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Probabilistic safety assessment of sea dikes in Giao Thuy-Nam Dinh

Probabilistic safety assessment of sea dikes in Giao Thuy-Nam DinhAcknowledgments

First of all I would like to send our sincere thanks to: Assoc.Prof.Mai Van Cong

and Prof. Radu Sarghiuta - my supervisor from ULG and TLU - for their concern,guidance, enthusiasm, valuable advice and assistance with so much warmth and

My high appreciation goes to all the teachers who have taught and armed me with

such a valuable knowledge to my future career in my country, my colleagues,

friends and my classmates for their support, assistance and for making my stayhere filled with joys and memories.

Pham Tien HungULG-TLU-2016

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My name: PHAM TIEN HUNG

Major: Sustainable Hydraulic structureStudent Number: 14§ULG07

Thereby declare that i am the person who conducted this master thesis under the‘guidance of Assoc.Prof Mai Van Cong and Prof.Radu Sarghiuta with the research

topie “Probabilistic safety assessment of sea dikes in Giao Thuy-Nam Dinh”

This is a new res ch topic which does not overlap with any dissertation before, sothere is no copy of any public dissertation. The contents of the thesis are presented inaccordance with regulations, the data resources and materials used in research arequoted sources.

If there is any problem with the contents of this thesis, I would like to take fullresponsibility as prescribed,

Hanoi, August 15, 2016Applicant

PHAM TIEN HUNG

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<small>‘TABLE OF CONTENT</small>

TABLE OF CONTENT

Acknowledgments 1REASSURANCES. 2

‘TABLE OF CONTENT 3(CHAPTERI: INTRODUCTION 1uu Background and justification 1

<small>12, Aimefsudy, 1</small>

13. Saudy approach 214, Outline of sud. 2(CHAPTER 2: GENERAL OVERVIEW OF STUDY AREA, 324 (Current status of sea dike system in Giao Thuy - Nam Dinh 322. Some features of Giao Thy’ sea dikes 3

<small>23. General assessment of current situation of sea dike system in Giao Thuy ieriet...9</small>

24. Some natural boundary condition in Giao Thuy - Nam Dinh lơ

©) Sliding o dike stope .

<small>‘CHAPTER 4: Applying probabilistic reliability analysis to safety assessment in Giao Thuy ~ Nam)</small>

Dink. 36

4 Wave overtopping 36

<small>42. Instability of armour revetment: 40</small>

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<small>‘TABLE OF CONTENT</small>

43. Toe foot instabilities 444 Piping “

<small>45, Sliding of dike slope 3</small>

46. Probability dike system failure 5s(CHAPTER 5: Conclusions and recommendation

<small>5a ‘Conclusions on safety ofthe sea dikes in Giao Thuy ø</small>

5.2. Recommendations ái

References 6

<small>Appendix I: The parameters of Giao Thuy sea dike design according to technical standards in sea</small>

dike design 2012), “4) Grade of stueture “

<small>b) Design water level 6</small>

© Deep-vater wave. 6s

4) Design wave neat the te ofthe dikes a

©) Require free board by wave overtopping. »

1) Design revetment thickness (technical standards in sea dike design 2012) n

Appendix 2: Geotechnical document use in calculating siding of dike slope T4“Appendix 3: Determining failure probability of sea dike system in Giao Thuy by using OpenTA,

software 76

Appendix 4: Fragility curve a

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<small>LIST OF FIGURES</small>

Figure 2.1: Map of sea dike system in Giao Thuy- Nam Dinh 3Figure 2.2: Erosion in dike slope of Giao Thuy sea dike 4Figure 2.3: The flood eaused serious damage on field region in Giao Thuy-Nam Dinh 5Figure 2.4:Damage of dike section dục to Typhoon 1 (2016) 6Figure 2. 5: Dike section improved by funding of PAM 7

<small>Figure 2.6: Sketch of double sea dike system at Giao Thuy disict(ceg- mai 2001), 7</small>

igure 2.7: Representative cross section of sea dike in Giao Thuy-Nam Dịnh (ceg_mai_2008) 8

Figure 2. 8: Main seasonal wind directions in northern Vietnam GB

Figure 3. Frame work of risk analysis (ce CUR HH, 1990). 6

igure 3.2:Defintion ofa failure boundary Z0, „

Figure 3.3 Definition of probability of failure and reliability index 2

<small>igure 3.1: Fault tee of Giao Thuy sea dike E</small>

Figure 3.5: Damage of sea dike caused by wave overtopping 26Figure 3,6 Pore pressure in the subsoil daring wave run-down (Piatczyk e al, 1998). By

<small>Figure 3.7: Schematization of scour mechanism at Nandlnh revetment. 31</small>

