How unique was Hurricane Sandy? Sedimentary reconstructions of extreme flooding from New York Harbor

Abstract

The magnitude of flooding in New York City by Hurricane Sandy is commonly believed to be extremely rare, with estimated return periods near or greater than 1000 years. However, the brevity of tide gauge records result in significant uncertainties when estimating the uniqueness of such an event. Here we compare resultant deposition by Hurricane Sandy to earlier storm-induced flood layers in order to extend records of flooding to the city beyond the instrumental dataset. Inversely modeled storm conditions from grain size trends show that a more compact yet more intense hurricane in 1821 CE probably resulted in a similar storm tide and a significantly larger storm surge. Our results indicate the occurrence of additional flood events like Hurricane Sandy in recent centuries, and highlight the inadequacies of the instrumental record in estimating current flood risk by such extreme events.

Figure 1. The Field Site.

(a) Hurricane Sandy's storm surge, based on an interpolation between USGS high water measurements (black dots) using ArcGIS 10.0. Note that the offshore contours are extensions of these onshore observations, with uncertainty increasing with distance offshore. Upper scale bar is 100 km. Box shows the area indicated in b. (b) Location of Seguine Pond on the southern coast of Staten Island. Lower scale bar is 5 km. Brown area indicates the extent of the terminal moraine. Numbers are selected USGS high water marks for Hurricane Sandy given in meters above NAVD88. Box shows the area indicated in c and d. (c) Landsat satellite image of Seguine Pond in 2010 with core locations shown. Scale bar is 50 m. (d) Seguine Pond on Nov. 4, 2012, 6 days post-Sandy. Scale bar is 50 m.

Figure 2. Core SG2 Age Constraints.

(a) Optical photograph of SG2 showing red event beds, with the Hurricane Sandy deposit at the surface. (b) X-radiograph showing density variations in the core. White areas are denser than black and generally correspond to event deposits. (c) Mercury (Hg) and (d) Zinc (Zn) abundances. The upper dashed line indicates the base of the Hurricane Sandy deposit or 2012 CE. The lower dashed line marks the initial rise in heavy metals accompanying the onset of the Industrial Revolution (1850–1900 CE). In d, the yellow triangles indicate three more dating horizons: the 1963 peak in ¹³⁷Cs abundance due to atmospheric nuclear weapons testing, the 1954 onset of ¹³⁷Cs, and a radiocarbon date indicating an age range between 1451 and 1629 CE.

Figure 3. Bayesian analysis using chronological constraints from core SG2.

The 1-σ age range is shown in medium gray and the 2-σ age range in light gray. ¹⁴C age probabilities are shown in dark gray just above the x-axis. The dark gray bar corresponds to the 1850–1900 onset of industrial heavy metals and circles indicate known ages based on ¹³⁷Cs and the depth of the 2012 Sandy deposit (Fig. 2). Vertical dashed lines correspond to the dates of significant surge events in New York Harbor and the horizontal dashed lines indicate their most likely deposit (seen in the x-radiograph). The 1893 event (red dashed lines) does not have a corresponding deposit.

Figure 4. Sedimentary characteristics of storm deposits in core SG2.

(a) Optical photograph of SG2 showing red flood derived deposits. (b) X-radiograph showing increased density of deposits. Deposits are indicated with orange stars. (c) Percentage of coarse material in each deposit. The percentage greater than 63 μm is shown in green and the percentage greater than 38 μm is shown in gray. (d) Median grain size (D₅₀) for deposits greater than 63 μm. The dashed blue line is the 63 μm sieving limit.

Figure 5. Core Transect from Seguine Pond.

X-radiographs and Zn abundance (red lines) of (a) Core SG1, the core closest to the barrier, (b) core SG2, (c) core SG3, and (d) core SG4. Deposits associated with Hurricane Sandy and the 1821 hurricane deposits are indicated with the dashed lines.

Figure 6. Hurricane Sandy vs. 1821 Hurricane Deposition.

(a) Percent coarse of the Hurricane Sandy and 1821 hurricane deposits in each core taken from Seguine Pond. The percentage of material >63 μm is in color and the percentage >38 μm is in gray. Core order starts closest to the barrier and extends landward (left to right). (b) D₉₀ grain size of the Hurricane Sandy (striped) and 1821 (solid) deposits in each core. Colors correspond to the same cores as in (a) and are shown in relative distance from the barrier.

Figure 7

SLOSH model results of storm surge (i.e. not adjusted for tides) for (a) the 1821 hurricane and (b) Hurricane Sandy. Maps were generated using the SLOSH Display program (1.66a). (c) Tides (dashed lines) and storm tides (solid lines) for the 1821 hurricane (blue) and Hurricane Sandy (red). The beginning of the inundation for both storms is set to 0.