Dam SafetyDecember 1992 Summary Report

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Norwegian Water Resources and Energy Administration

Water System Management Association

NORWAY

THE NORWEGIAN WATER RESOURCES END ENERGYADMINISTRATION

The Norwegian Water Resources and EnergyAdministration (NVE) is a governmental bodyresponsible for the management of Norway'swater and energy resources for the optimumeconomical benefit of the nation. An objectivfor NVE has been to obtain this at the lowestpossible cost to the environment and with thehighest possible saety to the public.

NVE's Dam Safety Section NVE-T approvesdesings and construction plans and supervisesdams and other river works in order to ensurepublic safety. Today more than 2500 dams aremonitored by the section. The section draftsregulations and guidelines for constructionand operation of such works. To maintain anddevelop knowledge within dam technology,hydrology and hydraulics, NVE initiates andtakes part in a variety of studies andresearch projects.

THE WATER SYSTEM MANAGEMENT ASSOCIATION

The objective of the members of Water SystemManagement Association is power productionthrough water system development. The majorityof the contry's power companies anddevelopement and owner associations aremembers.

Above all the responsibilities of theAssociation include matters of legal, economicand technical/scientific nature relating tothe planning and operation of hydro-electricpower stations and storage dams. Some examplesare legislation and concessions technicaloperatingexperiencesenvironmental questions,hydrology, and the coordination of theresearch and development for which theAssociation is responsible.

The Association reviews matters of commoninterest for its members, and advances thesematters towards the authoritis.

Photo: Kilen Dam, Sira-Kvina Power-Company

DAM SAFETY

SUMMARY REPORT

THE NORWEGIAN WATER RESOURCES AND ENERGY ADMINISTRATION

THE WATER SYSTEM MANAGEMENT ASSOCIATION

NORWAY

Contents Page

Dam Construction and Safety 1

Target Groups 2

Reports from the Project 3

Responsibility and Division of Work 8

Quality Control 9

Risk Analysis Methods 11

Evaluations of Dam Failure Capacities 11

Safety related to impact, classificationsystems for dams 11

The Need for Changes in the Dam Regulations 12

Need for Rehabilitation 12

Dam Safety during the Operational Phase 13

Planning and Construction Documentation 14

Operational Routines 15

Emergency Planning 15

Inspection Routines 16

Instrumentation 17

Safety Considerations 18

Reporting of Damage and Accidents 19

Registration of External Impacts 20

Flood Safety 20

Spillway Analysis 22

Obstruction of Spillways 22

Operational Safety of Gated Spillways 23

Leakage from Embankment Dams 24

Overtopping of Embankment Dams 26

Ageing of Concrete Dams 26

Landslides and Reservoirs 28

SUMMARY REPORT

Dam Construction and Safety

Modern dam construction in Norway startedabout the turn of the century when theexploitation of our water-power resourcesgained momentum. Masonry and concrete damswere predominant in the early stages whilelarge embankment dams came on the scene on alarge scale after 1950. During this timeNorwegian dam construction technology hasmaintained high standards, and the dams haveattained a favourable reputation as far asquality and safety are concerned.

The water reservoir behind a dam represents anenormous potential of energy capable ofprovoking a catastrophic situation in the caseof dam failure.

It will be a matter of course to everybodyinvolved in dam construction and operation,that safety standards of the structure arehigh enough to render inconceivable the ideaof dam failure.

Nonetheless we know that dam failurecatastrophes do occur, and it has happened inour time and in technologically highlydeveloped countries. No large scale accidentshave been caused by dam failures in Norway.There has nevertheless been a series offailures on smaller dams below 15 metres inheight.

A dam can never be considered as an absolutelysafe structure. However, realizing that risk-factors are present makes it possible tostrive for reduction and elimination ofindividual factors, so that total risk can bekept to acceptable levels.

During the period of extensive hydropowerexploitation the country has undergone inrecent decades, the perception of dam safetywas largely related to planning andconstruction, where calculation methods,loads, material properties and design were thekey words. But dam safety also depends a greatdeal on how dams are controlled, operated andmaintained, and to what extent differentincidents and situations emerging during theoperation phase are perceived and preparedfor. This was the reason that the NorwegianWater Resources and Energy Administration

(NVE), in 1987 initiated a cooperation projecttogether with the Water System ManagementAssociation (VR) and the dam owners on damsafety.

The report "Risk Analysis for Dams" was issuedin 1987 as a result of a preliminary project,while the main project commenced byestablishing a Board of Supervisors in theautumn of 1988. The Project Manager startedhis work in April 1989, and from then on thepractical work was launched. The project wasterminated in 1992.

The Board of Supervisors consisted of:

Chief engineer Bjarne Nicolaisen, NVE(president).Chief engineer Jan Daleng, VR.Professor Dagfinn Lysne, the HydraulicInstitute of the Norwegian Instituteof Technology (NTH).Chief engineer Thorleif Hoff, theNorwegian Energy Corporation (Statkraft).

