Probabilistic reliability analysis of red river dike system protecting giao thuy nam dinh master thesis major sustainble hydraulic structures coastal engineering and management code 62 58 02 0

Before the adverse effects of weather variations and changes in an abnormal phenomenon of natural disasters due to climate change, coupled with the requirement to ensure a higher level o

Reassurances

Name: Pham Viet Dinh

Major: Sustainable Hydraulic Structures

Student ID # 148ULG010

This is my thesis with the topic "Probabilistic reliability analysis of Red river

dike system protecting Giao Thuy - Nam Dinh" under the guidance of Assoc Prof Mai

Van Cong - Thuyloi University and Prof Radu Sarghiuta - University of Liege

I hereby declare that this thesis is the scientific study of my own The results and data in the thesis are honest and no one published it in any other research

Ha Noi, October 2016

Pham Viet Dinh

Acknowledgments

After a period of research, my master thesis with the topic "Probabilistic reliability

analysis of Red river dike system protecting Giao Thuy - Nam Dinh" has been

completed with the enthusiastic help of lecturers, friends, colleagues and my family

To obtain results as today, I would like to express deep gratitude to the Assoc Prof Mai Van Cong from Thuyloi University and Prof Radu Sarghiuta from University

of Liege those who were enthusiastic to guide and provide knowledge, documentation, scientific information as well as contribute valuable comments during the implementation

of this thesis

I also sincerely thank the help, the assistance in terms of expertise and experience

of the teachers teaching in class, undergraduate and postgraduate training office; classmate in Thuyloi University and all my family, friends have motivated, inspired, create favorable conditions in all aspects to completing this thesis

In the process of implementation of the thesis, due to the time and limited knowledge, there are surely unavoidable mistakes, so I wish to receive the comments of teachers, colleagues to help me improve knowledge in terms of learning and researching

Sincere thanks!

Ha Noi, October 2016

Pham Viet Dinh

Table of contents

Chapter 1: General introduction 1

1.1 Reasonable of this study 1

1.2 General information 2

1.3 International related research applications 3

1.4 Present situation of flood risks in Vietnam 4

1.5 Study objectives 5

1.6 Study approach 6

Chapter 2: Boundary conditions & description of study area 7

2.1 Description of dike systems in Vietnam 7

2.2 Influence factors 8

2.2.1 The influence of hydrology 8

2.2.2 The influence of the tide 9

2.2.3 The influence of irrigation planning, traffic and construction 9

2.2.4 The influence of the protected area 10

2.2.5 The influence of other factors 10

2.3 Natural conditions 11

2.3.1 Location 11

2.3.2 Topographical characteristics 11

2.3.3 Hydrology 12

2.3.4 Climatic condition 12

2.4 Present situations of Huu Hong river dikes 13

2.5 Practical issues in application of the standard criteria and norms: 17

2.6 Overview of traditional design methods: 17

2.6.2 Shortcoming in traditional design methods 19

2.7 Overview of probabilistic design method 19

2.7.1 General background 19

2.7.2 Historical development of probabilistic design method in the world 20

2.7.3 The approach of the method of probabilistic design 21

2.8 Conclusion chapter 2 21

Chapter 3: Safety assessment & reliability analysis of Huu Hong river dike 23

3.1 Background theory 23

3.2 Theoretical basis 24

3.2.1 Risk analysis 24

3.2.2 Reliability analysis of components in system 27

3.2.3 Mathematical basis of probabilistic design 28

3.2.4 Reliability analysis of system 36

3.3 Application of the probabilistic design in reliability of river dike system of the Huu Hong river dike 37

3.3.1 Case study: Safety assessment of existing river dike system in Giao Thuy district 38

3.3.2 Possible failure mechanisms of the river dike 39

3.3.3 Fault Tree Analysis 46

3.4 Conclusion chapter 3 46

Chapter 4: Application of probabilistic reliability analysis to assess safety of Huu Hong river dike system 48

4.1 Failure mechanisms to consider 48

4.1.1 Overflowing mechanism 48

4.1.2 Instability revetment mechanism 51

4.1.3 Scouring mechanism at dike toe 54

4.1.4 Piping mechanism 59

4.1.5 Inner slope instability mechanism 63

4.1.6 Fault Tree Analysis 65

4.2 Establishing fragility curve 68

4.3 Conclusion chapter 4 72

Chapter 5: Conclusions and recommendations 73

5.1 Achieved results 73

5.2 Existing problems 74

5.3 Recommendations 74

5.4 Further research directions 74

References 76

Appendix 1 77

Parameters and documentation used to calculate for the Huu Hong river dike 77

Appendix 2 86

Result of probabilistic reliability analysis 86

List of figures

Figure 2-1: Storms, natural disasters affect the coastal areas in Vietnam 8

Figure 2-2: Administrative maps of Nam Dinh province 11

Figure 2-3: Location of Nam Dinh province on the map (from google map) 13

Figure 2-4: Overall system map dikes in Giao Thuy district - Nam Dinh province 14

Figure 2-5: Representative cross-section of Huu Hong river dike 15

Figure 2-6: Several photos of current situation of Huu Hong dike 16

Figure 3-1: Framework of risk analysis (CUR 141 – 1990) 26

Figure 3-2: Reliability function is shown in the plane RS 28

Figure 3-3: Definition of the probability of failure and the reliability index 28

Figure 3-4: Diagram of failure analysis of serial system and parallel system 37

Figure 3-5: Diagram of flood defense system of Huu Hong river dike – Giao Thuy district – Nam Dinh province 39

