Prepared in cooperation with Federal Emergency Management Agency Flood Magnitude and Frequency of the Delaware River in New Jersey, New York, and Pennsylvania Open-File Report 2008–1203 U.S. Department of the Interior U.S. Geological Survey Cover. USGS stream-gaging station 01457500, Delaware River at Riegelsville, New Jersey, during the April 4, 2005, flood. Photograph taken by Amy McHugh, USGS. Flood Magnitude and Frequency of the Delaware River in New Jersey, New York, and Pennsylvania By Robert D. Schopp and Gary D. Firda Prepared in cooperation with Federal Emergency Management Agency Open-File Report 2008–1203 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey Mark D. 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Suggested citation: Schopp, R.D., and Firda, G.D., 2008, Flood magnitude and frequency of the Delaware River in New Jersey, New York, and Pennsylvania: U.S. Geological Survey Open-File Report 2008–1203, 7 p. iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Description of Study Area ............................................................................................................................1 Streamflow Data ............................................................................................................................................3 Trend Analysis ................................................................................................................................................3 Flood Magnitude and Frequency Computation .........................................................................................5 Summary..........................................................................................................................................................6 Acknowledgments .........................................................................................................................................6 References Cited............................................................................................................................................7 Figures 1. Map showing location of U.S. Geological Survey streamflow-gaging stations on the Delaware River ..............................................................................................................................2 2. Plot showing annual flood peaks recorded on the Delaware River at Trenton, N.J., water years 1898-2006 .................................................................................................................4 Tables 1. Streamflow-gaging stations on the main stem Delaware River. ..........................................3 2. Change in estimated forest cover for selected locations along the Delaware River, 1894 to 1992. ...................................................................................................................................5 3. Flood magnitude and frequency for streamflow-gaging stations on the main stem Delaware River ..............................................................................................................................6 iv Conversion Factors Multiply By To obtain Length inch (in.) 25.4 millimeter (mm) inch (in.) 0.0254 meter (m) Area square mile (mi2) 259.0 hectare (ha) square mile (mi2) 2.590 square kilometer (km2) Flow rate cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) Flood Magnitude and Frequency of the Delaware River in New Jersey, New York, and Pennsylvania By Robert D. Schopp and Gary D. Firda This report presents the methodology and resulting flood Abstract magnitude and frequency values, including 2-, 5-, 10-, 25-, 50-, 100-, and 500-year recurrence intervals, for eight active From September 2004 to June 2006, the Delaware River streamflow-gaging stations (stations) on the Delaware River in New Jersey, New York, and Pennsylvania experienced and presents the results of the trend analysis. three major floods that caused extensive damage. The Federal The U.S. Army Corps of Engineers (the Corps) and Emergency Management Agency (FEMA) needed updated FEMA flood-insurance studies have been using a set of information on the flood magnitude and frequency for the flood magnitude and frequency values that were initially eight active streamflow-gaging stations along the main stem computed in 1962 (U.S. Army Corps of Engineers, 1962) and Delaware River in New Jersey, New York, and Pennsylvania were last updated in 1984 (U.S. Army Corps of Engineers, that included the three recent floods in order to update its 1984). The effects of reservoir storage were removed from flood insurance studies. Therefore, the U.S. Geological Survey the observed flood peaks using a HEC-1 computer program (USGS) computed updated flood magnitude and frequency (U.S. Army, Corps of Engineers, Hydrologic Engineering values following the guidelines published by the Interagency Center, 1973) with the assumption that the reservoirs were Advisory Committee on Water Data in its Bulletin 17B. The full at the beginning of the floods. Both analyses (1962 and updated flood-frequency values indicate that the recurrence 1984) were developed using a mixed population analysis in interval of the September 2004 flood ranged from 20 to 35 which non-hurricane floods and floods caused by tropical years, the recurrence interval of the April 2005 flood ranged storms and hurricanes are analyzed separately. The results from 40 to 70 years, and the recurrence interval of the June of the two analyses were then combined, and the combined 2006 flood ranged from 70 to greater than 100 years. Exami- results were readjusted from unregulated conditions to the nation of trends in flood discharges indicate no statistically current regulated conditions. The flood-frequency values were significant trends in peak flows during the period of record for then graphically combined with USGS computations using any of the eight streamflow-gaging stations. observed flood-peak discharges. Introduction Description of Study Area During the 22-month period from September 2004 to The study area is the main stem of the Delaware River in June 2006, the Delaware River experienced three major the States of New Jersey, New York, and Pennsylvania. The floods that caused extensive damage. The Federal Emergency Delaware River has its headwaters in New York State. The Management Agency (FEMA) needed updated informa- river has two headwater streams, the East Branch and the West tion on flood magnitude and frequency for the eight active Branch; both rise in the Catskill Mountains. The two branches streamflow-gaging stations along the main stem Delaware join at Hancock, N.Y. From there the river flows downstream River in New Jersey, New York, and Pennsylvania that to Trenton, N.J., which is at the head of tide. The eight active included the three recent floods in order to update its flood- and two inactive streamflow-gaging stations include stations insurance studies for the Delaware. Therefore, the U.S. on the main stem downstream to and including Trenton, N.J. Geological Survey (USGS) computed new flood magnitude Stations farther downstream on the main stem are tidally and frequency values for the eight stations, and examined affected and, therefore, are outside the study area. The trends in flood discharges. Possible reasons for the occurrence drainage area of the station that is farthest downstream is of the three high-magnitude floods in a 22-month period were 6,780 mi2 and contains nine major reservoirs (fig. 1). also examined. 2 Flood Magnitude and Frequency of the Delaware River in New Jersey, New York, and Pennsylvania 76° 75° EXPLANATION *# U.S. Geological Survey streamflow-gaging station Delaware River Basin boundary iver are R law State boundary De h nc a r r e B v i t s R e e W r a w a el New York D Cannonsville anch t Br Reservoir as E Pepacton Reservoir 42° ! Hancock Neversink CatskillReservoir Mountains *#*#Callicoon General Edgar Jadwin Reservoir Lackawaxen Prompton River Reservoir *# Barryville Neversink River Pennsylvania Lake ! Wallenpaupack Port Jervis*# Montague *# Delaware River Francis E. Walter Reservoir *# 41° Delaware Water Gap Lehigh River Beltzville Lake *# Belvidere New Jersey Easton! *# Riegelsville 0 5 10 MILES Lake 0 5 10 KILOMETERS Nockamixon Lambertville *# Delaware River Sch uyl kill Riv er *#! Trenton Figure 1. Location of U.S. Geological Survey streamflow-gaging stations on the Delaware River. Trend Analysis 3 others, 1990). None of the gage records showed a statistically Streamflow Data significant trend at the 95-percent confidence