Dam Safety Guidelines Brief Overview - MD

8/6/2019 Dam Safety Guidelines Brief Overview - MD

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Canadian Dam Association, Dam Safety Guidelines:

ABrief Overview

April 2010

Mustafa Darwash


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Preface

This purpose of this paper is to give a very brief overview of the major points presented

in the CDAs Dam Safety Guidelines report. This paper will attempt to shed light on the

risk areas that dam projects entail. This paper is in no way to be taken as a complete

reference for dam safety guidelines and does not act to substitute the original document

by the CDA.


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Dam Safety Management

Many factors can contribute and play a major role in rating a Dam projects risk, whether

a new or existing dam. Hence, it is critical to identify all elements that can influence the

potential consequences of undesirable events.

Potential Consequences

When referring to potential consequences or damages it is important to classify the type or

category ofthese consequences that may arise as a result of an undesirable event, such as a

dam failure or breach;

y Life Safety/loss

y Economic Impacts

y Environmental Impacts (must be evaluated in terms of whether restoration of env. is

feasible and long it would take)

y Cultural loss

y Etc.

Furthermore, it is also important to evaluate each of the latter based on magnitude and duration

of impact. Factors that can affect severityinclude:

y Flow rate

y

Depthy Topography

y Transportation routes

y Mobility of the population

y Site access

y Advance warning

y Dam Failure Inundation Zone (DFIZ)

y Population At Risk (PAR): all persons directly exposed to floodwaters within the DFIZ.

I.e. For topography:

- Flat terrain: failure could inundate a huge are and large PAR, but would be slow-moving

and shallow. Hence, Potential Loss of Life (PLOL) would be minimal.

- Steep mountainous: PLOL is greater even though PAR is much less


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Dam Components

To properly evaluate a dam project one must also identify and evaluate all of its major

components individually. These components include:

y Dam

y Spillway

y Foundation

y Abutments

y Reservoir

y Tailraces

y Etc.

Dam Function

A dams function is vital information that must be known prior to rating a dam project

because it can play a great role in identifying potential consequences of any undesirable

event and also help in dam classification;

y Water storage

y Mining tailings impoundment

y Power generation

y Flood control

y Recreation

y Navigation

y Etc.

One of the factors that will help define the risk level of any dam project is the retained

substance by Dam:

y Water

y Water with any other substance

y Fluid waste


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y Fluid tailings

y Contaminants

y Etc.

The retained substance will determine the potential consequences of a seepage, leaks

or dam failure based on whether there will be release of fluids that can harm the public

or the environment.

It is important to identify the potential failure modes (See Table 1 in appendix) of a dam

and assess the consequences by estimating the resulting discharge.T

he resulting

discharge can help quantify the consequences of both downstream and upstream

damage, release of contaminants, cascades effects, etc. Hence, it is important to know

the discharge hydrograph (flow rate), flood wave routing (rout flood waves to an area

where the effects would be negligible) and inundation map (flood area).

Dam Classification [see Table 1-1 in appendix]

Dam classification should be based of magnitude, or severity, of consequences of

failure:

y Loss Life

y Injury

y Disruption of life

y Property damage

y Contamination

y Etc.


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Dam Breach

A dam breach shape and timing can depend on the type of dam:

1. Concrete gravity dam: tend to have partial breach, monolith sections formed during

construction are forced apart. Also, Concrete dams with abutments are resistant to

erosion and can withstand overtopping without serious damge.

2. Embankment dam: do not tend to have complete or sudden failure. Breach will erode a

portion of dam slowly. Also, embankments are more likely to fail in case of overtopping

than for concrete dam.

3. Concrete arch dam: more likely to fail suddenly and completely.

4. Buttress Dam

Design

In order to define design requirements (return periods) the consequences of a dam

failure should be analyzed for a random (i.e. earthquake, mis-operation, or other event)

failure as well as for a flood scenario. Therefore, the maximum flood for which the

structure is to be designed or evaluated, the inflow design flood (IDF) should be

selected on the basis of the potential consequences of failure. The Earthquake Design

Ground Motion (EDGM) should be derived by site-specific seismic hazard assessment

and should be based on the consequences of dam failure. See Table 5-1.