Figure 3-6: The failure mechanisms can occur in river dikes (CUR/TAW 1995) 40

Figure 3-7: Overflowing scheme 41

Figure 3-8: Instability revetment scheme 42

Figure 3-9: Scouring mechanism at dike toe scheme 43

Figure 3-10: Rupturing scheme Figure 3-11: Sand flowing scheme 44

Figure 3-12: Slope sliding scheme 45

Figure 3-13: Fault Tree diagram of flood defenses system – Huu Hong dikes 46

Figure 4-1: Real measured data of dike crest level 49

Figure 4-2: Distribution of dike crest level based on real measured data by using BESTFIT software 49

Figure 4-3: Real measured data of flood water level 50

Figure 4-4: Distribution of flood water level based on real measured data by using BESTFIT

software 50

Figure 4-5: Influence of random variables to the overflowing mechanism 51

Figure 4-6: Influence of random variables to instability revetment mechanism 54

Figure 4-7: Real measured data of flood flow 56

Figure 4-8: Distribution of flood flow by using BESTFIT software 57

Figure 4-9: Influence of random variables to scouring mechanism at dike toe 58

Figure 4-10: Real measured data of upstream water level 60

Figure 4-11: Distribution of upstream water level based on real measured data by using BESTFIT software 60

Figure 4-12: Influence of random variables to the rupturing mechanism 62

Figure 4-13: Influence of random variables to sand flowing mechanism 63

Figure 4-14: Results of inner sliding stability by using Geostudio 2007 software 64

Figure 4-15: Diagram of Fault Tree Analysis for Huu Hong river dike system 66

Figure 4-16: Result of failure probability after using OpenFTA software 67

Figure 4-17: Graph of the iterative period with the highest water level at Ba Lat hydrology station 69

Figure 4-18: Fragility curve of correlation between Pfsystem and FWL 71

Figure 6-1: Iterative period according to highest water level at Hon Dau, Ba Lat hydrology station 78

Figure 6-2: Diagram of height of wave 81

Figure 6-3: Figure A1- Appendix A1 - TCVN 8421-2010 – Graph of determining wave factors causing by the wind in deep water and shallow water 82

Figure 6-4: Figure A2- Appendix A1 - TCVN 8421-2010 – Graph of determining coefficient ki83 Figure 6-5: Result of inner sliding stability by using Geostudio 2007 software 85

Figure 6-7: Result of instability revetment mechanism in case FWL = 3.62m 86

Figure 6-8: Result of scouring mechanism at dike toe in case FWL = 3.62m 87

Figure 6-9: Result of rupturing mechanism in case FWL = 3.62m 87

Figure 6-10: Result of sand flowing mechanism in case FWL = 3.62m 88

Figure 6-11: Result of inner slope instability mechanism in case FWL = 3.62m 88

Figure 6 12: Diagram of Fault Tree Analysis in case FWL = 3.62m 89

Figure 6-13: Result of overflowing mechanism in case FWL = 2.0m 91

Figure 6-14: Diagram of Fault Tree Analysis in case FWL = 2.0m 91

List of tables

Table 4-1: List of random variables according to the overflowing mechanism 50Table 4-2: The probability of the failure and the influence coefficient of the random variables to the overflowing mechanism 51Table 4-3: List of random variables according to the instability revetment mechanism 53Table 4-4: The probability of the failure and the influence coefficient of the random variables to the instability revetment mechanism 53

Table 4-5: List of random variables according to the scouring mechanism at dike toe 57

Table 4-6: The probability of the failure and the influence coefficient of the random variables to the scouring mechanism at dike toe 58

Table 4-7: List of random variables according to the piping mechanism 61

Table 4-8: The probability of the failure and the influence coefficient of the random variables to the rupturing mechanism 61Table 4-9: The probability of the failure and the influence coefficient of the random variables to the sand flowing mechanism 62Table 4-10: The probability of the failure of the inner slope instability mechanism 64Table 4-11: Results of failure probability of Huu Hong river dike 65Table 4-12: Table of the iterative period with the highest water level at Ba Lat hydrology station 69Table 4-13: Probabilistic calculation with difference value of FWL 70

Chapter 1: General introduction

1.1 Reasonable of this study

Vietnam has about 3260 km of coastline, mainly consists of coastal lowland is protected

by a sea dike system, natural dunes, and mountains More than 165 km of coastline in the Red River delta, populated areas where there are significant changes and dynamic impact destroyed with intense frequently from the sea (storms, changes in sea level, flow, etc.) Therefore, dikes are important structures which are built, maintained and protected through generations to prevent flood water, sea water and to protect the lives and property of the government and people, to promote social and economic Social development, linked to national defense, security, sovereignty, and national benefits The process of formation and development of the dike system always linked to the life and productive activities of the people from generation to generation Most dikes are now combined as roads in which many dikes pass through tourist areas, urban areas and residential areas During the development processes, requirements for dike system as well as the direct impacts of human on the dike are growing and increasing in complex evolution

