Morphological Evolution of the Scour Pit After Danjiangkou Dam

Heightening Project

Ying Chen,Xian-zheng Wang, Hong-xia Zhang and Zhi-gang He

Hanjiang Bureau of Hydrology and Water Resources Survey, Changjiang Water Resources Commission, China

Keywords: Danjiangkou Dam Heightening Project, Scour Pit, Mophological Evolution, Empirical Orthogonal Function,

Autumn Flood in Hanjiang in 2021.

Abstract: Danjiangkou Reservoir is the largest artificial fresh water lake in Asia, the water source of the middle route

of the South-to-North Water Diversion Project. This paper analyzes the process and the reasons of the scour

pit under Danjiangkou Dam,then particularly analyzes its morphological evolution after the Autumn flood

in Hanjiang in 2021. After the flood in 2021, there are big increases in the range and depth of the scour pit.

The results shows that the area of 75m contours of the spit increased from about 1.2×10

4

m

2

to 1.4×10

4

m

2

.

Correspondingly, sediment volume of the scour pit drastically decreased, especially the volume between the

75m and 60m contours. Furthermore, the Empirical Orthogonal Function (EOF) is applied to analyze the

morphological evolution of the scour pit under the Danjiangkou dam. The topographic data are separated

into independent spatial eigenfuntion along with temporal eigenfuntion by the EOF analysis. The first three

modes provide a detailed, comprehensive description of the morphological evolution of the scour pit. The

first mode accounts for 58.5% of the overall evolution patterns, it shows the trend of the scouring

downstream of the scour spit. The second mode reflects that the scour pit achieved a dynamic balance of

sediment. And the third mode indicates that the sediment source of the center of the scour pit. This paper

explains that the EOF analysis can be used into the evolution of river-bed, and provides the theory basis for

the safety operation of the Danjiangkou dam.

1 INTRODUCTION

Riverbed erosion and accretion are a major part of

the evolution in which many researchers are very

interested. Researchers have never stopped studying

on it (Nitsche et al., 2006 and Hu et al., 2009).

Research results can be used to analyze the variation

trend, so that appropriate measures can be taken to

protect hydraulic structure or riverbed and avoid

harm. Due to the impact of the flood release from the

dam, scouring below the sluice continues, and it is

subjected to severe erosion damage(Li et al, 2003).

There are many factors leading to scour pits under

dams. A submerged hydraulic jump downstream of

an apron tend to cause the evolution of local

scour(Akiyama et al., 2010), resulting in the shape of

the scour pits similar. The geometrical similarity of

scour pits downstream of a sluice gate is related to

geological factors(Hamidifar et al., 2017).

The establishment of water conservancy

projects in river channels changes the characteristics

of the flow field, and the energy of the discharge

under the spillway is much larger than the normal

specific energy of the flow in the downstream river,

so the scouring damage to the downstream river bed

is more obvious.( Shang., 2012))

The downstream river bed erosion is an

engineering issue of wide concern in the hydraulic

engineering industry. Physical and Numerical models

have been widely employed to explore the

characteristics of river bed erosion downstream of the

dam. Zhang and Liu (2020) developed a physical

model, finding that the special station, such as

downstream apron and the right bank will be scoured

under the condition of the special flow. The research

shows the shape optimum design of overflow weir is

a factor affecting the location of scour.

Hao and Li(2019) build the overall hydraulic

model test of Chaiping Hydropower station, draw a

conclusion of the size of the hydropower station is

optimized to weaken the downstream riverbed scour

and reduce the influence on the overall stability of

the dam, so that the hydropower station can meet the

operation requirements of the project.

A 2D water-sediment mathematical mode is

established by Zou and Tang(2018), for calcutating

308

Chen, Y., Wang, X., Zhang, H. and He, Z.

Morphological Evolution of the Scour Pit After Danjiangkou Dam Heightening Project.

DOI: 10.5220/0012024900003536

In Proceedings of the 3rd International Symposium on Water, Ecology and Environment (ISWEE 2022), pages 308-318

ISBN: 978-989-758-639-2; ISSN: 2975-9439

Copyright

c

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

the possible downstream scour of the dam for the

project to be caused after the complete of Bazizui

Project.Meanwhile, in order to accurately predict the

range and depth of scour pits in downstream riverbed,

a numerical model of local riverbed scour was

established based on the dynamic grid technology in

Fluent software by Fan(2021).