Figure 3.8: Mechanism of piping at sea dike 2

Figure 4, 1: The normal distribution of MHWL based on BESTEIT software ”Figure 4.2: Contribution of parameter to overtopping lalure move in eursent dike 3”

<small>Figure 4 3: Contribution of parameter to overtopping failure mode in dike according to design</small>

standard 2012 40Figure 4.4: Contribution of parameter to instability armour of revetment in currently để

<small>Figure 4 5: Contribution of parameter to instability armour of revetment according to design</small>

standard 2012 44Figure 4 6; Contribution of parameter to foe foot istasiities 4s

<small>Figure 4, 7:Contribution of parameter to piping failure condition | sỉ</small>

Figure 4.8 Conibuion of parameter to piping failure condition 2. 2Figure 4.9: Safety factor of slope stability calculation in Outer slope SEmin=l 50 34

<small>Figure 4. 10: Safty factor of slope stability calculation in Inner slope SFmin=1.335 s:</small>

Figure 4. 11: Fault ioe analysis of Gino Thuy sa dike system for present situation, 56Figure 4, 12: Fault wee analysis of Giáo Thuy sea dike ccording to Dike Design Standard (2012)

Figure 413. Fragly curve a a function of the design wave height (H)-Appendix 4, page 93. 59

<small>Figure AI, 1: The Mean High Water Line in Giao Thuy - Nam Dinh 6</small>

Figure AI, 2: Plan of regions used to determine the parameters of deep-water wave 66

Figure A4. I: Fragility curve as a funetion ofthe design wave height (Hs) 8s

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<small>LIST OFTABLES</small>

“Table 2.1: Sediment load composition onthe shoreline of Giao Thuy "

‘Table 4.1: Stochastic variable for mochanism of wave overtopping 38

<small>‘Table 4.2: Failure probability ofthe dike due to overtopping 38</small>

Table 4.3: Contribution of parameter to overtopping [are mode. »‘Table 4.4: Stochastic varie for instability armour of revetment 2

<small>“Table 4.5: Failure probability and contribution of parameters to instability armour revetment in</small>

curtealy by using VAP. đãTable 4,6: Failure probability and contibution of parameters to instability armour rẻte0nenfin.dike acconding to dike design standard 2012 by using VAP. 4a‘Table 4. :Stochasie variable for mode oft Foot instabilities. a7Table 1 8: Failure probability and contibution of parameters to (oe foot instabilities by using VAP

var. 3a

Table 4. 13: Overall probability of failure at Giao Thuy sea dikes 56

‘Table AI, [: Safety standard and grade of sea dike “

<small>‘Table AI, 2: The parameters of deep-water wave in region Hai Phong Ninh Bình a</small>

Table AI, 3 The results of wave tansportation by using SWANID software )Table AL 4: Average overtopping rates are allowable according to Technical standards in sea dike

<small>design (2012) T0</small>

Table AI, Š The requted crest free board accorling to g=10 (im), +Table AL. 6 The erst level of Giao Thuy sea dike cording to safety design standard n

<small>‘Table A2. [Test result of physial and mechanical properties of soil layer 1 1</small>

‘Table A2 2: Test result of physical and mechanical properties of sil layer 2 ”%5. Test result of physical and mechanlcal properties of soil layer 3 T5-Tex result of physical and mechanical propertics of soi layer 4 75

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<small>(CHAPTER: INTRODUCTION</small>

CHAPTER: INTRODUCTION

1.1. Background and justific:

Vietnam is a typhoon prone country lo ted in the tropical monsoon area of the SouthEast Asia. The majority of Viet Nam population lived in the low lying river floodplains, deltas and coastal margins which involves mainly in agricultural and fisherysectors. In recent years, evolution of natural disasters and weather in Viet Nam was.‘complications, typhoons from the South China Sea bring torrential rainfall and highwinds to the coast and further inland. On average four to six typhoons attack the coastannually resulting in heavy damage, loss of life, and destruction of infrastructurefacilities and services. The reason why the water disasters are so serious is that mostof the population lives in areas susceptible to flooding. Thus the formation anddevelopment of defensive system are alway’ attached to life and production of peoplefrom generation through the generation.

‘Nam Dinh province in general, Giao Thuy district im particular constitutes part ofVictNam’s witha long line of dikes and sea defenses. Most of the sea dikes are built‘over the centuries mostly due to local initiatives and have generally an inadequate{design and are poorly constructed. Due to the bad state of the dikes significant partof the yearly funds has to be allocated to repairs and maintenance.

Although, before flood season in every year, several researches on the safetyassessment of the coastal defenses system have already been conducted but these

researches ’s weakness wete done based only on what already happened of the sea

defenses system in the previous years and the experiences on management of themonitors. As a result, the risk of the damages is still going on at the high rate andfrequently and establishing method for asses sing safety of sea dikes based onreliability analysis theory is necessary.