In recent years, natural disasters and climate changes in Vietnam have had more abnormal and complex In particular, storms and floods are two types of natural disasters which frequently occur and cause the most severe consequences, especially in the area of coastal estuaries However, most of the system of dikes and storm prevention, flood prevention existing today in Vietnam is designed and constructed based on the experience accumulated from many generations and applied safety standards which only suit economic situation - engineering conditions of the country in some decades ago Before the adverse effects of weather variations and changes in an abnormal phenomenon of natural disasters due to climate change, coupled with the requirement to ensure a higher level of safety of the protected areas to serve the sustainable development of economic - society, the research and development applications in reliability theory of optimal design system against storms, floods and on this basis build up the safety assessment criteria according to reliability theory and construction process safety assessment system of dikes in estuaries and coastal areas according to theory of reliability in Vietnam condition at the present time and the future is necessary

1.2 General information

Vietnam has a dense network of rivers and the coast stretching from North to South, so the system of dikes and bank protection structures play an extremely important role in the prevention, disaster mitigation and protecting the safety of the cultural center, the political, economic and residential areas stretching along the vast river basin, the coastal regions of the country

According to the general trend of development, nowadays coastal areas are a key economic zone dynamic contribution and increasingly more important role in the national economy and national security Therefore, the requirements for the protection of the population and economy against the destruction of hurricanes, floods, surges are becoming ever more urgent Besides the consolidation and upgrade the dike system has had, the planning for river bank protection, coastline and building the new dike system is in place in all three regions of the country

In the current issue of global climate change, Vietnam is one of the countries which is most severely affected In addition, the trend of development in Vietnam as well as the countries close to the sea in the world is seaward development, big cities concentrate on coastal So development of marine resources, tourism, and waterway transport are very important Therefore the system of dikes and bank protection structure have a very important role It not only is the task of protecting people and infrastructure, but also has the task of creating the resort location, beautiful natural landscape, creating refuge areas for ships, port protection when storm coming

In recent years, in the world, the risk of disaster in general and flood, in particular, has significantly increased trend of frequent occurrence and level of influence The events of the recent flooding disaster with considerable history as New Orleans, USA 2005; The UK and Eastern Europe (2007), Bangladesh and South Asia (2007), Pakistan (2008), and most recently the flooding disaster in the history of the capital Bangkok of Thailand (2011) The floods caused heavy damage to people and property So minimizing the risk of floods are now particularly interested in many countries around the world

1.3 International related research applications

Many hallmarks of scientific & the research program around the world related to flood prevention, dikes safety, application development of reliability theory in system and structure safety can be listed as follows:

Netherlands: Netherlands was known as a leading country in flood prevention since they

continually invested in research and technology related to this field One of their achievements was a reliability theory It has been put into the application to design the crucial components of Deltaplan program in the 70s Reliability theory continued to develop and extend its application and became a compulsory subject in civil engineering major in the 1985s After that, design standards following reliability theory were updated in 1990, 1995 and 2000 Then, with the purpose to update calculation technologies and safety simulation methods in a dike system of reliability theory to be high reliability and minimum margin of error, VNK project was implemented from 2001 to 2003 by Strategic Research Institute PNO, Deft Hydraulics and Deft University of Technology This project provides reliability criteria updated in design standards Next, VNK2 project (2007-2010) and SBW (2008-2011) were implemented by the same research group in VNK Project These projects focused on improving the accuracy of the models simulating with random loads and the durability of the flood prevention structure These are critical inputs of structure’s reliability analysis Besides, the effect of the dikes system length in safety assessment also investigated profoundly 3D model design following reliability theory also appeared In addition, flood risk standards were checked and updated according to not the only person, community and economy view, but also the culture, history, and the environment, etc

Britain and Europe: Britain and Europe inherited and developed the study of the

Netherlands to apply to their own country’s characteristics “Reliability of Flood Defenses and Integrated Flood Risk Management” project (FLOODSite) was implemented from 2005-2009 by

38 institutes & major universities from 20 countries in the region The project gave out the synthetic approach in the safety assessment of flood prevention, flood management, and flood mitigation systems Reliability theory was confirmed and developed as a core model for the safety assessment of the system and risk analysis of the flood prevention system Safety assessment models, optimal design systems and simulated models of loss due to flood were

developed and examined by the experimental application in the countries Integrated models, Solution 1& 2, were developed to apply in EU member countries Member countries came to an agreement to develop and use the same standard safety assessments in dike systems and flood prevention

U.S and Canada: These two countries developed the application of the reliability theory

in dam’s safety, especially high dams in the 90s Standard system was converted completely from traditional safety standard (safety factor) to safety methods following allowable reliability [β] Recently, a typical application in design standards according to reliability in U.S was a design project, planning in flood prevention system of New Orleans with system allowable reliability of [β] =4.2

Russia and China: Russia and China applied the reliability theory in quantitative

structure safety following the technical standards using allowable reliability [β] The application was mainly applied in dam design China limited the reliability of several specific structures by fixed reliability value For example, the reliability value of reinforced concrete was 3.6 ≤ β ≤ 4.2 Russia used reliability β to adjust several coefficients in design such as overloading factor, etc The standards of risk management and flood prevention structure safety assessment are constructed and applied in 5 recent years

1.4 Present situation of flood risks in Vietnam

To evaluate an overview of the work of dikes and issues of safety of flood prevention in Vietnam, we can cite several points stated in the "National Strategy for Prevention and Disaster Reduction 2020" as follows:

Vietnam is located in the tropical monsoon, one of five storms of the Asia - Pacific

region, often faced with this kind of natural disaster, floods and storms at most

In recent years, natural disasters occurring in all areas of the country have caused loss of life, property, infrastructure, economic, cultural, social and environmental adverse impacts In the recent 10 years (1997-2006) the disaster killed, missed nearly 7500 people and the damage is estimated at 1.5% of GDP Due to the impact of global climate change, Vietnam is among

countries most affected by sea level rise and other impacts of natural disasters increase in size as well as iterative period and unpredictable

