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Scientific Investigations Report 2014–5121

Prepared in cooperation with the Flood Control District of Maricopa County

Evaluation of the Magnitude and Frequency of Floods in Urban Watersheds in Phoenix and Tucson, Arizona

By Jeffrey R. Kennedy and Nicholas V. Paretti

Thumbnail of and link to report PDF (6.2 MB)Abstract

Flooding in urban areas routinely causes severe damage to property and often results in loss of life. To investigate the effect of urbanization on the magnitude and frequency of flood peaks, a flood frequency analysis was carried out using data from urbanized streamgaging stations in Phoenix and Tucson, Arizona. Flood peaks at each station were predicted using the log-Pearson Type III distribution, fitted using the expected moments algorithm and the multiple Grubbs-Beck low outlier test. The station estimates were then compared to flood peaks estimated by rural-regression equations for Arizona, and to flood peaks adjusted for urbanization using a previously developed procedure for adjusting U.S. Geological Survey rural regression peak discharges in an urban setting. Only smaller, more common flood peaks at the 50-, 20-, 10-, and 4-percent annual exceedance probabilities (AEPs) demonstrate any increase in magnitude as a result of urbanization; the 1-, 0.5-, and 0.2-percent AEP flood estimates are predicted without bias by the rural-regression equations. Percent imperviousness was determined not to account for the difference in estimated flood peaks between stations, either when adjusting the rural-regression equations or when deriving urban-regression equations to predict flood peaks directly from basin characteristics. Comparison with urban adjustment equations indicates that flood peaks are systematically overestimated if the rural-regression-estimated flood peaks are adjusted upward to account for urbanization. At nearly every streamgaging station in the analysis, adjusted rural-regression estimates were greater than the estimates derived using station data. One likely reason for the lack of increase in flood peaks with urbanization is the presence of significant stormwater retention and detention structures within the watershed used in the study.

First posted December 9, 2014

This publication is only available online

For additional information, contact:
Director, Arizona Water Science Center
U.S. Geological Survey
520 N. Park Avenue
Tucson, AZ 85719
http://az.water.usgs.gov/

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Suggested citation:

Kennedy, J.R., and Paretti, N.V., 2014, Evaluation of the magnitude and frequency of floods in urban watersheds in Phoenix and Tucson, Arizona: U.S. Geological Survey Scientific Investigations Report 2014–5121, 29 p., http://dx.doi.org/10.3133/sir20145121.

ISSN 2328-0328 (online)



Contents

Abstract

Introduction

Data Development

Flood Frequency Estimates at Streamgaging Stations

Summary and Conclusions

References

Figures (10)

Tables (4)


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