Based on the theoretical analysis and model

simulation results of riverbed scour under dam by

many scholars, it can be seen that the main

influencing factors of local scour under dam are

hydraulic factors such as the shape of overflow weir

and the size of downstream discharge. Taking

Danjiangkou Dam as an example, this paper

discusses the characteristics and evolution trend of

scour under danjiangkou dam after its heightening,

which provides technical support for dam operation.

Danjiangkou reservoir covers an area of 95,200

km

2

, accounting for 60% of the total area of the

Hanjiang River basin. Danjiangkou reservoir is the

largest artificial fresh water lake in Asia, the water

source of the middle route of the South-to-North

Water Diversion Project, the national first-class water

source protection zone, the important wetland

protection zone in China, and the national ecological

civilization demonstration zone. Danjiangkou dam

has attracted extensive attention both from both

inside and outside of the industry since it was set up.

Many scholars have also conducted a large number

of studies on the formation and evolution mechanism

of riverbed under the Danjiangkou Dam, and

analyzed the morphological evolution of scour

pits(Duan et al., 2007 and Zhang et al., 2008).

For Danjiangkou dam with the characteristics of

high water head, large discharge, the problems of

erosion damage are exceedingly serious. Virtually,

downstream riverbed has formed many scour pits

since the reservoir operation in the initial period of

the project. According to statistics, the duration of

flood charge in the deep hole 9-11 dam section is the

longest, the downstream bed received the certain

degree of flushing, and formed a prominent pit, now

the scour shows a trend of move to the upstream and

the East direction (Wang et al., 2007).

This paper mainly carried on the concrete

analysis to the scour pit under 9-11 dam section, and

analyzes the process and the reasons of the scour pit

under Danjiangkou Dam, then particularly analyzes

its morphological evolution after the Autumn flood in

Hanjiang in 2021. Therefore, the objectives in this

contribution are as follow: (1) to quantitatively

explore the morphological changes of scour spit in

area, sediment volume variations in different phase;

(2) to interpret the underlying factors associated with

morphological evolution and analysis the trend of the

scour pit.

2 STUDY AREA

2.1 General View of Danjiangkou Dam

Danjiangkou Reservoir is located in Xichuan County

of Henan Province and Danjiangkou City of Hubei

Province. Danjiangkou Reservoir is a large-scale

artificial reservoir with functions of water supply,

flood control and power generation as well as

irrigation and breeding. It is a multi-year regulating

reservoir. The initial project of Danjiangkou Dam

was started in September 1958 and completed in

1973. In the past 30 years, it has played a great role

and achieved remarkable economic and social

benefits. Then the Danjiangkou Dam heighting

project started in September 2005 and completed in

2013. In present, Danjiangkou reservoir dam has a

total area of 1050km

2

with a storage capacity of

29.05 billion m

3

when the normal water level is up

to 170m.

Danjiangkou dam is a wide-slit gravity dam,

which consists of dam sections 1-44 according to the

distribution characteristics of buildings a. According

to the frequency of operation, the spillway structure

is divided into two parts, normal spillway and

extraordinary spillway, which are arranged in the

dam section 8- 24. Among them, the dam sections 8-

13 is the deep hole dam section with a total length of

144m, which are composed of 11 deep spillway

holes. Dam sections 14-17 and 19-24 are overflow

dam sections with a total of 20 open overflow holes

of 8.5m in width. There are 8 holes in dam sections

14-17 as overflow holes, 12 holes in dam sections

19-24 as infrequently used overflow holes. Dam

section 18 is a longitudinal cofferdam with no-

overflow section (Wang et al., 2002).

2.2 Morphology of the Scour Pit Under

Danjiangkou Dam

According to the structural arrangement of the

spillway sections, it can be seen that the downstream

riverbed located at No. 8-13 dam sections and No. 9-

17 dam sections are vulnerable to be flushing. In

addition, combined with the statistics of the date of

the opening and closing of the gates (Table 1), the

duration of the flood discharge takes the longest in

the 9-11 dam section and the 15-16 dam section, so

the scouring situation of the downstream river bed in

this section needs to be focused on. With time the

Morphological Evolution of the Scour Pit After Danjiangkou Dam Heightening Project

309

riverbed has shown the water carved out a large

scour pit downstream of the 10-17 dam section. The

location and form of the scour pit are shown in

Figure 1.

