analysis of chinese dike breaches along the yangtze river and tributaries · 2014. 7. 29. · the...

Analysis of Chinese dike breaches along the Yangtze River and tributaries desk study

© Deltares, 2009

Henk Verheij, Paul Visser and Ulrich Förster

Title Analysis of Chinese dike breaches along the Yangtze River and tributaries Client Rijkswaterstaat / Centre for Water Management

Project 1001598-000

Reference 1001598-000-GEO-0003

Pages 48

Keywords dike breaches, China, inundation Summary In 1998, numerous dike breaches occurred in China. Within the framework of the MoU between the Chinese Ministry of Water Resources and Rijkswaterstaat, the Centre for Water Management requested the Chinese Research Centre on Flood and Drought Disaster Reduction (RCDR) to provide data from some of the 1998 dike breaches along the Yangtze river. These data could be extremely useful in improving Dutch dike breach models that have been implemented in inundation models such as SOBEK1D2D and HIS-OM. This report analyses the breach data provided by the Chinese authorities. The analysis shows that the information with respect to Zhaoyuan County Farm dike, Mengxi dike, and two breaches of the Tailai dike are particularly useful, since these cases give sufficient data on breach development, water levels, and discharges. For the other cases, data on breach development is limited to the maximum width of the breach, and information on discharges and/or water levels is missing for some cases. It is therefore recommended that inundation computations are carried out using the original Dutch breach models, together with improved models based on Chinese data for the following cases: Zhaoyuan County Farm dike, Mengxi dike, and Tailai County dike 2 and 3. References Rijkswaterstaat/Centre for Water Management contract WD-4924 dated 28 February 2008, Client project number 31008563

State final

i

Contents

1 Introduction 1 1.1 Introduction 1 1.2 Objective 1

2 Information provided 3

3 Cause of the dike breaches 5 3.1 Introduction 5 3.2 Location of the breach 5 3.3 Dike geometry 5 3.4 Subsoil and dike characteristics 6 3.5 Non-water-retaining structures in the dike profile 6

4 Models and equations for breach development 7 4.1 Introduction 7 4.2 Breach growth equations used in Dutch inundation models 9 4.3 Alternative dike breach equations 12

5 Analysis of dike breach data 15 5.1 Initial breach development 15 5.2 Breach width development 16 5.3 Scour hole development 17 5.4 Conclusions 19

6 Water levels, discharges, and volumes 21 6.1 Introduction 21 6.2 Water levels and discharges 21 6.3 Volumes 24 6.4 Conclusions 24

7 Conclusions and recommendations 25 7.1 Conclusions 25 7.2 Recommendations 26

8 References 28 Annex Memo: Visit to China and field visit of former dike breaches

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Analysis of Chinese dike breaches along the Yangtze River and tributaries

1

1 Introduction

1.1 Introduction In 1998, numerous dike failures occurred in China. Within the framework of a MoU between the Chinese Ministry of Water Resources and Rijkswaterstaat, the Centre for Water Management requested the Chinese Research Centre on Flood and Drought Disaster Reduction (RCDR) to provide data from some of the 1998 dike breaches along the Yangtze River. These data could be extremely useful for improving Dutch dike breach models, which have been implemented in inundation models such as SOBEK1D2D and HIS-OM. During mid 2008, RCDR selected some dike breach cases (Anonymous, 2008). In a memo dated August 2008, Deltares concluded that the quality of the information provided for the different cases differed markedly. In September 2008, representatives from Rijkswaterstaat/Centre for Water Management, Deltares and RCDR visited two breach locations (Förster, 2008; see Annex A). RCDR completed the report after the visit (Yao Qiuling et al, 2008). The information that was provided is the basis for the analysis reported in this document. The analysis has been carried out by Mr. H.J.Verheij of Deltares and Delft University of Technology, with specialist advice provided by dr. P.J.Visser of Delft University of Technology. Mr. U.Förster of Deltares was in charge of project management.

1.2 Objective The models applied by the Dutch government to predict inundation of polders after breaching of a dike use semi-empirical equations for the breach growth. These equations are based on limited data from prototype dike breaches and from small-scale laboratory experiments, where the latter mainly involve breaching in sand dikes. The objective of this study is therefore defined as follows: To analyse the collected dike breach data, and to investigate whether the data can be useful in improving Dutch breach models that have been implemented in the inundation models SOBEK1D2D and HIS-OM. If there is a realistic chance of success, the data will be used in a follow-up project for improving Dutch dike breach models that have been implemented in inundation models such as SOBEK1D2D and HIS-OM.

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2 Information provided

Based on the information provided and the September 2008 visit, the following cases seem to be useful for further analysis related to improving the Dutch inundation models SOBEK1D2D and HIS-OM: Zhaoyuan County Farm dike breach (Nenjiang trunk stream) Mengxi dike ring (Hudu River) Tailai County (Nenjiang trunk stream) Paizhou Dike; Henzheng dike ring (Yangtze River) City Defence dike of Jiujiang City (Yangtze River)

This chapter presents a short description of the cases. For further details, see Yao Qiuling et al (2008). To validate breach models against laboratory or prototype data, it is in general important to have information on hydraulic conditions, geotechnical data, shape of the dike cross-section, soil characteristics, and data from the breach development. As the data provided include information on the mentioned items, it is expected that they can be used to improve the Dutch breach models. In that respect, particularly information on the time-dependent development of the breach width is very important, as well as water level data in the river and in the inundated polder as a function of time.

Zhaoyuan County Farm dike breach (Nenjiang trunk stream) This case deals with two breaches. The dike failure was caused by overtopping, and the largest breach width was 530 m with a maximum scouring depth of 15.3 m. Development of the breach width over time is clearly described. Data are also available of water levels in the river, as well as the discharge through the breach as a function of time. Finally, extensive information on soil characteristics is available.

Mengxi dike ring (Hudu River) The process of dike breaching is described in detail, including time-dependent development of the breach width. The dike breach was caused by piping. Subsequent sliding of the dike’s inner and outer slopes resulted in a breach after 20 minutes, with a width of about 30 m. The final width of the breach was 185 m. Detailed data related to soil characteristics are also available.

Tailai County (Nenjiang trunk stream) Several breaches occurred in this case. In general, the amount and quality of the information is limited. The dike breaches were caused by overtopping. Water flowed over the dike at a certain moment, resulting in several breaches with widths up to 208 m. Some information on development of the breach width is available. As in other cases, extensive data related to soil characteristics are available.