1.2. Aim of study

Probability safety assessment of sea dikes in Giao Thuy-Nam Dinh. The aim of this

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<small>CHAPTERI: INTRODUCTION,</small>

study can be outlined as follows:

<small>= Establishing method for assessing safety of sea dikes system based on</small>

reliability analysis theory.

<small>= Setting up cases of risk in using reliabili</small> analysis of this sea dikes system,

- Suggesting the criteria to e imate safety according to reliability analysis

<small>= Finding fragility curve of Giao Thuy sea dikes</small>

~ Applying results achieved to assess safety ofa case study in Giao Thuy district.

1.3, Study approach

<small>~ Applying reliability analysis thesis</small>

~ Studying the documents involved designing and optimizing sea dikes based‘on reliability analysis theory at home and abroad.

~ Inheriting early researches relating to sea dikes,

1.4. Outline of study

~ The general information of the study is given in chapter |

<small>= In chapter 2, description of overview of Giao Thuy-Nam Dinh and some</small>

natural boundary condition,

<small>- The probability risk and reliabi</small> assessment and applying probabilityreliability analysis to safety assessment in Gino Thuy-Nam Dinh are presentedin chapter 3 and chapter 4, There will be investigated all kind of failure modes

which may occur and estimated which factors have the greatest impact to the

failure of sea dike

~ Finally, the conclusion and recommendations will be treated in chapter 5.

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<small>(CHAPTER 2: GENERAL OVERVIEW OF STUDY AREA</small>

CHAPTER 2: GENERAL OVERVIEW OF STUDY AREA

2.1. Current status of sea dike system in Giao Thuy ~ Nam Dinh.

Long ago, sea dike system has played vital role in the natural disaster consequencesprevention and mitigation. Furthermore, in the present recessionary conditions,defense system also has essential role as protection for residential and urban areas tocensure the sus tainable economic development.

Giao Thuy is a coastal district of Nam Dinh province, 35 km South away from NamDinh city. It is bounded on the Northwest by Xuan Truong district and Southwest byHai Hau district and also borders Thai Binh on the North and Northeast.

(Giao Thuy district has natural area of 230.22 square kilometers and population of256,864 people (counted in 2010), surrounded by 27 km of sea dikes from So estuaryto Hong estuary.

2.2. Some features of Giao Thuy sea dikes

In Giao Thuy sea dike, two major problem of defensive system are heavy damages

and serious erosion of the coastline. That failure due to the current design parameters

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<small>(CHAPTER 2: GENERAL OVERVIEW OF STUDY AREA</small>

were not sufficient while the actions of strong storm surges and typhoons are getting

stronger. The specific features can be listed as following:

~The sea dike system in Giao Thuy-Nam Dinh was renovated between 1997 and2004 with a length of 2580 m at the positions such as: K15.603 + K15.903; K20.350+ K22.267; K 23.685 + K23.935 located majority in Co Vay, Thanh Nien sluice, AngGiao Phong. Due to the innovation in a lot of times, Giao Thuy sea dikes is not

censured. In several places, dike sections (K22+400 + K27+161) are influenced by

erosion which narrowed down the cross-section dike to (1.5-2.0 m) at Giao Phong+Giao Lam.

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(CHAPTER 2: GENERAL OVERVIEW OF STUDY AREA,

<small>~ Along the coastline of Giao Thuy, the foreshore erosion causes loss of 0.5-0.7m</small>

thickness of sand in front of dike and the rate of structural erosion is from 10m to15m per year. If there are not sufficient and in-time counter methods, this problem

leads to fast retreat of coastline,

Figure 2. 3: The flood caused serious damage on field region in Giao ‘Nam Dinh.

Thuy-= The practical statistics showed that Nearly 40,000 ha cultivated land have been

affected by salt water infiltration and 70,000 tons of food was lost, salt mining fields,

and shrimp hatching ponds were also heavily damaged, according to figures from

‘Tuoitre Newspaper.

<small>= Strong storms with wind-strength of 9 to 12 cause houses to collapse, killing</small>

people and huge property loss. In 2012, the damage caused by the Typhoon 8 adds

tp to over 655 billion VND.

<small>= By some accounts, more than millions cubic meter of soil and thousands cubic</small>

meter of stone were taken away from the sea dikes due to Storms surge accompanied

with high tides.

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<small>(CHAPTER 2: GENERAL OVERVIEW OF STUDY AREA</small>

In the recent years, the dike slope and revetments were improved by funding of‘domestic and international organizations (PAM) to assure the storm’s a category 9and the average tide level of 5%, However, the landslides and dike break may happen

if Giao Thuy sea dikes have affected the design frequency exceeding above design.