The Project Manager was Civil EngineerSvein Larsen.

On the basis of actual conditions, the projecthas made recommendations on how to promote damsafety during the operational phase. Variousquestions relevant to dam safety in theoperational phase were likewise examined,phenomena related to the ageing of the dam, todiversion of floods, overtopping, leakages,security in the functioning of flood gates andpreparation for emergency situations. Data onactual operational experience, statistics ondam failure and risk-evaluations have alsobeen surveyed.

Target Groups

All categories of dam owners represented by the persons responsiblefor planning, building, operation,inspection and maintenance of dams.

Consultants assisting the dam owners.

Public authorities with administrativeresponsibility for dam safety.

Public authorities with responsibilityfor rescue operations during disastersituations.

Reports from the Project

The following reports have been issued by theProject:

Main Report

Summary Report

Report no 1: Ageing and Safety ofConcrete Dams.

Report no 2: Emergency Planning forAbnormal Situations.

Report no 3: Security of embankmentdams against leakage failures.

Report no 4: Obstruction of Spillways.

Report no 5: Overtopping of theimpervious core in embankment dams.

Report no 6,Parts I and II: Functional Security of

Flood Gates.

Report no 7: Alkaline Reaction inConcrete Dams.

Report no 1: Ageing and Safety of Concrete Dams

This report describes the construction ofconcrete dams in Norway in a historicalcontext, and traces their technologicaldevelopment. Various types failure arereviewed and safety consequences evaluated.The different types of dams are described interms of design, stability, safety andrehabilitation.

Characteristically damage to concrete damsappears clearly on the surface and visible tothe naked eye. Rifts, cracks, waterpercolation, limestone washing, larger or

smaller fractures and erosion are among themost common forms of damage.

How serious is such damage, will it haveconsequences for dam safety, how will itdevelop in the future, and what kind ofmeasures are required? Could there also be aninternal degradation, not visible on thesurface? Such questions arise naturally, andthese matters are evaluated and conclusionspresented in the report mow issued. Briefly itcan be said that concrete dams should notspring any major surprises on us from a safetypoint of view.

As mentioned already these dams readilydisclose their weaknesses, and there willgenerally be ample time to considerappropriate measures before damage has reachedunacceptable levels. Damage will is usuallyput right for aesthetic and economic reasonswell before it has discernible impacts onsafety.

Report no 2: Emergency Planning for Abnormal Situations.

Two examples of emergency planning arepresented in this report. The examples havenot been worked out in detail as far as sizeand layout are concerned and are should beregarded as merely parts of a process. Theidea is that dam owners should receiveimpulses and ideas to work out correspondingplans for own installations through theseexamples.

All emergency planning should be adapted toprevailing conditions at the individual damowner's site and to the consequences of damfailure. Planning will be important primarilyto those dam owners responsible for dams wherethe consequences of failure are large.

Emergency planning for dams bearing in mindcritical natural events constitute an integralpart of the total, and the scope of the workshould incorporate activities like:

dam owner's current programs for operation, inspection and maintenance ofhis dams;

emergency planning related to war-

situations, sabotage etc. for the dams;

the general emergency and rescue operations planning the for majoraccidents;

dam owner's emergency planning for othertypes of installations like power plants,factories etc.

Report no 3: Safety of embankment dams against leakage failures.

In Norway there have been several incidents ofinternal erosion causing increased leakage ofembankment dams. This phenomenon also figuresin international dam statistics as among themost frequent causes of damage and failure.

In Norway, however, the rockfill dam is themost common type of embankment dam, butfailures and serious damage typically occur inearthfill dams.

Leakage occurs frequently at the initialinundation of the reservoir. Relevantquestions are whether such leakage also candevelops in older dams, where abnormalconditions have not been noted previously, howleakage will develop, and how the safety riskcan be reduced by investigations andappropriate measures at the dam site. Thereport deals with all these questions.

Report no 4: Obstruction of Spillways.

This report reviews the results of model testsdesigned to reveal conditions leading toobstruction of permanent spillways. An attemptis also made to define and isolate thoseforces which impact on the debris in case ofaccumulation.

A complete evaluation of a spillway'sdiversion capacity should include an analysisof the situation at the obstruction. Theanalysis should answer the followingquestions:

What are the consequences of anobstruction of the spillway?

- What types and amounts of debris can be

expected to arrive?

- What potential is there for diverting orhalting the flow of debris?

The report will assist answering the abovementioned questions.

Report no 5: Overtopping of the Impervious Core in EmbankmentDams.

An investigation should be carried out intothe capacity of embankment dams to withstandcollapse when the reservoir's water levelovertops the impervious core. This should bedone in connection with PMF-analyses andemergency planning.