For decades, the investment of government and the effort of people have created the infrastructure system of prevention and mitigation of natural disasters and relatively uniform across regions The system of river and sea dikes on the 4500 km, large reservoirs serving the flood reduction, water regulation, electricity generation was based on the fixation of large river basins The irrigation-transportation structures, construction of residential flood prevention, prevent floods and erosion control projects, boats mooring zones to avoid the storm, forecasting warning system, communications, rescue aid victims have progressed, increasingly improving disaster prevention before coming Red river delta has been coping with flood frequency of 500 years, which is the level of regional guarantee Mekong Delta lives with floods increasingly proactive, continuous agricultural production and harvest stability during the past decade,

International cooperationdevelopment is of importance of prevention and mitigation of disasters Vietnam has actively participated in and contributed to the forums and international and regional commitments to the prevention and mitigation of disaster risk and climate change framework action Hyogo, Kyoto Protocol, Asian general agreement about the cooperation deal before the disaster The international community has helped Vietnam to train human resources, transfer of technology, experience, raise awareness, build models Especially ODA projects intended for flood prevention structure and mitigation of natural disasters; the non-refundable ODA projects for the local industry have brought very practical effect

1.5 Study objectives

- The purpose of this study can be presented as follows:

+ Safety assessment of existing river flood defenses of Giao Thuy

+ Determination of the reliability of Huu Hong Dike section for present condition

+ Establishment of the statistical probability distribution of loads (water level, discharge) and strength (soil properties) of the dike section

+ Quantifying the risk due to flooding in the protected area

In this research, the following approach will be used in the study:

- Collect the necessary data from all possible sources, including the subject

- Point out the future predictions of the probability of failure mechanisms for Giao Thuy dike

- Review of previous related studies that deal with Huu Hong river dike

- Review the existing dike design in Vietnam

- Evaluate and determine the safety of the river dike in Giao Thuy by applying reliability analysis

Chapter 2: Boundary conditions & description of study area

2.1 Description of dike systems in Vietnam

Vietnam is a country located in the tropical monsoon climate with abundant and diverse geomorphic, topography; West mountain, Northern-West mountain, and East mountain are surrounded by the sea and dense river systems Residential areas, cities, and agricultural areas often develop along the riverside areas and often are influenced by factors and risk of flood Dike system along the river branches is a solution of flood prevention which was used for a long time ago to protect residential area along riverside and entire delta region from the risk of flooding For a long time of development, the current dike system in the country is a large-scale structured system with about 13,200km of dike, of which about 10,600 km river dike and nearly 2,600km sea dike The main river dike system with over 2,500 km of the dike from grade III to special grade; remaining dike is under grade III and has not been yet decentralized In which:

- Dike systems of North and North Central: 5,620km of length, with flood protection is responsible for flood protection, ensuring safety for the Northern Plains and North Central

- River dike system, estuarine areas of Central and South Central: a total length of 904km

- The system of river dikes, embankments in Mekong Delta area: 4,075km of length

In terms of the socio-economic development of the country at present, the requirements for the protection of the population and economy against the destructive of hurricanes, floods, surges are becoming ever more urgent Besides the consolidation and upgrade the dike system has had, the planning for river bank protection, coastline and building the new dike system are set out in all three regions of the country

In the current issue of global climate change, Vietnam is one of the countries which is most severely affected In addition, the trend of development in Vietnam as well as the countries close to the sea in the world is seaward development, big cities concentrate on coastal So development of marine resources, tourism, and waterway transport are very important Therefore the system of dikes and bank protection structure have a very important role.Besides the task of

protecting people and infrastructure, it also has the task of creating the resort location, beautiful natural landscape, creating refuge areas for ships, port protection when having a storm

Figure 2-1: Storms, natural disasters affect the coastal areas in Vietnam

2.2 Influence factors

2.2.1 The influence of hydrology

Hydrologic factor influences markedly on the dike crest level for river dike and estuary dike Component affects to the calculation formula of the dike crest level consist of design flood water level; rising water level due to wind and wave

The design flood water level of the dike is determined to correspond to the guaranteed frequency of flood prevention design of the dike (to be taken according to the level of the dike by norm) Flood frequency curve is built from a series of annual measured flood data

Water rising due to the wind causes the influence of hydrology by these factors: wind direction; wind velocity; wind momentum; the depth of the water level in front of the dike

The height of wave run-up depends on many factors such as the parameters of the wave in front of the dike, dike slope coefficient, guaranteed wave run up levels, roughness and permeability of slope, wind velocity, depth of the water level in front of the dike, the direction of wave propagation… etc In which, the influence of the hydrologic factors including: wind

direction, wind velocity, the time the wind blows continuously, wind momentum, the parameters

of the waves in front the dike, the depth of the water level in front of the dike

2.2.2 The influence of the tide

The tide affects considerably to the crest level of the dike andestuary dikes Composition affects to the calculation formula dike crest level, including calculation sea level, height storm surge, and height of wave run-up

Sea water level is calculated froma guarantee frequency in the work place, including the astronomical tide and variable values due wave, flood, earthquake, fake tidal, water changing level, long-period variable, etc without regard to the storm surge

The height of the rising water level is determined by the frequency and latitude In this case, real measured data of tide is used to analyze the frequency If the tide has components including surges, no need to calculate this quantity anymore

