Comparison between 1D and 2D models to analyze the dam break

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Ignacio Valcárcel LencinaTRITA LWR Masters ThesisAP PEN DI X: DEV EL OPM E N T OF RESCU EACTIONS BASED ON DAM-BREA K FLOODANALYSIS (PRES EN T PROCED URE) .The previous part of this work is a description of twomodels developed with a solver software packagecalled Comsol Multiphysics. Once the validation ofthese models has been done, the first purpose shouldbe the application in real contexts. This Appendix hasbeen achieved to explain this practical approach in ageneral way, not using concrete software applied to aspecific dam.The enormous complexity of the dam break problemdoesn’t make possible to introduce simplifications inthe beginning. They are completely dependent of thespecific case analyzed. In addition, the constantevolution of the mathematical models makes verydifficult to recommend any specific model for ageneral use. Nevertheless, as a help for the selection ofthe model is recommended the study of the ICOLD(International Commission On Large Dams) about“Dam Break Flood Analysis” carried out by theSubcommittee of “Analysis of dam-break flooding andrelated parameters normally assumed” where 27existing models are described and evaluated. Actuallyonly a few are widespread and accessiblecommercially.The RESCDAM Project is a Grant Agreement (NoSubv 99/52623) signed between the EuropeanCommission and the Finnish Environment Institute. Itis actually the last complete study about the dam breakproblem, (after the study of the ICOLD) and it couldbe a sort of “guide” for the engineers involved withinthis task. Because dams and especially consequences ofa dam failure are quite similar in different countries,results and activities of the RESCDAM project shouldbenefit EU and associated countries.The main purpose of the RESCDAM project was todevelop emergency action planning for dams. A dambreak hazard (flood) analysis is a necessary aid for this.A risk assessment (analysis) was also included in theproject work. The pilot project of RESCDAM was theembankment dam of the Kyrkösjärvi reservoir locatedin Seinäjoki in Western Finland.According to the RESCDAM Project, a completeproject work should be divided into three subprojects:A) Risk Assessment (Analysis).B) Dam Break Hazard Analysis (DBHA).C) Emergency/Rescue Action Planning.Following the RESCDAM project steps, the actualprocedure to analyze entirely a specific dam-breakproblem is developed from now on. The second point(DBHA) is the principal focus of this ninth section ofthe thesis (numerical flow models, 1D and 2D). Sincethe comparison between one and two dimensionalmodels has been developed in terms of flood analysis,the first and third point are not the main purpose ofthis section; it has been developed in this appendix toexpound a complete dam break approach.This Appendix will focus on the scientific approachesthat have produced today’s engineering practices.After a very long period when the art of damconstruction remained empirical, but when there werenevertheless some remarkable inventions, dambuilding gradually took on a scientific basis, usingmathematical tools and techniques. Today, dambuilding calls for scientific and technical knowledge invaried disciplines.Each of these disciplines makes use of less specialisedtools provided by mathematics (continuum mechanics,materials strength), physics and chemistry. In terms ofsafety, it is in fact “rare” or “extreme” events that areof interest, that is, accidents. The usual approach istherefore to specify the design flood and designearthquake at the design stage.A) RISK ASSESSMENTConcept and bases of risk analysis for damsRisk assessment is the process of deciding whetherexisting risks are tolerable and present risk controlmeasures are adequate and if not, whether alternativerisk control measures are required. Risk assessmentincorporates, as inputs, the outputs from the riskanalysis and risk evaluation phases.The primary steps of a risk assessment consist of thefollowing (ICOLD, 1999):- Risk analysis.- Risk evaluation.- Risk management.Risk analysis.The risk analysis of a dam is the use of availableinformation to estimate the risk to individuals orpopulations, property or the environment, fromhazards. A risk analysis generally contains thefollowing steps:• scope definition• hazard identification• risk estimation.A risk analysis involves the de-aggregation ordecomposition of the dam system and sources of riskinto their fundamental parts.The scope and expectations of the risk analysis shouldbe defined at the outset. Definition of the scopeshould include (ICOLD, 1999):- description of the issues that initiated the riskanalysis,- description of the dam system to be analyzed,- identification of the point in time to which the studyrelates,24
