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 8 MOORE

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willing, at least at this stage, to accept even rare 'operation of the

auxiliary spillway unless a service spillway also is provided. One of

their principal reasons is that the service spillway provides a much

more rapid means of reservoir evacuation, to be prepared for subsequent

floods that may occur. The USBR sees this as a matter to be determined

by design judgment.

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He see it in a different light. Having accepted the auxiliary

spillway site as being competent for even a rare flood (perhaps once

in 1000 years), the principle of its competence is established and

acceptance of more frequent operations then would appear to become a

matter of economic optimization of damages vs. risk. In this process,

the frequency of auxiliary spillway use becomes of paramount importance.

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Frequency of Auxiliary Spillway Use

An analysis of the frequency of auxiliary spillway use reveals

some rather surprising results. They start with probabilities of various

reservoir levels being attained within,the joint usa pool (75,000

acre-feet) and within the exclusive flood control pool' (475,000 acre-feet).

We find, for instance,

a) that there is a 50-50 chance, that at anyone time,
the storage within the joint use pool would only
be 20,000 acre-feet.

b) that there is a 4-in-5 chance that only the joint
use pool would be full, 1. e. tha t 1,75,000 acre-feet
of exclusive flood control space would remain un-
f HIed.

n,ost importantly, that there is cnly a 50-50 chance
tha auxiliary spillway would be used once in 150 years,
even in the ab3ence of a service spilLway~

c)

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An economic study we have made, described in Chapter IV, demonstrates

that tvith median probah.ilities of flooding there is no economic justi-

fication of providing extensive protection to prevent damage to the

auxiliary spillway. Only under .:in extTemely ;Ibad leck sequence" of

high floods would there be. <1ny justification for suc.h protection.

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