The thesis looked into applications of Reliability Theory RT and Risk Analysis RA within a dam safety assessment, including downstream flood risks to provide accurate dam safety assessme
Trang 1MINISTRY OF EDUCATION
AND TRAINING
MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT THUY LOI UNIVERSITY
CAM THI LAN HUONG
RESEARCH ON APPLICATION OF RELIABILITY THEORY AND RISK ANALYSIS IN IRRIGATION RESERVOIR SAFETY
Trang 2This Thesis was completed in Thuy loi University
Supervisor 1: Prof.Dr PHAM NGOC QUY
Supervisor 2: Assoc.Prof.Dr MAI VAN CONG
Reviewer 1: Assoc.Prof.Dr Nguyen Van Vi
Reviewer 2: Prof.Dr Tran Dinh Hoa
Reviewer 3: Prof.Dr Pham Thi Huong Lan
The thesis will be defended before the Thesis Assessment Council at: Room 5 - K1 , Thuy loi University at 08:30 am on November 13nd, 2020
Thesis can be found at the library:
- National Library
- Library of Irrigation University
Trang 3INTRODUCTIONS
1 Rationale
According to the Ministry of Agriculture and Rural Development (MARD), there are 6,750 irrigation reservoirs, located in 45/63 provinces and cities, supplying a gross volume of water of about 14.5 billion m3, a significant contribution to Vietnam’s socio-economic development Those reservoirs have been built when the country’s economic conditions were underdeveloped; design and construction standards were at low level; budget for maintenance has ever been sufficient and the operations experienced many problems Of the 6,750 irrigation reservoirs, 1,200 have been degraded and are unable to release flood as required, threatening the safety of the works In recent years, climate changes has caused extreme and unusual rains and floods, seriously affected dam safety Since 2010 to date, up to 71 cases of dam and reservoir failures recorded, but it’s more often in three years: 2017 (in 23 reservoirs), 2018 (in 12 reservoirs and dams), 2019 (in 11 reservoirs), and a notable case was the failure
of Dam Thin reservoir in Phu Tho on May 28, 2020 For safe and efficient operations of the reservoirs, a thorough assessment of dam safety of those reservoirs became very urgent In Vietnam, the recent assessments of safety of reservoirs’ main structures were deterministic and did not take into account downstream flood risks Therefore, in many cases, dam safety assessments were not accurate, proposed inappropriate measures, and resulted in failures, particularly dam failures, caused heavy damages to the works, flooded assets and threatened lifes in downstream area
The thesis looked into applications of Reliability Theory (RT) and Risk Analysis (RA) within a dam safety assessment, including downstream flood risks to provide accurate dam safety assessments as a basis for dam safety improvements, upgrades and operations in a scientific, safe and effective manner under practical demands
2 Thesis’s Objective
Developing scientific dam safety assessment grounds and methodology for reservoirs in Vietnam, considering downstream floods with theory of reliability and risk analysis; applications in dam safety assessment for Nui Coc reservoir
in Thai Nguyen province
3 Thesis’s Subject and Scale
Thesis’s subject: The irrigation reservoir that is under operation with an independent basin, earth dam and downstream area
Thesis’s scale: Dam safety assessment is carried out for existing irrigation reservoirs throughout the country (1) Safety is assessed for dam and relevant works (earth dam, spillway and intake), only; (2) Only the flood impacts caused
by a single reservoir, not the upstream ones, incoming flows from other rivers,
Trang 4in-field rains and tides shall not be taken into study; (3) Case study is carried
out with Nui Coc reservoir in Thai Nguyen province
4 Thesis’s Approach and Method
The Thesis followed systematical, holistic, inheritance and advanced approach The main methods used in the Thesis are Reliability Theory (RT) and Risk Analysis (RA) and other scientific research methods, such as survey, data collection, statistics, analysis, literature review, mathematical modeling (for example MIKE 11, MIKE FLOOD used in flood mapping and downstream impact assessment)
5 Scientific and practical implications
Scientific implications: Scientific grounds are developed for rrigation reservoir
safety assessment, taking into account downstream floods through the statement and settlement of 3 problems: Determining probability of failure, determining required reliability of the reservoir’s headworks, using theory of reliability and downstream flood risk analysis; desing reservoir’s reliability
Practical implications: Dam safey risks are identified and analyzed for
irrigation reservoirs, dam safety is quantified, downstream flood risks are considered through determined probability of failure and required reliability, providing grounds for selection of reservoir’s safety options and downstream flood risk mitigation measures
