High-water marks in New Jersey following Hurricane Ida and associated floods, September 2021

The data contained within include high-water marks collected at 50 sites throughout the regions of New Jersey affected by significant flooding from Hurricane Ida during September of 2021. Each site contains between one to six associated high-water marks that were documented, photographed, and surveyed to datum. The datum represented by the elevations of the high-water marks is the North American Vertical Datum of 1988. All data (excluding photographs) associated with the high-water marks are contained within the data release associated with these metadata, as well as the USGS Short Term Network Flood Event Viewer (STN-FEV, U.S. Geological Survey (2021a)). Photographs can be found within the STN-FEV. A summary of the event leading to the collection of these data is provided below. Hurricane Ida was a large and destructive storm that originally made landfall on the Louisiana coast on August 29, 2021, causing extensive damage. Following landfall in Louisiana, the system weakened to a tropical depression as it moved northeast across the United States. As it approached the mid-Atlantic region, it interacted with a frontal system eventually becoming a strongly forced frontal low. As the low passed over parts of southeastern Pennsylvania, central and northern New Jersey, and southern New York, it produced rainfall totals nearing 10 inches in some localized areas during a 24-hour period, in addition to significant severe weather including tornadoes (National Weather Service, 2021). Two weeks prior, the region received significant precipitation from Hurricane Henri, and most larger river basins were still receding from that event. In New Jersey, the hardest-hit areas spanned from Hunterdon and Mercer counties northeastward through Somerset, Middlesex, Union, Essex, and Hudson Counties. Heavy rainfall rates associated with the remnants of Hurricane Ida caused flash flooding in several urban communities resulting in local stormwater drainage systems being overwhelmed. Small stream and larger river flooding also occurred along the storm path. In many cases, hydraulic structures such as bridges and culverts were quickly overwhelmed by the volume of stormwater which resulted in roads being overtopped by high-energy floodwaters and damaged or washed out. In Hunterdon County, the Wickecheoke, Swan, and Harihokake Creeks all experienced these effects. In Mercer and Somerset Counties, smaller stream basins such as Stony Brook, Beden Brook, Rock Brook, and Peters Brook were notable examples of streams that overtopped their banks during this event. Moving to the northeast in Union and Essex Counties, the Rahway, Elizabeth, and Second Rivers overwhelmed flood control projects and concrete-lined channels in urban areas resulting in some of the worst examples of flooding from Ida in New Jersey. Major flooding on other large rivers was somewhat restricted in New Jersey because precipitation produced by this event did not encompass the entire state. For example, northern basins such as the Ramapo did not receive as much precipitation as some central parts of the state. This in turn prevented catastrophic flooding on the lower Passaic River basin which has been notable during past widespread flooding events such as Hurricane Irene. The Raritan and Millstone River basins took the brunt of the effects of the storm, as most of the previously mentioned streams drain into these rivers. The provisional peak gage height at the U.S. Geological Survey (USGS) 01400500 Raritan River at Manville, NJ, streamgage exceeded the gage height recorded during the Hurricane Floyd flooding event that occurred in 1999, which was the period-of-record gage height peak. Main stem river flooding on the Raritan and Millstone Rivers caused significant flooding from backwater along many tributaries throughout the basin that in some cases took days to recede. Following the event, the USGS New Jersey Water Science Center deployed field crews to perform discharge measurements, assess damage to streamgages, and confirm provisional period-of-record peaks. The Federal Emergency Management Agency (FEMA) issued a Major Disaster Declaration for 13 New Jersey counties and requested assistance from the USGS in identifying, documenting, and surveying the depths and extents of flooding in several of the most affected communities within the disaster declared counties. The USGS deployed field crews to the communities of Milford, Stockton, Frenchtown, Lambertville, Clinton, Flemington, Rocky Hill, Somerville, Manville, Bound Brook, Plainfield, Ridgewood, Elizabeth, Cranford, and Rahway to flag and survey high-water marks. During the period from September 8-30, 2021, 139 high-water marks were documented and surveyed at 50 sites throughout the aforementioned communities. More information on the processes and techniques used to identity and survey high-water marks can be found at the USGS Water Science School page on High-Water Marks and Flooding (U.S. Geological Survey (2021b). References Cited: National Weather Service, 2021, "Event Review: September 1:Remnants of Ida", accessed October 25, 2021, at https://www.weather.gov/phi/eventreview20210901. U.S. Geological Survey, 2021a, Short-Term Network Flood Event Viewer for Tropical Cyclone Ida, accessed October 25, 2021, at https://stn.wim.usgs.gov/fev/#2021TropicalCycloneIda. U.S. Geological Survey, 2021b, USGS Water Science School: High-Water Marks and Flooding, accessed October 25, 2021, at https://www.usgs.gov/special-topic/water-science-school/science/high-wa….