Combined with the morphology of scour pits and the

exploration of riverbed geology under the dam (Cai

et al., 2002), it can be seen that the scour pit is

washed along three geological fault zones in the

plane at the same time. In this paper, three profile

lines are selected for qualitative analysis of scouring

and silting in the fault zone. The following sections

will elaborate on these ideas. The profile lines are

arranged based on the erosion zone and the positions

of the three erosion zones are shown in Figure 1.

Figure1: The location and form of the scour pit of the Danjiangkou Dam.

3 FORMMATERIAL AND

METHODS

3.1 Material

Topographic survey has been widely used to explore

river bed evolution. Based on digital contour DEM,

the sedimentary erosion characteristics of the scour

pit were studied. Topographic data are collected from

topographic maps from 2005 to 2021. All scales are

1:500, and Danjiangkou dam axis coordinate system

is adopted in the maps. Then kriging interpolation

technique is used to interpolate the digital data into a

grid with a spatial resolution of 2.5m × 2.5m to

reduce the interpolation error. In order to investigate

the morphological evolution of scour pits, the data of

discharge flow, water level difference between the

upper and lower reaches and opening position of dam

sections are collected and analyzed. The hydrological

data are all from the Hanjiang Bureau of Hydrology

and Water Resources Survey.

Table2 shows the statistics of flood discharge

from 2005 to 2021. Wusong height datum of China is

determined in the paper.

3.2 Methods

In order to analyze the spatial and temporal

correlations of elevation and explore possible

evolution patterns in different periods, the

multivariate analysis technique Empirical

Orthogonal/Eigen Function (EOF) is used in this

study. Empirical Orthogonal Function analysis

(EOF) method. EOF analysis was first introduced

into meteorological problem analysis (Yosef et al.,

2017). However, in recent years, many scholars have

applied this method to study topographic evolution

(Yuhi et al., 2017, Xia et al., 2005 and Horrillo-

Caraballo et al., 2009). Most of these scholars

analyze the evolution of typical sections by

processing topographic profile data, and some

scholars have applied this method to the evolution

analysis of the whole topographic region (Beckers et

al., 2003 and Dai et al., 2015). Through the research

of many scholars at home and abroad, it is known

that empirical orthogonal function analysis

methodcan be used to explain the causes of river bed

evolution. When EOF is used for terrain analysis,

feature vectors correspond to spatial samples, and

principal components correspond to time

ISWEE 2022 - International Symposium on Water, Ecology and Environment

310

coefficients, so it can be called EOF method for

time-space separation. Its principle is to decompose

the variable field that changes with time into several

mutually independent orthogonal modes through

spatio-temporal decomposition, and each mode has a

corresponding space function and time function. The

first several modes, which account for a large

proportion of the total variance of all variables at the

original space point, can be used to summarize the

most important information of the factor field.

Therefore, we can replace the study by studying the

rules of the change of these major components with

time and better explain the time changes of the field.

The advantage is that it can decompose the

irregularly distributed sites in a definite limited area,

and the result obtained by decomposition has more

complete physical significance, and can better reflect

the basic structure of the appearance.

In this paper, topographic data under

Danjiangkou Dam in recent years are given in the

form of elevation matrix A after anomaly

processing: A=X

m

×

n

. Where m represents spatial

data point and n represents time series length. Then,

the EOF analysis method is applied to decompose

the matrix

X

m

×

n

into orthogonal spatial characteristic

function V and time characteristic function Z.

X=VZ

(1

)

V is the spatial characteristic function,

reflecting the spatial variation of the scour pit area;

Z means time characteristic function, reflecting the

change of scour pit elevation over time.

The specific decomposition method is as

follows:

(1) Calculate the product of X and XT to obtain

a matrix R composed of correlation coefficients

R=X×X



(2

)

(2) Calculate the eigenvalues of the correlation

coefficient matrix R ( λ

1

, λ

2

... λ

m

) and the

eigenvector Vmxm, both of which should satisfy.

R=V

⋀

V



(3

)

∧ represents a diagonal matrix composed of

the eigenvalues of R.













