FLOOD04994

• County: Statewide
• Community: na
• Basin: Statewide
• Title: Guidelines on Selection of Acceptable Flood Capacity for Dams
• Date: 3/1/2000
• Prepared By: Australian National Committee on Large Dams
• Floodplain - Doc Type: Educational/Technical/Reference Information

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FOREWORD

In 1994 ANCOLD set up a Working Party to revise the 1986 Guidelines on Design Floods
for Dams in the light of moves to integrate risk assessment into dam safety procedures and
lCOLD developments in assessment of hydrologic safety.

The 1986 guidelines were a major step forward. and the first national guidelines on spillway
practice. The 1981 Workshop on Spillway Design had identified the need to rationalise the'
diversity of design flood practices used throughout Australia. ANCOLD also recognised
floods were a major cause of dam failure throughout the world and many older Australian
dams, often with no recorded design basis, had inadequate spillway capacity.

A number of new concepts were introduced in the 1986 guidelines, the major one being
incremental flood hazard categories for ranking the recommended design flood against
consequences of a dam failure. A pragmatic quick check spillway inadequacy ranking basis
was also introduced for existing dams to help owners identify priority dams for detailed
investigations and remedial works. In retrospect these procedures were a simplistic risk based
approach. At that time Economic Risk Analysis (ERA) was being considered for dam safety,
but the methodology was not well developed. As a result the 1986 guidelines, while referring
to ERA, did not propose ERA for dams where lives would be at risk. The 1986 guidelines
were complemented by the 1987 revision of Australian Rainfall and Runoff, which introduced
a chapter on estimation of extreme floods applicable to dams.

The subsequent publication of ANCOLD Guidelines on Risk Assessment, 1994, provided an
initial basis for integrating risk assessment into dam safety, with a draft design flood
guideline distributed in November 1995. It became apparent th~t procedures for risk-based
dam safety evaluation were still in the developmental phase world wide, and the 1994 risk
guidelines would need review. In addition, a review of extreme flood estimation procedures
was required to provide flood probabilities for application to risk assessment. Also ANCOLD
set up a Committee to prepare a quantitative, expanded basis for hazard categories to replace
the simple basis adopted for the 1986 flood guidelines. These Guidelines on Assessment of the
Consequence of Dam Failure, together with revision of the 1994 risk and 1987 flood
estimation procedures, are an important complement to the Guidelines on Selection of an
Acceptable Flood Capacity for Dams,

The application of risk assessment involves' a major philosophical addition to, and
enhancement of the deterministic standards developed over many years for appropriate safety
of dams. These Guidelines on Selection of an Acceptable Flood Capacity for Dams are issued
to provide more appropriate and consistent guidance within a risk process, for dam safety
evaluation under floods. In applying them, it must be remembered they are guidelines only to
what is considered current acceptable practice and allow owners and practitioners flexibility
to exercise professional judgment in all aspects. Indeed, without the application of such
judgment, the procedures themselves could lead to results that have serious shortcomings.
This is a rapidly developing area with on going research in many related aspects. In view of
this, ANCOLD will review the Guidelines periodically and, to that end, seek comments from
users at every opportunity.

Adrian Williams
Chairman, ANCOLD

ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams