Hoosic flood inundation Grids

A series of eight digital flood-inundation maps were developed for an 8-mile reach of the Hoosic River in North Adams and Williamstown, Massachusetts, by the U.S. Geological Survey (USGS) in cooperation with the Federal Emergency Management Agency (FEMA). The coverage of the maps extends from the confluence with North Branch Hoosic River to the Vermont State Line. Peak flows for the 20-, 10-, 4-, 2-, 1, 0.5, and 0.2-percent annual exceedance probabilities (AEPs) were computed for the reach from updated flood-frequency analyses. These peak flows and the flows associated with the 9.0 ft and 10.0 ft gage heights were routed through a one-dimensional step-backwater hydraulic model to obtain the corresponding peak-water-surface elevations and to place the flood of tropical storm Irene (August 28, 2011) into historical context. The hydraulic model was used to compute water-surface profiles for flood stages referenced to the streamgage and ranging from 9.0 ft (624.45 ft North American Vertical Datum of 1988 [NAVD88]) to 16.1 ft (631.59 ft NAVD88). The simulated water-surface profiles were combined with a geographic information system (GIS) digital elevation model (DEM) derived from 0.5-foot vertical accuracy light detection and ranging (lidar) data to create a set of flood-inundation maps. These maps delineate the areal