Flood of April 2007 in New Hampshire By Robert H. Flynn Prepared in cooperation with the Federal Emergency Management Agency Scientific Investigations Report 2008–5120 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. Myers, Director U.S. Geological Survey, Reston, Virginia: 2008 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Flynn, R.H., 2008, Flood of April 2007 in New Hampshire: U.S. Geological Survey Scientific Investigations Report 2008–5120, 53 p. iii Acknowledgments The author would like to thank Stephen Doyon from the New Hampshire Department of Environmental Services (NHDES) for providing peak-stage and peak-discharge data from the streamgages that NHDES operates, and David Knowles from the Federal Emergency Management Agency for providing review comments on this report. The author would like to express appreciation to U.S. Geological Survey staff—Kenneth Toppin, Scott Olson, Chandlee Keirstead, Richard Kiah, Sanborn Ward, Heather Sirotnak and Glenn Berwick— who assisted with collection and analysis of the data in this report; and to Scott Olson and Gene Parker, who provided technical reviews of this report. v Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Description of Study Area ...................................................................................................................3 Storm Characteristics ..........................................................................................................................3 Peak-Stage Data ............................................................................................................................................6 Peak-Discharge Data ..................................................................................................................................17 Flow-Frequency Analyses of the April 2007 Flooding ............................................................................18 Determination of the April 2007 Flood Discharge by Indirect Methods at Ungaged Sites ..............24 Comparison of the April 2007 Flood Data to Flood-Insurance Studies ...............................................26 Summary........................................................................................................................................................30 Selected References ...................................................................................................................................30 Appendix 1. HEC-RAS Model Calibrations to Estimate the April 2007 Flood-Peak Discharge from High-Water-Mark Data Collected for the Lamprey, Souhegan, and Suncook Rivers .........39 Figures 1. Map showing counties included in the Presidential Disaster Declarations of April 27 and May 10, 2007 ............................................................................................................2 2A–3. Screenshots showing— 2A. The 1-day observed precipitation totals ending on April 16, 2007, New Hampshire and Vermont ..........................................................................................................................4 2B. The 1-day observed precipitation totals ending on April 17, 2007, New Hampshire and Vermont ..........................................................................................................................4 2C. The 1-day observed precipitation totals ending on April 18, 2007, New Hampshire and Vermont ..........................................................................................................................5 3. The April 2007 monthly observed precipitation, New Hampshire and Vermont ........5 4A–5. Maps showing— 4A. Location of U.S. Geological Survey streamgage sites and indirect discharge- measurement sites having peak stage and discharge data for the April 2007 flood in Hillsborough County, New Hampshire .............................................................10 4B. Location of U.S. Geological Survey streamgage sites and indirect discharge- measurement sites having peak stage and discharge data for the April 2007 flood in Rockingham County, New Hampshire..............................................................11 4C. Location of U.S. Geological Survey streamgage sites and indirect discharge- measurement sites having peak stage and discharge data for the April 2007 flood in Merrimack County, New Hampshire ................................................................12 4D. Location of U.S. Geological Survey streamgage sites having peak stage and discharge data for the April 2007 flood in Belknap County, New Hampshire ..........13 4E. Location of U.S. Geological Survey streamgage sites having peak stage and discharge data for the April 2007 flood in Strafford County, New Hampshire ........14 4F. Location of U.S. Geological Survey streamgage sites having peak stage and discharge data for the April 2007 flood in Grafton County, New Hampshire ...........15 4G. Location of U.S. Geological Survey streamgage sites having peak stage and discharge data for the April 2007 flood in Carroll County, New Hampshire ............16 vi 5. Recurrence intervals for the April 2007 flood at streamgages in Hillsborough, Rockingham, Merrimack, Belknap, Strafford, Grafton, and Carroll Counties in New Hampshire .............................................................................................................23 Tables 1. Description of streamgages used in the April 2007 flood investigation, central and southern New Hampshire .........................................................................................................33 2. Peak-stage data and peak-discharge data during the April 2007 flood at 57 streamgage sites in central and southern New Hampshire .............................................7 3. Flow-frequency data and recurrence-interval estimates for 57 streamgages during the April 2007 flood in central and southern New Hampshire ................................19 4. Flow-frequency data and recurrence-interval estimates by indirect methods for four ungaged sites during the April 2007 flood in central and southern New Hampshire ......25 5. Flood-insurance study data for selected streamgage sites in central and southern New Hampshire .........................................................................................................27 6. High-water-mark data used in the determination of April 2007 flood discharge by indirect methods for four ungaged sites in central and southern New Hampshire ........29 Conversion Factors and Datum Multiply By To obtain Length inch (in.) 2.54 centimeter (cm) inch (in.) 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) Area square mile (mi2) 2.590 square kilometer (km2) Flow rate cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27). Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29) and North American Vertical Datum of 1988 (NAVD 88). Flood of April 2007 in New Hampshire By Robert H. Flynn Abstract and Strafford in New Hampshire (fig. 1; Federal Emergency Management Agency, 2007). Belknap County in New Hampshire was added to the disaster declaration on During April 16–18, 2007, central and southeastern May 10, 2007 (Federal Emergency Management Agency, New Hampshire experienced severe flooding as a result of 2007). In response to the disaster declaration, the U.S. up to 7 inches of rainfall from a storm that stalled off the Geological Survey (USGS), in cooperation with the Federal New England coast. As a result of the flooding, a Presidential Emergency Management Agency (FEMA), measured or Disaster Declaration was issued on April 27, 2007. On that computed flood characteristics at 57 streamgages and 4 day, disaster declarations were announced for Grafton, ungaged locations within and adjacent to the disaster areas. Hillsborough, Merrimack, Rockingham, and Strafford The April 2007 flood is documented in this report Counties. On May 10, 2007, Belknap County was added to through the presentation of flood data collected at 46 active the disaster declaration. and 11 discontinued streamgages and 4 ungaged locations in Following the flooding, the U.S. Geological Survey, central and southern New Hampshire. The flood data include in a cooperative investigation with the Federal Emergency peak-stage data, peak-discharge data, flow-frequency curves, Management Agency, determined the peak stages, peak and estimates of flood-recurrence intervals at each streamgage.