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 pred,pitable water flowing into the storm is usually estimated

from l2-hour persisting dewpoints in the warm moist air upwind from the storm

center, under the assumption that the air is saturated. The maximum amount

,of precipitable water that could occur at the same time of year as the storm

,idate is determined from the maximum l2-hour persisting dewpoints of record,

: again under the assumption that the air is saturated. A leeway of 15 days

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itowards higher dewpoints is often allowed under the assumption that a storm

Icould have occurred 15 days earlier or later without any appreciable change
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I:in ,its precipitation-producing effectiveness. The precipitable water is

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,I usually computed for a column of saturated air with base at the ground and
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Itop at the 200-millibarleve1.

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The selection of a surface dewpoint representative of the precipi-

'table water flowing into a storm often presents problems since the dewpoint

,must be in the warm moist air inflow at the ground and outside the rainfall

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,area in order to be representative.
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jthe selection of a dewpoint at a considerable distance from the storm center,
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loften 300 or more miles m,ay, It is difficult sometimes, for different
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l,ana1ysts to agree exactly on the same dewpoint. Also, they may not agree on
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I,the same elevation for the base of the moisture column, i. e., the average
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'il)eight of orographic barriers to moisture infle>l.

This requirement generally necessitates

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Plates 11-3 and 11-4 sho>l the maximizing factors used in the USBR

'study for transposing and maximi,ing the June 1964 }lontana storm. The de>l-

'points used in computing the adjustment factors >lere selected for points

roughly, 900 miles southeast from the observed and transposed storm centers,

',>lhich is a longer distance than usual, but the adjustment factors obtained

approximate those usually involved in maximizing the transposing outstanding

istorms. The reason for the remote dewpoint locations was that unsettled

iweather conditions precluded the selection of nearer deW'points.
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An innovation introduced in the USBR study '>TaS the use of two

adjustment factors, one for the rainfall at the higher elevations (1.47) in

ithe western portion of the project basin, and the other for the rainfall at

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IDAMES e MOORE

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