6 Thesis’s Outline
In addition to the opening, conclusion and recommendation, the Thesis has 04 Chapters: Chapter 1: Dam safety overview, dam safety assessment with reliability theory of and risk analysis; Chapter 2: Scientific grounds of RT and
RA in dam safety assessment; Chapter 3: Developing problem in application
RT and RA on irrigation dam safety assessment with downstream flood consideration; Chapter 4: RT and RA applications in Nui Coc dam safety assessment and downstream flood risk considerations
CHAPTER 1 DAM SAFETY OVERVIEW AND DAM SAFETY ASSESSMENT WITH RELIABILITY THEORY AND RISK ANALYSIS OVERVIEW
1.1 Dam safety overview, reliability theory and risk analysis
1.1.1 Dam Safety Concept, Reliability Theory and Risk Analysis
a) Dam safety concept: Dam safety means a comprehensive safety of a dam or reservoir’s dam, and relevant works that consititute the reservoir and downstream basin
b) Reliability theory means the application of systematical analysis and random theory in determining probability of failure (Pf) of component destruction mechanisms, and integrated probability of failure of the entire works Value of reliability index (β) is used in assessing the safety of the works
Trang 5c) Risk and Risk Analysis
An object's risk is the product of the object's likelihood of occurrence of and the consequences caused by a failure If it’s applied to a reservoir, the risk is defined as downstream flood, determined with the product of the probability of failure causing flood and the value of the flood’s damages
The theory of reliability and risk analysis method defines the scope of works based on an acceptable risk perspectives through the relationship established between the works' probability of failure and the value of corresponding damages caused by the failure with the risk function
1.1.2 Downstream flood problem
a) Downstream floods caused by reservoir’s releases: (1) Reservoir’s design
flood release does not cause damage Recent encroachment of downstream basin of some reservoirs (namely Nui Coc, Ke Go, Vuc Mau, Ayun Ha, Ia Ring, Dau Tieng, and so on) has impeded the reservoir’s design flood release, causing downstream floods in case of flood release; (2) Reservoir’s emergency flood releases (i.e., in case of rains and floods that are beyond the design probability; earthquakes in the basin are higher than the design standard; or other impacts that threaten the dam safety); (3) Dam failure: Various cases of dam failure
b) Downstream flood consequences
Downstream floods and inundations cause negative economic, social and environmental impacts, particularly in case of dam failure which impacts would
be the most significant
1.1.3 Some notable reservoir failures in the World
In 1975 floods, failure of Ban Kieu dam in Ha Nam province in China caused 175,000 deads and above 11 million homeless The failure of Machchu II dam
in India which is 29m in height, in August 1979, due to 3/18 gates stuck, caused 2,000 deads Recently, the failure of Sepien Senamnoi hydropower dam in Laos (on 23 July 2018) caused 26 deads, 1,300 households and 6,600 persons affected On 19 may 2020, both Edenville and Sanford dams in Michigan (USA) have failed, caused terrible floods and 10,000 evacuated
1.1.4 Dam safety of existing irrigation reservoirs in Vietnam
Recently, there are totally 1,200 reservoirs degraded and deteriorated, potentially threatening the reservoirs’ safety 200 reservoirs of that have been seriously degraded and required for urgent actions
1.1.5 Some recent irrigation reservoir failures
Since 2010 to date, 71 irrigation dam and reservoir failures were recorded, the typical cases were: Phan Lan dam (in Vinh Phuc province) failed on 5 August
2013 due to 1.5m high overtopping Auxiliary dam No.2 of Dam Ha Dong reservoir (in Quang Ninh province) failed on 31 October 2014 due to
Trang 6overtopping and gate stuck Recently, on 28 May 2020, Dam Thin dam (in Phu Tho province) failed when there was no rain and water level in the reservoir was below the normal water level
1.1.6 Causes of reservoir failures
Earth dams’ failures are floods, geological problems, seismics, seepage, structural problems, and stability Spillway’s failures include: hydraulic failures (insufficient discharge capacity, erosion, porosity, channel erosion, equipment, operation, materials, structures, geotechnics); Intake’s failures are: leaning intake’s tower, joints failure, culvert break, erosion, leakage and erosion, damaged stilling basin, jam and broken gates, seepage along the intake’s side walls
1.1.7 Some dam safety management strengthening measures
a) Non-structural measures: operation monitoring; forecast, warning capacity strengthening, capacity strengthening for reservoir management agencies and individuals, information, education and communication
