Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009
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Prepared in cooperation with the Federal Emergency Management Agency Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009 Scientific Investigations Report 2010–5127 U.S. Department of the Interior U.S. Geological Survey Cover. Photograph looking downstream at area of Suncook River avulsion, Epsom, New Hampshire. (Photograph taken on June 18, 2008) Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009 By Robert H. Flynn Prepared in cooperation with the Federal Emergency Management Agency Scientific Investigations Report 2010–5127 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2010 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov 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., 2010, Flood study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009: U.S. Geological Survey Scientific Investigations Report 2010–5127, 64 p. (Also available at http://pubs.usgs.gov/ sir/2010/5127.) iii Acknowledgments The author would like to express appreciation to the following U.S. Geological Survey personnel who assisted with collection of data in this report—Richard Kiah, Scott Olson, Thor Smith, James Degnan, Chandlee Keirstead, Heather Manzi, and Glenn Berwick. The author would like to acknowledge that the cost of generating the 1- and 4-ft contour interval data used in the creation of the flood mapping for this study was funded through a New Hampshire Department of Safety-Bureau of Emergency Management grant (Emergency Management Performance Grant) with in-kind matching funding provided by the New Hampshire Department of Environmental Services through its dam-maintenance efforts in the Suncook River watershed. THIS PAGE INTENTIONALLY LEFT BLANK v Contents Acknowledgments ........................................................................................................................................iii Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Purpose and Scope ..............................................................................................................................4 Description of the Study Area ............................................................................................................4 Flood Mapping and Flood-Plain Management Applications .........................................................6 Methods Used to Determine Flood Data ....................................................................................................6 Field Data Collection.............................................................................................................................7 Indirect Determination of Peak Flow for the Suncook River Flood of April 2007 .......................7 Hydrologic Analyses.............................................................................................................................8 Hydraulic Analyses ...............................................................................................................................8 Flood Elevations .............................................................................................................................................9 Summary........................................................................................................................................................10 References Cited..........................................................................................................................................10 Appendix 1. Suncook River 10-, 2-, 1- and 0.2-Percent Annual Exceedance Probability Flood Profiles (10-, 50-, 100-, and 500-Year Recurrence-Interval Floods, respectively) ................25 Appendix 2. Suncook River April 2007 Flood and 50-, 20-, and 4-Percent Annual Exceedance Probability Flood Profiles (2-, 5-, and 25-Year Recurrence-Interval Floods, respectively) 39 Appendix 3. Elevation Reference Marks ..............................................................................................53 Appendix 4. Suncook River Flood Map of the Floodway, and the 100- and 500-Year Flood from the Confluence with the Merrimack River to the Upstream Corporate Limit of Epsom, New Hampshire ..............................................................................................................................55 Figures 1. Map showing extent of the flood study of the Suncook River in Epsom, Pembroke, and Allenstown, NH ......................................................................................................................2 2. Pre-avulsion aerial imagery with the new and abandoned channels of the May 2006 Suncook River avulsion, Epsom, NH ..........................................................................................3 3. Map showing location of the Suncook River streamgage at North Chichester, NH (01089500), and indirect discharge locations for the determination of the April 2007 flood peak on the Suncook River ...............................................................................................5 Tables 1. Drainage areas and peak recurrence-interval flows for selected locations on the Suncook River in Epsom, Pembroke, and Allenstown, NH ....................................................9 2. Cross sections and the 10-, 50-, 100-, and 500-year recurrence-interval flood data for Suncook River in Epsom, Pembroke, and Allenstown, NH ............................................12 3. Cross sections and the 2-, 5-, 25-year recurrence-interval and April 2007 flood data for Suncook River in Epsom, Pembroke, and Allenstown, NH ............................................15 4. Surveyed and HEC-RAS-modeled April 2007 elevations of flood high-water marks for the Suncook River, NH .........................................................................................................18 5. 1978 FIS and 2009 flood study 100-year flood elevations for selected locations in the Suncook River in Epsom, Pembroke, and Allenstown, NH ..................................................19 6. Cross sections, 100-year recurrence-interval, and floodway data for Suncook River in Epsom, Pembroke, and Allenstown, NH .............................................................................20 vi Conversion Factors, Datum, and Abbreviations 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) Volume cubic yard (yd3) 0.7646 cubic meter (m3) Flow rate foot per second (ft/s) 0.3048 meter per second (m/s) cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32 Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F–32)/1.8 Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29). OTHER ABBREVIATIONS USED IN REPORT ADCP Acoustic Doppler Current Profiler FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Map FIS Flood Insurance Study GPS Global Positioning System HEC-RAS Hydrologic Engineering Center River Analysis System NFIP National Flood Insurance Program NHDOT New Hampshire Department of Transportation NWIS National Water Information System USACE U.S. Army Corps of Engineers USGS U.S. Geological Survey Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009 By Robert H. Flynn Abstract Introduction On May 15, 2006, a breach in the riverbank caused an The Suncook River drains a group of lakes to the south avulsion in the Suncook River in Epsom, NH. The breach of Lake Winnipesaukee and flows for approximately 30 mi in the riverbank and subsequent avulsion changed the estab- to the Merrimack River at a location between Concord and lished flood zones along the Suncook River; therefore, a new Manchester, NH (fig. 1). Floods typically occur along the flood study was needed to reflect this change and aid in flood Suncook and Little Suncook Rivers during the spring, fall, recovery and restoration. For this flood study, the hydrologic and winter. Some of the most severe flooding in early spring and hydraulic analyses for the Suncook River were conducted is a result of snowmelt and heavy rains in conjunction with