the Xijiang and

Dongjiang has a gradual increase trend from upstream

to downstream. However, due to the supporting effect

of the Fuchun River flow on the Puyang River outlet,

the flushing time of area below Meichi increased first

and then decreased from the upstream to the

downstream. At the same time, the influence of

discharge of Fuchun River on the flushing time

roughly extends to the middle and lower reaches of

the Xijiang and Dongjiang River. The discharge of

Fengqiao River has little effect on the flushing time

in the Xijiang River and area below Meichi, and the

influence of the discharge of Fengqiao River on the

flushing time of the Dongjiang River Basin is roughly

18km~25km away from the entrance of the

Dongjiang River. The influence of the discharge of

tidal intensity of Qiantang River on the flushing time

of the area below Meichi is within 5km of

downstream exit.

(2) Except for the negative correlation between

the discharge of Puyang River and the flushing time

of Puyang River Basin, and between the discharge of

Fengqiao River and the flushing time of the Fengqiao

River, each factor has a positive correlation with the

flushing time of the Puyang River Basin.

(3) The initial water level and the discharge of the

Puyang River are the main factors affecting the

flushing time of the river basin, and the discharges of

the Fuchun River and the Fengqiao River also have a

significant effect on the flushing time.

The results of this research can provide the basis

for the prevention and control of the water

environment of tidal rivers, and also provide technical

support for the ecological regulation of tidal rivers.

ACKNOWLEDGEMENTS

This experiment was supported by the Dean's Fund of

Zhejiang Institute of Hydraulics & Estuary. Thank

you here.

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Study on Inﬂuencing Factors of Flushing Time of Puyang River in the Upper Tributary of Qiantang River

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