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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DAME5i. 8: (M,OORE
...\)QU

-3-

The Inflow Design Flood (IDF)

With the above policy in mind, we'consider that the IDF should

be near the maximum flood calculated as being probable, that is, the

probable maximum flood (PMF).

We have checked the rationale of the USBR probable maximum

precipitation (PMP) together with their estimates ofPMF. l~e conclude

that the basic approaches are correct; the principal differences 'between

. '. '.' .

our estimate and theirs are, we believe, ,due to the greater amount of

time we have had in reviewing the details. With less time available to

them, the nec6ssary approximations by the USBR have 'been on the con-

servative side. The details of these differences are documented in the

attached report.

In considering the appropriate IDF for the Narrows Dam, all

of the facets involved should be considered in a systems context, i.e.

Pill', PMF, design of the dam and spillway, the risks to life through

failure, and the safety factors inherent in the calculations. Included

in the design aspects is the possibility of use 'of a parapet wall to

serve as part of the freeboard, Each factor needs to be con,sidered in

relation to the others, rather than in a step.-by-step process in which

each val.ue is determined separately. We know of no mathematical pro-'

cedure that will allow an'optimization of all of the factors involved

as tradeoffs between costs -and risks. Instead, we consider that only

judgment can be used to appropriately weigh the factors involved,'

Taking into account these factors with respect to safety of,
the dam,' we conclude:

1)

The dam should be desigried for anIDF having a
volume ,of 1,960,,000, acre-feet (75 percent of
USBR's IDF). This corresponds to a PMP of 94
percent of USBR PMl', as given on Table III-IS
i,n the body nf the report.