Location matters: Sandy’s tides hit some parts of the N.J. coast harder than others

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.

USGS notes that the USGS data for this study came from the network of more than twenty-five tide-gauges along the coast of New Jersey, combined with fourteen 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.

— Read more in Thomas P. Suro et al., Documentation and Hydrologic Analysis of Hurricane Sandy in New Jersey, October 29-30, 2012, Scientific Investigation Report 2016-5085, Scientific Investigations Report 2016-5085 (DOI:10.3133/sir20165085) (USGS, 2016)