FLOOD00005

• County: Statewide
• Title: Precipitation Records and Flood-Producing Storms in Cheyenne Wyoming
• Date: 10/23/2003
• Prepared For: Cheyenne
• Floodplain - Doc Type: Educational/Technical/Reference Information

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INTRODUCTION

Background

A massive thunderstorm that inundated downtown Cheyenne (fig. 1) with
up to 7 inches of rain and hail in just over 3 hours (Druse and others,
1986) caused the most devastating flood in Wyoming's history. Twelve
deaths, 70 injuries, and $61.1 million in damage resulted from the flooding.

As a result of the storm and flood, there is an increased awareness of
flood hazards. The City Engineer's Stormwater Advisory Group was formed to
assist with suggestions and planning to help alleviate drainage and flooding
problems. Recommendations of the Advisory Group include (D. S. Benner,
former Cheyenne Assistant City Engineer, written commun., 1986):
1) Designation of the flood plain of the 1985 flood as a no-building
area, and implementation of interim planning and building standards
until a comprehensive flood-prone area assessment can be
performed;
2) Installation of a flood-alert system;
3) Implementation of an urban-hydrology study for development of
design information; and
4) Implementation of a comprehensive master-drainage plan and funding
program.

The U. S. Geological Survey, in cooperation with the City of Cheyenne,
has begun an urban-hydrology study. One phase of this study examines the
precipitation record for Cheyenne and evaluates the storm of August 1, 1985,
in order to provide background information for the master-drainage plan and
urban-hydrology study. The data eventually will be incorporated into an
urban rainfall-runoff model for the Cheyenne area.

Purpose and Scope

This report examines existing precipitation data for Cheyenne and
compares three different data distributions to determine which one best
describes the Cheyenne data. New precipitation-frequency curves for
selected durations are developed for the Cheyenne area on the basis of data
collected through 1986. These curves are compared to similar curves
developed on the basis of data available before 1985 to determine the
influence of the August 1, 1985, storm on predictions of the magnitude of
precipitation for various recurrence intervals and durations. Hyetographs
for nine other flood-producing storms that occurred in the Cheyenne area
between 1904 and 1973 are examined and compared to the hyetograph for the
August 1 storm.

Acknowledgments

The author gratefully acknowledges the assistance of David S. Benner,
former Cheyenne Assistant City Engineer, for making helpful comments on
pattern index calculations; Brenda Brock, National Weather Service, for
assisting in researching old precipitation data; and Felicia A. Reid, U.S.
Geological Survey, for writing several preprocessor computer programs used
in the analysis of the incremental precipitation data.

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