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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DAMES e. MOORE

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

auxi.,liary spillway unless a service spillway also is provided, One of

their principal reasons is that the service spillway provi.des 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.

We 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.

frequency of Auxiliary Spillway Use

An analysis of the frequency of auxiliary spillway use reveals

some rather surprising results.' They startw,ith probabili.ties of various'

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

acre-feet) and wi, thin the exclusive flood control pool (475,000 acre-feet).

We fi.n.d, for instance,

a) that there is a 50-50 chance, that at anyone time,
the storage within the joi,llt use pool \'lOuld ouly
be 20,000 acre-feet.

b) t.hat there is a 1,-'in--5 chance that only the joint
use pool would be full, i.e. that 475,000 acre-feet
of exclusive flood control space would remai.n un-
fill ed.

c)

most importantly, that there is only a 50-50 chance
the auxiliary spillway would be used OnC'" in 150 years,
even In the absence of a service spilhlaY.

An economic study we -have made, des'cribed. in Chapter. IV,' demonstra tes

that with median probabilities of, flooding ,there is no, economic just,i--,

ficatlon of provrdi.ng extensive protection t.o preVent damage to the

auxiliary spil.L\-lay. Only .ll.nd.er an extremely tlbad luck sequence." of

h1.gh floods would there be any j'ustifica-tion for such .prote.ction.

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