USGS

Water Resources of Colorado

Analysis of the Magnitude and Frequency of Floods in Colorado

by J.E. Vaill

Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, Denver, CO 80225, USGS Water-Resources Investigations Report 99-4190, 35 p., 3 figs.

This document also is available in pdf format:Adobe Acrobat Icon WRIR 99-4190 (version 1, 364KB). Corrections have been made to this report subsequent to its distribution. The most current version of the report is version 3. (Requires Adobe Acrobat Reader)

Version 2 (379KB)
Errata sheet for version 2

Version 3 (384KB)
Errata sheet for version 3

Abstract

Regionalized flood-frequency relations need to be updated on a regular basis (about every 10 years). The latest study on regionalized flood-frequency equations for Colorado used data collected through water year 1981. A study was begun in 1994 by the U.S. Geological Survey, in cooperation with the Colorado Department of Transportation and the Bureau of Land Management, to include streamflow data collected since water year 1981 in the regionalized flood-frequency relations for Colorado. Longer periods of streamflow data and improved statistical analysis methods were used to define regression relations for estimating peak discharges having recurrence intervals of 2, 5, 10, 25, 50, 100, 200, and 500 years for unregulated streams in Colorado. The regression relations can be applied to sites of interest on gaged and ungaged streams. Ordinary least-squares regression was used to determine the best explanatory basin or climatic characteristic variables for each peak-discharge characteristic, and generalized least-squares regression was used to determine the best regression relation. Drainage-basin area, mean annual precipitation, and mean basin slope were determined to be statistically significant explanatory variables in the regression relations. Separate regression relations were developed for each of five distinct hydrologic regions in the State. The mean standard errors of estimate and average standard error of prediction associated with the regression relations generally ranged from 40 to 80 percent, except for one hydrologic region where the errors ranged from about 200 to 300 percent. Methods are presented for determining the magnitude of peak discharges for sites located at gaging stations, for sites located near gaging stations on the same stream when the ratio of drainage-basin areas is between about 0.5 and 1.5, and for sites where the drainage basin crosses a flood-region boundary or a State boundary. Methods are presented for determining the magnitude of peak discharges for sites located at gaging stations, for sites located near gaging stations on the same stream when the ratio of drainage-basin areas is between about 0.5 and 1.5, and for sites where the drainage basin crosses a flood-region boundary or a State boundary.


Table of Contents

Abstract

Introduction

Purpose and Scope

Previous Studies

Approach

Flood-Frequency Analysis at Streamflow-Gaging Stations

Regional Flood-Frequency Analysis

Regression Analysis

Multiple Regression and Drainage-Basin Characteristics

Generalized Least-Squares Regression

Limitations and Accuracy of Regression Equations

Estimating Magnitude of Peak Discharges

Gaged Sites

Sites near Gaging Stations on the Same Stream

Ungaged Sites

Summary

References Cited

Supplemental Data

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