The report contains guidelines and advice forthe undertaking of such an investigation.

It should be noted that appraisal of variousfailure mechanisms (erosion, slides) can helpdecide what is the maximum water level areservoir can sustain.

Norwegian rockfill dams with a reinforced damtoe of boulders can withstand large floodwater levels. There are, however, also somedams in Norway where downstream slopeprotection and the dam toe consists of earth,gravel or mixed tunnel rocks/ quarried rocks.Such dams will withstand only smaller floods,and reinforcing of the dam toe should beconsidered in such dams.

Report no 6: Functional Security of Flood Gates.

Spillways that do not function as designed cancause dam failure, and it is necessary toensure that spillways with sluicing deviceshave an adequate functional security.

Analysis of functional security can beperformed as part of this exercise so as todetermine diversion capacity at differentdegrees of gate operation failure.

The report demonstrates, with examples, howsuch analyses can be undertaken. The potentialfor operational and sluicing failures areexplained together with various measureswhich

can be taken to improve functional security.The report is divided into two parts forpractical reasons.

Report no 7: Alkaline Reaction in Concrete Dams.

The recognition that alkaline reactions cancause damage to concrete is relatively new inNorway, even although such reactions have beenknown over a period of many years. Anextensive research project on alkalinereactions has been going on at SINTEF FCB (TheNorwegian Centre for Industrial Research inTrondheim)(?) since 1989. Extracts of theconclusions from the research project as oflst January 1992 are presented in this report(Part 1).

Data is scarce on how large expansions havebeen caused by alkaline reactions in differentstructures in Norway. It has been possible toreconstruct the historical rate of expansionat Votna dam by carrying out regulardeformation measurements since the dam wasbuilt. This material is presented in part 2(Grøner). It is concluded that the expansionof the concrete over the last 10 years amountsto a total of about 0.4 per mille, and thatthe present expansion rate is about 0.06 permille annually.

Norwegian research on alkaline reactions hasso far focussed, to a very limited degree, onthe effects of reactions on forces and loadsin different types of structures and on whateffects this has for safety. This is reviewedin part 3 (Grøner).

The effects can be summed up in four parts:

The structure undergoes deformation whichcan cause blocking of gates, jamming ofoutlets etc.

The structure will crack at the surface,opening up for penetration by otherdestructive processes.

The properties of the concrete regardingpermeability, elasticity and resistance topressure, strain and section strength willbe altered.

The internal expansion of the concreteleads to additional strain in thereinforcing, concrete and cables. Theeffects are different from expansioncaused by temperature.

The expansion of the structure caused byalkali can be of the order of 0.5 per mille,and this alone results in considerableadditional strains in the reinforcing.Expansion will cause additional strains fordifferent types of structure, and a completereview of this can be obtained throughanalysis of the structure in question.

Responsibility and Division of Work

The dam safety promotion work can be dividedinto two main parts:

The first part concerns tasks directly relatedto a dam construction project. The othercomprises a general part which benefitsindirectly the safety promotion work in eachindividual project.

There is a natural link between the two parts,as experience from individual dams influencesand forms the background for the contents ofthe general part, concerning laws andregulations as well as standards andexperience.

On the other hand the contents of the generalpart will form the basis for safety promotionwork connected to each individual dam project.

To-day, two main parties are responsible fordam safety:

The dam owner.

The Safety and Emergency PlanningDepartment of NVE(NVE-T).

A crucial factor in dam safety is that thedistribution of functions, tasks andresponsibilities is well known and acceptableto both parties. Under these circumstancesthe actual distribution of functions is oftenfound not to correspond with formal

arrangements. Dam safety can be threatened ifthis is allowed to go too far.

Safety Promotion Work for the Individual Dam Project

The individual dam owner plays the main partin dam safety promotion work because:

The individual dam owner shall see to itthat dam safety is maintained at asatisfactory level for all dams in hisownership.

NVE-T's role is dictated by its specificobjectives:

NVE-T shall on behalf of the publiccontrol the dam owner and verify that thelevel of safety of his dams is adequate.

An important element in dam safety promotionwork is to develop standards for adequatesafety levels. Both parties are responsiblefor this independently of each other.

General Safety Promotion Work

NVE-T has the main responsibility for laws,regulations and the Norwegian Dam RegulationsPart II. These should, however be developedand maintained in close cooperation with thedam owner organizations, the dam owners andother agencies in the water management sector(consulting engineers, research institutes,suppliers and contractors).

All agencies in the sector participate indevelopment and maintenance of informalguidelines, standards and experience, but themain responsibility lies naturally with thedam owner organizations and NVE-T.

Quality Control

The water management sector and NVE havejointly initiated a system of Project QualityControl, to formulate guidelines for contentsand functions of a quality control system. Theguidelines will be presented in mid 1993.