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Figure 2. 5: Dike section improved by funding of PAM.

‘The structure typically of Giao Thuy sea dikes system shown in Figure 2.6, includedtwo dike layers in each section,

Figure 2. 6: Sketch of double sea dike system at Giao Thuy

‘The second dike of sea dike system help to limit flooding and will be the new firstline of defense when a breach takes place in the frst dike. In general, the second dikeis weaker than the frst dike because it mainly made of soil. These dikes must andwill be reinforced when the water reaches them and the distance between the dikes

varies roughly 200 meters, Inside of the dikes are the land areas divided into sections

varying between several hundred meters up to 3 km. When a breach occurs at the

front dike, the division into sections help to limit areas to be flooded.

As a matter of fact, the determining factor of durability of the dikes is the earth core

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<small>(CHAPTER 2: GENERAL OVERVIEW OF STUDY AREA</small>

consisted of material from local sand and clay resources. The revetments on top of‘outer slope was built on natural stones oF artificial blocks on a layer of clay.

“The typical parameters of representative cross section of this sea dike as Figure 2.7below:

es re

Figure 2. 7: Representative cross section of sea dike in Giao Thuy-Nam Dinh(eeg_ mai 2004)

~ Design of cross section:

<small>= Dike crest level: 5.5m</small>

~ Dike profile: seaside slope 1:4, landside slope 1:2

= The sea slopes are protected by pitched stone revetment:

~ Below DWL the thickness of 45 em for rock revetment, the average weight isapproximately 250kg.

<small>= Above DWL the thickness is of 30 cm</small>

‘Two functions of sea dike system in Giao Thuy are flood defense and protection ofinland from erosion. This means that the dikes must be stable in any case. However,

nearly all the dikes which were constructed in the past only based on the experiences

of the local people and designed by very old method, The dikes system seems to beinsufficient respect to the actual boundary conditions for the time being. Thesefailures caused flooding in the wide area along Giao Thuy coastline and as theconsequence, it leaded to loss of land, economic archives and even loss human’s life,

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General assessment of current situation of sea dike system in Giao Thuyistrict

Long ago, defens ystem played an important role in flood prevention works. Giao‘Thuy is a coastal district, not only affected the flow of the river, but also influencedby the sea. So that the sea dike system has an important task in contributing topolitical and economic stability of Giao Thuy district in particular and Nam Dinhprovince in general

‘The sea dikes in Giao Thuy was built from long years ago (about 250 years) on soft‘ground raised by Red river. This dike system affected directly by tide, typhoons andflood flow from rivers into South China sea, Especially, dike break and landslides‘often happen when heavy storm combined with high tide in Giao Thuy dike.

In the middle of dike system, sea gains on land caused of the los of dike and peoplein the area, Moreover, the landslides and dike breach occurred frequently bad an

adverse effect on production activities and life of the people in Giao Thuy. That

results caused by the low standard of soil on dike body, lack of reasonable of materialin armour revetments and toe dike, the deterioration and damage of old sluice

In addition, the level design standard of sea dike is low, therefore it does not meetflood prevention and control requirement in current situation of Giao Thuy. Thisproblem will cause serious damage when there was a combination of surge and high

In the recent times, the damage caused by natural disasters tends to increase inWorldwide, along with the global climate change. These storms tend to come upstronger, becoming super typhoon while the phenomenon, rain, winds, tornadoes also‘occur more frequently caused more serious consequence,

‘The activities in economic and social life of coastal area caused the changes in thenatural environment in the direction of disadvantage and increase the damage ofnatural disasters. In many areas, mangroves and coastal forests have been lost not‘only cause changes in the ecological environment in ways that are harmful, which

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also makes big waves hit straight into the sea dike, causing sea dike break.

It is clear that the damages of defensive system lead to many impacts on the socialand economic development in the area. In fact, some sections of new sea dikes hadbeen built by some efforts of the central and local authorities in order to keep undercontrol the possible adverse consequences. However, these efforts still keep limitedto lack of suitable design methodology as well as strategic and long-term solutions

and constrained budget.

