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Scientific Investigations Report 2009–5167

Prepared in cooperation with the New Jersey Department of Environmental Protection and the U.S. Army Corps of Engineers

Methodology for Estimation of Flood Magnitude and Frequency for New Jersey Streams

By Kara M. Watson and Robert D. Schopp

ABSTRACT

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Methodologies were developed for estimating flood magnitudes at the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year recurrence intervals for unregulated or slightly regulated streams in New Jersey. Regression equations that incorporate basin characteristics were developed to estimate flood magnitude and frequency for streams throughout the State by use of a generalized least squares regression analysis. Relations between flood-frequency estimates based on streamflow-gaging-station discharge and basin characteristics were determined by multiple regression analysis, and weighted by effective years of record. The State was divided into five hydrologically similar regions to refine the regression equations. The regression analysis indicated that flood discharge, as determined by the streamflow-gaging-station annual peak flows, is related to the drainage area, main channel slope, percentage of lake and wetland areas in the basin, population density, and the flood-frequency region, at the 95-percent confidence level. The standard errors of estimate for the various recurrence-interval floods ranged from 49 to 63.9 percent.

Annual-maximum peak flows observed at streamflow-gaging stations through water year 2007 and basin characteristics determined using geographic information system techniques for 254 streamflow-gaging stations were used for the regression analysis. Drainage areas of the streamflow-gaging stations range from 0.18 to 779 mi2. Peak-flow data and basin characteristics for 191 streamflow-gaging stations located in New Jersey were used, along with peak-flow data for stations located in adjoining States, including 25 stations in Pennsylvania, 17 stations in New York, 16 stations in Delaware, and 5 stations in Maryland. Streamflow records for selected stations outside of New Jersey were included in the present study because hydrologic, physiographic, and geologic boundaries commonly extend beyond political boundaries.

The StreamStats web application was developed cooperatively by the U.S. Geological Survey and the Environmental Systems Research Institute, Inc., and was designed for national implementation. This web application has been recently implemented for use in New Jersey. This program used in conjunction with a geographic information system provides the computation of values for selected basin characteristics, estimates of flood magnitudes and frequencies, and statistics for stream locations in New Jersey chosen by the user, whether the site is gaged or ungaged.

Revised, May 2010

First posted September 18, 2009

For additional information contact:
Director,
New Jersey Water Science Center
U.S. Geological Survey
810 Bear Tavern Road, Suite 206
West Trenton, NJ 08628

http://nj.usgs.gov/

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Suggested citation:

Watson, K.M., and Schopp, R.D., 2009, Methodology for estimation of flood magnitude and frequency for New Jersey streams: U.S. Geological Survey Scientific Investigations Report 2009–5167, 51 p.



Contents

Abstract

Introduction

Methods of Study

Development of Flood-Frequency Regression Equations

Regression Analysis and Resultant Equations

Flood Frequency and Magnitude at Gaged Sites

Estimation of Flood Frequency and Magnitude at Ungaged Sites

Estimation of Flood Frequency and Magnitude for an Ungaged Site Upstream or Downstream from a Gaged Site

Estimation of Flood Frequency and Magnitude for a Site Between Gaged Sites

StreamStats

Summary

Acknowledgments

References Cited

Appendix 1. Results of skew analysis for 119 streamflow-gaging stations in New Jersey and surrounding States

Appendix 2. Flood-frequency estimates computed from observed streamflow-gaging-station data, estimates predicted from regression equations, and a weighted average for streamflow-gaging stations



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