2.2.3 The influence of irrigation planning, traffic and construction

Irrigation planning, traffic planning and building planning transform boundary conditions

of the dike crest level computing, such as conditions of topography, hydrology, hydraulic In particular:

- The construction of the system of reservoir in upstream of the river basin will change the flow of water in the river to downstream;

- The construction of hydraulic structures, traffic on the river as the dam, bridge across the river changes the hydraulic regime of rivers;

- The planning of construction of transport routes at downstream will change the direction

of flood drainage, and it also affects the capability of flood drainage to the sea;

- The construction of urban planning, infrastructure of presidential area would reduce the flooding storage zone and narrow the section of flood drain

2.2.4 The influence of the protected area

In the approach of the traditional method, the scale of protected area affects directly to the level of the dike design Then it affects the guaranteed frequency and allowable safety for the dike

In the approach of the probabilistic method, the scale of protected area affects the level of damage when the failure occurs The level of damage of the failure affects the risk because the risk is a function of the failure probability and the consequences The dike crest level is defined

as the optimal balance plan between risk level and investment costs of constructing the system, or

in other words, the dike crest level is determined based on the probability or frequency of acceptable damage of protected areas Therefore, the scale of protected areas directly impacts the dike crest level

2.2.5 The influence of other factors

In addition to the above factors and other factors also affect the dike crest level, such as topography, geology, the cross-section shape of the dike, functions and tasks of the dike, etc.:

- Topography affects the wind factor, wave in front of the dike;

- Geology can affect the shape of cross-sectional structure, from that it affects to the wave factor of the dike In addition, the foundation of dike also affects to the settlement of the dike;

- Topography and geology also affect the changing of the river bed, river bank, coast, sediment and main flow;

- Shape cross-section of dike affects the wave factor in front the dike as the wave run-up;

- Functions of the dike also affect the dike crest level as wave-breaking, dike allows water

to spill over or does not allow water to spill over, the position and functions of dike protection system consisting of two or more dike axis (main dike; coffer dike…)

2.3.3 Hydrology

River system: Giao Thuy district has a complex river system Depend on the topographical characteristics, rivers flow from the North to the South Downstream of some big rivers such as Red river flow through the area so they have a wide river bed, not so deep and the flow speed is slower than the upstream The water level of rivers can be divided into 2 seasons: flood season and dry season

2.3.4 Climatic condition

Giao Thuy has an almost climatic condition of the Red River Delta area that is affected by

a tropical monsoon climate with four distinct seasons: Spring, Summer, Autumn and Winter

Temperature: the annual average temperature is 20°C, the highest is 39°C in June and the lowest is 5°C in December and January

Sunshine: Annually, there are about 250 sunny days, total sunshine hours are about

1650-1700 hours 3 months of the summer (May, June, July) have the most sunshine hours with the average of 170 – 200 hours per month November has the least sunshine hours with about 40-45 hours per month

Humidity: the humidity of the atmosphere is pretty high with an average of 75-80% and a large amplitude Sometimes the humidity is up to 90%, but sometimes it is lower than 30%

Rainfall: the average rainfall is 1400-1600mm The rainfall distributes irregular, rainy season from May to October make about 75% of the total rainfall in a year, especially in July, August, and September Because of that distribution, it usually has flood in the rainy season The flood has a bad effect on agriculture and the environment

Wind: the most common wind direction is Southeast but it changes due to the season The common wind direction in winter is North-East then changing into East direction, and the most common wind direction in summer is a West wind (the wind from Laos) Moreover, this area usually has a tropical storm with high-speed wind and heavy rain, which makes flood and

2.4 Present situations of Huu Hong river dikes

Nam Dinh province is a part of the coast of Vietnam with a total length of about 70 km, which is protected by a system of river dikes and sea dike The dike system has been severely eroded and severely damaged to the flood defense system There were many times of dike break which caused serious flooding and losses The present situation of Nam Dinh dikes can be considered a typical coastal area in Northern part of Vietnam In general, the erosion and damages of coastal defenses occur frequently, so it results in serious economic consequences as well as the social consequences of the relevant locations

Figure 2-3: Location of Nam Dinh province on the map (from google map)

Giao Thuy, a coastal district belong to the Nam Dinh province, locates at the edge of the Red River delta, and far 45 km from Nam Dinh City to the south Flood defense system of Giao Thuy district, including 31.161 km of sea dike from K0+000 to K31+161 and nearly 30km river dike in which 11.702 km length of Huu Hong dike segment locates from K208+000 to K219+702 (see figure 2-4) Dike crest level from +4.5m to +5.0m; width of crest surface from

4.5m to 5m which has been renovated and reinforced for many years This dike also is public roads for 6 communes and Ngo Dong town along the dike

- For the dike from K208 + 153 to K208 + 735: The current dike crest surface is a concrete road that is still relatively good

- For the dike from K208 + 735 to K210 + 670: The current dike crest surface is concrete roads which were heavily damaged, does not guarantee the load conditions and necessary to remove and reinforcement