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To-day a comprehensive set of regulationsexists within the field of dam safety. Herethe standards dam owners must comply withduring planning and construction of the damhave been established. Quality control systemswhich can contribute to the realization ofthese standards are clearly importantelements. Defining the dam owners' and NVE-T'sfunctions and roles in this supervisory taskwill be a prominent part of the QualityControl Project.

Regulations governing the activities of thedam owner during the operational phase areinsubstantial. The dam owner assumesresponsibility for dam safety during theoperational phase, but the implications ofthis are dealt with to a limited degree in thepresent regulations. For a quality controlsystem to work more detailed specificationswill have to be set out in a set ofoperational rules for dams.

This should be achieved under the establishedsystem of regulations, and not through rulesfor quality control. Quality control does notcomprise the establishment of specificrequirements to be complied with, but ratheras the development of a system to guaranteethat the requirements will be followed.

Quality control in its original concept alsoimplies that the supervising agency (NVE)shall shift its activity from the control ofdams to control of the dam owners' qualitycontrol systems.

The possible consequences for dam safety ofthis change should be more closely evaluatedbefore it is adopted as the official systemfor control. Our largest dams represent adisaster potential so vast that NVE shouldpresumably make independent securityassessments here on a periodic basis.

An evaluation of NVE's inspection strategywithin a future quality control system shouldbe made under the Quality Control Project. NVEshould use a term different from "QualityControl" for its strategy, in case futureinspection systems do not correspond withother inspection agencies' quality controlsystems. This should be done in order to avoidmisunderstandings.

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Risk Analysis Methods

Use of risk analysis methods in dam safetywork offers a sounder basis for the assesmentof dam-associated risks rather thantraditional methods for safety promotion.

Calculations of dam failure waves and theconsequences of dam failure were previouslyprepared by the previous Defence ElectricPower Organization (KSFN) now the EmergencyPlanning Section of NVE-T.

The dam owners themselves should assumeresponsibility for making such analyses. It isnot expected, however, that the dam ownerswill assume this task until NVE-T clarifiesthe situation.

Evaluations of Dam Failure Capacities

Dam safety has traditionally been analysed andexpressed through design standards and designloads. Safety related to individual events canbe given a more explicit expression byanalysis of failure situations, as individualdams may possess different abilities tosupport impacts in excess of the designstandards.

Assessment of failure capacity should bestandard for all new structures. Assessmentsshould be undertaken for existing structuresdepending on the what the consequences of damfailure might be.

Safety related to impact, classification systems for dams

In Norway it is estimated that about 2500 damspossess dam structures or reservoir ofsufficient size for them to be subject to NVE-T's inspection.

These dams vary from 2 metres to 140 metres inheight. There are reasons to believe thatfailure at one of our large dams with greatdamage potential would represent a disaster ofthe worst imaginable dimensions in Norway.Failure at one of our smaller dams could,

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however, occur without causing any majordamage. It seems evident for these reasonsthat all safety precautions for damconstructions should depend on the damagepotential of a failure.

The purpose of a classification system will beto group individual dam structures intodifferent categories, so that similarstandards could be established for dams withineach of these categories.

The Need for Changes in the Dam Regulations

The need for change in some of the regulationshavs been revealed in different aspects of theproject work. The results are presented inAnnex 1 as "Proposed Changes in the DamRegulations." It should be stressed that theproject has not made a complete revision ofthe dam regulations with possible amendmentsin mind.

NVE should initiate such a complete revisionof the dam regulations, and as a next steppropose amendments to them. The work should becarried out in cooperation with the watermanagement sector. Development of regulations,rules and recommendations for the operationalphase (the Dam Regulations Part II) should bethe main task.

Need for Rehabilitation

The following possible needs forrehabilitation of our dams can be listed onthe basis of safety considerations:

Changes/removal of bridges over permanentspillways.

Increased capacity of cross sectional areaof tunnels and shafts at spillways.

Construction of crests on older embankmentdams, or, alternatively, construction ofconcrete parapet walls.

Construction of dam toes made of boulderson existing embankment dams.

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Improve upstream slope protection onembankment dams.

Increase spillway capacity on gated dams.

Installation of several independentreserve systems for operation of floodgates.

Reconstruction of older wooden flashboarddams.

Rehabilitation of concrete dams.

Rehabilitation of wooden and steel framedams.

Dam Safety during the Operational Phase

The dam owner is responsible for adequatesafety at his dams.

Dam safety means assurance against anuncontrolled and damaging outflow of water(arising from operational errors or damfailure) or against damage to the reservoir bywater levels higher than anticipated.

In order to meet his responsibilities the damowner should work out, carry through andcontrol programmes for the activities heassumes to be necessary for safety during theoperational phase. The extent and content ofthe programmes shall be adapted to the type ofdam, safety level and the consequences of damfailure.