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Paizhou dike; Henzheng dike ring (Yangtze River)

The dike breaches were caused by piping. The data provided were submitted in a clear way with respect to breach development, breach shape, and water levels. Collapse of the dike resulted in a final breach width of 760 m, with a maximum depth of 32.1 m. Development of the breach width has not been sufficiently clearly described, although indications have been given. Time-dependent information on water levels in the Yangtze River and detailed data related to soil characteristics are not available.

City Defence dike of Jiujiang City (Yangtze River) The dike is situated in an urban area with a flood-wall on top of the dike. The cause of dike failure was piping with subsequent sliding of the inner slope, resulting in a final breach width of 64 m and a maximum scouring depth of 7 m. Development of the breach width is sufficiently described. Detailed data related to soil characteristics have been provided. Time-dependent data of water levels in the Yangtze River are not available.

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3 Cause of the dike breaches

3.1 Introduction For the cases presented, the causes of the dike breaches were:

1. overflow, or 2. piping

As a consequence of overflowing or piping, sliding occurred that resulted in an initial gap. This chapter investigates possible causes of the breaches.

3.2 Location of the breach The available information shows that three of the breached dikes were located in the flood plain at some distance from the main channel. However, the breaches in Mengxi dike and the City Defence dike were located directly adjacent to the main channel. This may have contributed to the water level inside the dike body as well as the piping process, as the initial water table was already high before the flood started. The breach in Paizhou dike was situated at a locally smaller cross-section of the flood plain. The water pressure was probably already higher, and may have contributed to the piping process.

3.3 Dike geometry Figures 3.1 and 3.2 show example cross-sections of the Zhaoyuan County Farm and Tailai County dikes respectively at the breach location. The dike at Zhaoyuan County Farm was constructed on farmland with an unusual shape. The dike crest level was probably too low as the dike was described as a “critical levee section”, and breaching was caused by overflowing. The Zhaoyuan County Farm dike is a sand dike.

Figure 3.1 Cross-section of dike breach Zhaoyuan County Farm dike; Pangtoupao reach

4~5m

2.5~5.8

142.21~141.12

1:2 1:2.5sand

clayriverb

ank protection

sand

4~5m

2.5~5.8

142.21~141.12

1:2 1:2.5sand

clayriverb

ank protection

sand

Figure 3.2 Tailai County dike: Alaxin location

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Note that Figure 3.2 shows the cross-section of the Tailai County dike at the Alaxin location. The figure shows a sand dike. The dike body consists of clay at the three other Tailai County dike locations. In general, dike geometry is not considered to have caused breaching of the dikes, except for the City Defence dike. This dike was located in a city, and consisted of an embankment with a vertical wall at the riverside measuring approximately 3.5 m in height. The top level of this wall was between 0.8 m and 1.1 m higher than the crest level of the dike at the land side. The vertical wall is probably the cause of the breaching, after piping resulted in a reduction of the resistance forces related to the dike mass.

3.4 Subsoil and dike characteristics Piping was responsible for three of the breaches: Mengxi dike, Paizhou dike and City Defence dike. Settlement and sliding occurred due to the piping. The piping process may be induced if a sandy subsoil acts as a dike base. For example, the Paizhou dike consists of loamy soil on a sand base. The City Defence dike also partly consists of a concrete wall. Zhaoyuan County Farm dike is a sand dike with a clay cover, and Tailai County dike is a clay dike. Neither dike suffered piping, and the cause of breaching in both cases was overtopping.

3.5 Non-water-retaining structures in the dike profile The information provided does not indicate the presence of non-water-retaining structures in, on, or near the dikes, such as trees, small storage buildings for dike repair or maintenance material. Such structures cannot therefore have caused breaching of the dikes. The absence of trees in 1998 was confirmed during the field visit in September 2008 (Förster, 2008); see Annex A.

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4 Models and equations for breach development

4.1 Introduction This chapter describes the models and equations that have recently been developed in the Netherlands for the prediction of breach growth. Section 4.2 presents an overview of the breach growth equations that have been implemented in the Dutch inundation models Sobek1D2D and HIS-OM. A number of other breach equations are also discussed. In sections 4.3 and 4.4, the observed developments of the breach width are compared with predictions obtained using Dutch breach equations. In section 4.5, a new equation is derived for predicting the volume of the maximum scour hole as a function of the water power (discharge times water head). Finally, section 4.6 contains a number of conclusions. The relationship between different models and equations will first be discussed. A dike breach may result in inundation of a polder. The dimensions of the breach, combined with the duration of the high water level in front of the dike, determine the amount of water flowing into a polder. It is obviously essential to understand and be able to predict the process of breach initiation and time-dependent development breach development. Figure 4.1 shows a schematic of relevant events during an inundation computation.

Figure 4.1 Relationship between empirical breach growth equations and inundation models SOBEK1D2D

and HIS-OM

Two inundation models are used in the Netherlands: SOBEK1D2D developed by Deltares/Delft Hydraulics, and HIS-OM developed by Rijkswaterstaat. These models

High water level

Initial breach

Development of breach

Water flows through breach

Inundation of polder

Prediction of local water levels

Selection of time and location of the breach

Empirical breach growth equations

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implement simple breach growth equations based on available breach data. The models are applied to inundation studies by using various scenarios in which breaches are assumed at different locations. It is currently impossible to compute the location as well as the point in time of breach initiation. These aspects have to be selected beforehand on the basis of experience and expertise. The final output of the inundation models are the water levels at different locations in the inundated area. This obviously requires data on surface levels, locations and levels of line elements such as roads, and locations and dimensions of buildings. In addition to the simple, empirical breach growth equations, several validated numerical models are also available for breach development in homogeneous sand dikes. These models are also applied for dikes with cohesive material by adding empirical correction factors. The two breach models are: BRES developed at Delft University (see Visser, 1998), and BREACH developed at Deltares/Delft Hydraulics. These models can also predict the growth of the breach width, and the discharge through the breach as a function of time. Both models consider the development of a breach as five consecutive phases, which are shown in Figure 4.2. For computational purposes BRES uses all phases, while BREACH starts at t = t3 assuming an infinite short period of time between breaching initiation and the start of width development at t3. In principle, it is possible to implement the BRES/BREACH models in the inundation models, but this requires more data before an inundation computation can be executed. This report focuses on the possibility of improving the simple, empirical prediction equations as implemented in SOBEK1D2D and HIS-OM.