So a suggestion, in this thesis, the reliability analysis theory combined the analysis ofthe factors affecting on flood defense system will be implemented according 10research and development of methodology design, A case study of research in Giao.‘Thuy-Nam Dinh is chosen for demonstration of the method and calculating,

2.4, Some natural boundary condition in Giao Thuy - Nam Dinh

a) Delta topography

Delta topography of Giao Thuy District has flat topography with an altitude vary from10-15m to mean sea level (MSL), gradually sloping from Northwest to Southeast. Inthe middle of the delta, mountains and bills can be found, linked to the geologicalformation under the alluvial sequences

b) Soil characteristics and Geological features

Soil in Giao Thuy district has alluvial characteristics because this area has beenformed by the rivers in Red River system. The action of waves and tide current causesthe coastline shaving and the erosion is taking away the small grains causing thecoarsening of the grain size of the beach,

According to the geology investigation document of the Hydraulics EngineeringSurvey and Design Service of Namdinh, strata structure of Giao Thuy coast has 5following layers:

Layer 1: backfill soil of dike: sandy gray clay, brown clay, yellow gray clay,plastic to stiff with thickness of 1.5 10 2.0 m,

0

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~ Layer 2: sandy brown gray clay, pink brown clay, plastic to soft with thickness

0f 2.0-5.0m

Layer 3: sandy black gray cla5.0 to 6.4m.

gray clay, plastic to liquefacient, thickness of

Layer 4: sandy brown gray clay, brown clay, plastic to soft with thickness of 6.5t09.2m

Layer 5: fine gray sand, fine black gray sand, soft with thickness of 9.2 to 15.0m

We can easily realize that Giao Thuy has a vulnerable beach from above structure ofstrata. Therefore, the stability of the dike will be seriously threatened if the upperlayer is washed away

©) Sediment transport conditions

From sediment investigations in Giao Thuy, it is clear that sections of the beachsituated relatively far from the river mouth in the range of 10 km are not nourishedby river sediment caused by The sediment supplied by rivers is accumulated in therear shore zone close to the river mouth and is not transported along the shore in anysignificant amounts.

Sand — Aleuile | Clay

d) Climate and Meteorology

Giao Thuy is situated in tropical climate area with a pronounced maritime influence.“The winter is cool and dry, with mean monthly temperatures varying from 160C to

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21°C. Fine drizzle is frequent in early spring, after which the temperatures rise rapidlyto a maximum of 40°C in May. The summer is warm and humid, with averagetemperatures varying from 27°C to 29°C, The prevailing winds are Northeast in the‘winter, and South and Southeast in the summer.

‘The average annual rainfall is 1600 to 1800 mm, 85% of which occurs during therainy season (April to October). The heav rainfall occurs in August and

September, causing intensive flooding in the delta due to overflow of the riverbanks.

‘Typhoons and tropical storms are frequent between July and October. During theperiod from 191 1 to 1965 the region withstood 40 typhoons. However, the frequency‘of storms and typhoons appears (0 have increased in recent years, Typhoon stormsusually come from the west pacific, through the Philippines or Eastern Sea. They thenshoot into the coastal areas of South China and Vietnam. Among the typhoons that‘occurred from 1954 to 1990, strong winds with grade 12 were observed for 31 cases‘The annual average number of typhoons is about 5, but more than 10 were observedin 1964, 1973 and 1989,

‘Typhoons also bring about periods with heavy rains, (over 100 mnvday, possibly300- 400mm/day) causing severe flooding. The rains, which affect areas in radius of200 ~ 300 km, may become terrible natural calamities. When such storms break overthe main Jand, a huge amount of water is released, damaging the sea dikes (rainfallerosion), and flooding the coastal areas.

©) Oceanography

he sea at Giao Thuy is open sea (there is no offshore island) so the wind fetch islong enough for wave growth and approaches the shoreline without any obstawhich can cause considerable damage to shoreline and sea dikes. According to

previous observation, waves at Giao Thuy had following characteristics:

<small>~ In winter (from September to March): In the winter, the sea was much more</small>

rough sea than in the summer, Wave height is about 0.8m ~ 1.0m, with periodsvarying from 7 to 10 seconds. Predominant wave direction was northeast, and,

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<small>(CHAPTER 2:GENERAL OVERVIEW OF STUDY AREA</small>

makes angles of about 30° to 45° with the shoreline.

~ Inthe summer (from April to August): In the summer there are less rough sea

{days but strong storms usually happen in this season causing severe damage to.the dike system. Average wave height varies from 0.65m to 1.0m with period.ging from 5 to 7 seconds, The prevailing wave direction is south andsoutheast.

Witter MONSOOD cuuuox.

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gì Waves

“The sea at Giao Thuy is open sea (there is no offshore island) so the wind fetch islong enough for wave growth and approaches the shoreline without any obstacles,which can cause considerable damage to shoreline and sea dikes. According to‘observation, the waves at Giao Thuy has following characteristics

= In winter (from September fo March): In the winter, the sea was much morerough sea than in the summer, Wave height is about 0.8m ~ 1.0m, with periodsvarying from 7 tol0 seconds, Predominant wave direction was northeast, andmakes angles of about 300 to 450 with the shoreline.

