FLOOD02473

• County: Weld
• County: Logan
• Basin: South Platte
• Title: A Post Evaluation of Rainfall Reports Associated with Pawnee Creek Flood of July 29-30, 1997 in Eastern Weld and Western Logan Countie in Northeastern Colorado
• Date: 8/1/1998
• Prepared For: Weld County/Logan County
• Prepared By: CSU
• Floodplain - Doc Type: Flood Documentation Report

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Introduction and Background

On the night of July 29-30, 1997 exactly one day after the devastating Fort Collins,
Colorado flash flood ofJuly 28, 1997, a similar extreme rainfall event occurred over
portions of eastern Weld and western Logan counties in northeast Colorado, The storm
developed in an extremely moist air mass which spawned other heavy rain events in other
portions of Colorado over a period of several days in late July and early August. The
resulting flood on Pawnee Creek miraculously claimed no human lives but destroyed
numerous roads, bridges and property before inundating farmland and large portions of
the town of Atwood and the city of Sterling, Colorado.

Within a few days of this flood on Pawnee Creek and its tributaries, teams were organized
to gather rainfall data from the affected areas of Weld and Logan counties. The purpose
of this effort was to determine how much rain had fallen to produce such a large and
dramatic flash flood. Team members consisted of officials from the Logan County
Cooperative Extension Office in Sterling, Colorado, the U.S. Army Corps of Engineers
from Omaha, Nebraska, and the Colorado Water Conservation Board in Denver. A total
of at least six individuals participated in this survey. In addition, several other officials
concentrated their efforts on documenting flood damage and peak flows. The results of
these surveys were assembled into a draft report prepared by Tom Browning of the
Colorado Water Conservation Board in October 1997 entitled, "Pawnee Creek, Logan
County Colorado -- Flood ofJuly 1997." This preliminary analysis showed that 12 to 15
inches of rain fell within six to eight hours over an area of several square miles near the
core of the storm. These excessive rainfall totals, if accurate, make this one of only a
handful of extreme storms of this magnitude in the recorded history of Colorado.

Because of the extreme rainfall totals, keen interest in this storm can be expected for many
years to come by both hydrologists, meteorologists and engineers. It could potentially
influence design criteria for dams and spillways and other critical structures in Colorado,
It is very important that this storm be documented thoroughly for scientific, engineering
and historical purposes while quantitative data about the storm are still readily available.
In June 1998, the Colorado Climate Center was approached by the Colorado Water
Conservation Board (Department of Natural Resources) to assist in this documentation
effort. Funds were approved in July 1998 through an existing contract with Riverside
Technology, Inc. in Fort Collins, Colorado for evaluation of 1997 Colorado flooding.

The basis for this post-evaluation were the tabulations of names of residents, locations and
rainfall totals assembled for this sparsely populated grassland area by the original survey
teams and printed in the October 1997 draft report. The goal of this effort was to verify
rainfall reports for the evening of July 29-30, 1997 and evaluate their accuracy and
confidence, seek additional reports and supporting information, produce a map depicting
the storm rainfall pattern, and detennine how much rain may have fallen in the days
immediately preceding the July 29-30, 1997 event,

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