Flood-Inundation Maps for the West Branch Susquehanna River near the Boroughs of Lewisburg and Milton, Pennsylvania, 2013 By Mark A. Roland and Scott A. Hoffman Scientific Investigations Report 2014-5094 Abstract: Digital flood-inundation maps for an approximate 8-mile reach of the West Branch Susquehanna River from approximately 2 miles downstream from the Borough of Lewisburg, extending upstream to approximately 1 mile upstream from the Borough of Milton, Pennsylvania, were created by the U.S. Geological Survey (USGS) in cooperation with the Susquehanna River Basin Commission (SRBC). 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 the estimated areal extent and depth of flooding corresponding to selected water levels (stages) at the USGS streamgage 01553500, West Branch Susquehanna River at Lewisburg, Pa. In addition, the information has been provided to the Susquehanna River Basin Commission (SRBC) for incorporation into their Susquehanna Inundation Map Viewer (SIMV) flood warning system (http://maps.srbc.net/simv/). The National Weather Service (NWS) forecasted peak-stage information (http://water.weather.gov/ahps) for USGS streamgage 01553500, West Branch Susquehanna River at Lewisburg, Pa., 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. Calibration of the model was achieved using the most current stage-discharge relations (rating number 11.1) at USGS streamgage 01553500, West Branch Susquehanna River at Lewisburg, Pa., a documented water-surface profile from the December 2, 2010, flood, and recorded peak stage data. The hydraulic model was then used to determine 26 water-surface profiles for flood stages at 1-foot intervals referenced to the streamgage datum ranging from 14 feet (ft) to 39 ft. Modeled flood stages, as defined by NWS, include Action Stage, 14 ft; Flood Stage, 18 ft; Moderate Flood Stage, 23 ft; and Major Flood Stage, 28 ft. Geographic information system (GIS) technology was then used to combine the simulated water-surface profiles with a digital elevation model (DEM) derived from light detection and ranging (LiDAR) data to delineate the area flooded at each water level. The availability of these maps, along with World Wide Web information regarding current stage from USGS streamgages and forecasted stream stages from the NWS, provide emergency management personnel and residents with information that is critical for flood response activities, such as evacuations and road closures, as well as for post-flood recovery efforts. DISCLAIMERS: This report, identified as SIR 2014-5094, has been approved for release and publication by the Director of the U.S. Geological Survey. Although this database has been subjected to rigorous review and is substantially complete, the U.S. Geological Survey reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the U.S. Geological Survey nor the U.S. Government may be held liable for any damages resulting from its authorized or unauthorized use. Although these data have been processed successfully on a computer system at the U.S. Geological Survey, no warranty expressed or implied is made regarding the display or utility of the data on any other System, or for general or scientific purposes, nor shall the act of distribution constitute any such warranty. The U.S. Geological Survey shall not be held liable for improper or incorrect use of the data described and/or contained herein. 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 information product, for the most part, is in the public domain, it also contains copyrighted materials as noted in the text. Permission to reproduce copyrighted items for other than personal use must be secured from the copyright owner. Disclaimer for Flood-Inundation Maps: Inundated areas shown should not be used for navigation, regulatory, permitting, or other legal purposes. The SRBC and USGS provide these maps “as is” for a quick reference, emergency planning tool but assume no legal liability or responsibility resulting from the use of this information. Uncertainties and Limitations for Use of Flood-Inundation Maps: Although the flood-inundation maps represent the boundaries of inundated areas with a distinct line, some uncertainty is associated with these maps. The flood boundaries shown were estimated on the basis of water stages/flows at USGS streamgage 01553500. Water-surface elevations along the stream reaches were estimated by using steady-state hydraulic modeling, assuming unobstructed flow, and using discharges and hydrologic conditions anticipated at the streamgage. The hydraulic model reflects the land-cover characteristics and any bridge, dam, levee, or other hydraulic structures present as of the date of the published map. Unique meteorological factors(timing and distribution of precipitation) may cause actual discharges along the modeled reach to vary from those assumed during a flood and lead to deviations in the water-surface elevations and inundation boundaries shown. Additional areas may be flooded as a result of unanticipated backwater from major tributaries along the main stem or localized debris or ice jams, which can be a common occurrence during floods in the winter months. The flood-inundation boundaries depicted on these maps are based on DEMs of varying resolution. Additional uncertainties and limitations pertinent to this study are described elsewhere in this report. The series of flood-inundation maps may be used in conjunction with NWS river forecasts that may have additional uncertainties inherent or factored into NWS forecast procedures. The NWS uses forecast models to estimate the quantity and timing of water flowing through selected stream reaches in the United States. These forecast models (1) estimate the amount of runoff generated by a precipitation or snowmelt event, (2) simulate the movement of floodwater as it proceeds downstream, and (3) predict the flow and stage (and water-surface elevation) for the stream at a given location (AHPS forecast point) throughout the forecast period (every 6 hours and 3 to 5 days out in many locations). For more information on AHPS forecasts, please see http://water.weather.gov/ahps/ pcpn_and_river_forecasting.pdf. CONTENTS: This report consists of a document and 26 on-line maps, each with data files and metadata. Datafiles for inundation layers: A shapefile shows the water-surface inundation extent. All 26 inundation layers associated with USGS streamgage 01553500-West Branch Susquehanna River at Lewsiburg, PA are contained within the following shapefile, referenced to water-surface stage level. LewPA.shp Depth grids for use with ArcGIS show inundation extent and water depth range. The number in the name represents the gridID number and can be cross-referenced with the shapefile (LewPA.shp). lewpa_01 lewpa_02 lewpa_03 lewpa_04 lewpa_05 lewpa_06 lewpa_07 lewpa_08 lewpa_09 lewpa_10 lewpa_11 lewpa_12 lewpa_13 lewpa_14 lewpa_15 lewpa_16 lewpa_17 lewpa_18 lewpa_19 lewpa_20 lewpa_21 lewpa_22 lewpa_23 lewpa_24 lewpa_25 lewpa_26 INSTRUCTIONS AND DOCUMENTATION FOR U.S. GEOLOGICAL SURVEY SCIENTIFIC INVESTIGATIONS REPORT 2014-5094 To access the data: The report and data files can be downloaded via the web from http://pubs.usgs.gov/sir/2014/5094/. The main product is a Portable Document Format (.pdf) report which requires Adobe Acrobat for viewing. Acrobat software runs on a variety of systems, and is available for download free of charge from Adobe at http://www.adobe.com. To access the inundation layers and depth grids, AcrGIS 10.2.2 is preferred, older versions may work also. References to non-U.S. Department of the Interior (DOI) products do not constitute an endorsement by the DOI.