= Inthe summer (from April to August): In the summer there are less rough sea

days but strong storms usually happen in this season causing severe damage tothe dike system. Average wave height varies from 0.65m to 1.0m with periodranging from 5 (o 7 seconds, the prevailing wave direction is south andsoutheast.

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CHAPTER 3: Probability risk and reliability assessment3.1. General introduction

Over the past few decades. flood defense system design has recorded breakthroughsints development. In traditional method, dike crest level is based on maximum floodwater level recorded in history. This water level is determined based on statistic datacalled design water level defined based on design frequency.

“The design frequency of design water level is widely applied as a safety standard forprotected area according to probability of flooding. However, this theory is true in‘case of the break dike caused by the flood level exceeded design water level and nottrue in case of flood level smaller than design water level

The safety standard depended on traditional deterministic approach is designfrequency of load and safety factor based on possible failure mechanisms. Inthe wayof reliability analysis theory, the safety standard is limit states of failure probabilityof system regarded as combination of failure probability of components in the systemWhich connected closely to exceed frequency limit of load. The probability offlooding is determined if the damage causes can be listed and failure probability of‘each component can be determined. From the above arguments, idea of dike safetyassessment based on failure probability analysis of all the relevant factors is feasible.

Principle of probability of a structure system must base on by each component thosefailures creative by mechanisms and model failure. Calculating the probability of‘each component is very important when anticipated probability analysis of one

‘There are three factors when assessed probability. They are Thread ~ mechanism =failure probability. Firstly, we must list threads and mechanism. A failure mechanismis defined as a form in which structure must effected by thread.

Advanced of probability analysis method is the probabilistic approach results in aprobability of failure of whole system taking account of individual element (cross

1s

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section or structure). So the probabilistic approach isan integral design method forthe whole system.

3.2. General background of probability theory

a) Risk analysis

In every floodplain, the accepted probability of flooding isnot the same. It depends‘on the expected lss in cas of failure, the nature ofthe protected area and the safety

standards of the country. However, risk is the product of the probability and a power

‘of consequence, therefore this reason accepted risk is a better measure than anaccepted failure probability: Risk = (probability) * (consequence)n. The power n is‘depended on the situation of th ‘a natural risk approach and impliesthe calculation of expected value while n>1 denotes the risk aversion,

Figure 3. 1:Frame work of risk analysis (see CUR 141, 1990)

“The elements of risk analysis in the probabilistic approach are shown in Figure 3.1At first, before making an inventory of all the failure modes and the possible hazards,the flood defense system has to be described as a configuration of elements such asdike sections, sluices and other structures. Due to the miss of a failure mode can

6

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seriously influence the design safety, this step plays an important role in the analysis,

In the next step, the quantifying of the impacts of failure for all possible ways offailure is carried out. The probability of the failure and consequences form are a part‘of the risk. The design can be evaluated when the risk is calculated with the eriteriaavailable such as a maximum acceptable probability of a number of casualties, Aframe of reference is necessary for determini wg the acceptable risk. The national

safety level aggregating all the activities in the country can also get to be this frame

of reference. The evaluation of the risk will decide whether the work can decide toadjust the design or to accept it with the remaining risk,

b) Reliability analysis

‘The reliability function is Key elements ofthe probabilistic calculation of ascertainingthe probability of failure. The function of reliability Z is formed concerning the limitstate considered, in such a way that positive value of Z corresponds to non-failureand negative values to failure (see Figure 3.2). The probability of failure thus berepresented as P(Z<O). The reliability function isa function of a number of stochasticvariables,

Figure 3. 2:Definition of a failure boundary Z=0

‘These methods define failure probability about predetermined reliability functiondistributed some levels as following

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Level |

Nowadays, the design works are based on design standards and guidelines. In thatway, the parameters of durability are adjusted by the coefficient characteristics andthat of load are adjusted by load coefficients. Shown by the following formula:

D en

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R: DurabilityS: Load

{yas Safety coefficient of durability

Ys: Safety coefficient of load

‘The characteristic value of durability and load coefficient are calculated by formula

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progressions need to do.

‘The general form of reliability function Z=R-S is considered. In which R and § arefunctions of strength and load respectively and both considered following normaldistribution. This implies thatthe statistical parameter of reliability function Z can be‘oblained through:

Wher: Bi etait index: p=

{4 (-B): Standard normal distribution for the variable

In general Z in equation (3.10) will be a function of more than two variables. Thesevariables do not have to be normally distributed and Z does not have to be linear.Only if Z is a linear funetion and all variables are normally distributed (andindependent) the second equation in (3.10) is indeed equality and not an

Z may be a function of n stochastic variables XI, X2,....X , as both the "load", S,and the "strength", R, may depend on more than one variable. In order to perform a

20

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level II calculation, the variables XI, X2,...X have to be independent and it must bepossible to linearize the reliability function Z in all point of Z. Suppose the reliabilityfunction, Z, fulfils the requirement and the variables X are all normally distributedand independent.