Figure 4.2 Five phases in the development of a dike breach (Visser, 1998)

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4.2 Breach growth equations used in Dutch inundation models Figure 4.3 shows two black-box relationships that have been implemented in the Delft inundation models SOBEK1D2D and HIS-OM to predict the breach width as a function of time, and the soil type of the dike’s core material:

Sand dike: 67 log0.145sand

tW (4.1a)

Clay dike: 20log0.08clay

tW (4.1b)

where: W = breach width at time t (m) t = time after breach initiation (hrs) Figure 4.3 Observed breach width as a function of time and dike material (note: the vertical axis shows the

breach width W)

A more recent breach growth equation developed by Verheij-vdKnaap predicts the growth of the breach width W (m) as a function of the difference in water levels at both sides of the dike at the breach location (H in m), and the critical flow velocity for the dike material (uc in m/s). This breach equation has been implemented in the Delft package SOBEK1D2D and HIS-OM for inundation modelling:

0,5 1,5

21

.log 1c c

f gg HW f tu u

(4.2)

with H = hupstream – hdownstrean where:

0

50

100

150

200

250

0 10 20 30 40 50 60

t (hr)

B (m

)

Zalk 1926Papendrecht 1953Nieuwkuijk 1880ZwinWL Basin tests 1996TU Basin Tests 1996Wieringermeer 1945 Kleiachtige dijkenLog function sandLog function clay

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W = breach width at time t (m) uc = critical flow velocity depending on the soil characteristics (m/s) f1 = coefficient (-) f2 = coefficient (-) hup = upstream water level (m) hdown = downstream water level (m) t = time (s) g = acceleration of gravity (m/s2) Figure 4.4 shows Equation (4.2) with the data.

0

50

100

150

200

250

0 10 20 30 40 50 60

time in hours

bre

ach

wid

th (

m)

Bres gemeten

B = 1.3 * g^0.5 * H^1.5 / uc * log(1+0.04*g* t / uc)

Figure 4.4 Breach width W on the vertical axis as a function of time t in hours on the horizontal axis for

sand dikes

The following default values and ranges can be applied for the input variables: Table 4.1 Default values Equation (4.2)

Parameter Description Default Range f1 coefficient 1.3 0.5 – 5 f2 coefficient 0.04 0.01 – 1 Uc critical flow velocity 0.2 m/s 0.1 - 10 m/s

Equation (4.2) contains the critical flow velocity uc for the dike material. An appropriate value should be selected to compute accurate breach widths. Methods to select a value for the critical flow velocity are presented below. Table 4.2 shows characteristic values for various soils based on earlier research by Verheij (2002).

observed breach widths -------- Equation (4.2) - - - - - Equation (4.1a)

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Table 4.2 Strength characteristics of various soil types Type of soil uc (m/s) c (Pa) cE (m-2s-2) grass, good 7 185 0.01.10-4 grass, moderate 5 92.5 0.02.10-4 grass, bad 4 62 0.03.10-4 clay, good (compact; undrained = 80-100 kPa)

1.0 4 0.50.10-4

clay with 60% sand (firm; undrained = 40-80 kPa)

0.80 2.5 0.60.10-4

good clay with less structure 0.70 2 0.75.10-4 good clay, heavily structured 0.60 1.5 1.5.10-4 bad clay (loose; undrained = 20-40 kPa) 0.40 0.65 3.5.10-4 sand with 17% silt 0.23 0.20 10.10-4 sand with 10% silt 0.20 0.15 12.5.10-4 sand with 0% silt 0.16 0.10 15.10-4 Note: c = critical shear stress; cE = strength coefficient However, instead of using Table 4.2, it is also an option to relate the erosion resistance to the plasticity index Ip and the liquid limit wl. The problem then arises that this cannot be used for sand because for sand Ip = 0. Based on the work of Mirtskhoulava (1991), Hoffmans and Verheij (1997) simplified the expression for cohesive sediments and presented the following equation for the critical flow velocity:

fasa

c Cgdd

hU 6.04.08.8log

in which Cf (= 0.035c) is the fatigue rupture strength of clay, c is the cohesion, da (= 0.004 m) is the size of detaching aggregates according to Mirtskhoulava, and h is the water depth. The Mirtskhoulava equation results in conservative values. An alternative has been presented by Kamphuis & Hall (1983) via a relationship with the percentage of clay:

, 1 0.01c c sand clayu u P where: uc,sand = critical flow velocity for sand (m/s) Pclay = percentage clay (%)

= coefficient (-) De Vroeg (2002) added the influence of the void ratio v, resulting in the following equation:

, 1 0.01 0.65c c sand clayu u P v where: = coefficient (-)

v = voids ratio (-) Recommended values for the coefficients en are 15 and 1 respectively. A value of 0.2 -0.3 m/s is assumed for uc,sand .

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The voids ratio v is a measure for the compactness:

1

nvn

where: n = porosity (-) An average n value for sand is 0.4, which results in a voids ratio of v = 0.65. Possible values for clay are: “stiff clay” v = 0.25 and “fairly compacted clay” v = 0.5.

4.3 Alternative dike breach equations In addition to the Equations (4.1) and (4.2), two other breach growth equations are available. It cannot be excluded that Equation (4.2) will in future be replaced by one of these equations. Therefore, both equations will be discussed. Zhu (2006) presented a model for breach growth in clay dikes. His equation for the prediction of width growth is as follows:

1 1

2( )sin cos

sl

sl

dEdW tdt d E dt

(4.3)

where: d = water depth in the breach (m) Esl = lateral erosion rate (m/s)

1 = slope angle side walls of the breach (degrees) t = time (s) Bernitt (2006) and Bernitt and Madsen (2008) recently presented an equation for the growth of breach width in sea dikes. They also presented a figure with data from observed breach widths as a function of time. As the figure and the equation differ, the authors of this report adjusted the equation to obtain the values presented in the figure. The resulting equation reads:

1/ 284.34 10dW t

g Fdt

(4.4)

where: F = threshold level of the foreland (m) g = constant of gravity (m/s2)

= water level (m) The results of the equations of Zhu (2006), Bernitt (2006) and Bernitt and Madsen (2008) have been compared with the Verheij-vdKnaap Equation (4.2). Curves can be constructed by assuming some data, for example a water depth d in the breach of 4 m ( - F = d =4m). The result is seen in Figure 4.. In principle, Equation (4.3) shows the same trend as the Deltares/Delft Hydraulics Equation (4.2) but with a smaller breach width. The reason for this is that Equation (4.2) is derived for sand dikes, while Equation (4.3) is based on clay dike results. Equation (4.4) shows a linear relationship between time and breach width and was

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derived on the basis of data from sand dikes. This explains the small difference with Equation 4.2 for the first 48 hours, but it increases for longer durations. However, a linear relationship as in Equation (4.4) is not realistic.