Figure 2-4: Overall system map dikes in Giao Thuy district - Nam Dinh province

Dikes of Giao Thuy district were built a long time ago (about 250 years) on a weak foundation (sediment accretion of the Red River system) Dikes stretch from the estuary of Red River in the North (beginning of the line) to So River (Ha Lan estuary) in the South (end of the line) that has a complex terrain with terrain conditions and geological conditions change frequently Not only being directly influenced by tides and winds of the storm from the South China Sea but also being affected by flood flows into the South China Sea from inland waterways So the coastline in Giao Thuy became complicated for some years ago Middle of

coastline which is faced to the sea was occurred seriously erosion

Figure 2-5: Representative cross-section of Huu Hong river dike

Filled soil in dike body and the foundation is low quality in many dike segments, which almost is sand and sandy soil, so it is easy to be slided by rain and wave The important location was protected by dike revetment but often being ruined because revetment structure and the toe

of revetment were not suitable (revetment was constructed of dry masonry) Some sluice was built about 40 years ago, and some of the sluices were shorter than dike body, so the form and structure were outdated or even has been damaged and degraded Therefore, it does not meet the requirements of present flood defense

Although existing dikes were built and upgraded through periods in order to meet the requirement of flood prevention safety standards at 1/100 years for river dike The fact is that the failure still frequently occurs at critical locations along the river dikes

In recent times, along with the global climate change, the damage caused by natural disasters tends to increase around the world Those storms have a strong upward trend, and they can become the destructive heavy typhoon The phenomenon of heavy rain, gust, tornadoes also occurs more frequently In many parts of the world, the phenomenon of earthquakes and tsunamis also occur frequently and cause more serious consequences immensely

The socio-economic activities in coastal and seaside cause changing in the natural environment in the direction of disadvantage and increase the damage of natural disasters In many areas, mangroves and coastal forests have been lost that lead to not only cause changing in the ecological environment in ways that are harmful, but also make billow hit straight into the sea dike, causing sea dike breaking and flooding

Figure 2-6: Several photos of current situation of Huu Hong dike

Although there have been some reports on the safety assessment of the coastal defense system before the flood season in each year, but those reports are made based on management experience and history occurred in the defense system in the prior year The risk of damage is still intense and frequent Therefore, the safety assessment of the existing defense system and analysis of the current situation based on the latest design are necessary

2.5 Practical issues in application of the standard criteria and norms:

The standard system and norms in dike design in our country is old and cannot yet update the actual situation of the current situation The classification of the dike is incomplete for all regions in Vietnam Dike classification is an important regulation, which affects the design criteria and scale of structure

Nowadays, there are no design standards for river dike (new draft river dike design standards in 1999 that have not been enacted) In particular, the standard 14TCN 130-2002

"Design guidelines for sea dike" only applies to sea dike structure

The design of the estuary dike stills difficult because there are no specific standards to apply Calculating just only follows characteristic that can be applied partly according to the sea dike

The regulations, standards and norms only guide to calculate waves and surges but fragmented, inadequate and unsystematic; tide combination, storm and flood for the river dikes in the some river sections, which are affected by tide, still not regular to be specified for each region yet

2.6 Overview of traditional design methods:

2.6.1 Background information

Current traditional design of the structure is calculated according to the deterministic method In this method, the design values of the load and the parameters of strength are considered to determine, corresponding to each case and design combinations The designer selects limited condition corresponding to its suitable design combination load This limited corresponds the characterized strength of the structure

Structures are considered safe when the distance between load and strength is large enough to ensure satisfying of each limit state of all components of the structure

Calculating in this way, just only solves two problems which are overall stability and strength stability of the structure

The content of the traditional design method as follows:

a Allowable stress method

According to this method, strength conditions of the form:

Where:

+ σmax: The largest calculated stress at a point, determining from the most

disadvantaged load combination;

+ [σ]: Allowable stresses, obtained according to documents and criteria

b Calculating according to safety factor method:

This method is usually used in the calculation of stability The checking formula is:

c Calculating according to safety limit states method:

Specific characteristics of the calculating according to safety limit states method is using a group of safety factor with statistical characteristics as load combination coefficient nc, working conditions coefficient m, reliability coefficients Kn, load deviation coefficients n, the safety factor for materials KVL This group of coefficients is replaced by a general safety factor K This method is classified into 2 calculation groups: first limit state and second limit state

nc.Ntt ≤ mR/Kn (2.3)

Where:

+ Ntt: Calculated numeric value of summing load;

+ R: Calculated numeric value of strength of structure

2.6.2 Shortcoming in traditional design methods

The hydraulic structure is affected by natural factors, which mainly are random factors The hydraulic structure design according to traditional solutions can not consider the randomly of impact factors as well as the factors that make the load of structure, so this method is still limited

In many cases, the structure was calculated with the largest load, the maximum loaded, great selection safety factor, but the structure still occurs the incident The case like this based on the theory of traditional design cannot explain

Some limitations of traditional design methods can be cited as:

+ The failure probability of each components as well as the whole system canot yet determine;

+ Overall properties of a completed system has not yet mention;

+ In the design, it has not yet mentioned the influence of the scale of system (length of dikes );

+ Can not compare the strength of the various sections such as shape and position;

+ Can not give the failure probability and the extent of damage to the protected area

2.7 Overview of probabilistic design method

2.7.1 General background

The evolution of logic methods in structural design has been summarized as follows Initially, they were calculated according to the deterministic methods (according to the allowable stresses and safety factor), the premise is the load calculation and strength has been the default during the working process of structure In fact, the load function and durability impact of many different factors, and laws change randomly So the previous fixing of the calculated value of them during the working time is not satisfactory yet On the other hand, to increase the level of

reserve safety, we must reduce the value of allowable stress, or increase the safety coefficient This increase and decrease are unavoidable to subjective factors

