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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2~08

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centers Band C so that the northern and southern portions of the pattern

would better fit the general orientation of the mountains in the project

basin. This modification of the pattern was justified.

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The isohyetal map for the May 4 to 8, 1969 storm (Plate II-2)

suggests that Center A of the transposed Montana storm pattern could have

been placed over the Buckhorn Mountains about 15 miles to the east. However,

this would have necessitated steepening the isohyetal gradient in the eastern

portion of the center, since much more rainfall would then be indicated for

the Fort Collins area than could reasonably be expected from this particular

type of storm. The isohyetal pattern for the April, 14 to 16, 1921 storm,

which was included in the USBR study, indicated that Center A could occur

where shown on Plate II-l. Since any detailed changes or distortions in the

transposed pattern could not at this time be based on factual information,

the isohyetal pattern as transposed in the USBR study is acceptable for the

purpose intended, i.e., a prospective design storm for the South Platte Basin

above Narrows Damsite.

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The maximum depth-area-duration (D-A-D) values for the 1964

Montana storm, for durations up to about 48 hours over the various subbasins

of the project basin, are shown in the upper part of Plates II-3 and II-4.

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Moisture Maximization of the Transposed Storm

The standard procedure for maximizing storm rainfall is based on the

assumption that an observed storm would have produced more precipitation if

the atmospheric moisture drawn into the storm had been greater than observed

and that the increase in precipitation would be proportional to the increase

in atmospheric moisture, Storms, of course, are not all equally effective

in converting atmospheric moisture (precipitable water) into precipitation.

However, there is yet no reliable method for evaluating the maximum precipi-

tation-producing effectiveness of storms, and it is generally assumed that

outstanding storm rainfalls were produced by storms of near-maximum efficiency,

Consequently, the maximizing procedure usually consists solely of moisture

maximization.

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