b) Structural measures: dam repairs, safety improvements for 1,200 deteriorated reservoirs that fail to meet flood protection requirements under applicable standards and regulations
1.2 Overview of reliability theory and risk analysis applications in irrigation field and dam safety assessment
The RT applications started in 1920 and rapidly developed in 1970s USA and Canada were the two pioneers in study and application of RT in large dam safety assessments since 1990s The research on the combination of RT and RA
in dam safety assessment has been carried out in Germany, the Nethelands and Australia since 1996 and immediately became popular in the Europe and America, a main topic for many workshops held by International Committee of Large Dams (ICOLD) since 2000
In Vietnam, the RT has been developed since 2000 in irrigation field, particularly
in flood protection, open sluices, canals and on-canal works, irrigation reservoirs’ headworks Representative authors are: Nguyen Van Mao, Nguyen Quang Hung, Pham Hong Cuong, Mai Van Cong, Le Xuan Bao, Tran Quang Hoai… In the field of dam safety, Nguyen Lan Huong has developed a dam safety assessment methodology for irrigation reservoirs; some ODA projects have also taken into account dam safety risk such as: “Viet Nam - New Zealand Partnership on Dam Safety”, “Dam Safety Rehabilitation Project - WB8”, and many others Determining downstream floods is standardized in TCKT 03:2015
It meant in Vietnam, the emphasis of almost studies was placed on the applications of RT and RA in dam safety assessment; and there was no comprehensive and integrated research on the problem of reliability of headworks and downstream flood risks
Trang 7Recently, there are 3 programs globally popular for computation of reliability, namely: Bestfit, VaP and Open FTA In Vietnam, DCT2007 is used for assessment of quality of irrigation system under theory of reliability level II; SYPRO2016 determines reliability of works (earth dam, spillway and culvert)
at levels II and III
1.3 Outstanding issues in dam safety studies in Vietnam
In Vietnam, assessments of headworks are normally undertaken with deministic method, flood risks have been taken into account, but at very limited level, normally focused on one of two contents: (i) Safety assessment of headworks, regardless the relations between reliability of reservoir and the downstream flood risks; (ii) Developing downstream flood map, surveying losses and damages and proposing appropriate reservoir operations as well as flood responses and mitigation measures without consideration of coherance of current conditions of headworks
For these reasons, the recent dam safety assessments presented such limitations as: assessment has been made for headworks only and with simplified general layout, not sufficient and comprehensive to cover failure mechanisms of individual components and the entire headworks Assessment results were therefore not objective and thorough on the status of the works; assessment of losses and damages, and downstream flood risks is a must, but it has ever been
a concern; studies and researches have been done separately, not taken into account the connection between dam safety and downstream flood risks
1.4 Research orientations and issues to be addressed in the Thesis
The combination of RT and downstream flood RA is an advanced method by which the outstanding issues in dam safety assessments in Vietnam are addressed The thesis’s selection method was developing dam safety assessment and downstream flood risk consideration methodology
1.5 Chapter’s conclusion
Through an overview of dam safety, applications of RT and RA in irrigation field and safety assessments of reservoirs in Vietnam and the World, advantages and disadvantages of some case studies and conditions of existing irrigation reservoirs in Vietnam, some happened failures; an analysis of dam safety failure causes, Chapter 1 pointed outstanding issues of national studies Thereby, it oriented the Thesis’s emphasis on developing dam safety assessment and downstream flood risk consideration methodology through analyzing the reliability of reservoir headworks and downstream flood risks; proposed optimum dam repairs and safety improvements in response to downstream flood risks
CHAPTER 2 SCIENTIFIC GROUNDS OF RELIABILITY THEORY AND RISK ANALYSIS IN DAM SAFETY ASSESSMENT
2.1 Reliability Theory in Dam Safety Assessment
Trang 82.1.1 Reliability of a failure mechanism
a) Definition of failure mechanism: Failure mechanism means a type of works’
failure caused by the interaction between boundary conditions and the works’ physical-mechanical process The failure mechanism is simulated by two
quantities, resistent load capacity (R) and active load (S)
b) Reliability function of a failure mechanism: Reliability function (Z) means
