FLOOD08812

• County: Morgan
• Community: Fort Morgan
• Stream Name: Narrows Dam
• Basin: South Platte
• Title: Flood Study Narrows Dam South Platte River
• Date: 6/1/1975
• Prepared For: CWCB
• Prepared By: Dames & Moore
• Floodplain - Doc Type: Floodplain Report/Masterplan

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DAMES B MOORE

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2)

These conclusions involve a significant amount of
conservatism, in keeping wit~ current professional
practice in the design of major dams in populated
areas.

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3)

A parapet wall involves no more risk than the present
scheme and should be investigated for possible cost
reduction.

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The l1agnitude of the IDF

The magnitude of an estimated IDF often raises questions;

therefore, some discussion is needed of the definition and purpose of

an IDF, together with some facts on relationships between floods that

have actually occurred.

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An IDF starts with estimates of a probable maximum flood (Pl1F).

The Pl1F is the estimated flood that would result if all factors which

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could contribute to the generation of a flood were to reach their most

critical values that could occur concurrently. The PMF is thus an

estimate at the boundary between possible and impossible floods. An IDF

is the flood used to design the structure'8.nd may be less than the PI1F,

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depending on the risks involved.

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It is comrrioli "practice for major dams

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in populated areas to use an IDF equal to the PI1F.

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The PHF ts an estimate, and as such is subj ect to the. same

limitations as all estimates: inadequacy of data and inaccurac~es in

procedures. These limitations cause us to think of the p~w as being 3

range of possible. values, rather than a single vci.lue. The range re-

presents the possibilities of error in the estimate and can be thought

of as a "confidenc~ bandll of values. The IDF r..}'e have recommended is

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in the lower part of the confidence band but not as the extreme lowest
end.

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It is a natural t.endency for laymen, as well as for many

individuals who are highly ski1led in other techni"a1 areas but without

experL,nce in floed bydroiogy, to question the magnitude of a PHF.

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