0

100

200

300

400

500

600

0 50 100 150

Time (hr)

Bre

ach

wid

th (m

)

Eq.(4.2)Eq.(4.3)Eq.(4.4)

Figure 4.5 Comparison of prediction equations for the breach width

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5 Analysis of dike breach data

5.1 Initial breach development For the cases presented, there is unfortunately limited information available about the initial breach width. Breach initiation data are only presented for Zhaoyuan County Farm dike and Mengxi dike. For Mengxi dike (clay dike), the data includes the development of a breach width measuring 30 m within 20 minutes, finally resulting in a maximum width of 185 m. For Zhaoyuan County Farm dike (sand dike), the initial development was:

Time (hrs)

Width (m)

0 0 5.75 4.1

11.75 81.4 Figure 5.1 presents a comparison of the initial breach width data using the Verheij-vdKnaap formula for the Zhaoyuan County Farm dike and the Mengxi Dike. It can be clearly seen that the initial development for the Mengxi dike fits closely, whereas data from the Zhaoyuan County Farm dike does not. These results are rather confusing. The Zhaoyuan County Farm dike is a sand dike and is expected to follow Equation (4.2), but even after nearly 12 hours the width is less than predicted by Equation (4.2). The Mengxi dike, however, is a clay dike and should show a smaller breach width. Note that the initial erosion of dike material is based on the head cut migration mechanism.

Breach initiation

020406080

100120140160180200

0 2 4 6 8 10 12 14 16

Time (hr)

Bre

ach

wid

th (m

)

Eq.(4.2)Zhaoyuan dikeMengxi dike

Figure 5.1 Initial breach growth of Zhaoyuan County Farm dike and Mengxi dike

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5.2 Breach width development

An example of the final shape of the breach is shown in Figure 5.2. Similar figures are available for the dike breaches in all cases.

115

120

125

130

135

140

100 200 300 400 500 600 700 800

hor i zont al di st ance( m)

elev

atio

n(m)

127.8 127.7

430m530m

134.66 Dike crest

Landside groundRiverside ground 16.4m

6.96m

9.44m

115

120

125

130

135

140

100 200 300 400 500 600 700 800

hor i zont al di st ance( m)

elev

atio

n(m)

127.8127.8127.8 127.7127.7

430m430m530m

134.66 Dike crest

Landside groundRiverside ground 16.4m

6.96m

9.44m

Figure 5.2 Breach shape of Pangtoupao reach of Zhaoyuan county farm dike

However, information about time development of the breach width is only available for the Zhaoyuan County Farm dike, and for the initial growth in the first 20 minutes in the case of the Mengxi dike (see Section 5.1). The available information is summarised in Table 5.1 for Zhaoyuan County Farm dike (sand dike) and in Table 5.2 for the other dike breaches. Since no data were available for the length of time that elapsed until the maximum width was observed, the duration was based on experience. This was assumed to be 120 hours. The results have been compared with Equation (4.2), see Figure 5.3. Table 5.1 Breach development in Zhaoyuan County Farm dike (sand dike)

Time (hrs) Width (m) 0 0 6 4.1 12 81 18 122 30 340 54 366 78 530

Table 5.2 Maximum breach width of all breaches (duration 120 hours)

Dike breach Max width (m)

Remarks

Mengxi dike 185 30 m after 20 minutes, clay dike Tailai County dike 90.5

208 170 120

Alaxin (1) – sand dike Laojuzi (2) – clay dike Banzishan (3) – clay dike Guangrong-Yanjian (4) – clay dike

Paizhou dike 760 Loamy soil on a sand base City Defence dike 64 Clay with gravel

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Breach growth

0

100

200

300

400

500

600

700

800

0 20 40 60 80 100 120 140

Time (hr)

Bre

ach

wid

th (m

)

Eq.(4.2)Zhaoyuan dikeMengxi dikeTalai dike 1Talai dike 2Talai dike 3Talai dike 4City defence dikePaizhou dike

Figure 5.3 Comparison of the breach width development using Equation (4.2)

For all dikes except for Paizhou dike and Zhaoyuan County Farm dike, the final breach width is less than predicted by Equation (4.2). Paizhou dike is a clay dike (more specifically, loamy soil) and the large breach width therefore cannot be explained. The result for the Zhaoyuan Country Farm dike, which is a sand dike, also cannot be explained since the observed width growth is larger than for the clay dikes. Note the slow initial development in the first 6 hours followed by rapid breach width growth. The predictions for the other dikes are more or less in line with expectations, although the observed breach width of Tailai dike 1 is rather low for a sand dike. It has to be concluded that the correlation between the data and the predictions given by Equation 4.2 based on Dutch breach observations is rather poor. However, it should be kept in mind that no information is available about the point in time of breach initiation for the Chinese breach. For example, the differences will be small if we consider a duration of 72 hours instead of 120 hours. The foregoing means that the Dutch breach growth in Equation (4.2) can be improved by extending the database with the Chinese breach data (except for Paizhou dike, if the large difference cannot be explained), and then carrying out a new fit. However, the scatter in the data is considerable and this will influence future fits.

5.3 Scour hole development Scour holes were observed far below the original bed level up to a depth of 32.1 m. The flow velocities through the breach were obviously high enough to erode the base material. Table 5.3 presents breach data from all cases.

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Table 5.3 Breach data of all breaches Dike breach Max width

(m) Max scour

depth (m)

Water head (m)

Max Discharge

(m) Zhaoyuan County Farm dike

530 15.3 4 4700

Mengxi dike 185 14 8 n.a. Tailai County dike - 1 90.5 8.8 3.3 n.a.