The dam owner shall establish an organizationwith relevant professional competence andfinance to enable him to carry out the abovementioned tasks. Some tasks will call for in-house capacity, while for others outsideexpertise will have to be hired. The damowner's schemes for promotion of safety mustbe adapted to the structure, size and specificcapabilities of his organization. The need foroutside assistance will emerge as a part ofthe total picture.

The dam owner's programmes must also beconceived so as to provide the basis for NVE-T's inspection.

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NVE-T's Inspection

NVE-T is responsible for monitoring thatthe dam owner fulfils his safetyobligations.

NVE-T can impose different degrees ofmonitoring. NVE-T should adopt a monitoringstrategy geared to the consequences of damfailure. Dams where failure will have majorconsequences should be more intensivelymonitored by the NVE-T than dams whereconsequences are small.

Planning and Construction Documentation

Documentation clarifying all aspects ofsignificance for the dam should exist from theplanning and construction phases. It is ofcourse important that documentation alsocovers all modifications added during theconstruction phase of the dam. Suchmodifications can be potential sources ofdamage or dam failure.

Documentation should exist in various forms.

For dams constructed before the introductionof the Dam Regulations, documentation may bevery scarce in some cases.

The extent to which new documentation shouldbe provided will depend on its usefulness andthe need for it. Supplementary documentationshould be restricted to a minimum for olderdams with small failure consequences.

Dam safety is partly based on detailed,concrete public safety requirements, andpartly on what the dam owner considers to beadequate levels of safety. The dam ownershould see to it that proper documentationexists showing that all detailed, specificofficial safety requirements have been met.

NVE-T should establish a similar system inorder tomake a register of instructions givento and documentation received from the damowner.

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Operational Routines

The purpose of the dam owner's currentoperations may comprise production of hydro-electric power or delivery of drinking water.Operational routines for these will be verynumerous.

The concept 'operational routines' in thisreport, however, cover only activitiesconnected to the dam's operational safety.

This comprises operational routines related toregistration of water levels and flooddiversion. The following operational routinesshould be included in the programme:

Principal operational rules related toflood diversion.

The main requirements can in some cases becontradictory. Principal operational rulesshould set out the main flood diversionoptions open to those responsible for suchsituations.

The programme must be realistic and wellconceived. Principal operational rules cannotcover every possible eventuality in detail.

It should be noted that one reason for thedifficulties in setting out operational rulesis contradictory demands in a flood situation.Setting out such rules reveal that demands arecontradictory.

Emergency Planning

The concept 'emergency planning' is a generalone and can be applied in a variety ofcontexts. In the water management sector theconcept used to be related to the work carriedout by the former Defence Electric PowerOrganization (KSFN), which has now beentransferred to the Emergency Planning Sectionof the NVE (NVE-TB). The purpose of itsemergency planning was initially to secure aneffective energy supply in war situations aswell as under emergency conditions (periodsprevious to war).

A main aim of any hydropower company will be

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to secure effective energy supply duringperiods of storm and bad weather. It must,however, be kept in mind that the hydropowercompany may also be responsible for dams withan extremely large damage potential in case ofdam failure, and that the safety of these damswill be at risk in the same situations.

Only emergency planning for dam safety relatedto incidents caused by natural forces has beenanalysed by the project. It is important,however, that all associated emergencyplanning be coordinated.

How to Improve Dam Safety through Emergency Planning

The dam owner can improve dam safety throughemergency planning in the following ways:

The dam owner's staff, administration andorganization can, through participation inplanning and by developing an operationalemergency plan, better tackle criticalsituations and avoid dam failure and otherunwanted incidents.

Weak points can be detected throughplanning, and measures taken beforepotentially critical situations arise.Such weak points can take different formsas follows:

Technical design of construction.Parts of the operational emergencyplan.Available resources for emergencysituations.

The dam owner presents his emergencystrategy to NVE-T. This gives NVE-T anopportunity to evaluate his strategy in anoverall assessment of the safety of thedam and to comment on potentiallyinadequate safety levels.

Inspection Routines

Dam owners' inspections constitute animportant part of the planning of construction safety. Inspections have always been undertaken, but the task has been performed in

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a more systematic way after introduction ofthe Dam Regulations in 1981.

The current practice of inspections is largelysatisfactory, and the Project has not foundany reason to pay particular attention tothis aspect in its work.

It has to be said, however, that underwaterinspections and material analyses should beconsidered in many cases in addition to visualinspections.

The need for inspections, however, variesgreatly, depending on the category of dam, itsgeneral condition and it state of safety. Thisshould be reflected in the inspectionprocedures.

Instrumentation

Instrumentation of dams for measuring ofdeformations, pore water pressure, tensions,water percolation, temperature or otherphysical standards will form part of a damowner's safety monitoring.

