ARE WE SIGNIFICANTLY UNDERESTIMATING THE CURRENT RISK OF EXTREME FLOODING TO NEW YORK CITY?

Extreme storm surge events, such as Hurricane Sandy in 2012, are infrequent and difficult to predict. Historical tide gauge records can be used to calculate the reoccurrence interval of different magnitudes of flooding, however, the brevity of these records places significant uncertainties on assessing the return frequency for the most extreme events. Here we utilize flood deposits preserved within coastal ponds to assess how often a flood event like Hurricane Sandy has impacted the New York City region. In terms of their sedimentological imprints, deposition associated with Hurricane Sandy and a hurricane in 1821 CE clearly rank as the two largest floods to impact New York Harbor since European colonization. The 1821 deposit is significantly coarser but also thinner than deposition from Hurricane Sandy. This is consistent with the 1821 hurricane being of greater intensity, yet shorter duration. Numerical simulations for the 1821 event are consistent with depositional trends, and support historical accounts of a storm surge of roughly 4 m for the 1821 event, compared to a surge of just 2.8 m for Hurricane Sandy. The return period of Hurricane Sandy’s 3.4 m storm tide is estimated to be near or greater than 1000 years based on probabilities obtained from existing tide gauge data and simulated hurricane climatology. However, the probability of two 1000-year storm tides occurring over two successive centuries (i.e. similar storm tides for Hurricane Sandy and the 1821 hurricane) is quite low at near 2%. Results indicate current return period estimates for flood events like Hurricane Sandy may significantly underestimate the true risk of extreme flooding to New York City.