Tailai County dike - 2 208 14 3.8 1040

Tailai County dike - 3 170 20.3 5.3 1920

Tailai County dike - 4 126 n.a n.a 471

Paizhou dike 760 32 7.2 n.a. City Defence dike 64 7 5 n.a. n.a. = not available The energy or power P in the water flowing through the breach creates a scour hole with a volume V. This jet-type scour also occurs when an opening forms in a pipeline. Based on this experience, a relationship can be developed between volume V and power P. The power P is defined as: P gQH (5.1) where P = power (kW) = density of water (kg/m3)

g = gravitational constant (m /s2) Q = discharge (m3/s) H = water head (m) The volume V can be estimated using:

scour scourV y A with 2 2scour

W LA (5.2)

where V = volume of the scour hole (m3) yscour = maximum depth of the scour hole (m) Ascour = area of the scour hole (m2) W = breach width (m) L = length of the scour hole (m) The length L is unknown, but as a first estimate based on experience it is assumed that L = 2 W . Furthermore, the value of the coefficient is 0.33 for a cone-shaped scour hole and 1.0 for a cylinder-shaped scour hole. A value of 0.6 is assumed as a first estimate. The values of P and V can now be computed using the data from the four cases where the discharge is known, see Table 5.3. The results are shown in Figure 5.4. The resulting equation is: 18.1V P (5.3)

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y = 18,071xR2 = 0,7848

0,0E+00

5,0E+05

1,0E+06

1,5E+06

2,0E+06

2,5E+06

3,0E+06

3,5E+06

4,0E+06

4,5E+06

0,0E+00 5,0E+04 1,0E+05 1,5E+05 2,0E+05

Power (kw)

Scou

r vol

ume

(m3 )

Figure 5.4 Scour volume versus power

The derived relationship is relevant for situations where the foundation of the dike erodes. Until now, this aspect has not been included in the Dutch formulas. However, more data are required in order to increase the accuracy of the equation. The volume of the scour hole at Paizhou has been estimated to be 16,000,000 m3. Since the discharge is unknown, the power cannot be estimated using Equation (5.1). The scour volume therefore cannot be used for validation. However, Equation (5.3) can be applied to estimate the power P, resulting in a value of approximately 889,000 kW (= 16,000,000 m3 / 18). This value can be compared to a power computed with Equation (6.1), using a computed discharge with a relationship that will be derived in Section 6.2 where the observed and the calculated discharges are compared. Based on this comparison, it can be concluded that the observed discharge is approximately 50% of the computed discharge (see Equation 6.1). For the Paizhou breach, this means a discharge of some 16,000 m3/s. The computed discharge now corresponds with Equation (5.1): P = 1,130,000 kW. Comparing this value with the value of 889,000 kW gives a factor 1.3 . This is considered to be a good resemblance, taking into account all inaccuracies.

5.4 Conclusions The breach growth data provided for the breaches along the Yangtze River differ from values predicted using Dutch breach growth Equation (4.2). In that respect, the data provided add new information for improving the Dutch breach growth equations, since the Dutch equations were derived for sand dikes (whereas the Chinese data are mainly for clay dikes). One of the reasons for the difference is that the point in time of breach initiation is not known. The authors of this report have assumed a duration of 120 hours, but 72 hours is also possible. However, the differences are small. The final dimensions of the scour hole (depth yscour and length L) are also relevant, although only data for the scour depth of the observed breaches are available. Nevertheless, this aspect is relevant for determining the volume of water flowing through the breach into a

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polder. This aspect is in principle included in the Dutch models, but it can be improved. It should be noted that more sophisticated inundation models such as Delft3D, which use three-dimensional modelling of the flow and erosion of dike material in the breach, already take this aspect into account.

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6 Water levels, discharges, and volumes

6.1 Introduction The water volume flowing into the polder is relevant for inundation of the polder. This is a function of the discharge through the breach, the dimensions of the breach, and the water levels at both sides of the breach. The water levels have been provided for four cases, and discharge data are also available for three cases. However, no such information is available for the City Defence dike. An overview is presented in Table 6.1 Table 6.1 Available data regarding inundation characteristics Case discharges water levels Zhaoyuan County Farm dike + + Mengxi dike + + Tailai County dike - 1 - - Tailai County dike – 2 + + Tailai County dike – 3 + + Tailai County dike - 4 + + Paizhou dike - + City Defence dike - -

6.2 Water levels and discharges Water levels as a function of time are relevant for determining the discharge through a breach. The water levels for two of the breaches at Tailai County dike are presented as an example, and are compared with water level gauge recordings upstream. These data have not been analysed because this requires the use of a numerical model, and this is outside the scope of this study.

129,00

130,00

131,00

132,00

133,00

134,00

135,00

1 5 9 13 17 21 25 30date

wat

er le

vel(m

)

laolongkoudalaifarm

Figure 6.1 Water level process lines of the farm station (interpolated from the data of Laolongkou and Dalai station); Tailai County dike

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Examples of the resulting discharges as a function of time are shown in Figures 6.2 and 6.3.

Figure 6.2 Breach discharge curve of Pangtoupao reach of Zhaoyuan County Farm dike breach

0200400600800

1000120014001600180020002200

0 50 100 150 200 250 300 350 400 450 500 550 600

t i me( hour )

brea

ch d

isch

arge

(m3/

s) l aoj uzi

banzi shan

sout h of guangr ong vi l l age

Figure 6.3 Breach discharge Tailin: Laojuzi, Banzishan and Guangrong~Yanjiang

Relevant data about the discharges and breach dimensions are presented in Table 5.3. These data have been used to compare the observed discharges with discharges computed using breach or inundation models. A comparison with a numerical model (not for one of the considered cases) is shown in Figure 6.4 (Zhu, 2006).

3

date(mon/dat)

disc

harg

e£m̈

3 /s

£© 5000

4500

4000

3500

3000

2500

2000

1500

1000

500

08.13 8.20 8.27 9.3 9.10 9.17

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Figure 6.4 Validation of model for clay dikes (TU Delft test T3) The observed and computed discharges for the available data have been compared. The discharge Q can be computed using: 1/ 2 3 / 2(2 )Q WHu g WH (6.1) where W = width of the breach (m) H = water head (m) u = flow velocity in the breach (m/s)

= discharge coefficient (-) For the discharge coefficient , a value of 0.5 was applied based on experience and expertise. Different aspects were taken into account, such as large flow velocity differences over the cross-section of the breach, and the unknown breach width at the point in time that the discharge is maximal. The result for the four breaches is (see Figure 6.5): 2.1calculated observedQ Q The accuracy of the equation is rather high, with R2 = 0.88. Furthermore, it is relevant to mention that the highest discharge is related to a sand dike, and the other discharges relate to clay dikes. The result seems to be consistent.