The dam owner should set up a program for hismeasurements. The program has to be adapted tothe category of dam, age and condition. Theprogram should have a clear concept andpurpose behind the measurements. Themeaurements must be followed up and analysedby qualified personnel. The extent of themeasurements should be revised at intervals of10 to 20 years. The need for measurements willdecrease after the first 10 years of the dam'slife span.

The need for new measurements should also beassessed in connection with any damage orirregular damconditions. Special requirementsfor measurements can arise with very old dams,and technological developments give rise tonew opportunities for measurements.

The purpose of dam instrumentation isgenerally be to record the state of the dam,and constitutes a part of the strategy topreserve safety of the construction.

Instrumentation also has a place in the

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programme for operational safety. The purposeof such instrumentation is to record the waterlevel in the reservoir, the position of gatesand other technical details regarding theoperation of gates.

Safety Considerations

The overall security of a structure vis-a-visa threat or a load consists of the originalsecurity dating from the time of itsconstruction and the change in this over itslife-span.

Inspection routines and instrumentationpractices can reveal changes in the state ofthe structure, but this, in isolation, cannotgive a correct estimate of the securitysituation. The dam owners must undertake riskassessments on the basis of current safetystandards in order to complete the picture.

The various risks or

Security assessments can be directed towardsdifferent kinds of risks and loads or as anassessment of total security. A number of suchrisks/loads were analysed in the dam safetyproject as issues related to individualelements of the structure.

The dam owner should clarify and give reasonsfor which revisions of security that arepresumed necessary, and then discuss this withNVE-T.

The dam owner may perhaps find it difficult tounderstand that revisions are needed in somecases. It can be difficult to determine whatis necessary or not. In situations like thiswhat other dam owners are doing andestablished practice can give useful pointerson what to do. Changes in practice may,however, be hard to introduce.

A change in practice might be achieved byclarifying the benefits from and needs forsafety assessments, and by emphasising the damowner's responsibility to clarifiy the need(or lack of need) for such assessments.

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NVE-T itself however should assess the need torevise safety provisions for specific risksituations (for example blocking of spillwaysor a slide in the substructure of arch dams).It should instruct, in a planned and co-ordinated manner, groups of dam owners toundertake such revisions where considerednecessary.

Amendments to Regulations

A change in practice so that the dam ownerhimself undertakes the revision of security toa larger extent can be achieved by introducingthis into the regulations.

The present system is so conceived that theDam Regulations deal with the followingissues:

What kinds of security assessments need tobe carried outThe current acceptable security level.

These regulations cover both planning andconstruction.

It can be decreed that documentation onsecurity shall be submitted for previouslyconstructed and approved dams in cases wherethe revised dam regulations contain paragraphsthat are either new or imply substantialdeviations from earlier practice.

Reporting of Damage and Accidents

Reports on damages and abnormal incidentsduring operation and the follow-up of suchirregularities constitute an important elementof dam safety. Such reporting has severalpurposes.

Adequate handling of the matter internallyis guaranteed.

It ensures that NVE-T is guaranteeddetails and influence on the matter

It ensures that information on damage andaccidents reaches a central register ofdam safety.

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Reporting of damage or abnormal situationsshould be done through three main routines asfollows:

Normal reporting governing inspection,operation and maintenance.

Reporting of acute damages andirregularities.

Reporting for damage statistics.

Registration of External Impacts

The principal loads a dam is supposed towithstand are determined by theoreticalcalculations. Both the data sources and thetheoretical model may, however, be subject tosubstantial uncertainty. Recording the loadthat a dam structure is really exposed to isimportant in detecting miscalculations indeterminingload factors.

The following story from Switzerlandillustrates the importance of recording oflarge recurrent floods:

A dam was exposed to a great flood after 20years of operation. The dam was overtopped,collapsed partially and 17 persons werekilled. The size of the flood was calculatedat three times the design flood. A closerreview of the data showed, however, that thedesign flood had been exceeded six timesaltogether in the course of the 20 years lifespan of the dam.

The dam was afterwards rehabilitated andprovided with a larger flood diversioncapacity.

The recording of wind and wave data should beinitiated for a sample of dams so that variousdistricts are covered. NVE-T should coordinatethe work and instruct a selected group of damowners to undertake it.

Flood Safety

Flood safety in this context denotes securityagainst dam failure caused by flood. A failure

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of this kind normally develops in the form ofovertopping. Events of this kind have evolvedat a number of dam failures, and overtoppingby flood can be regarded as one of the majorpotential causes of dam failure. A failure canalso develop from erosion in the floodchannel.

From a purely technical point of view,overtopping can be caused by:

Recurrent floods that are of larger thananticipated at construction (larger waterflow, more debris, a more difficultsituation totally).

The dam cannot support predicted waterlevels.

Spillways not having the prescribedcapacity.

The reasons the spillway has less capacitythan anticipated may be:

Actual capacity of the spillway issmaller than calculated technically.

The spillway has been blocked.

Gates, flashboards etc. in thespillway have not opened as anticipated.