ILis supposed that the reliability function can be linearized, so tangent plane in a point‘on its surface can be expressed by a first order Taylor expansion:

VX0+ Vú =X) „(2u

Z0,

“--=Linearized reliability funedon of Z in {X:°)

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gz) Pe, Mz cor

Figure 3. 3:Definition of probability of failure and reliability index

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Hf mean values Xj =/4„„„...X; =,g„„ ate situated, so called mean value

approximation of the probability of failure is obtained. If the failure boundaryis nonlinear, a better approximation can be achieved by linearization of thereliability function in the Design Point, The Design Point is only defined if thevariables are normally distributed (or are transformed to normal distributedvariables). The Design Point is defined as the point on the failure boundary in which

the Joint (normal) probability density is maxima.

“The design point is given:

Level 3:

‘The fundamental of failure probability calculation in level 3 is mathematicalsimulation of subsets of failure probability

If the probability density function f,,(R,S) of R (durability) and

S (load) is

determined, the failure probability canbe calculated by integration method:

3.3. Probabilistic reliability analysis of sea dikes system in Giao Thuy-NamDinh

‘The flood defense system consists of many components made up a closed system to

<small>2</small>

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protect residential areas. Safety factors depend on safety standard when designed and.constructed, The different level of safety standard is provided in national standardand applied in design work.

1) Issues that concem flood defense system includes,

2) Is protected area safe?

3) Is current safety standard enough? And if no, how to improve safety standard in

‘The safety of defense system in Giao Thuy- Nam Dinh is assessed by determining

the failure probability of each component, whole system and probability of flooding.First of all, we need mapping system, identifying failure mechanisms. The mainfailure mechanisms of sea dikes in Giao Thuy are listed below:

<small>= Overtopping</small>

~ Toe foot instability of revetment

<small>~ Instabilities of armour layers</small>

<small>- Piping</small>

- Sliding of dike slope

<small>24</small>

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<small>vig || Pn] [ Sing a) Tos Ra) sai Tam</small>

Figure 3. 4: Fault tree of Giao Thuy sea dike

Overtopped sea water exceeds he critical value of wave overtopping too much,

Due to wave overtopping damages dike crest level, the following failures could,be occurred:

Erosion of crest sides,

Erosion of inner slope due to too much wave overtopping,

At some places where the dike body was not well protected by cover layers, the‘material of filter layer is washed.

Damage of outer slope protection due to impact of wave or erosion at the toe.

<small>25</small>

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<small>= Damage of upper part of revetment due to the return flow of overtopped seawater.</small>

{tis clear that, the crest level of the dikes and the strength of erest wall are the most

Important parameters to avoid these failures due to wave overtopping.

“The design water level is the first component which mainly contributes to crest height,

‘ofa dike. This water level corresponding to design frequency and design lifetime ofstructures should be the highest water level which may occur at the location

Normally the design water level includes mean water level, tial level, storm surges,

wind setup, wave setup

Wave run-up level is the second important component contributed by wave height at

the location near the toe of the dikes. It is so called local design wave height for the

dikes. Wave run-up depend on the outer slope, which can consist of materials by

different roughness. If there is @ berm, the berm width is also a parameter which caninfluence the magnitude of the run-up and overtopping of incident waves, and theimpact on slope protection.

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‘The reliability function of overtopping:

Z, :Crest evel ofthe dike

Zn. Maximam water lev acts on the dike (including wave run-up level)

‘The failure presents when Z0, therefore the probability of overtopping failure modeis PZ<0),

b) Mechanisms of instability of armour layers of revetment

In VietNam, one of the most regular failure is instabilities of the armour layer of the

revetments. There are some main reasons which caused the failure. At first, the

thickness of the cover layer is not sufficient to the hydraulic condition due to the factthat most of the dike des \s were applied old method of the year 60s. Secondly, thequality of the constructions was not good becaus

by hands.

the revetments implemented mostly

‘The minimum thickness of the armour layers of revetment should be at least thick‘enough in order to remain the safety for under layers of revetment and dike's body.