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y = 2,1254xR2 = 0,8789

0100020003000400050006000700080009000

10000

0 1000 2000 3000 4000 5000

Qobserved (m3/s)

Qca

lcul

ated

(m3 /s

)

Line of perfect agreement

Figure 6.5 Calculated versus observed discharge

6.3 Volumes The total water volume flowing into a polder can be estimated using the breach dimensions (breach width, scour depth) and the water levels in the river and polder as a function of time. However, sufficient data to compute the total inflowing volume of water are only available for the Zhaoyuan County Farm dike breach, but the observed volume is unknown. This aspect has therefore not been analysed.

6.4 Conclusions The data available on water levels and discharges allow numerical simulations to be performed using breach models for some of the studied breaches. The data allow these models to be improved. It is also believed that missing data (for instance, related to discharges as a function of time) can be derived from the results of numerical simulations using the available water level data.

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7 Conclusions and recommendations

7.1 Conclusions Data from several dike breaches along the Yangtze River in 1998 have been analysed. The analysis has been carried out within the framework of the MoU between the Chinese Ministry of Water Resources and Rijkswaterstaat. The objective of the study was to investigate whether the dike breach data can be used to improve Dutch breach equations, which are implemented in the inundation models SOBEK1D2D and HIS-OM. The overall conclusion from this study is that some of the data provided are useful with respect to:

1. growth of breaches in sand and clay dikes, and 2. inundation of the polder.

Data from Zhaoyuan County Farm dike and Mengxi dike are particularly useful, since both cases give time-dependent information on breach development, water levels, and discharges. For other cases, the breach data are limited to the maximum width of the breach. These data are also dependent on the case data of discharges, and/or water levels are missing. Table 7.1 presents an overview of available data and usefulness. Table 7.1 Available data regarding inundation characteristics case discharges water

levels water head

breach width

scour depth

dike material

usefulness

Zhaoyuan County Farm dike

+ + + as a function of time

+ sand ++

Mengxi dike - + + maximum * and initial width

+ clay ++

Tailai County dike 1

- - + maximum * + sand -


+ + + maximum * + clay +


+ + + maximum * + clay +


+ + - - - clay -

Paizhou dike - + + maximum * + loamy soil

-

City Defence dike

- - + maximum * + clay + gravel

--

Note: *) = the duration for development of the maximum breach width was not known In principle, the data from breaches in the Zhaoyuan County Farm dike, the Mengxi dike, and from breaches 2 and 3 in the Taila County dike are promising with respect to improving the Dutch breach models and breach growth equations. The “benefit” of carrying out a project that includes numerical simulations is that it will yield more reliable predictions of breach growth and the inflowing volume of water. This is because three of the cases involve clay dikes, the weak point of both the Dutch breach growth equations and the Dutch breach models. However, it should be kept in mind that the available data show a considerable scatter.

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More detailed conclusions regarding particular aspects are as follows:

The growth of breach widths can be satisfactorily predicted by Dutch semi-empirical breach growth equations if the duration for breach width development is assumed to be approximately 72 hours or less. The difference can be considerable if the duration is longer, in particular for sand dikes.

The data available on the depth of the scour hole allowed a preliminary relationship between the volume of the scour hole and the power of the inflowing water to be derived. This can be relevant for determining breach discharges when scour holes are formed at depths lower than the dike foundation. It is thereby relevant for determining the volume of water flowing into a polder through a breach.

Data from observed discharges show a reliable and consistent agreement with calculated discharges. It is believed that missing data for the discharges as a function of time can be derived from numerical simulations using available water level data.

7.2 Recommendations Based on the results, it is recommended that inundation computations are carried out using the original Dutch breach growth equations and breach models, together with versions of these equations and models that have been improved by calibration and validation with data from the following cases (also see Table 7.1):

Zhaoyuan County Farm dike Mengxi dike Talai County dike 2 Talai County dike 3

This will involve the following phases: 1 Improving the Dutch breach growth equations by incorporating the Chinese data into the

database, and subsequently determining new values for the coefficients. 2 Implementing an equation in the Dutch breach models for the development of scour

depth, based on the relationship between the power of the breach discharge and scour volume. However, this first requires improvement of the relationship by collecting other data.

3 Executing simulations with the Dutch inundation models SOBEK1D2D and HIS-OM using (i) the old breach equations, and (ii) the improved breach equations resulting from phase 1.

To improve the results of a future project, it is recommended that the following additional data is collected in China: > Point in time of the maximum breach width (after the initial breach). > The development of the breach width over time. > Shape of the scour hole, specifically the length L as a function of the width W. > Final volume of the scour hole. > The dike material of the Mengxi dike. The documents mention clay, although the dike

was selected during the field visit because of its sand core.

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Paizhou dike is not mentioned as a possible case for a future project. This is because there is doubt about the observed maximum width of the breach (760 m), and because it is unclear whether the type of dike material was loamy soil or clay. If these two aspects can be resolved, Paizhou dike would also be an interesting case.

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8 References

Anonymous (2008a): Breach of Tailai County Nenjiang Trunk Stream dike, case 5. Note, China Institute of Water Resources and Hydropower Research Anonymous (2008b): City Defense Dyke of Jiujiang City, Jiangxi Province, case 1. Note, China Institute of Water Resources and Hydropower Research Anonymous (2008c): Paizhou dike (or named Hezheng dike ring) in Yangtze River, case 3. Note, China Institute of Water Resources and Hydropower Research Anonymous (2008d): Zhaoyuan County Farm Dike Breach Located on Nenjiang Trunk Stream of Upper Reach of Songhuajiang River. Note, China Institute of Water Resources and Hydropower Research Bernitt, L. (2006): Risk Analysis of Dike-Protected Areas (in Danish) Kystdirektoratet, Danmark Bernitt, L and H.T.Madsen (2008): Temporal development of a sea dike breach. Proc. ICCE-2008, Hamburg, Germany Förster, U. (2008): Visit to China and field visit of former dike breaches. Memo, Deltares, Delft, the Netherlands Hoffmans, G.J.C.M and H.J.Verheij (1997): Scour Manual. Balkema Publishers, Rotterdam, the Netherlands Kamphuis, J.W. and K.R.Hall (1983): Initiation of erosion of consolidated cohesive materials by unidirectional current. J. of Hydraulics (ASCE), Vol 109, pp 49-61 Mirstkhoulava, T.S. (1988): Basic physics and mechanics of channel erosion. Gidrometeoizdat, Leningrad.