Analyses of dams' security against failurecaused by large floods should containassessments of all the aforementioned issues.Unfortunately flood and capacity evaluationshave often been restricted to assessments ofwater flow through unobstructed channels. Anextremely large flood is a catastrophe

situation

which has to be analysedspecifically.

Assessments of Safety

It is recommended that dam owners undertakecomplete analyses of failure risks caused byfloods for all new structures and all existinginstallations. The extent of the analysesundertaken should be related to theconsequences of dam failure. In particular

this concerns analyses of obstruction

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situations and of failures in operation ofmechanical flood regulating equipment.

It is of particular importance that revisionsare undertaken for installations that arevulnerable to increases in flood-loads. Thiswill primarily concern:

Installations comprising tunnel/shaft inspillways.

Installations comprising gated spillways.

Spillway Analysis

A spillway analysis should comprise thefollowing elements:

Evaluation of capacity.

Evaluation of erosion.

Evaluation of obstruction.

Evaluation of operational safety for mechanical flood regulating equipment

Tunnel/Shaft/Spillway

Installations where the spillway is composedof a tunnel/shaft system beneath a fixedspillway or gate, are installations with lessthan the normal security reserves. Betterstandards of security than originally usedwhen the installation was planned could now beintroduced.

A re-evaluation of the capacity of suchspillways should therefore be undertaken inconnection with a flood safety assessment.

Obstruction of Spillways

Blocking of spillways by debris is common inflood situations. Landslides from steep slopescan cause accumulation of large quantities oftrees with roots and branches in the riversystem, and reference is made to what happenedat Palagnedra Dam in Switzerland. Tales from

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the large flood "Storofsen" in Norway in 1789say that entire parts of valley sides withhouses, forest etc. slid down and into theriver Lågen.

There are also examples from Norway where snow- and rock-slides have blocked spillways.

Obstruction is one of the most important riskfactors and leads to the reduction of thepredicted capacity of the spillway andeventually to dam failure.

An obstruction assessment should form part ofan assessment of the risk from of accidentfrom exceptional loads, and the first stepshould be to evaluate the consequences of acomplete obstruction.

Operational Safety of Gated Spillways

There is always a possibility that thecapacity of the spillway will be less thanpredicted because of operational failures.

A complete analysis of the cpacity of gatedspillways must therefore include assessmentsrelated to failures during operation.

The Project has undertaken examples of suchanalyses and these are presented in Report 6,sections I and II.

The purpose of the analysis.

A functions and safety analysis of a gatedspillway will be a part of several processes.

* Emergency plans.

The analysis could be included in theemergency strategy and serve as the basis foraction planning in case of operationalfailures.

* Reserve capacities.

The analysis can clarify how the reservecapacities of the installation can beimproved.

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Technical design, organization.

The analysis can serve as a basis for decisions on changes in technical design orfor planning of actions in flood situations.

Safety evaluation.

The analysis can contribute to a betteroverall picture of total safety related toovertopping. The total probability ofovertopping will in general consist of twofactors:

The probability of overtopping withall gates open.

The probability of overtopping withdifferent kinds of functional failures.

The analysis can point to the principal typesof functional failures and indicate levels ofprobability.

Operational regularity of energy production.

Concern for dam safety is the basis of theProject and the reason behind the proposedanalyses of functional safety. Such analyses,however, can also help reduce operationallosses in economic terms.

Design of Analysis

An analysis should be undertaken by theoperational staff who know the particulars ofthe installation, in co-operation with theexperts (external or internal) who can viewthe issues in question in a fresh light. Somehelp from personnel skilled in analyticalmethods may be desirable for larger evaluationtasks.

Leakage from Embankment Dams

International statistics indicate that cracksor damage caused by leakage are responsible asignificant share of the accidents/problemsrelated to embankment dams.

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Incidents of irregular leakage have alsooccurred at Norwegian dam installations, andthe dam failure at Roppa in 1976 was caused bya leakage.

International statistics nevertheless seem toconfirm that no large rock-fill dams havefailed even when big leakages have occurred.The main reason for this is that the actualleakages have been limited in scale, and thatdownstream dam-toe was able to divert theleakage without becoming subject toinstability.

International statistics show that failure hasoccurred in a number of earth-fill dams wherethe cause was considered to be internalerosion. In most cases the failure occurredduring the initial inundation of thereservoir. But failures have also taken placeafter many years of operation, which clearlyshows that internal erosion can also developover a long time.

In Norway there is also a substantial numberof smaller earth-fill dams in addition torock-fill dams. These are particularly exposedto failure by leakage.

It is recommended that the owners undertakeassessments of the security of all embankmentdams for failure caused by leakage. Suchassessments should be standard in theconstruction of new dams. Thought should alsobe given to carrying out assessments forexisting dams.

Systems for registration of leakages.

