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Estimates of the Magnitude and Frequency of Flood Flows in the Connecticut River in Connecticut

U.S. GEOLOGICAL SURVEY
Open File Report 2005-1369

By Elizabeth A. Ahearn

Abstract

Annual peak-flow data from three U.S. Geological Survey (USGS) gaging stations on the Connecticut River—at Thompsonville, Conn. (USGS station 01184000), at Hartford, Conn. (USGS station 01190070), and near Middletown, Conn. (USGS station 01193000)—were used to estimate flood flows for annual exceedance probabilities of 0.1, 0.04, 0.02, 0.01, and 0.002 (10-, 25-, 50-, 100-, and 500-year recurrence intervals, respectively). Flood flows range from 198,000 cubic feet per second (ft3/s) at Thompsonville to 211,000 ft3/s at Middletown at the 100-year recurrence interval.

Flood-frequency analysis of annual peak flow through September 30, 2004 was performed using the procedures described in the publication "Guidelines for Determining Flood-Flow Frequency," commonly referred to as Bulletin 17B, published by the Interagency Advisory Committee on Water Data in 1982. The 100-year flood estimates at all three stations have large uncertainties, as represented by the 90-percent confidence interval (about 50,000 ft3/s). Long-term records (1929-2004) at the three stations represent relatively constant watershed conditions based on results of statistical trend tests and analysis of frequency curves in hydrologically similar basins. Further study is needed to estimate the effects of flood-plain storage, channel storage, and flow regulation by dams on peak flows in the Connecticut River.

Table of Contents

Abstract

Introduction

Data Used to Determine the Magnitude and Frequency of Flood Flows

Annual Peak-Flow Records

High-Flow Measurements, April 2005

Estimates of the Magnitude and Frequency of Flood Flows in the Connecticut River in Connecticut

Flood-Frequency Analysis for Thompsonville, Hartford, and Middletown

Stage-Frequency Analysis for Hartford

Confidence Limits

Evaluation and Statistical Analysis of the Flood-Flow Estimates

Flood Control in the Connecticut River Basin

Comparison of Frequency Curves in Hydrologically Similar Basins

Trend Analysis

Skew Analysis

Mixed Population Analysis

Summary

References

Appendix 1. Data for U.S. Army Corps of Engineers flood-control dams in the Connecticut River Basin


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