the residual value of the load capacity (R) under the impact of external active load (S) The function (Z) is set at the limit state where its negative values correspond to the destructing/failure state of the mechanism and vice versa, the positive value of Z corresponds to the safe working state and is represented as follows:
Z = R - S (2-1)
c) Reliability function solution of a failure mechanism: The reliability function
is solved at the following levels: Level I: Calculating with permissible factors
of safety; Level II: (approximate method) the reliability function is linearized and probability density function is replaced by functions with a normal distribution; Level III: Solving the complete random problem if the probability density function remains unchanged as in the approximate method
d) Reliability index β means the value used as a substitution for Failure
Probability Pf :
β = -1
(1 -P f) (2-41)
in which, -1
is the inverse of the normal distribution function
2.1.2 Failure Tree Diagram
a) Definition: A graph that illustrates the relations between structural failure
mechanisms in a system is called as failure tree diagram
b) Failure tree establishment
(i) System analysis: refers to the description of the system's functions, its constituent parts and the relations between the components The failure tree describes the logical sequence of events that result in the same unexpected event
called the "final failure"
c) Failure tree diagrams is described with a system of conventional symbols
representing failures and associated gateways representing failure relationships
2.1.3 Reliability Function of a System
There are 2 ways of describing the relationship between component failure tree diagrams of the system: parallel coupling or serial coupling
Figure 2-5: Typical failure tree diagram
Trang 9a) Failure probability of serial system
Failure probability of a serial system shall be higher than the maximum failure probability of a component and smaller gross failure probability of all components
Ditlevsen’s approximate formula is as follows:
If Level II method is applied in calculation of component failure probability, the margin of failure probability of a system that have (n) components is: ( ( )) ∑ ( ) (2-43)
Figure 2-6: Determining method of failure probability of a serial system
b) Failure probability of serial system
The system fails if all the system’s components fail Failure interval means: (2-45) Failure probability of system is
( ) (( | ) ( | ) ( | )) (2-46)
2.2 Downstream reservoir flood risk analysis
a) Downstream reservoir flood risk definition
For reservoirs, downstream flood risk is defined as follows:
RR = Pf Cn (2-48)
In which, RR: downstream flood risk; Pf : downstream flood proability; Cn: total costs of damages caused by downstream floods
b) Risk Analysis Purpose:
Figure 2-8: Downstream reservoir flood risk analysis diagram
Purpose of downstream flood risk analysis is providing scientific grounds for making decision on works’ management, operation that guarantee the safety for
Trang 10the works and downstream areas, and promote the reservoir’ benefits Downstream flood risk analysis was carried out under dam failure scenarios Downstream flood risk analysis was compared to standard risks or established risk values If required, technical specifications of headworks, downstream flood protection works shall be adjusted to have risk values satisfactory to the standards
Acceptable risks or risk value limits (standard risk) means the risk value that is equivalent to optimum point where the total cost is the lowest
b) Risk analysis principle and steps are presented in Figure 2-8, 2-9:
Figure 2-9: Main steps of downstream reservoir flood risk analysis
2.3 Downstream reservoir flood consequences and determined downstream reservoir flood damages
2.3.1 Downstream reservoir flood consequences means economic, social and
enviornment damages and losses, particularly in case of dam failure, which are groupped into two groups, including: group of life damages and asset damages which are categorized as direct and indirect damages; tangible and intangible damages
2.3.2 Downstream reservoir flood damage assessment
There are common downstream damage assessment, including: (i) Statistic method (based on available data and information) and (ii) simulation modeling
in association with verification via survey data, this method is commonly used through establishing damage curve (damage function) based on flood map; verified with monitored historical data
There are many flooding process simulation programs that integrate GIS and allow visual presentation of flood characteristics such as MIKE URBAN,
Trang 11MIKE FLOOD developed by Denmark Hydraulics Institute (DHI); ISIS
developed by Halcrow (England) and others
Gross downstream damage is determined with the following formula:
∑ ( ) (2-49)
In which: D: gross downstream damage