Mirstkhoulava, T.S. (1991): Scouring by flowing water of cohesive and non-cohesive beds. Journal of Hydraulic Research 29(3), 343-353 Pan Shuibo and E.Loukola (1993): Chinese-Finnish cooperative research work on dam break hydrodynamics. National Board of Waters and the Environment, Helsinki, Finland Visser, P.J. (1998): Breach growth in sand dikes. Delft University of Technology, Faculty of Civil Engineering and Geosciences. PhD- Thesis, Delft, the Netherlands Vroeg, H.J. de, G.A.M.Kruse and M.R.A. van Gent (2002): Processes related to breaching of dikes, Erosion due to overtopping and overflowing.

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Delft Hydraulics, Delft, the Netherlands Yao Qiuling, Xie Jiabi and Sun Dongya (2008): Dike breach case investigation and dike breach modeling China Institute of Water resources and Hydropower Research, draft, 1st version, Yonghui Zhu (2006): Breach Growth in Clay Dikes Delft University of Technology, Faculty of Civil Engineering and Geosciences. PhD- thesis, ISBN-10: 90-9020964-6, ISBN-13: 978-90-9020964-7, Delft, the Netherlands

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A Memo: Visit to China and field visit of former dike breaches

From: Ulrich Förster To: Remco Schrijver Date: October 20, 2008 Subject: Visit to China and field visit of former dike breaches 1 Introduction Numerous dike breaches occurred in China in 1998. The State Flood Control and Drought Relief Headquarters (SFCDRH) - part of the China Institute of Water Resources and Hydro Power (IWHR) - has indicated that the dike breaches have been documented in detail. Within the framework of the MoU between Rijkswaterstaat and China, Rijkswaterstaat requested SFCDRH to provide information from 5 to 10 cases so that it can be used to improve Dutch dike breach models. These are implemented in inundation models such as SOBEK1D2D and HIS-OM. In July 2008, SFCDRH provided information from the following cases: - Zhaoyuan County Farm dike breach (Nenjiang trunk stream) - Dorbod Mongolzu Zizhixian dike (Nenjiang trunk stream) - Tailai County (Nenjiang trunk stream) - Paizhouwan dike; Henzheng dike ring (Yangtze River) - Mengxi dike ring (Hudu River) - City Defence dike of Jiujiang City (Yangtze River) - Jiujiang Jiangzhou dike (main stream of Yangtze River) - Anzao polder Dongting lake area of Hunan Province - Hubei Mengxi polder between Hudu River and Songzi River - Hubei Peizhouwan polder (Yangtze River) An initial assessment of the information provided was summarised by Deltares in the memo “Chinese documents on dike breaches”, dated August 27, 2008. 2 Presentations and discussion On Friday 12 September, a Dutch delegation (Pieter Janssen, Peter Blommaart, Remco Schrijver, and Ulrich Förster) took part in a meeting with the SFCDRH in Beijing, China. Representatives of the SFCDRH were Prof Ding Liuqian, Dr Sun Dongya, Ms Yao Qiuling, Mr Zhao Jinyong, Dr Xie Jiabi, who were later joined by Dr Cheng Xiaotao. Ms Yao Qiuling first gave a general introduction about dike breach cases in China since 1998, where the above cases were presented in more detail. A great deal of information was added to the reported cases, for example time-dependent water level data and translations of Chinese symbols. Mr Ulrich Förster gave an overview of dike breach modelling in the Netherlands. The Chinese party was interested in the use of SOBEK and Delft 1D.

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It was concluded that four cases would be worthwhile for further research:

a) Zhaoyuan County Farm dike breach (Nenjiang trunk stream); b) Tailai County (Nenjiang trunk stream); c) Paizhouwan dike; Henzheng dike ring (Yangtze River); d) Mengxi dike ring (Hudu River),

Cases a) and d) were selected because of the well-delivered information, cases b) and c) because these dikes are clay dikes. It emerged during the discussion that Dr Xie had compiled information concerning the dikes in Zhaoyuan and Tailai County, and that he had already visited these dikes. Dr Xie will also complete information concerning these dikes. Ms Yao has compiled information concerning the other two dike breach cases. 3 Field visits Between 15 and 18 September, Mr Schrijver and Mr Förster visited the dike breach location along the Mengxi dike and the Paizhouwan dike, accompanied by Ms Yao and Mr Zhao. The following information was gathered: Mengxi dike: The Hudu River has its origin from a dike breach along the Yangtze River. The Mengxi dike-ring encloses an area of 258 km², protecting 3 towns and 150,000 people. The original dike crest level was 43 m. The incline of the inner slope was 1:3. Many ponds were observed at the rear of the inner slope. The pond nearest to the dike was 7 m deep. Only grass covered the dike before the dike breach. According to the local authorities, no trees were standing on or behind the dike. During the flood on August 7 1998, the water level in Zhakou (nearest measuring station, 6 km downstream) was 44.67 m, which is 42.90 m at the dike breach location. The discharge of the Hudu River was 4,000 m³. As a result of piping, the crest settled and the breach developed with a flow rate of 3 m/s. Almost the whole dike ring area was flooded (258m²). The maximum width of the breach was 188 m. The depth of the breach was 14 m plus 7 m as a result of scouring. A new pond was formed as a result of the scouring process. The pond still exists.