Emergency PreparednessEmergency response/preparedness plan should exist for construction and operations

phases of a dam and especially where significant cofferdams are required. For a new

dam, the final plans should be in place prior to first filling of the reservoir.


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Dam Maintenance

Maintenance of the equipments and systems of a Dam is very important to ensure

operational availability, safe operation and integrity whether for an existing or new dam

project. Maintenance varies according to the type of dam structure.

For concrete structures:

- Regular cleaning of foundation and internal drainage systems

- Maintenance of surface and joint sealing systems

- Sealing of cracks.

Hence, a concrete related risk that should be emphasized is loss of structural integrity

due to pattern cracking. Alkali Aggregate Reaction must be taken into consideration and

diagnosed for as it can cause concrete to lose strength.Where cold climates exists,

freeze-thawing cycle can also become a risk to take into consideration.

For Embankment Structures:

Vegetation removal and mowing; establishment of desirable vegetation cover

Burrowing animal control

Riprap deterioration

Settlement of crest and freeboard

Seepage induced slumping

Drainage system cleaning.

For existing dams some findings which typically necessitate followup include:

Failure or unusual movement, cracking, subsidence, or settlement of any part of dam


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Unusual concrete deterioration or cracking, including development of new cracks or the

lengthening or widening of existing cracks

Piping, boils, slides, settlement of the dam, abutment, dike, or embankment

Significant slides or settlements of materials in areas adjacent to reservoirs

Significant damage to slope protection

Unusual instrumentation readings New seepage or leakage or significant increase in preexisting seepage or leakage

Sinkholes

Significant instances of vandalism or sabotage

Natural disasters, such as floods, earthquakes, or volcanic activity;

Other signs of instability of a dam component where failure might be the expectedoutcome.


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Appendix

Note 1: There is no identifiable population at risk, so there is no possibility for loss of life other than byunforeseeable misadventure.

Note 2: People are only temporarily in the dambreach inundation zone: for example, passing through on

transportation routes, or voluntarily participating in recreational activities.


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Note 3: The population at risk is ordinarily located in the dambreach inundation zone, e.g. as permanent

residents. Three classes (High, Very High, Extreme) are proposed to allow for more detailed estimates ofpotential loss of life to assist decisionmaking if the dam owner is able to do the appropriate level of

analysis.

Note 4: In general, loss of life is a random variable characterized by a probability distribution bounded by

0 and the PAR (i.e. one probability of X fatalities, a different probability of Y fatalities, etc., over therange from 0 to PAR). The shape of this distribution depends almost entirely on assumptions in the

estimation process. In specific cases, a certain value of this random variable could be selected to provide a

singlevalue estimate of loss of life.

Note 5: The appropriate level of safety required at a dam where people are temporarily at risk depends on

the number of people, the exposure time, nature of activity and other conditions. The requirements couldcorrespond to a higher class. However, the design flood requirement, for example, might not be higher if

the temporary population is not likely to be present during the flood season.

Notes:

1. As defined in Table 11, Suggested Dam Classification Scheme.2. Depends on the number of people exposed, length of exposure, seasonal nature of flooding and

consequences, and other considerations.

3. For purposes of probabilistic risk analysis, the annual exceedance probability (AEP) of that flood maybe assumed as 104.


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4. Extrapolation of flood statistics beyond 1/1000 year flood (103 AEP) is discouraged. The flood

defined as 1/3 between 1/1000year and PMF should be used instead of any previously estimated1/10,000 year flood.

5. For purposes of probabilistic risk analysis, the AEP of that flood may be assumed as 105.

6. For purposes of probabilistic risk analysis, the AEP of the PMF may be assumed as 106.

7. AEP levels are to be used for the mean rather than the median estimates of the hazard.


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Dam Safety Guidelines Brief Overview - MD