Fi: Area of block (i) of the total downstream area that is dividied into (n) blocks
f(hi): Value of damage function is equivalent to height of flood (hi) of block (i)
Damage results are presented in the form of map and the damage table at
various levels of flood
2.4 Dam safety assessment in consideration of downstream flood risks
2.4.1 Downstream flood risk assessment
a) Downstream reservoir flood risk assessment: refers to comparison of risk
value against current safety standard of acceptable downstream risk values
b) Acceptable risk value: refers to the highest potential risk limits in the
downstream area in terms of impact significance and probability which provide
the grounds for deciding the design safety level
b) Principle of acceptable risk determination from economic perspectives
According to economic point of view, allowable failure probability [Pf] is
optimized in terms of cost and downstream flood risk
Total cost of a system (Ctot) is determined by Total investment cost (IPf) in the
system’s repairs and upgrades to achieve the safety higher than Pf and
downstream flood risk (RPf) Therefore, optimum failure probability is
determined with general target function as follows:
In which: IPf: Total investment cost in repairs and upgrades to have the higher
system’s safety, including initital construction investment cost (Io); investment
in construction of new or system repairs or upgrades for higher satety (i.e.,
failure probability [Pfi] is lower); Then, optimum failure probability is
determined with the following general target function, maintenance cost for the
repaired and upgraded works PV(MPf); i; n: identifical number and number of
investment options (new construction or upgrades) of headworks under
The solution of equation (2-51) is acceptable risk value at allowable failure
probability [Pf] of the reservoir headworks, symbol Pf-opt determined by two
methods, analytical and trial-and-error
Trang 12
Figure 2-12: Optimizting failure probability in economic perspective
2.4.2 Dam safety assessment
Comparing failure probability of headworks (Pf) and allowable failure probability [Pf] determined by acceptable downstream risk If the current risk value of headworks is accessive the acceptable risk of the downstream area the system should be repaired and upgrade to ensure the safety
2.5 Chapter’s conclusion
In Chapter 2, scientific discussions of RA and downstream flood RA have been systemized and consolidated: clarifications of definition, contents and application methods of RT and downstream flood RA in dam safety assessment for calculating and determining: downstream flood risks, economically acceptable risk value; allowable failure of probability [Pf] or required reliability
of reservoir headworks Based on this fundamental theory, the Thesis’s emphasis was placed on develping application problems, such as: headworks’ risk analysis, assessment of reliability of individual failure mechanisms, individual components and the entire reservoir headworks in Chapter 3
CHAPTER 3 DEVELOPING PROBLEM IN APPLICATION OF RELIABILITY THEORY AND RISK ANALYSIS ON RESERVOIR SAFETY ASSESSMENT WITH DOWNSTREAM FLOOD CONSIDERATION
3.1 Diagram of relations between headworks and downstream area
3.1.1 Diagram of headworks
a) Definition of headworks: Works built in the same site and share tasks of water resource use and disaster prevention solutions called as irrigation headworks Reservoir headworks includes the following items: dam, spillway, intake and associated works such as hydropower plants (combined), navigation lock, wooden boat waterway, fish passages, mud and sand sluice, tourist works…
b) Reservoir headworks layout
Reservoir headworks layout is diversified, and the most popular one is: dam, spillway and intake
Trang 13a Spillway is on the dam’s shoulder,
intake is inside the dam body
b Spillway is separate to dam, intake is inside dam body
c Several spillways are separate to the dam, one spillway is on shoulder of main dam, culvert is inside main and auxiliary dams (1 Main dam; 2 Spillway; 3 Culvert; 4
Reservoir channel; 5 Auxiliary dam) Figure 3-1: Some general layouts of reservoir headworks in Vietnam
3.1.2 Diagram of downstream reservoir flood area
Figure 3-2: Diagram of downstream reservoir flood area
In case of headworks failure, flows will accumulate in basins downstream the main dam At that time, a flood area occurs in downstream area If reservoir’s flows overtop the main dam, break the auxiliary dam or flows are released through main and auxiliary spillways, and intakes, if failed, do not lead flows into the river where the main dam runs across In such case, there will be more than one flood zone in downstream area
3.1.3 Connection between headworks’ safety and downstream floods
Figure 3-4: Diagram of connections between headworks’ safety and downstream
reservoir floods