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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For regulatory purposes to the 1986 Guidelines, to provide consistency in assessing the potential need
for upgrading, the first assumption was generally considered the most appropriate. In the current risk
assessment context, for estimation of DCF, the second assumption, that the dam is overtopped
without it failing, is the more realistic case and is preferred for extrapolation of the spillway rating
curve.

Notwithstanding the above considerations, it should be noted by definition, the spillway rating curves
will be identical upto the crest level of the dam, and consequently the outflow frequency curves to the
point ofDCF should be coincident. The AEP of the DCF should be the same regardless of the method
adopted for extrapolation of the spillway-rating table beyond this point.

However, in practice, if a sufficient number of points is not used to define the outflow frequency curve
(particularly in the area of the curve proximate to the DCF discharge), the method of extrapolation
adopted will resuh in different values of AEP of the DCF. This is particularly so if the outflow
frequency curve is derived by applying the shape factors directly to outflow floods rather than by
routing a range of inflow floods through the storage, It should be noted that for this reason it is
recommended that outflow frequency curves should be obtained by the routing of inflow floods rather
than by the direct application of shape factors to selected ordinates of the outflow frequency curve.

4, The result should be qualified on the assumptions made for initial reservoir level.

5, The paper, "Improved Estimates of Hydrologic Risk for Dams -Impacts of the New Flood
Guidelines", Hill, Nathan, Weinmann & Green, 1999 ANCOLD Conference on Dams, Proceedings,
pp 83-91, provides information on the various factors in the revised flood estimation procedures.

Fig A1.1 Flood Frequency Cmves

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10" lO,j 10"" 10.1 IO.a

ANNUAL EXCEEDANCE PROBABILITY

ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams