Flood inundation maps

Digital flood-inundation maps for a eight-mile reach of the Hoosic River from the confluence with North Branch Hoosic River to the Vermont State Line, in North Adams and Williamstown, Massachusetts, were created by the U.S. Geological Survey (USGS) in cooperation with FEMA. The inundation maps, which can be accessed through the USGS Flood Inundation Mapping Science Web site at http://water.usgs.gov/osw/flood_inundation/, depict estimates of the areal extent and depth of flooding corresponding to selected water levels (stages) at the USGS streamgage at Hoosic River near Williamstown, MA (01332500). Current conditions at the USGS streamgage may be obtained on the Internet. The NWS forecasts flood hydrographs at many places that are often collocated at USGS streamgages. The forecasted peak-stage information, also available on the Internet, may be used in conjunction with the maps developed in this study to show predicted areas of flood inundation. In this study, flood profiles were computed for the stream reach by means of a one-dimensional step-backwater model. The model was calibrated using the most current stage-discharge relations at the Hoosic River near Williamstown gage and documented high-water marks from recent floods (if available). The hydraulic model was then used to determine water-surface profiles for floods with annual exceedance probabilities of 1% and 0.2% referenced to the streamgage. The flood-peak inundation area was modeled in a GIS by combining [high-water mark data, and/or steady-state hydraulic modeling] and available [Lidar] digital elevation model (DEM) data as part of the U.S. Geological Survey study of the flood of August, 2011 in Williamstown and North Adams, MA. Information about the study, floods, and methods used can be found in the USGS Scientific Investigations Report 2014-5236.