=Λ

m

λ







00

0

00

2

1

(4)

Arrange the eigenvalues in non-ascending

order. Each eigenvalue has a list of eigenvectors

corresponding to it, also known as EOF. The

eigenvector corresponding to the k eigenvalue λk is

the k column of the eigenvector matrix V.

)(:,KVEOF

k

=

(5)

V is projected onto the matrix X composed of

terrain data, so as to obtain the corresponding time

coefficients of all spatial feature vectors

XVZ

T

×=

(6)

(4) Calculate the contribution rates of each

mode. The formula for calculating the corresponding

contribution rate Rk of the k is as follows:

[]

1

1, 2,..., ( )

k

m

i

R

kppm

λ

=

== ≤



(7)

(5) The cumulative contribution rate of each mode can

be obtained according to the formula below

)P

1

mG

m

i

p

i

<=



=

（

λ

(8)

Through the numerical calculation software

MATLAB according to the EOF analysis of the

calculation principle of programming, calculate the

time characteristic function and space characteristic

function of each mode, as well as the corresponding

eigenvalue. Therefore, elevation change of scour pits

under Danjiangkou dam can be transformed into a

series of characteristic values to reflect the pattern of

its morphological changes.

Table 1: The duration of the flood discharge of the dam section in 2017-2021.

Time

(h）

Dam Sections

8 9 10 11 12 14R 14L 15R 15L 16R 16L 17R 17L

2017 89.5 140.0 547.5 558.1 454.0 257.1 352.0 413

2018 52.2 139.5 434.9 242

2019 90.2 110.8 115.3 15.7 50.9 59.4 22.9 27.8

Morphological Evolution of the Scour Pit After Danjiangkou Dam Heightening Project

311

2020 23.0 109.2 167.8 3.0 24.5 171.0 142.7

2021 596.8 866.1 1185.5 1043.8 577.9 115.1 913.3 593.4 46.6 375

Sum 851.8 1226 2016.1 1620.6 628.8 166.0 456.0 1110.2 796.9 736.1 655 74.4 375

Table 2: The flood discharge data of the Danjiangkou Dam in 2005-2021.

Year

Maximum

reservoir

water

level

(

m

)

Maximum water

level under the

dam(m)

Water

Head

Difference(m)

Maximum

discharge（m

3

/s）

The number of

discharge sluice

hole(deep/overflow)

Days of

flood

discharge

2005 156.95 95.99 60.96 14300 9 5 32

2007 151.15 92.66 58.49 6870 6 1 27

2009 153.04 91.35 61.69 2450 1 16

2010 154.95 93.71 61.24 7280 7 2 55

2011 157.29 95.78 61.51 12800 9 4 29

2012 155.44 90.91 64.53 2460 2 6

2017 167.00 94.70 72.30 8040 4 4 34

2018 165.43 91.77 73.66 3380 4 23

2019 166.51 94.32 72.19 7260 5 3 10

2020 164.7 91.89 72.81 3680 1 2 19

2021 170.00 95.03 74.97 11100 5 4 75

4 EVOLUTION

CHARACTERISTICS OF

SCOUR PITS UNDER

DANJIANGKOU DAM

Based on topographic data of the Danjiangkou dam

from 2005 to 2021, DEM model was established to

analyze the evolution characteristics of scour pits

under the Danjiangkou dam in recent years.

4.1 Horizontal Changes

According to the profile of scour pits, scour pits are

basically extending downstream along the three

scour zones. In order to qualitatively analyze the

scour and silt shape of scour pits, scour pits are

divided into three levels according to contours of

60m, 75m and 85m for analysis.

Changes in the area of the scour pit in the 60m,

75-60m and 85-75m contours can be seen in figure

2. The 60m contours area can be used to describe the

variation of scour and silting at the bottom of the pit,

75-60m can reflect the evolution at the slope of the

pit, and 85-75m was set as the sediment scour and

deposition at the boundary of the pit. Considering

the implement of the Danjiangkou dam heightening

project in 2013 and intensive autumn flood in 2021,

the temporal variations of the morphology can be

generally divided into three phases: before the

heightening project (phaseⅠ: 2005~2013), after the

heightening project (phase Ⅱ : 2013~2019) and

through the autumn flood in Hanjiang in 2021

(phaseⅢ: 2019~2021). Elevation changes according

to the sediment erosion and accretion in each period

in figure 3, which the 75m contour line in 2005 can

better showcase the specific variation position of the

scour pit (bottom or slope).

