WSP03196

• File Number: 8443.600
• Description: Narrows Unit - Studies
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 6/30/1975
• Author: Dames and Moore
• Title: Flood Study Narrows Dam South Platte River
• Water Supply Pro - Doc Type: Report/Study

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DAM ES 8- MOORE

-4-

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2887
2)

These conclusion
conservatism, in
practice in the
areas.

involve a significant amount of
keeping with current professional
esign of major dams in populated

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3)

A parapet wall i
scheme and shoul
reduction.

volves no more risk than the present
be investigated for possible cost

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The Magnitude of the IDF

The magnitude of an e timated IDF often raises questions;

therefore, some discussion is n eded of the definition and purpose of

an IDF, together with sonie fact on relationships betl.cen floods that

have actually occurred,

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An IDF starts with es imates of a probable maximum flood (PI1F).
The PMF is the estimated flood hat would result if all factors which

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could contribute to the generat on of a flood were to rea~h their most

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critical values that could occu concurrently. The PHF is thus an

estimate of the 'boundary betwee possible and impossible floods. N.l IDF

, '
is the flood nsed to design the'structure and may be less th'Hl the Pill',

depending on the risks involved

It is common practice for major dams

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in populated areas to use an II)' equal to the PMF.

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The Pr-1F is an estimat" , and as such is subject to the same

limitations as all estimates:

.nadequacy of data arId inacctt:i:'acies in

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procedures. Th'2.se limitations ause us to think or -th~ PH}" as' being a

range of possible values, rathe than a single value. The range re-

presents thepossibi.lities of e ror in the estimate and can he thought

of as a "confidence oandtt of va.ues. The IDF we have recommend.ed is

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in the 10l'er part of the confid nee band but not as the extreme low<est

end.

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It is a natural rende cy for laymen, as '..ell as for ffi'my

,individuals who are highly skil' cd in other technical areBS but wi.thou::

experience in flood hydrology, 0 question tilE,,: Tik'lgnitude of aPNF.

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