Nghiên cứu ứng dụng lý thuyết độ tin cậy và phân tích rủi ro trong đánh giá an toàn hồ chứa thủy lợi việt nam tt tiếng anh

MINISTRY 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 ASSESSMENT IN VIETNAM Major: Hydraulic engineering Major code: 9580202 SUMMARY OF TECHNICAL PHD THESIS HANOI, 2020 This 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: Assoc.Prof.Dr Nguyen Mai Dang The thesis will be defended before the Thesis Assessment Council at: Room - 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 INTRODUCTIONS 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 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 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, in-field rains and tides shall not be taken into study; (3) Case study is carried out with Nui Coc reservoir in Thai Nguyen province 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) 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 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 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 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 c) 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 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 overtopping 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 Recently, there are 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 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 SCIENTIFIC GROUNDS OF RELIABILITY THEORY AND RISK ANALYSIS IN DAM SAFETY ASSESSMENT 2.1 Reliability Theory in Dam Safety Assessment 2.1.1 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 -Pf) (2-41) -1 in which,  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 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 a) 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 = P f C n (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 the 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, a 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; Spillway; Culvert; Reservoir channel; 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, 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 11 Based on the above connection diagram, for different cases, there are diagrams of headworks and downstream reservoir zones as follows: a) b) c) Figure 3-5: Diagram of headworks and downstream reservoir zones a) Reservoir has downstream flood zone V1; b) Reservoir has independent downstream flood zones V1, V2; c) Reservoir has or more than independent downstream flood zones V1, V2,…Vn Each diagram in Figure 3-5 is integrated with downstream flooding factors, including: flows from other river basins, in-field rains and tidal influences in Figure 3-3 Based on analysis of the relations between headworks and downstream zones mentioned above, a general failure tree diagram is established as follows: Figure 3-6: General failure tree of downstream reservoir flood zones 3.1.4 Case study limits The Thesis looked into and developed reliability theory and risk analysis application problems in dam safety assessment under downstream flood risks for common cases in Vietnam such as: (i) the headworks has components: earth dam, spillway and intake; (ii) downstream area has only one flood zone, i.e., in case of failure in headworks’ components, flows in the reservoir will be released through those components and communicate in the river basin where the main dam goes cross; (iii) downstream zones are affected by flood releases in case of headworks’ failure 3.2 Establishing failure tree diagram Failure in dam, spillway, intake or other associated structures of the headworks shall directly or indirectly cause emergency flood release through spillway or dam failure and downstream floods, of which dam failure shall cause the most significant damages and losses Thus,“downstream floods” means “the final failure” of the failure tree diagram 12 Figure 3-8: Failure tree diagram of downstream floods in the case study 3.2.1 Dam’s failure mechanism and failure mechanism tree diagram Figure 3-9: Dam’s failure tree diagram 3.2.2 Spillway’s failure mechanism and failure mechanism tree diagram Figure 3-10: Spillway’s failure tree diagram 3.2.3 Intake’s failure mechanism and failure mechanism tree diagram Figure 3-11: Intake’s failure tree diagram 13 3.3 Establishing reliability function of reservoir’s failure mechanism a) Principle Operations of individual components of headworks follow physical and mechanical laws, interactions between water environment, foundation and the works itself This rule is considered to determine the load function (S) and the strength function (R) for establishing the reliability function b) Application conditions: The object of the study is reservoir headworks simulated by serial connecting system c) Solving the reliability function: The reliability functions are solved in level by Monte - Carlo random simulation (MCS) 3.4 Problem 1: Determining failure probability and analyzing reliability of reservoir headworks 3.4.1 Objective: Assessing the current safety status of reservoir headworks through identifying and comparing reliability of headworks against applicable safety standards 3.4.2 Problem’s contents a) Implementation procedures: Determining reliability of failure mechanism of headworks’ associated components; analyzing reliability of components of headworks, proposing dam safety improvements and downstream flood risk mitigation b) Problem solving steps: able 3-5: Failure Matrix of Reservoir Headworks Failure mechanism Headworks components Failure Failure Failure Failure … … i n (0) (1) (2) (i) (n) Main dam p11 p12 … p1i … p1n Auxiliary dam No p21 p22 … p2i … p2n … … … … … … … Auxiliary dam No j pj1 pj2 … pji … pjn Spillway No … … … … … … … … Spillway No k Pk1 pk2 … Pki … pkn Intake No … … … Intake No l Pl1 Pl2 … Pli … Pln Other components (m) Pm1 pm2 … pmi … pmn Sum up PSC1 PSC2 … PSCi … PSCn Total P1 P2 … P3 … P3 Pl Pm PHT - Step 1: Describing tasks, structure, scale, status of headworks components; determining relations between components - Step 2: Assessing causes of failures in different mechanisms; listing potential failures that may occur in individual components and the headworks; 14 - Step 3: Developing failure tree diagram for headworks components and the entire headworks - Step 4: Establishing and solving reliability function of failure mechanisms for determining failure probability - Step 5: Analyzing failure tree diagram, integrating failure probability for individual components and the entire headworks in a failure matrix - Step 6: Determining reliability index (β) of individual failure mechanisms and the entire system T3.4.3 Results and implications of Problem Problem results help (i) identifying components that are in the most danger of failure (Pj max) and require for repairs and upgrades; (ii) determining failure mechanism with maximum failure probability (PSCi max) that needs for repairs and upgrades for improved safety for the headworks 3.5 Problem 2: Determining required reliability of headworks under downstream flood risks 3.5.1 Objective Determining required reliability or allowable failure probability [Pf] of headworks under acceptable downstream reservoir risks and optimum economic perspective 3.5.2 Contents of Problem a) Determining economically optimum failure probability of headworks Pf-opt Pf-opt is determined with downstream safety probability at minimum point of cost curve (C) b) Selection of allowable failure probability [Pf] of headworks The selected reliability of headworks is not only economically optimum, but also comprehensively considered in terms of political, social and environmentally comprehensive The failure probability [Pf] is determined as below: (3-13)  Pf   Pf opt  Pf In which, Pf depends on downstream risk factors (R) that have not been considered such as: intangible damages and losses to life, religious and historical values, living disturbances; required protections measures for important public works in downstream area; the multipurpose reservoir operation… c) Problem solving steps Step 1: Developing investment curve (I) representing relation between headworks investment value and headworks safety probability Step 2: Developing risk curve (R) representing relation between downstream flood risk value at the headworks safety probability, including establishing flood map; determining damages; establishing damage map Step 3: Determining allowable failure probability or reliability of headworks: 15 Step 4: Selecting failure probability [Pf] and making conclusions on headworks’ safety - Selecting failure probability [Pf] as per formular (3-13) - Conclusions: (i) If the current failure probability of the reservoir headworks Pf ≤ [Pf] the reservoir is safe; (ii) If current failure probability of the reservoir headworks is Pf ≥ [Pf] the reservoir is not safe 3.5.3 Results and implications of Problem (i) Allowable failure probability [Pf] is the critical value which would be used for comparison and conclusion of safety of headworks under downstream flood riks; (ii) It reflects systematics, relation between works’ safety and the downstream reservoir area, thereby, reservoir repairs and upgrades are assessed and recommended 3.6 Problem 3: Design of headworks under required reliability and downstream reservoir flood risks 3.6.1 Objective of Problem Upgrading headworks according to flood tolerability of downstream areas through allocating required probability [Pf] by individual components of the reservoir headworks 3.6.2 Contents of Problem Allocating required probability by individual components of headworks in Problem is undertaken in reverse steps of Problem Allocating proportion depends on the level of impact of individual failure mechanism on the system’s failure probability and determined as follows: a) Statistic method: Under this method, reliability of individual component failure mechanism and the entire system is considered as a random quantity It’s highly accurate and used if the monitoring data series is sufficient long and synchronous b) Allocating method of allowable reliability according to level of impact of individual failure mechanism Step 1: Determining the level of impacts caused by individual failure mechanism and individual components on the system’s failure probability according to the results of Problem Figure 3-16: Reliability allocating by failure tree diagram 16 Step 2: Allocating probability by levels of failure tree diagram and the factors determined in Step P1i = K1i [Pf] In which: K1i is the first allocating factor; i: various components of dam, spillway, intake, for example: Pdam = K1dam [Pf] Similarly, calculations are made for other components and allocation is carried out by levels 2, n with factors K2j; K3l… Knm Step 3: Repeating for determining design value of the influencing variables: With Pfi of individual components and allocated mechanisms, using Monte – Carlo method for repeating calculations for determination of design value of main variables that ensure Pfi < [Pfi] 3.6.3 Results and Implications of Problem Problem presents metodology for design of reservoir repairs and upgrades that meet the required reliability or allowable failure probability [Pf] based on existing conditions of the headworks and flood tolerability of downstream reservoir area 3.7 Dam safety improvement and downstream flood risk mitigation method If failure probability (Pf) ≥ [Pf] the reservoir is not safe, and downstream is likely in the risk of flood At that time, dam safety improvements should be in place 3.7.1 Group of downstream flood risk mitigation measures (Top - Down) impact on downstream area: Developing natural disaster response plan, emergency response plan, constructing works to protect dykes against floods and river embankments 3.7.2 Group of downstream flood risk prevention measures (Bottom - Up) impact on headworks: Investing in repairs, upgrades of headworks at required probability for downstream flood risk mitigation 3.7.3 Combined downstream flood risk mitigation and prevention measure The combination of downstream flood risk mitigation and prevention measure is based on the principle of synchronous affect on headworks and downstream area Figure 3-17: Diagram of approach of dam safety improvements 17 Based on management requirements, "design” points and failure probability [Pf] is selected: (i) Selecting “design point” on the left to “optimum point” if accepting higher risk (R) and lower investment capital (I) or (ii) selecting „design point” on the right if selecting higher investment (I) for minimized risk (R) as in Figure 3-18 Figure 3-18: Selecting design point 3.8 Conclusions of Chapter The established problem has pointed out the relations between headworks’ safety and downstream flood risks; developing scientific grounds for determining current failure probability and reliability index (β) of the headworks; designing reservoir headworks that meet the required reliability at flood tolerability of the downstream area Thereby, proposing reservoir safety improvements and downstream flood risk mitigation measures, pointing out how to select the design point in response to the management practices CHAPTER RELIABILITY THEORY AND RISK ANALYSIS APPLICATIONS IN NUI COC RESERVOIR SAFETY ASSESSMENT WITH DOWNSTREAM FLOODS 4.1 Introductions to Nui Coc reservoir Nui Coc reservoir is located within Cong river basin, Southwestern to Thai Nguyen province, having been exploited since 1982; storage capacity is 176 mil m3, which main task is irrigating 12,000 of crop lands, supplying water for industry and municiple at 40 ÷ 70 mil m3/year, flood detention for Cau river, facilitating fisheries, hydropower and tourism 4.2 Nui Coc reservoir satety assessement with downstream flood risk 4.2.1 Nui Coc reservoir safety assessment approach diagram (Figure 4-5) 4.2.2 Determining current failure probability and reliability index of Nui Coc reservoir a) Diagram of headworks and Nui Coc reservoir downstream area Nui Coc reservoir headworks comprises of: 01 main dam, 07 auxiliary dams, 01 main spillway, auxiliary spillway and intakes (01 intake in main dam body; 01 intake in auxiliary dam no1 body) 18 In case of headworks’ failure, Nui Coc reservoir’s flows released through main, auxiliary dams and spillways fall into Cong river, located behind dam Thus, Nui Coc reservoir’s downstream area is illustrated in Figure 4-6 Figure 4-5: Problem solving approach diagram Figure 4-6: Diagram of Nui Coc reservoir headworks b) Establishing failure tree diagram Figure - 8: Nui Coc reservoir headworks failure tree c) Determining failure probability and assessing reliability of failure mechanisms and reliability analysis in Table 4-1, Figure 4-10 19 Figure 4-10: Level of impacts on Nui Coc reservoir safety failure mechanism Table 4-1: Level of impacts of failure mechanisms on headworks’ reliability (1) Calculations made at maximum water level in 20 year monitoring series: Value of failure probability is smaller than the design flood protection probability: Pf =0.0067 < P1% = 0.01 but failure probability is higher than the check flood pretection probability Pf = 0.0067> P0,2% = 0.002 Thus, headworks is likely failed, which cause is mainly the downstream slope slides (P2 = 22.45% Pf) 20 (2) Calculations made with design flood level, updated with hydrological and reservoir bed deposition data: Value of failure probability is higher than the design and check flood probability: Pf = 0.00693 > P0,2% = 0.002 Reliability index of reservoir at design flood level decrees from 2.425 down 1.4164 (reducing 42%) in comparison to the maximum annual water level At that time, failure probability of overtopping mechanism, downstream slope slides, outlet erosion and cavity in dam foundation increases The main failure of headworks is ovretopping (P1= 60.05% Pf) and downstream slope slide (P2 = 16.40% Pf) Seepage and hydraulic