Peak-Flow Frequency Estimates for U.S. Geological Survey Streamflow-Gaging Stations in Connecticut Water-Resources Investigations Report 03-4196 Prepared in cooperation with the CONNECTICUT DEPARTMENT OF TRANSPORTATION and the CONNECTICUT DEPARTMENT OF ENVIRONMENTAL PROTECTION U.S. Department of the Interior U.S. Geological Survey Peak-Flow Frequency Estimates for U.S. Geological Survey Streamflow-Gaging Stations in Connecticut By Elizabeth A. Ahearn U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 03-4196 Prepared in cooperation with the CONNECTICUT DEPARTMENT OF TRANSPORTATION and the CONNECTICUT DEPARTMENT OF ENVIRONMENTAL PROTECTION East Hartford, Connecticut 2003 U.S. DEPARTMENT OF THE INTERIOR GALE A. NORTON, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information write to: Copies of this report can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Information Services 101 Pitkin Street Building 810 East Hartford, CT 06108 Box 25286, Federal Center http://ct.water.usgs.gov Denver, CO 80225-0286 CONTENTS Abstract ............................................................................................................................................................... 1 Introduction ......................................................................................................................................................... 1 Annual Peak-Flow Data ...................................................................................................................................... 2 Peak-Flow Frequency Analysis at Streamflow-Gaging Stations ........................................................................ 2 Assumptions of Frequency Analysis .......................................................................................................... 4 Generalized Skew Coefficient .................................................................................................................... 4 Peak-Flow Magnitude and Frequency of Floods from Log-Pearson Type III Distribution ...................... 5 Flows Altered by Flood-Control Dams and Reservoirs .................................................................... 8 Multiple-Site Station Records ........................................................................................................... 8 Accuracy of Peak-Flow Frequency Estimates ........................................................................................... 8 Mixed-Population Analysis ........................................................................................................................ 9 Maximum Known Peak Flows ............................................................................................................................ 11 Summary and Conclusions .................................................................................................................................. 12 References Cited ................................................................................................................................................. 13 Appendix 1. Description of U.S. Geological Survey Streamflow-Gaging Station Locations, Connecticut ....... 22 Contents iii FIGURES Figure 1. Map showing U.S. Geological Survey streamflow-gaging stations with at least 10 years of annual peak streamflow data, Connecticut .......................................................................................3 Figure 2. Map showing station skews plotted at the centroid of each basin for geographic and topographic patterns, Connecticut .................................................................................................... 6 Figure 3. Graph showing frequency curves for mixed-population analysis, Pomperaug River at Southbury, Connecticut ....................................................................................................................................... 10 Figure 4. Graph showing relation of maximum known peak flow to drainage area for streamflow-gaging stations in Connecticut....................................................................................................................... 11 Figure 5. Graph showing relation of discharge for 100-year recurrence interval to drainage area for streamflow-gaging stations in Connecticut........................................................................................ 12 iv Figures TABLES Table 1. Peak-flow frequency estimates for streams in Connecticut for selected recurrence intervals .......... 16 Table 2. Arithmetic mean and standard error of prediction for 36 U.S. Geological Survey streamflow- gaging stations used to develop a generalized station skew for Connecticut ................................... 7 Table 3. Length of record needed to be within 25 percent of the correct peak-flow frequency value 95 and 80 percent of the time ............................................................................................................ 9 Tables v CONVERSION FACTORS CONVERSION FACTORS Multiply By To obtain mile (mi) 1.609 kilometer square mile (mi2) 2.590 square kilometer cubic foot per second (ft3/s) 0.02832 cubic meter per second vi Conversion Factors Peak-Flow Frequency Estimates for U.S. Geological Survey Streamflow-Gaging Stations in Connecticut by Elizabeth A. Ahearn ABSTRACT Connecticut Department of Environmental Protection to provide peak-flow frequency estimates at streamflow-gaging stations. Annual peak-flow data from 128 U.S. Geological Survey streamflow-gaging stations in This report presents the peak-flow frequency Connecticut with at least 10 consecutive years of estimates for 1.5-, 2-, 10-, 25-, 50-, 100-, and 500-year record were used to estimate peak-flow recurrence intervals for selected streamflow-gaging stations in Connecticut. Information about the annual magnitudes for 1.5-, 2-, 10-, 25-, 50-, 100- and peak-flow data and computational procedures for peak- 500-year recurrence intervals (exceedance flow frequency analysis is included in the report. probabilities of 0.67, 0.50, 0.10, 0.04, 0.02, 0.01, Graphs are presented that illustrate the magnitude of and 0.002, respectively). Peak-flow frequency floods by drainage area. analyses of annual peak flows through the 2001 This report updates and supersedes previously water year were performed using the procedures in published peak-flow frequency estimates for the publication “Guidelines for Determining streamflow-gaging stations in Connecticut (Weiss, Flood-Flow Frequency,” commonly referred to as 1983; see also the section “Floods” in the series of Bulletin 17B, by the Interagency Advisory reports, Water Resources Inventory of Connecticut, Committee on Water Data (1982). A generalized Parts 1 to 10) by incorporating 20 additional years of skew coefficient of 0.34, with a standard error of streamflow data and updated methods of peak-flow prediction of 0.51, was developed to improve frequency analysis. The additional data and peak-flow frequency estimates in the state; this development of a generalized skew for Connecticut replaces the generalized skew coefficients for provide more accurate peak-flow frequency estimates Connecticut shown in Bulletin 17B. for streamflow-gaging stations than peak-flow frequency estimates from earlier reports. The author gratefully acknowledges the INTRODUCTION following USGS personnel: Toby Feaster and Andral Caldwell, for their assistance in providing quality control and quality assurance of the annual peak-flow The magnitude and frequency of peak flows are data and in the application of several computer needed to design hydraulic structures, delineate flood- programs used in a review of the peak-flow data; hazard zones, minimize property damage, and reduce William H. Kirby, for his technical help in analyzing the loss of life caused by floods. In an effort to provide and reviewing the mixed-population data; Glenn updated information for regulatory, planning, and Hodgkins, for his assistance in developing the design activities in Connecticut, the U.S. Geological generalized skew; and Claudia Tamayo, for her Survey (USGS) performed a cooperative study with the valuable help in reviewing peak-flow data and in Connecticut Department of Transportation and the compiling results. Abstract 1 ANNUAL PEAK-FLOW DATA records extend back to 1683 (Connecticut River at Hartford). Historical information on the floods used in The USGS streamflow-gaging network provides this analysis, such as the date of occurrence and height valuable information on high flows and historical of the water surface, can be obtained from USGS files floods that is important to the understanding of available at the District Office in East Hartford, hydrologic events. The basis for characterizing the Connecticut, and in Thomson and others (1964). frequency of floods is the annual peak-flow data. The A comprehensive review of the NWIS peak-flow annual peak flow is defined as the maximum database was an essential element of the study. Several instantaneous flow occurring in a water year (October 1 computer programs (C.L. Sanders, Jr., U.S. Geological to September 30). Annual peak flows range from flows Survey, written commun., 1999) were used to perform that barely exceed the natural stream banks and have the QA/QC review of the peak-flow data. The QA/QC minimal effects to flows that inundate