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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Appendix 1: Estimation of Dam Crest Flood (DCF) and associated
Annual Exceedance Probability (AEP)
Refer sub-section 4.3.3.

The AEP of the outflow discharge corresponding to the specified still water reservoir level (sub-section
43.1.) is determined by first deriving an outflow frequency curve. The steps involved in deriving the
outflow frequency CUIVe are:

1. Derive inflow floods for a range of AEP's, (say: I in 50, I in 100, I in 1000, I in 10 000, I in 100
000, I in I 000000), and a range of durations.

2. Route the inflow floods for all selected AEP's and durations through the storage assuming either a
single initial reservoir level (such as FSL), or a range of levels selected through a joint probability
analysis of inflows and storage contents. Adopt a sufficient range of durations to identifY the
maximum outflow discharge at each selected AEP; note that the duration of the storm that produces
the maximum outflow is likely to vary with AEP.

3. Derive an outflow frequency curve using the envelope of the critical outflow peak discharges for each
of the selected AEP's,

4. Determine the AEP of the DCF by simply reading off from the outflow frequency CUIVe the AEP
which corresponds to the discharge which occurs at the specified still water reservoir level.

Example.

Figure ALl below demonstrates the method described above. The inflow and outflow frequency curves for
an existing embankment dam have been derived. The outflow discharge corresponding to the reservoir level
at the lowest point of the embankment has been determined from the spillway rating curve as the discharge

corresponding to DCF, in this example 15000 m3ts. The AEP of the DCF can then be read off the outflow

frequency curve as being 8 x 10-6 or I in 125000.

Comments

The following comments are provided for consideration when undertaking an assessment of the AEP of the
DCF.

I. There are a range of possible approaches that can be adopted for deriving flood frequency curves. It is
recommended that when undertaking spillway adequacy assessments for a portfolio of dams, the same
procedure be adopted for all dams to provide consistent results.

2. When routing extreme floods through a storage it is usually necessary to extrapolate the spillway
rating curve above the crest of the dam. In the past, for assessment of the Imminent Failure Flood
(IFF) for the 1986 Guidelines IFFIPMF preliminary spillway adequacy check, it has been common
practice to extrapolate the curve using one of two assumptions:

. The first assumes that of an infinitely high bank where all the discharge is contained within the
confines of the spillway.
. The second assumes that overtopping of the dam occurs without it failing once flow reaches the crest
of the dam.

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