During the phase Ⅰ :In 2005-2006, the

maximum discharge flow reached 15,100m

3

/s. The

area at the pit bottom increased significantly with

the scour area at 60m contour of 1619m

2

.

Meanwhile, the area of 75m contour almost

remained unchanged, with the area of the 85m

contour line decreased slightly. It shows that the

scour in this period mainly downward scours. In

2006-2013, there was a small change in the area

surrounded by contour lines below 60m and 75-60m,

while the area of 85-75m contour increased

significantly with an area increment of 1828m

2

,

ISWEE 2022 - International Symposium on Water, Ecology and Environment

312

indicating that scour pits were mainly developed and

expanded to the periphery during this period.

During the phaseⅡ: in 2013-2015, changes in

the area of scour pit were not evident. During this

period, discharge was decreasing, and scour pit

basically did not develop further. After the

Danjiangkou Dam heightening project, experienced

the autumn flood in Hanjiang in 2017, the area of

85-75m and below 60m contours decreased, while

the range between 75-60m showed an increasing

trend, indicating that the anti-scour layer had

gradually formed at the pit bottom during this

period, and there was no further scouring. The scour

sand at the pit tail fell back to the pit bottom, leading

to accretion of the pit bottom. As a result, the range

of 60m contour is reduced, and the scour tends to

extend downstream and to the east and west sides,

and the larger the downstream discharge, the larger

the range of expansion.

During the phase Ⅲ: in 2019-2021, Hanjiang

River experienced the autumn flood in 2021. The

discharge flow was as high as 11100m

3

/s with the

water head difference was 74.97m, the highest in

recent years. The water level in front of the dam was

170m, reached normal water level for the first time.

This means the jet trajectory length increased. The

effect goes further downstream. Therefore,

remarkable scour zone can be seen in the tail of the

scour pit（tail means downstream part of the scour

pit).

In order to study the elevation and position

changes of the lowest point of the scour pit, the 60m

contour line and the position and elevation of the

lowest point in 2005, 2011, 2017, 2019 and 2021

were superimposed and plotted in figure 5. As can

be seen from the figure, the 60m contour line is

separated into left and right parts after 2019, while

the middle part is silted, indicating that the scour pit

develops along the scour zone. The main scour site

is located near the right river bed of the scour pit,

and the minimum elevation of the scour pit is also

distributed along the scour zone and presents a

downward trend year by year. After the

implementation of the Danjiangkou dam heightening

project, the minimum elevation of the scour pit does

not move down further, but the elevation is still

reduced by 3.6m in 2021, which is mainly related to

the magnitude of the discharge flow.

Figure 2: Changes in the area of the scour pit in the 60m,

75-60m and 85-75m contours.

4.2 Longitudinal Changes

Sediment volume of the scour pit was calculated to

explore the erosion or deposition regime during the

2005-2021. The change of sediment volume, as well

as the area, was explored by dividing into three

levels (Figure. 4).

Changes in sediment volume displayed that the

region below the 85m contour received a net erosion

with the amount of 5.18×10

4

m

3

over the phaseⅠ

(2005-2013), with an average erosion rate of 6.48×

10

3

m

3

/yr. Furthermore, the amount of sediment

erosion during the 2006~2009 in the zone was the

most dominant over the phase I, which amount was

2.28×10

4

m

3

, accounting for 44.0% of the period. In

vertical, the erosion mainly occurred in the area

below 60m with the rate was 1.03 × 10

4

m

3

/a in

2005-2006. During this period, the maximum flood

discharge was 15100m

3

/s, which impact played a

dominant role on the scour spit evolution below the

60m contour. The rate of erosion below the 60m

contour in 2005-2006 was obviously higher than the

other periods of the phase I. However, the flood

discharge in 2009-2011 was as high as 13000m

3

/s,

but the elevation of the bottom of the scour pit did

not decrease further. In summary, the period from

2005 to 2011 was the main development period of

scour pit. The initial stage expresses the continuous

downward flushing of the scour pit which the

erosion occurs mainly below the 60m contour. Then

the second stage shows that the anti-scour layer

gradually appears at the bottom of the pit, which

leads to the downward flushing rate of the bottom of

the pit slowing down, and the effect of the discharge

on the surrounding slope becomes relatively

obvious. The sediment volume in 2011-2013 was

Morphological Evolution of the Scour Pit After Danjiangkou Dam Heightening Project

313

relatively in equilibrium and even slight sediment

accumulation was found in the period.

After the execution of heighting project, as a

result of the decrease of the discharge, the scour pits

turned into silting with the amount of 0.67×10

4

m

3

in 2013-2015. Subsequently, with autumn flood in

2017, Danjiangkou dam applied 167m high water

level for the first time after heighten the dam with

the discharge was 7750m

3

/s, the difference between

the upper and lower water levels of the dam reached

72.93m. The increase of water flow resulted in the

scouring trend of the scour pit, but the area from

60m to the bottom of the pit still showed a trend of

silting, which was mainly caused by the sediment at

the slope of the pit falling back to the bottom. In

2017-2019, the fluctuation of area alteration was

much insignificant due to the weak hydrodynamics.

During the period of 2019 to 2021(phase Ⅲ),

autumn floods occurred in the Hanjiang, and the

flood discharge days of the sluice reached 75 days,

and the discharge exceeded 10,000 m

3

/s. With the

hydrodynamic factors changed, the scour pit under

the dam presents a scouring trend, mainly within the

range of 75-85m. As shown in figure 3, the scour is

mainly located on the downstream tail slope. To sum

up, the morphology of the scour pit is

unstable.Considering the increasing discharge, the

scour pit in the morphological changes has the

potential to show an increasing trend. The scour

position of the scour pit is prone to locate the slopes

on both sides and downstream.

The section changes of the three scour zones in

the scour pit over the years were analyzed, as shown

in figure 6. As can be seen from the figures, scour

basically develops along the erosion zone ① . In

2011, the scour depth reached the maximum with an

elevation of 52.6m, which was more than 6m lower

than that of the same area in 2005. Subsequently, the

erosion zone①did not wash down further, and the

lowest elevation gradually increased, but the slope

near the downstream showed a trend of continuous

scour. With an elevation of 73m, the distance in

2019 was scrubbed nearly 20m compared with that

in 2011. The scour zone mainly showed downward

scour from 2005 to 2011, downstream scour from

2011 to 2019, and the scour zone mainly showed

downward scour and slope collapse from 2019 to

2021, with a collapse amplitude of 11m.

The lowest point of erosion zone②occurred in

2013, which was 54.5m. Then the pit bottom showed

a trend of backsilting with erosion zone②, and the

downstream slope showed a trend of scour,

especially in 2021, the slope scoured down about

15m. The erosion zone③ is relatively higher than

the other two, with the lowest elevation of 58.7m.

The scour range at the bottom of the pit and on both

sides of the slope is not large. The maximum scour

occurred in the transverse direction in 2009, which is

relatively stable in recent years. To sum up, scour

mainly occurs in the area of erosion zone①, while

erosion zone ② and ③ is relatively weak. The

amplitude and morphological changes of the scour

pit are strongly correlated with the discharge and

opened dam sections of the Danjiangkou dam.

Figure 3: Elevation changes of the scour pit. Positive values indicate net deposition while negative values indicate net

erosion.

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314

Figure 4: The erosion (negative values) and deposition (positive values) rates of the scour pit.

Figure 5: Horizontal changes of the scour pit in the 60m contours and the state of the lowest elevation.

Morphological Evolution of the Scour Pit After Danjiangkou Dam Heightening Project

315

Figure 6: Changes in the profile of the erosion zone①-③.

5 EMPIRICAL ORTHOGONAL

FUNCTION ANALYSIS

Based on the principle of empirical orthogonal

function (EOF) analysis, this section calculate the

contribution rates of different modes and

corresponding spatial and temporal parameters

below the 85m contour area of scour pits under the

dam, so as to accurately and scientifically analyze

the evolution characteristics of the scour pit under

the dam. The contribution rates of the first three

modes are 58.5%, 14.0% and 9.6%, respectively.

Therefore, the spatial and temporal distribution

characteristics of the first three modes are mainly

introduced according to the proportion.

Spatial function data were processed by contour

map, positive value indicated deposition(use warm

colors) while negative value represents erosion(use

cool colors) for making the spatial function more

intuitive to show the evolution of the scour pit. As to

the sedimentation area, scour is in the increasing

stage of time function and sedimentation is in the

decreasing stage. For the scour area, the trend is

opposite.

