Flood study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Abstract

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/pubprodTo 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. AcknowledgmentsThe 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. AbstractOn May 15, 2006, a breach in the riverbank caused an avulsion in the Suncook River in Epsom, NH. The breach in the riverbank and subsequent avulsion changed the established flood zones along the Suncook River; therefore, a new flood study was needed to reflect this change and aid in flood recovery and restoration. For this flood study, the hydrologic and hydraulic analyses for the Suncook River were conducted by the U.S. Geological Survey, in cooperation with the Federal Emergency Management Agency.This report presents water-surface elevations and profiles determined using the U.S. Army Corps of Engineers onedimensional Hydrologic Engineering Center River Analysis System model, also known as HEC-RAS. Steady-state watersurface profiles were developed for the Suncook River from its confluence with the Merrimack River in the Village of Suncook (in Allenstown and Pembroke, NH) to the upstream corporate limit of the town of Epsom, NH (approximately 15.9 river miles). Floods of magnitudes that are expected to be equaled or exceeded once on the average during any 2-, 5-, 10-, 25-, 50-, 100-, or 500-year period (recurrence interval) were modeled using HEC-RAS. These flood events are referred to as the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year floods and have a 50-, 20-, 10-, 4-, 2-, 1-, and 0.2-percent chance, respectively, of being equaled or exceeded during any year. The 10-, 50-, 100-, and 500-year flood events are important for flood-plain management, determination of floodinsurance rates, and design of structures such as bridges and culverts. The analyses in this study reflect flooding potentials...

Analysis of the transport of sediment by the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, after the May 2006 flood

“…Less than a year after the 2006 breach, during April 16-18, 2007, southeastern New Hampshire experienced severe flooding due to a storm that stalled off the New England coast. The peak flow of April 2007 (10,600 ft 3 /s) was the second highest flood recorded at the Suncook River streamgage (Flynn, 2008). The highest flood of record at the Suncook River streamgage (12,900 ft 3 /s) occurred in March 1936.…”

“…Valley and belowwater portions of the cross-section data were obtained from field measurements (using Acoustic Doppler Current Profiler (ADCP) and a surveying total station with level loop) that were referenced to the NGVD 29. Additional valley crosssection data were obtained using contour-interval data created from aerial photogrammetry collected in May 2007 (Eastern Topographics, 2007) and in April 2008(Eastern Topographics, 2008. All bridges, dams, and culverts in the study reach were field-surveyed to obtain elevation data and structural geometry.…”

“…Field-surveyed cross sections and flagged flood highwater marks from the April 2007 flood (Flynn, 2008) that had been used in calibrating the Suncook steady-state flow model (Flynn, 2010) were referenced to NGVD 29, in feet, using closed level-loop surveying methods from either a known reference datum or from a reference datum established using Leica System 1200 Global Positioning System (GPS) with 3-millimeter (mm) (0.01-ft) horizontal accuracy and 10-mm (0.03-ft) vertical accuracy in static mode (Leica Geosystems, 2008). Between the Short Falls Road Bridge and the Buck Street Dams, cross-section and profile data were also collected using a GPS in conjunction with an ADCP based on techniques described by Muller and Wagner (2009).…”

“…Where needed, additional valley cross-section data were obtained using 1-and 4-ft contour-interval data created from aerial photogrammetry collected in May 2007 (Eastern Topographics, 2007) and in April 2008(Eastern Topographics, 2008. These data were referenced to the horizontal North American Datum of 1983 (NAD 83) and vertical NGVD 29.…”

Flood of April 2007 in Southern Maine

“…Flood-frequency estimates from Hodgkins (1999) were updated for this report with an additional 9 years of data where available for stations in southern Maine. (Flynn, 2008). Several New Hampshire streamflow-gaging stations close to the Maine border had peak-flow recurrence intervals of between 100 and 500 years, including the Isinglass River at Rochester Neck Road near Dover (USGS station number 01072870) and the Oyster River near Durham (USGS station number 01073000) (Flynn, 2008).…”