Comparison between 1D and 2D models to analyze the dam break wave - Appendix- formulation of the objective of the risk analysisbased on the main concerns identified,- identification of the decisions to be made, thedecision-making criteria and the decision-makers,- description of the proposed risk analysis processincluding statement of the assumptions andconstraints governing the analysis.Risk analysis for dam systems can be categorized bythe nature of the hazards leading to the consequencesof concern. The hazards in question may be groupedinto four general categories, namely:- hazards due to natural conditions (normal loads,earthquakes, floods, debris, winds, etc.);- operational hazards (spillway reliability, operatorerror, etc.);- internal hazards (aging, alkali-aggregate reaction,internal erosion, metal fatigue, etc.); and- social hazards (war, sabotage, etc.).Failures are often categorized as static, hydrologic orseismic. Some of the various factors, are presented inthe figure below:Figure 57. Some factors to cause dam failures.Risk estimation entails the assignment of probabilitiesto the events and responses identified under riskidentification. The assessment of appropriateprobability estimates is one of the most difficult tasksof the entire process. The outcome of this step is acalculation of the risk of failure.Risk evaluation.Risk evaluation is the process of examining andjudging the significance of risk. The risk evaluationstage is the point at which values (societal, regulatory,legal and owners) and judgements enter the decisionprocess, explicitly or implicitly, by includingconsideration of the importance of the estimated risksand the associated social, environmental, andeconomic consequences, in order to identify a range ofalternatives for managing the risks.Risk mitigation is a selective application of appropriatetechniques and management principles to reduceeither likelihood of an occurrence or its consequences,or both (ICOLD, 1999).Risk acceptance is an informed decision to accept thelikelihood and the consequences of a particular risk(ICOLD, 1999). Risk acceptance is coupled to the riskreduction evaluation and is a basis for deciding whatresidual risk will be accepted for the affectedcommunity and structures.Risk managementRisk management is the systematic application ofmanagement policies, procedures and practices to thetask of identifying, analyzing, assessing, treating andmonitoring risk (ICOLD, 1999). Having received therisk information, and knowing the risk valuationcriteria, a decision-maker must come to a decision. Anumber of good practice steps to be taken to aid thedecision process are given by the ICOLD. These stepsshould include e.g. consultations with stakeholders andcommunity, insurance issues, legal defensibility ofdecisions, risk information to decision-maker, and tothe public, clarification of the role of decision-makerand documentation of the decision and its rationalimposes a discipline that is helpful to sound decisionmaking.Causes and probabilities of dam incidentsThe data, which have become available in the last 10 -20 years, provide a foundation for the ideasrecommended to be used in risk analysis. This studyaddresses risks associated with the construction,operation, and failure of dams, excluding indirectenvironmental or resettlement problems. The purposeof the study is to identify the main real risks associatedwith each type and height of dam, for allcircumstances. It may be of help in several respects:- in extensive risk analysis of very large dams, tosubstantiate reliably the probabilities chosen in eventtrees;- in simplified risk analysis of smaller dams, to focuslow-cost risk analysis on a few main risks;- in identifying possibilities for reducing these risksthrough low-cost structural or non-structuralmeasures.When studying the possibilities of risk assessment thereported failure circumstances can be classified asfollows:• Exceptional circumstances: floods, upstreamdam-break wave, earthquakes, and war. Riskassessment for exceptional circumstances islogically based on the probability and impactof such circumstances.• Normal circumstances: first filling, aging(after two years operation) and internalerosion, and construction. For dams innormal circumstances the safety is essentiallybased on human behaviour, and riskassessment should pay a great deal ofattention to this point.B) DAM BREAK HAZARD ANALYSISIntroductionDam break modelling consist of first, prediction of theoutflow hydrograph due to dam breach and latter,routing the hydrograph through the downstream valley25
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Comparison between 1D and 2D models to analyze the dam break

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