instability mechanisms have small failure probability and take minor proportion of the system’s failure probability Based on the failure cuve of the two mechanisms, overtopping and downstream slope slide as in Figure 4-11, it’s recognized that: - Slope of downstream slope instability curve is small and failure probability P2 changes slightly when water level changes from +46.51 to +48.7 This means the dam has been consolidated for 40 years and dam slope is quite stable However, the failure probability increases rapidly when reservoir water level changes from +48.7 to +50 - Slope of downstream slope instability curve is small and failure probability P1 is small and changes slightly within maximum annual water level range +46.5 to +47.5 However, the slope of the curve is large, failure probability P1 increases gradually and changes largely within the water level range from +47.5 and higher This means dam is in the risk of overtopping if water level in the reservoir is above +47.5 Figure 4-11: Fragility curve of overtopping (a) and downstream slope instability (b) 4.2.3 Determining required reliability of headwroks with downstream flood risks a) Developing headworks investment cost curve Ipf according to the scenarios in Figure 4-12 b) Determining value of damages and value of risks with downstream flood map Damage map in scenarios in Figure P2-5 c) Determining required reliability of Nui Coc reservoir headworks d) Selection of allowable failure probability [Pf] of headworks Figure 4-14 shows that Ctot is minimum at scenarios and at the point which value Cmin are approximately equal each to other and optimum reliability Pf-opt =1/5,000 21 Figure 4-12: Scenarios for determination of required reliability of Nui Coc reservoir Figure P2-5: Damage map at various flood level (Scenarios 3) Table 4-5: Investment cost in Nui Coc reservoir upgrades at various scenarios Unit: bil.VND Figure 4-14: Curve of investment in headworks upgrade with downstream Nui Coc reservoir flood risks 4.2.4 Nui Coc reservoir safety assessment with downstream Nui Coc reservoir floods Pf updated design water level=0.0693 > Pf extream annual water level > [Pf] = 0.0002 = 1/5,000 It means Nui Coc reservoir headworks (designed at check flood probability of QCVN 04-05:2012/BNNPTNT P = 0.2% = 1/500 year) is likely failed and cause downstream floods and need repairs and upgrades to fix hydrological and hydraulic changes, as well as flood tolerability of downstream area which is reduced in comparison to the design 4.2.5 Analysis and selection of Nui Coc reservoir repairs and upgrades For reservoir’s repairs and upgrades, Option is selected: Rising normal water level up +47; adding an emergency free spillway which B=100 m; enlarging spillway for B = 80 m, keeping Zng = +41,2 unchanged 22 4.2.6 Design of Nui Coc reservoir at required reliability Based on calculation results in Table 4-6, calculated section is larger than the original one of main dam Thus, main dam of Nui Coc has been upgraded to higher reliability Table 4-6: Main parameters of main dam at allowable failure probability 4.3 Conclusion of Chapter Calculation results within the study scope in Chapter IV give the current reliability: Pf updated design water level=0.0693 > Pf extream annual water level=0.0067 > [Pf] = 0.0002 = 1/5.000 Value of downstream Nui Coc reservoir flood risk at current reliability Pf = 0.0067 is 2.261 bil vnd, 26.6 times flood tolerability of downstream area This presents the existance of Nui Coc reservoir with the downstream area of Song Cong City, Pho Yen town, industrial zone and other are likely in the danger of floods in case of headworks’ failure Recently, Nui Coc reservoir was designed at reliability index β = 2.425 < [β]= 3.5 compared to the dam construction time, in the downstream area, many infrastructure added, thus, value of affected area increases, living standard increases, leading to higher safety (in other words, downstream area at this point of time accepts lower risk value in comparison to that in the time of reservoir construction) Thus, current safety standard fails to meet downstream flood prevention, and there is a need for upgrading headworks which emphasis is placed in the measures for preventing overtopping CONCLUSIONS AND RECOMMENDATIONS Thesis’s results (1) An analysis and overall assessment of reservoirs in the World and in Vietnam, pointed outstanding isses of national reservoir studies; emphasized advantages of the combined RT and RAin dam safety assessment (2) Established scientific grounds for application of RT in determining reliability (Pf) of headworks and required reliability of irrigation reservoir (3) Developed dam safety assessment methodology with downstream flood risks through the establishing and solving problems (4) Propsed risk mitigation measures in Bottom - Up approach (from downstream to headworks) in order to minimize downstream flood impacts through downstream measures; the top-down prevention measure (from headwroks to downstream) is taken through investments in headworks safety improvements to achieve required reliability at the flood tolerability of the downstream area or combined measures 23 (5) Applied the developed methodology in Nui Coc reservoir safety assessment in Thai Nguyen province which results are practical