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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2910

II-9

the lower elevations in the eastern portion (1.38). This departure from
standard procedure appears reasonable, and this opinion was expressed on
page 12 of the Hydromet review of the USBR study.

In the Hydromet review, different dewpoints and inflow barrier

heights were used to compute adjustment factors for maximizing and trans-

posing the June 1964 Montana storm. The dewpoints Hydromet selected were

for locations 310 miles east-southeast of the storm centers and for different

elevations. Also, one adjustment factor was used for both the high-elevation

and low-elevation rainfall, The adjustment factor obtained for the time of

storm occurrence, i.e., June 7 to 8, was 1.66, which would increase the

maximized values given at the bottom of Plates 3 and 4 by 13 and 20 percent,

respectively. This adjustment seems high for such an outstanding storm.

(It should be noted that the Hydromet report shows 1.47 as the adjustment

factor, but this was found to be in error and was later corrected to 1.66),

Qualitative Assessment of the PMP Level

A common procedure for qualitatively evaluating the level of a

PMP estimate for a specific basin is to compare the estimate with those for

adjacent or nearby basins, or with values indicated by a generalized PMP

chart for the region of interest. Such a comparison is presented in

Table 11-2, much of which was taken from page 13 of the Hydromet review.

The comparison shows good agreement between the June 1964 storm, as trans-

posed and maximized in the USBR study (line A) and the PMP estimates for the

Upper Arkansas Basin (line F) and the extrapolated PMP estimates for the

Chatfield Project (line G). The generalized estimates of line H are for the

relatively flat regions to the east of the Upper South Platte River drainage,

and are not so comparable.

The above comparison suggests that what the USBR study refers to as

the recommended design storm approximates the general level of PMP for the

area of the project basin, i.e., 9955 square miles. This opinion was

expressed on page'15 of the Hydromet review. However, if the June 1964 storm

values are adjusted by the factor of 1.66 indicated in the Hydromet review,

DAMES & IWOORE