The turning to limit state method is a step forward on the path of improving the structure design method Limit state method essentially is quasi-probabilistic, where the partly safety factor (nc, Kn, m, KVL) is determined according to statistical probability

The next move followed the turning to probabilistic approaches within the framework of the reliability theory This theory considers the frequently changing of the load and impact, material properties, structure self-weight

2.7.2 Historical development of probabilistic design method in the world

The late 1960s and 70s of the XX century, the world has made the published research on the application of reliability theory in the field of construction The concept of "probability ensures no failure" as well as calculating this probability has become familiar in the field of construction

The 1970s to 1990s of the XX century appeared series research on structure reliability The research was focused on building the calculation reliability methods which can be applied to technical problems and developing the optimum design according to reliability This problem was growing both in the Soviet Union and European countries, even in America

Reliability theory has also been applied to the field of computing the hydraulic structure since the 90s of the XX century The probabilistic design and the risk design were developed quite strongly in the field of sea and shore protection structure

In Vietnam, the theory credibility has also been infiltrated from the 1960s, since then ithas been continuously developed The first, the theory was disseminated by the books, lectures, teaching curriculum in the university Next is the research within the framework of the master thesis, doctoral thesis in the transport industry, construction, hydraulic structure, dike, and structure to protect the river banks On the structure of construction sector had the original regulations for the structural reliability In comparison with the world, this theory application in

2.7.3 The approach of the method of probabilistic design

In the case all causes of possible failure of structure can be listed, and the probability of possible failure certainly can be identified, so in principle it is able to determine the probability of the failure Hence, we can give a new approach in the structure design with the idea "Need to consider extent possible to build structural safety standards based on risk analysis for all relevant factors" This is the basic reason of the developing method "structure design according to probabilistic theory and reliability analysis"

The probabilistic design method is a design method based on basis of statistical probability mathematics to analyze the interactions between the random variables of the load and the resistance in the failure mechanism under the limit state of the structure In a probabilistic design, all the failure mechanisms are described by mathematical modeling or simulation models respectively Calculating the failure probability of a part of structure or the structure is based on the reliability function of each failure mechanism

In the theoretical basis and application of the method of probabilistic designs, I am going

to study and analyze in Chapter 3

2.8 Conclusion chapter 2

Long time ago, the system of dikes and bank protection structures play an important role

in the flood prevention, disaster mitigation In the present situation, dike system and protection structures have also added a more important role, such as: ensuring the safety of residential areas, urban areas, catering to the task of economic development sustainable socio-economic development in the whole of the society

The system standards and norms are old, sporadic and incomplete Design method has many limitations, some issues have not been mentioned (such as rising sea levels due to global climate change) The determination of the calculated parameters is lacking scientific basis or not consistent with the actual situation

On the method designed base on deterministic model, the lacking is unavoidable and can not mention to the possibility of occurring the load exceeds or smaller than the design load The

cause of this lacking comes from choosing a value of specific design load This is a serious lacking in the estimation of the level of failure of the structure

Structural analysis according to probability theory in the framework of reliability theory is

a logical development, gradually developed from the safety factor method, quasi-probabilistic method, to analyze the load boundary, the load capacity of the material, structure characteristics and working conditions of the structure

The research of applying probabilistic design method in dikes and flood defense system in general and for the Huu Hong river dike in Giao Thuy district - Nam Dinh province, in particular,

is a right way and suitable with the current trend



Chapter 3: Safety assessment & reliability analysis of Huu Hong

river dike

3.1 Background theory

In recent decades, the design embankments and flood prevention structures have been developed On previous traditional methods, dikes were designed mainly based on experience Accordingly, the dike crest level is determined based on the biggest flood level of historical flood events might be recorded In many parts of the world, the design of dikes, sea, and river dikes are based on the concept of water level corresponding to design frequency For the dikes, the water level is determined based on the statistics and called the design the water level, determined by a design frequency or appearance frequency

The appearance frequency of the design water level was established to widely apply as a safety standard for areas protected by dikes; it was built based on the probability of flooding However, this theory is true only for the case when the incident of dike occurs due to the flood exceeds the design water level; it is not suitable in the case of flood level smaller than the design water level

Safety standards for each type of structure based on the traditional approach to design is frequency design of load and general safety factor and each component of the structure, for each mechanism failure According to a probabilistic approach and theory of reliability, safety standard is limited in terms of the probability of the incident of the whole system structure and this damaged of systems are considered random combinations damaged of the components of the system according to the possible failure mechanism The probability of failure of the structural system is related closely to the exceeding frequency of impact load However, the two concepts are not identical and cannot replace each other

In principle, we can determine the probability of flooding occurrence in the case of all the causes of dike failure can list and the probability of each failure can certainly be determined Because this current calculation cannot be done easily in application design, so dikes designing still determine the design frequency (frequency of exceeding the load of the main parameters) based on frequency acceptable occurred flooding

Based on the above issues, the probability of occurrence of the load parameters built in design criteria and was chosen as the safety assessment criteria flood prevention In Vietnam, the frequency the water level designing around 1/20 to 1/100, frequency designing discharge flow (for river dikes) ranges from 1/50 to 1/1000, this value depends on the level of importance of protected areas

According to the idea of the methodology above, we may create a new approaching method in the construction design with the idea: It is necessary to consider the level of standard structure design based on risk analysis of all related factors

This is the fundamental reason for the development of "Structure Design according to probabilistic theory and reliability analysis."

