Skip Links

USGS - science for a changing world

Scientific Investigations Report 2012–5130

Prepared in cooperation with the U.S. Army Corps of Engineers

Development of Regional Skews for Selected Flood Durations for the Central Valley Region, California, Based on Data Through Water Year 2008

By Jonathan R. Lamontagne, Jery R. Stedinger, Charles Berenbrock, Andrea G. Veilleux, Justin C. Ferris, and Donna L. Knifong

Abstract

Flood-frequency information is important in the Central Valley region of California because of the high risk of catastrophic flooding. Most traditional flood-frequency studies focus on peak flows, but for the assessment of the adequacy of reservoirs, levees, other flood control structures, sustained flood flow (flood duration) frequency data are needed. This study focuses on rainfall or rain-on-snow floods, rather than the annual maximum, because rain events produce the largest floods in the region. A key to estimating flood-duration frequency is determining the regional skew for such data. Of the 50 sites used in this study to determine regional skew, 28 sites were considered to have little to no significant regulated flows, and for the 22 sites considered significantly regulated, unregulated daily flow data were synthesized by using reservoir storage changes and diversion records. The unregulated, annual maximum rainfall flood flows for selected durations (1-day, 3-day, 7-day, 15-day, and 30-day) for all 50 sites were furnished by the U.S. Army Corps of Engineers. Station skew was determined by using the expected moments algorithm program for fitting the Pearson Type 3 flood-frequency distribution to the logarithms of annual flood-duration data.

Bayesian generalized least squares regression procedures used in earlier studies were modified to address problems caused by large cross correlations among concurrent rainfall floods in California and to address the extensive censoring of low outliers at some sites, by using the new expected moments algorithm for fitting the LP3 distribution to rainfall flood-duration data. To properly account for these problems and to develop suitable regional-skew regression models and regression diagnostics, a combination of ordinary least squares, weighted least squares, and Bayesian generalized least squares regressions were adopted. This new methodology determined that a nonlinear model relating regional skew to mean basin elevation was the best model for each flood duration. The regional-skew values ranged from –0.74 for a flood duration of 1-day and a mean basin elevation less than 2,500 feet to values near 0 for a flood duration of 7-days and a mean basin elevation greater than 4,500 feet. This relation between skew and elevation reflects the interaction of snow and rain, which increases with increased elevation. The regional skews are more accurate, and the mean squared errors are less than in the Interagency Advisory Committee on Water Data’s National skew map of Bulletin 17B.

First posted July 24, 2012

For additional information contact:
Director, California Water Science Center
U.S. Geological Survey
6000 J Street, Placer Hall
Sacramento, California 98519
http://ca.water.usgs.gov

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.


Suggested citation:

Lamontagne, J.R., Stedinger, J.R., Berenbrock, Charles, Veilleux, A.G., Ferris, J.C., and Knifong, D.L., 2012, Development of regional skews for selected flood durations for the Central Valley Region, California, based on data through water year 2008: U.S. Geological Survey Scientific Investigations Report 2012–5130, 60 p.



Contents

Abstract

Introduction

Rainfall Flood Data

Cross-Correlation Model of Concurrent Flood Durations

Flood-Frequency Analysis

Regional Duration-Skew Analysis

Summary

References

Appendix 1. Unregulated Annual Maximum Rain Flood Flows for Selected Durations for all 50 Sites in the Central Valley Region Study Area, California

Appendix 2. Ancillary Tables for Regional-Skew Study in the Central Valley Region of California

Appendix 3. Methodology for Regional-Skew Analysis for Rainfall Floods of Differing Durations


Accessibility FOIA Privacy Policies and Notices

USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2012/5130/
Page Contact Information: Contact USGS
Page Last Modified: Thursday, January 10, 2013, 07:57:41 PM