This confirmed RT and RA and application problems are reliable and applicable to Vietnam The Thesis’s new contributions (1) Developed diagrams and algrothism for solving RT and RA problems in determination of reliability index of irrigation reservoir headworks with downstream flood risk (2) Case study (Nui Coc reservoir): failure probability and value of reliability are quantified with downstream flood risks, providing basis for design of reservoir upgrades and repairs Outstanding issues and development orientations 3.1 Outstanding issues (1) Factors of optimum reservoir operations in response to the multipurpose tasks of water supply, irrigation, power generation, tourism, aquaculture, were not considered (2) Non-material damages were not considered in the risk function 3.2 Development orientations Continuing studies and researches for final reliability theory and risk analysis application in a comprehensive and practical manner, such as: (1) Study on quantifying effects of upstream reservoirs and the statistical relationship on safety of reservoirs in the same river basin (2) Considering failures of other works of the headworks in the failure tree diagram; Analyzing correlation between failure probability, combined correlation between failure mechanisms in case of many problems with the same cause (3) Research on determining the required reliability for multipurpose reservoirs such as flood control, power generation, water supply (4) Research on the non-material damage factor in the risk function (5) Study on scaling up the application for spillway, dams and newly built reservoirs and reservoir upgrades and repair to meet socioeconomic development planning of upstream and downstream reservoir areas Recommendations Irrigation reservoir safety assessment with downstream flood risks through risk analysis and reliability theory is a new method that overcomes the shortcomings of the deterministic method However, the calculation is a problem of trial with large volume, requiring long monitoring series of input data to ensure reliability The practical dam and reservoir safety management in Vietnam shows that the biggest difficulty in applying risk analysis and reliability theory for reservoirs were: (1) in adequate and insufficient monitoring data, the series of data was not long enough, affecting the accuracy of calculation results; (2) Statistical documents on downstream damage and flood damage were not available and have not been archived systematically Therefore, it is necessary to set up a database system for construction monitoring, specialized hydro-meteorological monitoring and flood damage data in the downstream of the reservoir with long and synchronous monitoring series to improve the accuracy in determining the reliability of an irrigation reservoir in Vietnam./ 24 LIST OF PUBLICATIONS [1] Pham Ngoc Quy, Cam Thi Lan Huong “Celebration of 75 years of Vietnamese irrigation tradition occasion, think about dam and reservoir safety”; Water resources journal, Vietnam Irrigation Association; No 03 (08/2020), ISSN 1859-3771, pp.14-17, 2020 [2] Cam Thi Lan Huong, Mai Van Cong, Pham Ngoc Quy, “Study and determining required confidence index for reservoir headworks with downstream flood risks – application for Nui Coc reservoir in Thai Nguyen province”; Water resources journal, Vietnam Irrigation Association; No 02 (04/2020), ISSN 1859-3771, pp.53-63, 2020 [3] Cam Thi Lan Huong, “Study and determining flood risks in the downstream Nui Coc reservoir area, Thai Nguyen province”; Irrigation engineering and environment science journal, Water Resources University; Số 68 (3/2020), ISSN 1859-3941, pp.43-50, 2020 [4] Cam Thi Lan Huong, “Study and determining irrigation reservoir headworks’ confidence index according to confidence theory – applications in Nui Coc reservoir in Thai Nguyen province”; Irrigation Sciences and Technology Journal, Vietnam Academy of Water Resources; Edition 58 (02/2020), ISSN 1859-4255, pp102-108, 2020 [5] Pham Thi Huong Lan, Cam Thi Lan Huong, “Study on downstream Nui Coc reservoir safety assessment under emergency flood release scenarios”; Proceedings of Science - Technology Club Conference of the technical universities 54, pp118-130, 2019 [6] Hoang Van Thang, Dana Cork, Pham Ngoc Quy, Cam Thi Lan Huong, et al, Guide to quick inspection of earth dam, MARD and Smart Infrastructure Mekong Project (SIM), Dan Toc Publish house, 2017 [7] Pham Ngoc Quy, Cam Thi Lan Huong, et al, Earth dam safety assessment criteria, Xay Dung Publish house, Hanoi, 2016 [8] Cam Thi Lan Huong, Mai Van Cong, Pham Ngoc Quy and Le Xuan Bao, "Study on assessment of downstream flood damages in case of flood release or dam failure, applications to Dau Tieng reservoir in Tay Ninh province" Proceedings of the Conference of Natural Sciences of the Water Resources University, pp.582-584, Hanoi, 2016 [9] Cam Thi Lan Huong, “Summary of hydraulic dam failure in Vietnam in recent years, causes and lessons learned”, Irrigation Sciences and Technology Journal, Vietnam Academy of Water Resources, No 13, pp.67-70, 2013 ... 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... province 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... 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: Assoc.Prof.Dr Nguyen Mai Dang The thesis