- Definition 1: Risk is the probability of an unwanted failure in a process or an object

- Definition 2: Risk is a consequence of an unwanted failure

- Definition 3: Risk is a multiplication of the probability of failure and the consequences

The third definition gives a better comparison of risks However, because the probability

of the failure is dimensionless, but the consequences of a failure often are dimensions and different about properties, so the risk can not be described in just a number

b General risk formula

The most general definition of the risks of natural disasters and floods was recognized by international scientific organizations and now being widely adopted in many countries as follows:

Risk= Probability × Consequence

In order to minimize the risk of flooding, many countries have made on the application of the definition of the combination of two main groups of solutions:

1) Group of solution 1 – Reduce probability of failure

We need to reduce the probability of flooding by measuring to improve the safety of the flood protection system such as increasing stabilized, reinforcing and upgrading of dikes, dams, and systems of flood prevention structure

2) Group of solution 2 - Reduce consequences

Minimizing consequences of the damage when the risk of flooding occurs, through measures such as planned an emergency response, salvage and rescue in time; land planning, rational use of space, enhance the accuracy of forecasting systems and early warning etc

According to common trend in the world, flood prevention safety issues and river dike system safety are now understood in a broad sense involves the following aspects:

- River dike system: Including two main components:

+ Dikes and dike segments create “a closed arc protection” for residential areas + Protected areas of the dike system

- Safety dike system: Including safety stability of dike and dike segments; safety flood prevention of protected areas

Thus, safety assessments of dike system would include the safety assessment of the stability of closed arc protection and the assessment of the appropriateness of existing flood defenses to ensure the protection of the dikes This assessment can be done through applying the

general definition of risk above with a purpose: potential risk which was threatened by the flood

to protected areas is as small as possible

c Diagram of process of risk analysis

The process of risk analysis a system by a probabilistic method includes the following steps:

+ Description of all components of the system;

+ List the types of risks and failures that may occur;

+ Quantitative consequences for all the failures which are capable happened; + Identifying and assessing the risk;

+ Conclusions on the results of risk analysis

Diagram of general approach to evaluation, risk analysis applications for flood prevention system is shown in the figure below:

Figure 3-1: Framework of risk analysis (CUR 141 – 1990)

Objective of risk analysis

Frame of reference Discription of the

system

Possible harzards and failure modes

Probability of failures

Probability part of the risk Quantifying

concequence of failure

Probability of consequences Risk

Risk acceptance

The figure above describes the components in the risk analysis of a system by probabilistic method The first is the description of the components in the system of flood prevention structure such as dikes, drains, conduit, sluice and other components of the structure Next is the list the types of threats and failures can occur This is an important step in the risk analysis because of without a failure type (a failure mechanism) can also seriously affect the safety of the design Next step is to quantify the consequences for all failures can happen

3.2.2 Reliability analysis of components in system

The limit state has been the state just before the failure occurred Reliability is the probability that limit state cannot exceed Limit states often use to establish the reliability function The general formula of the limit state function as follow:

Where: + R: Strength / resistance to damage;

+ S: Load / capacity of causing damage

The calculation of the failure probability of a component is based on the reliability function of each mechanism failure Reliability function Z is established based on the limit state corresponding to failure mechanism which is considered, and it also is a function of many variables and random parameter Accordingly, Z < 0 is considered to be failure occurs; otherwise, failure does not occur if Z gets the other value (Z ≥ 0)

The limit state is a state where the Z = 0 in RS plane; It is considered to be the boundary

of failure

The probability of failure is determined by: Pf = P(Z ≤ 0) = P(S ≥ R)

Reliability is identified as: P(Z > 0) = 1 - Pf

In a simple case, reliability function is linear with basic random variables of the normal distribution The probability of the failure is calculated through the standard distribution function

reliability function

Figure 3-2: Reliability function is shown in the plane RS

Figure 3-3: Definition of the probability of failure and the reliability index

The point which is located in the failure zone with the greatest probability density is considered the "design point" Normally, this point belongs to the failure boundary "Design point" is important in estimating the probability of failure

3.2.3 Mathematical basis of probabilistic design

There are three methods available for calculating the probability of failure when we have

a given reliability function and given variables and random parameters International technical has accepted the following methods:

R

Z<0 Failure zone

Z>0 Safety zone Z=0 Failure boundary

S

The design is based on the standards and design guidelines In which reliability parameters are adjusted by the coefficient characteristics, parameters load is increased by the factor of the load Expressed by the formula:

S

R

S R

: Safety coefficient of load

In more specific, the characterized of strength parameter and load are calculated by formula:

S S S

R R

R R R

V k

V S

S

V

V k R

*

*

βαγ

βαγ

(3.5)

Safety coefficient of strength depends on the standard deviation of both strength and load:

2 2

S R

R R

σσ

σα

Depending form of reliability function and distribution of basic random variables, cases of calculation include:

a) Case 1: Linear reliability function with basic random variables normal distribution:

In this case, the calculation of the probability of failure is simple by using expected value and standard deviation of the basic variables The reliability value is determined by formula:

If the basic random variablesX X1, 2, ,X follow the normal distribution law, Z is also n

normal distribution function Probability Z <0 is determined through standard distribution functions:

b) Case 2: Nonlinear reliability function:

In the case reliability function is a nonlinear function of several independent basic variables with a normal distribution, the function is not standard distribution Using Taylor's methods expansion to determine the approximate reliability function Z Approximate expressions

µµ