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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"The delineation of lower and upper limits to PMP is somewhat
analogous to the confidence bands used in statistical work.
It would be nice if a confidence band could be placed about a
PMP estimate in an objective manner, similar to the standard
statistical methods, but this is not possible because PMP is
not estimated by formal statistical methods. This limitation,
however, does not invalidate the concept of a confidence band,
but it means that its limits must be based in considerable
measure on judgment, as is the PMP itself. Factors influencing
such judgment are the same as those for assessing the general
level of PMP."

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The Narrows Damsite Design Storm and PMP

Design storms generally do not equal PMP for all durations and

subareas within a basin. In the Narrows Damsite study, the chief interest

is on volume of streamflow rather than on peak discharge, as will be demon-

strated in Chapter III. Consideration of basin size and temporal distribu-

tion of precipitation in major storms in the basin, or transposable to the

basin, indicated that the critical duration was 24 hours. Hence, the

emphasis was on a design storm that would approach or equal the 24-hour PMP

and would have a temporal rainfall distribution determined by that of the

maximized observed storm that would equal or approximate the 24-hour PMP.

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Storm Transposition

The headwater areas of the South Platte Basin are in a region

whose topography is about as complex as any to be found east of the Conti-

nental Divide. In the absence of any detailed reliable procedures for

evaluating PMP in this region, the transposition of major storms from outside

the project basin to supplement storms that have occurred in the basin is

justified.

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Storm transposition is a generally accepted procedure, and presents

no difficult problems over plains areas. Transposition of mountain storms,

however, requires various adjustments of the transposed isohyetal pattern

because the topography of the storm site and that of the basin to which the

pattern is transposed are never exactly the same. (There is no universally-

accepted procedure for making the required adjustments). Consequently,

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