Inthe year 60s, the limited fund leads to not popular in using the concrete blocks

However, the suitable rock size is chosen in Viet Nam generally and in Giao Thuyspecifically for the cover layer of revetments very limited, difficult to find the bigrocks, which have the diameter larger than 30cm because the rocks have beenexploited by explode

According to Dikes and revetments, Krystian W Pilarezyk editor, when the watermoves on a revetment structure itcan affect the subsoil, especially, when this consistsof sand. This effect is considered within the framework of the soil- mechanicalaspects and can be of importance to the stability of the structure, see Figure 3.6

‘There are three aspects that will be discussed within the framework of mechanical aspects: elastic storage

soil-level The background information can be found CURI69/RWS (2001).

softening (liquefaction); and drop in the water

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‘© Elastic storage in the subsoil is connected with the permeability and stiffness of

the grain skeleton and the compressibility of the pore water (the mixture of‘water and ar in the pores of the grain skeleton). Because of these characteristics,‘wave pressures on the top layer are passed on delayed and damped to the subsoilof the revetment construction and to deeper layers (as seen perpendicular to theslope) of the subsoil. This phenomenon takes place over a larger distance ordepth as the grain skeleton and the pore water are stiffer. Elastic storage canJead to the following damage mechanisms (Stoutjsdijk, 1996)

= Lilfing of the top layer:

~ Patil stiding ofthe top layer:

~ Sliding ofthe top layer:

<small>= Sliding ofthe subsoil (treated in geotechnical related stability section)</small>

For the stability of the top layer, clastic storage is particularly of importance if the toplayer is placed directly on the subsoil without granular filter.

Because the revetment construction con is of a top layer on a filter layer, the

thickness of the filter layer may in these diagrams be partially or completely(depending on the type of revetment) added to the thickness of the top layer. The

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‘equivalent thickness is defined as:

Deg = is the equivalent thickness ofthe top layerD_ isthe real thickness of the top layer

bb is the thickness ofthe filter layer

At sis the relative mass (weight) under water ofthe top layer

<small>‘© Liquefaction: The difference between liquefaction and elastic storage is that</small>

with liquefaction, water overtension is connected with a plastic deformation of agrain skeleton instead of an elastic deformation, Water overtension throughsoftening occurs when the grain skeleton deforms plastically to a denser packing.With regard to liquefaction, according to Dutch criteria, with a top layer on a

granular filter there is generally no danger of liquefaction,

© Drop in the water level: A drop in the water level may occur as a result of tideor a ship passing through a waterway or canal. As with placed stone revetments, theresulting uplift especially dangerous when the top layer sanded up duc to Whithe permeability of the top layer may decrease in time.

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bfis thickness of the filter layer (m);

kris permeability of the filter;

top is permeability of the top layer;

Dis thickness of the top layer

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“The common form of reliability function of armour layer of revetment is

A: is relative density of applied material

Ds is the size of elements

©) Toe foot instabilities

‘The maximum scour depth in front of the toe structure is used to determine theprotective required depth for the toe structure obviously. During the life time of+ the maximum depth of scour is regarded to an equilibrium scour depth.From practical experience, that depth should lie from 0.5 to 1.0 times significant waveheight (Pilarezyk et al, 1998), Basing on both model of mathematics and physics,there are several researches on scour development near and around the toe of marine

<small>a</small>

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FO) with /(6)<03-17e5 20)

In which

h The depth of water infront ofthe te

Ho: The height of wave infront ofthe toe

«: The angle of outer slope

‘The reliability function of this situation is defined as follow:

1, The dike te depth

1h: The seour depth

Piping occurs when underground of dike washed due to seepage entrancementcollapsed dike body. The reason of this failure due to one or many soil layers contactdirectly with unequal water environment, Fi ly, itis going up foundation of dike atdownstream. Continuously it is expansion of underflow of dike material. Pipinghappen when material of foundation erosion due to seepage increase, it makes sand‘of dike foundation move continuously to downstream. Processing happen for a longtime will lead to appear a sand low on dike foundatior it makes empty dikefoundation and threads the reliability of dike body.

Figure 3. 8: Mechanism of piping at sea dike

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‘The failure mechanism of piping occurred when two conditions must be satisfied:

= The clay layer under the dike must be ruptured (1)

‘There is transport of sand (2)

<small>„</small>

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The first condi n: foundation clay layer is rupture when pressure of seepage flowexceeded saturated volume weight. So the reliability funetion of the first condition is

Z= pgd=p,gAi 622)

In which

cis saturated volume weight of foundation layerwis volume weight of water

dis clay layer depth from toe ofthe dike to layer of sand

zis acceleration of gravity

Allis hydraulic pressure head

“The second condition: Based on the Bligh’s criterion in the reliability Function of

<small>piping is:</small>

In which:

I=L’ +12 +B +d (see Figure 38)

constant depending on soil type, according to Bligh

Hs the difference in water levels between sea side and inland

‘m: model parameter uses to calculate distribution of experimentinvestigated

©) Sliding of dike slope

A fixed 4 of conditions and material parameters are considered to be a base for‘computing the factor of safety in the deterministic slope stability analyses. The slopeis considered to be unstable or susceptible to failure, ifthe safety factor is greaterthan the unity

Reliability function for this mechanism is

<small>M</small>

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