Dam Hazard Category

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IntroductionPart 1 is a guide for assessing the hazard category of

dams in the Auckland Region. It applies to both

existing and proposed structures. The rating system

described is based on the New Zealand Society of

Large Dams (NZSOLD) Dam Safety Guidelines 1995.

Some additional categories have been added to the

NZSOLD Hazard Category system to take into

account the environmental, cultural and historical

issues associated with dam hazards. This system

does not replace the NZSOLD categories, - they are

additions only.

1.0 Definitions

Two terms are usually associated with structures or

natural phenomena that have the ability to cause

damage or threaten life. These are hazard and risk.

The hazard potential of a structure can be defined as

the potential for that structure to cause damage in

the event of failure. For a dam this mainly refers tothe potential damage from uncontrolled release of

water.

The risk associated with a structure is the extent to

which the hazard can be realised. This is often

thought of as the probability of an event occurring.

The extent to which society will accept risk is often

termed the tolerable level of risk. This level is

determined by good practice and societys

acceptance of risk.

Another term often used in association with hazard

and risk is vulnerability. This describes the ability or

inability of the area or people that would be affected

by the hazard to withstand the event. In the case of

dams this describes the ability of a community or

region downstream of the dam to withstand a dam

break flood. Included in this is the state of readiness

of the community, or its ability to prepare for

response.

The hazard category of a dam (or similar structure) as

defined in this guideline is based mainly on its

hazard potential.

2.0 Why apply a hazard category?A hazard category needs to be applied to dams in

order to ensure that appropriate levels of

investigation, design, construction control,

maintenance and operation are undertaken by the

owner and his or her representatives.

Higher hazard dams need a higher level of

investigation, design input and optimisation,

construction testing monitoring, and ongoing

performance monitoring. The application of an

appropriate hazard category helps councils ensure

enough effort is put into these components of dam

building and operation.

A dams hazard category determines the frequency

and magnitude of ongoing internal and external

performance reviews. It may also affect bonds and

legal responsibilities.

3.0 Hazard vs risk

Hazard and risk are interrelated in dam engineering,as with most contexts. A higher hazard structure

dictates a greater level of design input in order to

reduce the level of risk to the community, as shown

in Figure 1.1.

The hazard potential of a dam can change with the

passage of time, usually in response to land use

changes downstream or upstream. Dams and

Figure 1.1: Hazard vs Risk

dam safety guidelinesPart 1: Assessment of dam hazard category


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dam safety guidelinesGuidelines for construction, maintenance and monitoring

2Part 1: Assessing dam hazard category

procedures may need to be modified in order tomaintain a tolerable level of risk in relation to hazard

potential.

Training and Civil Defence strategies can reduce the

vulnerability of a community, but these are not always

seen as effective. Such approaches may not always

provide a permanent solution, so effective

engineering solutions such as strengthening are

often preferred.

4.0 The hazard categoriesMany factors influence a dams hazard potential.

Most reflect the physical attributes of the structure

and its storage volume, but others are less tangible.

The main factors of interest are:

dam height

water depth

stored volume

downstream valley profile

physical location

downstream conditions and values

nature of stored material.

1 Dam height

The higher the dam, the more potential

energy contained in the stored liquid, the

faster it will escape and the more damage it

will do.

2 Water depth

This has similar implications to dam height

and in most cases the two are

interdependent. In this document they are

considered separately for two reasons:

a to maintain consistency with the

Building Act 1991

a occasionally dam height and stored

depth are significantly different.

3 Stored volume

The volume stored behind a dam

determines the duration of a dam break

flood and the extent of associated damage.

Stored volume and dam height together

make the largest physical contribution to a

dams hazard category. This concept is

schematically shown in Figure 1.2.

This guideline uses the classes of storage

volume set out in the NZSOLD Guideline,

except for the 20,00Om3 lowest cut-off

limit. This is defined in the Building Act1991, above which the dam requires a

Building Consent.

4 Downstream valley profile

The shape and gradient of the valley below

the dam influences the rate at which flood

water dissipates. Narrow and/or steep

valley systems tend to sustain high flow

energies, conserving the damage potential

of the flow for longer and confining the

extent of flooding. Wide and/or flat valleysystems have the opposite effect.

5 Physical location

Every locality has characteristics that

separately or together are unique.

Hydrological, geological and topographical

controls influence both risk and hazard

category.

6 Downstream conditions and values

The presence of communities, amenities or

industries downstream must beconsidered, as well as the sensitivity and

value of ecological communities and

cultural and historical sites.Figure 1.2: Dam Height and Storage


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dam safety guidelinesGuidelines for construction, maintenance and monitoring

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7 Nature of stored material

Not all dams store water. Dams are also

used to store many other substances such

as mine waste, effluent, toxic materials.

Many of these will represent a greater

hazard than water alone and may have

unique storage requirements.

To avoid a very cumbersome system with many

levels of hazard, the NZSOLD guideline utilise three

levels of hazard category:- high, significant and low.

This guideline uses the same categories, with thelow category subdivided into minimal and low:

high

significant

low

minimal

The minimal category comprises dams which do not

require a building consent under the Building Act

1991. This is equivalent to the previous ARC criteria

for dams considered to be a Permitted Activity.

A third classification that may influence the definition

of a minimal hazard dam isgiven in the Ministry ofCommerce discussion paper (May 1996). In this

document values of 5.0m for dam height and 20,000

m3 to 50,000 m3 are used as the division between

dams not requiring and those requiring control

through permits or consents. The Auckland Regional

Council has adopted the more rigorous standard

outlined in the NZSOLD guideline.

Minimal impact dams are permitted activities

provided their design, construction, operation,maintenance and monitoring complies with Part 2 of

this guideline.

All other hazard category dams require resource

consents.

5.0 Determining hazard categoryTo simplify the determination of hazard categories for

dams, the folllowing flow chart has been developed

which defines the main criteria. Explanatory notes for

each criterion are also appended to the chart. The

first half of the flow chart is defined by the NZSOLD

Hazard Category, and is therefore based on

parameters that produce the hazard potential of the

structure. The second section includes parameters

that will often incorporate a component of risk in

addition to hazard. This is required to provide the

necessary inputs into the Auckland Regional Councils

more detailed hazard category.

The less tangible categories shown, particularly

geological, hydrological, historical, ecological or

archeological issues, may not necessarily result in a

higher hazard category. Lack of information may

mean that a higher rating needs to be selected

initially, though additional investigations reduce this.

Figure 1.3 is a worked example of selecting a hazard

category. In this example, possible geologicalconcerns are raised, so a higher hazard category is

selected. Additional investigations and design input

may alleviate these concerns for both existing and

proposed dams.

6.0 Summary

The system described in this guideline details the

methodology of and reasons for applying a hazard

category to existing and proposed dams. The system

has been developed within the framework of the

NZSOLD Dam Safety Guidelines 1995 and the

Building Act 1991.

The simple flow chart attached helps determine an

appropriate hazard category for a dam or dams. It will

also identify areas of concern for further

investigation.

The implications of each hazard category for the dam

owner, his or her representatives and regulatory

authorities is discussed in Part 2.


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Figure1.3

:Exampleof

EvaluatingaHazardCategory


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Figure1.4

:EvaluatingaHazardCategory

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Dam Hazard Category