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Scientific Investigations Report 2014–5078

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Prepared in cooperation with the Federal Emergency Management Agency

Estimation of Flood Discharges at Selected Annual Exceedance Probabilities for Unregulated, Rural Streams in Vermont

By Scott A. Olson

With a section on Vermont Regional Skew Regression
By Andrea G. Veilleux

Thumbnail of and link to report PDF (11.9 MB)Abstract

This report provides estimates of flood discharges at selected annual exceedance probabilities (AEPs) for streamgages in and adjacent to Vermont and equations for estimating flood discharges at AEPs of 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent (recurrence intervals of 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-years, respectively) for ungaged, unregulated, rural streams in Vermont. The equations were developed using generalized least-squares regression. Flood-frequency and drainage-basin characteristics from 145 streamgages were used in developing the equations. The drainage-basin characteristics used as explanatory variables in the regression equations include drainage area, percentage of wetland area, and the basin-wide mean of the average annual precipitation. The average standard errors of prediction for estimating the flood discharges at the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent AEP with these equations are 34.9, 36.0, 38.7, 42.4, 44.9, 47.3, 50.7, and 55.1 percent, respectively.

Flood discharges at selected AEPs for streamgages were computed by using the Expected Moments Algorithm. To improve estimates of the flood discharges for given exceedance probabilities at streamgages in Vermont, a new generalized skew coefficient was developed. The new generalized skew for the region is a constant, 0.44. The mean square error of the generalized skew coefficient is 0.078. This report describes a technique for using results from the regression equations to adjust an AEP discharge computed from a streamgage record. This report also describes a technique for using a drainage-area adjustment to estimate flood discharge at a selected AEP for an ungaged site upstream or downstream from a streamgage.

The final regression equations and the flood-discharge frequency data used in this study will be available in StreamStats. StreamStats is a World Wide Web application providing automated regression-equation solutions for user-selected sites on streams.

First posted June 19, 2014

  • Appendix 9 XLS (19 KB)
    Streamgages Evaluated for Development of the Generalized Skew Used in Estimating Flood Flow
    Frequency in Vermont and Surrounding Areas

For additional information, contact:
Office Chief, New England Water Science Center
U.S. Geological Survey
New Hampshire-Vermont Office
331 Commerce Way, Suite 2
Pembroke, NH 03275
(603)226-7800
http://nh.water.usgs.gov

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Suggested citation:

Olson, S.A., 2014, Estimation of flood discharges at selected annual exceedance probabilities for unregulated, rural streams in Vermont, with a section on Vermont regional skew regression, by Veilleux, A.G.: U.S. Geological Survey Scientific Investigations Report 2014–5078, 27 p. plus appendixes, http://dx.doi.org/10.3133/sir20145078.

ISSN 2328-0328 (online)


Contents

Abstract

Introduction

Flood Discharges at Selected Annual Exceedance Probabilities for Streamgages

Characteristics of Streamgage Drainage Basins

Regression Equations for Estimation of Flood Discharges at Selected Annual Exceedance Probabilities for Ungaged Stream Sites

Vermont StreamStats

Summary

Vermont Regional Skew Regression

Selected References

Appendix 1. Descriptions of Streamgages in Vermont and Vicinity Used to Develop the Regional Regression Equations

Appendix 2. Summary of Data Used in the Frequency Analysis of Annual Peak-Discharge Data

Appendix 3. Flood Discharges for Selected Annual Exceedance Probabilities for Selected Streamgages in Vermont and Vicinity

Appendix 4. Maximum Recorded Annual Peak Discharge at Streamgages in Vermont and Vicinity Used to Develop the Regression Equations

Appendix 5. Basin Characteristics Tested for Use in the Regression Equations

Appendix 6. Basin Characteristics Used to Develop the Regression Equations

Appendix 7. Example Application

Appendix 8. Variance of Estimates (Vs) at Selected Annual Exceedance Probabilities for Streamgages in Vermont and Vicinity

Appendix 9. Streamgages Evaluated for Development of the Generalized Skew Used in Estimating Flood Flow Frequency in Vermont and Surrounding Areas