The reporting of leakages in dams is importantfor the timely recording of abnormalconditions within the dam.

The dam owner must establish a system ofrecording which serves the necessary purpose.Embankment dams, the failure of which couldhave significant impactss should possesssystems for automatic registration ofleakages, permitting the remote transmissionof records.

Embankment dams that have experienced largeand sudden leakages should also have such an

26

automatic, remote transmission system forrecording.

The dam owner's system for recording ofleakages should consist of reporting andassessment routines as well as the purelytechnical setup for recording data.

Overtopping of Embankment Dams

Dam failures due to overtopping are amongstthe commonest type of dam failure in theworld.

This type of failure is also of such a naturethat the probability for it occurring does notreduce with the age of the dam.

Assessments of safety in embankment damsshould therefore comprise analyses of theresistance of such dams against failure fromovertopping (ultimate state). Such analysesmay be undertaken in a number of situations:

As an assessment of the extent to which adam can support current risk loads withoutfailure. Risk loads could for instance bewater levels higher than design floodlevels as a result of estimated maximumflood or obstruction of spillway.

Assessment of the water level at which thedam will collapse constitutes part of theemergency planning.

A risk evaluation of dams will alsocomprise an assessment of the failurelimit (ultimate state) for dams.

Analyses of overtopping should be undertakenfor all embankment dams in order to determinethe water level the dam can support withoutfailure.

Ageing of Concrete Dams

The concept 'ageing of dams' indicates thatdams undergo changes over the years. The damstructure and the foundations will be exposedto forces of disintegration, which also can

27

cause changes in loads. The disintegrationprocess can be slow or rapid depending on thecharacter of the concrete and the foundationsand the impacts these are exposed to.

Age in itself gives no indication of a dam'scondition and safety. This can only bedetermined through assessment of:

The dam's condition and safety when itcame into use.

The forces of disintegration to which thedam has been exposed.

The speed of these processes and how farthey have gone.

The importance of the forces of disintegration for the functioning andsafety of the dam.

Discovering hazardous ageing processes.

It is essential that the dam-owner discovershazardous development so as to be able toinitiate measures to combat hazardous ageingprocesses.

This has to be done through an inspectionprogramme set up by the dam-owner. Frequentlythis programme will be restricted to visualinspection. Closer examinations will only beundertaken when visual inspections lead one tosuspect that hazardous disintegration forcesare at work.

It is recommended that more systematicinvestigation into conditions in concrete damsbe introduced. Neither must those elements ofthe structure lying submerged for years beneglected. Underwater inspection by divers orvideo-recorder should be elements in a totalinspection programme.

In order to obtain a comprehensive overview,dam owners as a group should monitordevelopments in a selected sample of Norwegianconcrete dams. They could do so in co-operation with the Water System ManagementAssociation (VR) and in collaboration withNVE-T, through a systematic, permanentinvestigation programme.

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Landslides and Reservoirs

The potential for slides into reservoirs or inthe river basin behind will affect dam safety.This can be because of the destruction to thedam that waves from a slide can cause, or bymalfunctioning of spillways caused by slidematerial or debris dumped into the riversystem.

Loen and Tafjord are Norwegian examples oflandslides which caused large waves. Theseslides could have caused dam failure with evengreater destruction of life and property ifthey had slid into dammed reservoirs.

The slide of an entire mountainside into thereservoir of the Vajont dam in Italy isparticularly well known. The dam withstood an80 metres high wave over the crest, but some2600 lives were nonetheless lost downstream.

The dam owner must assess the potential riskfor slides while NVE-T (possibly incooperation with dam owner's experts) drawsthe final conclusion on to what extent the dammust be able to withstand slides into thereservoir. Slides into the reservoir have tobe considered as an exceptional load, so thatdams where the consequences of failure arevery great should be designed to sustain suchextensive loads.

rosjekt damsikkerhet Dam SafetyReport

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PublishedReports

Prosjektdamsikkerhet

NVE op (111.-,1

.ffindniengulowlene•for.."

Norwegian Water and Energy Administration Water System Management Association

P.O. Box 5091, Maj P.O. Box 145

N-0301 Oslo Norway N-1371 Asker Norway

SIntvt NHt.

Tilstopping av flomlopAldring og sikkerhet av

betongdammer

Sikkerhet av fyllingsdammer mot lekkasjebrudd

Nor,s ..oteskniske InsMou

Beredskapspianlegging for

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Rapport nr. 5

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Rapport nr. 6 ,

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Prosjekt damsikkerhetRapport nr. 7

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P'jekt damsikkerhetRapport nr. 1

Prosjekt damsikkerhetRapport nr. 2

Prosjekt damsikkerhetRapport nr. 3

Prosjekt damsikkerhetRapport nr. 4

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Funksjonssikkerhet ved flomluker

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Funksjonssikkerhet ved flomluker

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Alkarireaksjoner

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