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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2907

II-6

between the rainfall patterns of these storms leave little doubt as to the
transposability of the Montana storm. Plate 11-1 shows the isohyetal pattern'
for the June 1964 Montana storm in place, while Plate 11-2 shows the May 1969
Colorado transposed storm pattern. The latter pattern was revised slightly
for differences in orientation of the Montana mountains and that of the
Colorado mountains to which it was transposed.

In the placement of the transposed pattern, as shown in Plate II-I,

an attempt was made to locate the rainfall centers on slopes approximately

those on which they were located in Montana. The degree of success achieved

in this attempt is indicated by comparison of the slopes roughly estimated as

the differences between the highest elevation in each storm center and the

highest elevation 25 miles upwind, i.e., to the east. These differences in

elevation are compared in Table II-I. The differences agree reasonably well

6/
except for Center D, and the Chatfield report- suggests that the location of

the storm rainfall center in that part of the project basin is not necessarily

fixed by the orography.

It should be noted that the June 1964 storm rainfall pattern trans-

posed was one drawn by the USBR. There are several quasi-official versions

of the geographic distribution of the rainfall in this storm. The variations

result from differences in judgment required in the interpolation of rainfall

amounts between gages. While differences in the various isohyetal patterns

could result in appreciable errors over certain small areas, differences in

average rainfall over areas as large as 10,000 square miles would be relatively

small if the maps were constructed by experienced analysts.

Assessment of the Transposition i

The only adjustment made by the USBR up to this point in the transpd-

sition of the Montana storm pattern was,to "bend" the original pattern betwee1

6/ U,S. Weather Bureau, "Probable Maximum Precipitation over South Platte I
- ,
River, Colorado, and Minnesota River, Minnesota", Hydromet Report 44, I
1969,

DAMES e. MOORE