Location Matters: Sandy’s Tides Hit Some Parts of the New Jersey Coast Harder Than Others
USGS researchers ground-truthed Hurricane Sandy's October 2012 storm tides in New Jersey and found northern coastal communities had significantly higher storm tides than southern ones did, though flood damage was widespread in both areas. The findings suggest that some southern New Jersey communities may be underestimating their future flood risks.
While Hurricane Sandy produced record flooding and damage in most areas along the coast of New Jersey, storm tide levels in many northern New Jersey locations were not only higher than in Southern New Jersey, they were significantly higher than the federal flood insurance program’s latest “base flood” – the flood that has a one-in-100 chance of happening in any given year.
That is a key finding of a new U.S. Geological Survey study, published soon after the fourth anniversary of Hurricane Sandy’s landfall in New Jersey October 29-30, 2012, that reveals local variations in the storm’s flood tides. The researchers found that peak storm tide elevations along the northern coast of New Jersey and Raritan Bay generally ranged from about 9 to 15 feet. Along the central and southern coast from Ocean to Cape May counties the peak water levels generally ranged from about 5 to 8 feet.
USGS researchers and Federal Emergency Management Agency officials often describe storms’ highest flood tides in terms of their odds of occurring at a specific place in any given year. A flood that has a one-in-100 chance of occurring in any given year may be described as a “one percent flood” or a “base flood,” and a flood that has a one-in-500 chance of occurring any year as a “0.2 percent flood.”
How extreme was the flooding caused by Hurricane Sandy? In some northern areas, the highest storm tides were two feet or more higher than that one-in-100 flood risk, the one percent flood, and six inches or more higher than the one-in-500 flood risk, or the 0.2 percent flood.
In southern New Jersey, the pattern is different, the new USGS research shows. In some parts of southern New Jersey peak storm tide levels did not reach the one-in-100-chance flood level, although many communities in these areas experienced severe damage from the storm.
The new study, done by the USGS in cooperation with FEMA, is important because FEMA has recently released new flood insurance studies for New Jersey coastal communities, with new estimates of areas at high risk of flooding. The new estimates guide community planners and property owners and will be used to set flood insurance rates. They define the areas at high risk of flooding as those that will be affected by a peak storm tide with a one-in-100 chance of occurring each year. The study suggests that in some southern communities, extreme flooding may occur at peak storm tide levels that are lower than FEMA’s one-in-100 projection.
“It was clear that the northern communities and those along Raritan Bay experienced the most extreme water levels, as compared to those along the southern coast of New Jersey and Delaware Bay,” said Thomas Suro, a USGS surface-water specialist, and lead author of the study. “This suggests that some southern communities may be underestimating their future flood risks.”
“This is important information for residents, community officials and planners,” Suro said. “In Southern New Jersey, homes and businesses that came through Hurricane Sandy generally survived peak storm tide elevation less than about 9 feet. In some parts of Atlantic and Cape May counties, these peak storm tide elevations were actually 1 to 2 feet lower than FEMA’s new estimates of the areas at the highest risk. And if the storm had changed track, flood levels could have been a lot worse.”
The researchers also compared Hurricane Sandy to other historic storms along the New Jersey coast, and found that in most coastal locations, Sandy’s peak water-surface elevations exceeded those set by the Great Nor’easter of December 1992, the March 1962 storm, and the Great Atlantic Hurricane of 1944 in most places.
The USGS data for this study came from the network of more than 25 tide-gauges along the coast of New Jersey, combined with 14 special storm-tide sensors and post-storm high-water-mark data from 169 sites. In cooperation with FEMA, the USGS deployed special sensors in advance of the hurricane, to record storm tide elevations along the New Jersey coast. The information is used to determine the extent and depth of flood waters, help test and calibrate computer models’ flood projections, make damage estimates, and improve future estimates of coastal flood risk.
The researchers used FEMA’s HAZUS modeling software program, which can estimate various types of disaster losses, to calculate the dollar value of buildings damaged by the hurricane’s floodwaters along the New Jersey coast. They found the model estimated a dollar value of about $19 billion in building damages. They then used high-water marks collected by USGS field crews on the scene to adjust the estimate of the floodwaters’ extent. Using the model’s same cost projections, the adjusted survey yielded nearly $27 billion in building damage, nearly $8 billion more than the model showed. The large gap between the two estimates highlights the importance of ground-truthing modeled estimates of flood depth and extents, Suro said.
The publication, “Documentation and Hydrologic Analysis of Hurricane Sandy in New Jersey, October 29-30, 2012, Scientific Investigation Report 2016-5085” by Thomas P Suro, Anna Deetz and Paul Hearn, is available online at https://pubs.er.usgs.gov/publication/sir20165085.
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