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Picture 1 Mengxi dike (at the location of the dike breach, change-over of old and

reconstructed dike in the background)

Picture 2 Mengxi dike at the location of the dike breach (change-over of old and

reconstructed dike)

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Picture 3 Mengxi dike (platform inner slope of the reconstructed dike)

Picture 4 Mengxi dike (platform inner slope at the location of the dike breach)

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Picture 5 Mengxi dike: pond formed by scouring

Picture 6 Mengxi dike (at the location of the dike breach)

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Picture 7 Mengxi dike (at the location of the dike breach) Paizhou dike: The old Paizhou dike was a farmer’s dike, situated on the right bank of the Yangtze River. This type of dike is extremely typical for the Yangtze River. The Paizhou dike ring is part of the Yangtze main dike. The dike ring protects 100,000 ha of farmland and 60,000 people. The crest of the new dike is approximately 32 - 33 m high. The platform height of both the old and the new dike is 26 m. The position of the centre line of the reconstructed dike has shifted to the foreland, which is about 400 - 500 m wide. According to the local authorities, fewer trees covered the foreland before the dike breach. The difference in water level between Paizhou Town and the location of the dike breach is about 1 m. The breach measured a maximum of 760 m wide and 32 m deep (-7.8 m below zero height). The scouring area was 14.88*104 m². According to the authorities, the incline of the original dike slope was greater than the current slope. This is not in accordance with the delivered sketches of the dikes, where both the old and the reconstructed dike have an incline of 1:3 on the inner and the outer slope. This information will be checked by Ms Yao. The new dike is protected by a geo-textile, strengthened by a filling of sand and cement. Ms Yao had a great deal of information, both in oral form as well as copies of sketches and reports by the local authorities. It was unfortunately not possible to follow the whole discussion. Ms Yao promised to translate the information that had been provided into English. On 18 September, a visit was made to the county council to gather information about the Paizhou dike breach, including a contour map of the scour. A visit was also made to the design institute for water resources in Wuhan, where all dikes and sluices of Hubei Province are designed. Ms Yao gained further insight into the information that was available (maps, descriptions of dike breaching processes, etc.).

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Picture 8 Paizhou dike: reconstructed dike

Picture 9 Paizhou dike: reconstructed dike

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Picture 10 Protective layer on reconstructed Paizhou dike

Picture 11 Protective layer on reconstructed Paizhou dike

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Picture 12 Change-over of old and reconstructed dike

Picture 13 Foreland of Paizhou dike

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4 Return to SFCDRH for evaluation of the field visits On Friday 19 September, Mr Remco Schrijver and Mr Ulrich Förster visited the SFCDRH for a final résumé of the field visits. The Dutch party presented a table with an overview of the availability of information (see appendix). This overview was discussed with the Chinese party. Ms Yao intends to translate all information she received during the field visit into English. In addition, further agreements concerning the format and delivery time of the report were made: a draft report will be sent to Rijkswaterstaat at the end of October. The final report will be delivered at the end of November 2008. Dr Xie provided a number of photographs of the dike in Zhaoyuan. Photographs of the Tailai will be added to the report. 5 Remarks Dr Sun mentioned that his institute will execute several laboratory tests (in a flume: 10 m long, 2 m wide) and field tests next spring, involving breach experiments and research into the piping mechanism. Dr Sun suggested that there should be cooperation between the Dutch and the Chinese parties regarding the exchange of data. Ms Yao Qiuling is also working on a PhD thesis concerning piping. Ms Yao gave a presentation, “Study on the Mechanism of Piping and the Effect of Suspended Cutoff in Dike Foundation”, at the 4th International Symposium on Flood Defence in Toronto, Canada. It is promising that there is now greater interaction with the SFCDRH within the scope of the SBW piping research carried out by Rijkswaterstaat and Deltares. Dr. Xie is carrying out a reliability analysis for a Chinese project, and is therefore extremely interested in the use of PC-Ring.

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Appendix: Available information on dike breach cases (including field visit)

Category Parameter Zhaoyuan County

Farm Dike

Tailai County

Paizhou Dike

Mengxi Dike Ring

Foreland +/- +/- + #

Outer slope +/- +/- + +

Crest + +/- + +

Inner slope +/- +/- + +

Geometry

Hinterland +/- +/- + +

Foreland ? ? ? +

Outer slope ? ? + +

Crest ? ? + +

Inner slope ? ? + +

Characteristics

Hinterland ? ? + + (pond)

Material type + + + +

Thickness layer ? ? + +

Subsoil

Grain size distribution + + + -

Material type +/- + + + Dike

Grain size distribution +/- + + +

Maximum level + Discharge only (?)

+ +/- Water level in river

Change in time + Discharge only (?)

+ +/-

Starting time + + + +

Maximum discharge through breach

+ ? Maximum water level

only

Maximum water level

only

Change in time of discharge through breach

+ ? - -

Final geometry of breach + + + +

Breach

Change in time of geometry of breach

- - - -

Normal level - - - -

Maximum level 0 0 0 0

Water level in hinterland

Change in time 0 0 0 0

+: is available -: is not available ?: availability unknown 0: availability less relevant #: not relevant

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Geometry: Foreland: level, presence of ditches etc. Outer slope: slope, presence of berms etc. Crest: width, height etc. Inner slope: slope, presence of berms etc. Hinterland: level, presence of ditches etc.

Characteristics: Foreland: presence of trees and undergrowth etc. Outer slope: covering etc. Crest: presence of road etc. Inner slope: covering etc. Hinterland

Material type: clay sand clayey sand sandy clay homogeneous / not homogeneous one layer / layered grain size distribution if possible. If not, characterisation will suffice (‘course’, ‘fine’ etc.)

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Remarks and questions to be discussed on Friday September 19 Zhaoyuan Country Farm Dike Explanation of table 3.3 is needed: are the given parameters for dike body or foundation

zone? Figure 3.6, height of foreland and platforms are not given Figure 3.4, translation not clear Which data are used for calculating water level of sheet 1.3 of presentation? What is the meaning of the discharge curve?

Tailai Country A total of 4 breaches are described:

o No longitudinal section for Alaxin dike breach o How should the discharge curve be read, figure 5.6?

Water levels (maximum level and change in time) are not in the report Explanation of table 5.5 is needed

Paizhou Dike The presentation states ‘The dike embankment is made up of loamy soil which is the same

as top ground layer of 3.3 m in thickness’. What is meant by this exactly? In the field, it was stated that the slope of the old dike was steeper than the slope of the

reconstructed dike. However, the presented cross- section shows that both slopes are equal. Which information was given during the field visit?

The technique used for reconstructing the dike is unknown in the Netherlands. Could you

please give us more information about the technique? Mengxi Dike Ring According to the title in the presentation table, water levels should be presented from July 30

to August 13, although they are only presented up to August 1 (dike breach was on August 7).

The cross-sections of the dike breach, closing cofferdam, and reconstruction of the dike do not match.

What was the time span of the dike breach? How should the table with physical characteristics of dike and foundation materials be used ? What happened to the stone revetment built in 1988?

General remarks Final draft by email Comments by Rijkswaterstaat by email on final draft Hard copy report (2 reports)

analysis of chinese dike breaches along the yangtze river and tributaries · 2014. 7. 29. · the...

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