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 results (line D, Table 2) exceed all PMP values mentioned above except

the generalized estimates of line H. They exceed the values of line A by

15 percent. It thus appears that the PMP values shown on lines D and H may

be somewhat higher than the general level of PMP, i.e" in the upper part

of the PMP confidence band, although not necessarily at its upper limit.

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Discussion
The design storm (Table I1-3) recommended in the USBR study is
certainly ample, and there is very little chance that it will ever be
exceeded appreciably. If, however, a somewhat greater risk of exceedance
is acceptable, there is a good basis for a design storm of slightly lower
magnitude. The factors which form this basis are discussed in the following
section.

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Seasonal Transposition Considerations

The June 1964 storm was transposed about 500 miles south-southeast,

a latitudinal displacement of roughly 8 degrees. The low-pressure systems

associated with heavy general rains at high elevations on the eastern slopes

of the Continental Divide and on intervening mountain barriers to moisture

flow from the Gulf of Mexico tend to weaken as the season progresses from

spring to summer. This weakening first begins at the lower latitudes, and

progresses northward as the season progresses.

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This seasonal weakening of storms in the area of interest was recog-i

nized in the Hydromet review, which pointed out that, while the standard pro-

cedure for deriving PMP estimates allows a seasonal transposition of 15 days

toward the warmer season when computing a moisture adjustment, exceptions

would be storms in the transition season from winter to summer. The Hydromet

report states:

"Mechanisms important to spring and early summer storms, which
cannot be quantified, play an important part in precipitation
production in these months. Vigorous storm systems more typical
of winter but extending into spring would be one type of these
mechanisms. Transposition into the warmer season increases the
moisture adjustment and suggests we would be taking the storm
farther away from such mechanisms",

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DAMES C. I'Io1iIOORE