Event-Specific Flood Extent and Depth Maps for Pennsylvania - Hurricane Ida, 2021

On August 29, 2021, Hurricane Ida made landfall in Louisiana. Weakening into a post-tropical cyclone, Ida moved north, passing over Pennsylvania. Widespread areas of Central and Southeastern Pennsylvania received 5 inches of rain, with over 10 inches recorded in some localities. Rainfall intensity was particularly intense, with rates exceeding 2.0 inches per hour. Leading up to the event, streamflow values were at the 75th percentile across much of the affected region, leading to catastrophic flooding on September 1st and 2nd, 2021. As a result of rain from the remnants of Ida, 19 streamgages recorded new record gage height peaks across the state. Computed flood frequencies estimated that 9 streamgages had flows greater than 1-percent Annual Exceedance Probability (AEP), with three of these gages exceeding 0.2-percent AEP (Stuckey and others, 2023). Following the flooding, the USGS documented almost 300 high-water marks, surveying over 200 of them into North American Vertical Datum of 1988. These HWMs were published to the USGS Flood Event Viewer. Using geospatial methods presented in Stuckey and others (2023), flood extent and depth maps were generated for 4 localities with sufficient density of HWMs and favorable hydrologic and hydraulic conditions for flood mapping. This data release contains flood extent polygons, modelled boundary polygons, HWM points, and flood depth rasters for flooding associated with Hurricane Ida for 4 Pennsylvania localities: Conestoga River near Lancaster, PA Frankstown Branch Juniata River near Williamsburg, PA Paxton Creek near Harrisburg, PA Raystown Branch Juniata River near Saxton, PA Previously published USGS products mapped flooding associated with Hurricane Ida in Southeastern Pennsylvania. Computed flood frequencies were also published in this previous work. The corresponding products including a data release, scientific investigations report, and geonarrative are linked in Related External Resources section of this page. References: Stuckey, M.H., Conlon, M.D., and Weaver, M.R., 2023, Characterization of peak streamflows and flooding in select areas of Pennsylvania from the remnants of Hurricane Ida, September 1–2, 2021 (ver. 1.1, September 28, 2023): U.S. Geological Survey Scientific Investigations Report 2023–5086, 28 p., https://doi.org/10.3133/sir20235086.

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