The first mode is most considerable for

morphological evolution, contributing to 58.5% of

the elevation variability (fig 7a).It can be seen that

the main trend of scour and accretion area of the

scour pit under Danjiangkou dam: the bottom of the

scour pit is gradually silted, and the slope (especially

tail of the erosion zone) are gradually scoured. On

the whole, the scour is the main with the maximum

scour depth is 0.065m while the maximum

sedimentation thickness is 0.02m. According to

figure 9b, the time function corresponding to the

first mode shows a rising trend, indicating that the

bottom of the scour pit near the apron presents a

continuous silting trend, and the tail channel of the

scour zone presents a continuous scouring trend. The

most intense areas of scouring and silting are located

in the erosion zone①, which is consistent with the

analysis results of the evolution characteristics of the

scour pit in the previous chapter. Moreover, it is

found that the evolution intensity is directly

proportional to the gradient of time function. The

change gradient of the time function from 2019 to

2021 is large, indicating that the erosion of the scour

pit has the greatest impact during this period. The

first mode reflects, to a certain extent, the impact of

hydraulic factors (including the intensity of

downstream discharge and the impact of hydraulic

push caused by the water head difference and ect.)

on the scouring and silting of the riverbed.

The contribution rate of the second mode is 14.0% (

figure 7c), and that of the third mode is only 9.6% (

figure7e). Compared with the first mode, the

contribution rate of the second mode is much lower,

but it can also show the evolution characteristics of

the scour pit under the dam to a certain extent. The

scour/silt degree of the second and third modes is

relatively balanced. The scour of the second mode

mainly occurs at the slope of scour pit (mainly

upstream, left and right) and the bottom, and the

scour range is small but the depth is large. Siltation

mainly occurs downstream of the scour pit.

Incorporating with the distribution of alternate time

function of "W" and "M" patterns of the second

mode and the evolution of scour pits over the years (

figure7d), it can be seen that the second mode is an

alternate scour and silting state of the scour pit, and

the degree of scour and silting is basically balanced,

indicating that the siltation in the downstream of

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scour pits is mostly from the sediment scoured down

from the upstream of the scour pit.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 7: The time-space variation of the scour pit by the

EOF analysis.

From the perspective of the spatial distribution of

the third mode, the scour area mainly occurs in the

slope and erosion zone①, and the siltation area is

concentrated in the center and the left of the scour

pit. According to the thickness distribution of scour

and siltation, variations are basically the same, that

is to say, the local morphological adjustment of the

scour pit is mainly caused by the exchange of the

center of the scour pit and the slope of the scour pit.

Due to the shape of the scour pit, the sediment of the

slope is prone to deposit in the middle.

6 CONCLUSIONS

During the period of 2005 to 2021, the scour pit

under Danjiangkou dam was experienced

significantly morphological evolution. From the

horizontal plane, the scour pit gradually increased in

scope, mainly reflected in the extension of the

downstream area. The scour pit was mainly scoured

downstream along the three erosion zones, mainly

occurring in the erosion zone①. For the longitudinal

perspective, the phase Ⅰ (2005-2013) is the main

development period of the scour pit, and the scour

pit shows downward flushing depth. Scour volume

in this period is 5.18×10

4

m

3

accounting for 48.1%

of the total amount of scour pit, and the lowest

height of scour pit decreases by about 6m.

After the implementation of the Danjiangkou

dam heightening project, for the phase Ⅱ (2013-

2019), the scour pit developed slowly. The scour

area extended downstream mainly by scouring the

downstream slope. Meanwhile, the scour pit bottom

showed a silting trend instead of being scoured

further. The scour pit was in the redevelopment

stage for the phaseⅢ(2019-2021). The scour pit was

significantly deeper downstream, mainly due to the

change of hydrodynamic factors, such as the

significant increase of downstream discharge and the

increase of water head distance. In 2021, the normal

water level of the Danjiangkou dam was up to 170m

the first time, and the water head difference reached

74.97m. The scour area was moved to downstream

significantly. Meanwhile, the scour pit slopes

(mainly the upstream near the dam apron, the right

bank area and the downstream area of the erosion

zone) were still be eroded.

ACKNOWLEDGMENTS

This work was supported by National Natural

Science Foundation of China (Grant No.

U20A20317).

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