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Scientific Investigations Report 2008-5206

Prepared in cooperation with the
New Hampshire Department of Transportation

Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

By Scott A. Olson

ABSTRACT

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This report provides estimates of flood discharges at selected recurrence intervals for streamgages in and adjacent to New Hampshire and equations for estimating flood discharges at recurrence intervals of 2-, 5-, 10-, 25-, 50-, 100-, and 500-years for ungaged, unregulated, rural streams in New Hampshire. The equations were developed using generalized least-squares regression. Flood-frequency and drainage-basin characteristics from 117 streamgages were used in developing the equations. The drainage-basin characteristics used as explanatory variables in the regression equations include drainage area, mean April precipitation, percentage of wetland area, and main channel slope. The average standard error of prediction for estimating the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year recurrence interval flood discharges with these equations are 30.0, 30.8, 32.0, 34.2, 36.0, 38.1, and 43.4 percent, respectively.

Flood discharges at selected recurrence intervals for selected streamgages were computed following the guidelines in Bulletin 17B of the U.S. Interagency Advisory Committee on Water Data. To determine the flood-discharge exceedence probabilities at streamgages in New Hampshire, a new generalized skew coefficient map covering the State was developed. The standard error of the data on new map is 0.298. To improve estimates of flood discharges at selected recurrence intervals for 20 streamgages with short-term records (10 to 15 years), record extension using the two-station comparison technique was applied. The two-station comparison method uses data from a streamgage with long-term record to adjust the frequency characteristics at a streamgage with a short-term record.

A technique for adjusting a flood-discharge frequency curve computed from a streamgage record with results from the regression equations is described in this report. Also, a technique is described for estimating flood discharge at a selected recurrence interval for an ungaged site upstream or downstream from a streamgage using a drainage-area adjustment.
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.

For additional information contact:
Director
U.S. Geological Survey
New Hampshire-Vermont Water Science Center
361 Commerce Way
Pembroke, NH 03275
(603) 226-7807
http://nh.water.usgs.gov

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Suggested citation:

Olson, S.A., 2009, Estimation of flood discharges at selected recurrence intervals for streams in New Hampshire: U.S. Geological Survey Scientific Investigations Report 2008–5206, 57 p.



Contents

Abstract

Introduction

Purpose and Scope

Description of Study Area

Flood Discharges at Selected Recurrence Intervals for Streamgages

Peak Discharge Data Used in this Study

Determination of the Magnitude and Frequency of Flood Discharges for Streamgages

Generalized Skew

Record Extension

Combined Records of Nearby Streamgages

Magnitude and Frequency of Flood Discharges for Streamgages

Maximum Recorded Floods and Envelope Curves

Characteristics of Streamgage Drainage Basins

Regression Equations for Estimation of Flood Discharges at Selected Recurrence Intervals for Ungaged Stream Sites

Regression Analysis and Final Regression Equations

Limitations and Sensitivity

Accuracy of the Regression Equations

Accuracy Analysis of Individual Estimates from the Regression Equations

Standard Error of Prediction

Equivalent Years of Record

Prediction Intervals

Use of Regression Equations at or Near Streamgages

Drainage-Area-Only Regression Equations

New Hampshire StreamStats

Summary

Selected References

Appendix 1. Mean April Precipitation

Appendix 2. Wetland Areas

Appendix 3. Example Application


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