Sandy’s Storm Surge Surprise

Courtesy of TheAtlantic.com

Sandy has been a harrowing October surprise for some and has been devastating for too many who have lost loved ones, or been forced from their homes. Fox News described the unprecedented impact of this record-breaking storm. An example of the personal impact came when 20 babies had to be evacuated from the neonatal ward at Tische Hospital where they were under intensive care. Backup generators failed in the hospital after lower Manhattan lost power. This is the largest loss of electricity in city history so such stories are all too common.

Storm damage usually comes from two sources and Sandy offered these in spades: high winds toppled trees and power lines or hurtled objects at dangerous speeds, and excessive rainfall saturated the ground and overwhelmed storm water systems. Rainfall up to 8” was forecast and West Virginia braced for up to 3’ of snow. But Sandy added a third punch that was equally damaging and disruptive: storm surge.

When a hurricane hits, the intense low-pressure system brings counter-clockwise winds (in the Northern Hemisphere) that pile up ocean water at the coast. This is the storm surge. Higher winds make for larger surge. Long duration winds make for a larger surge. And a long fetch (the distance over which the winds blow) makes for a larger surge. Sandy offered all three. A huge, slow-moving low-pressure storm barreling straight into the coast for several days over the vast North Atlantic with winds exceeding 90 miles per hour.

What else could you ask for in a ‘perfect storm?’ It turns out Sandy offered more. Sandy’s surge did its greatest damage at the time of high tide in New York City. The National Oceanic and Atmospheric Administration (NOAA) predicts tides for about 3,000 locations along the coastlines of the US including seven in New York harbor. The station at The Battery is a primary location where the harmonics used in these models are calculated directly from historical records. The tidal forecast at the time of Sandy’s landfall looks like this (Figure 1). Just as winds were piling up water, the tides rose too.

Figure 1. Calculated tidal range at The Battery in New York harbor during the storm surge of hurricane Sandy.

Two high tides occurred on October 29. The first, at about 8am, was the greater of the two. With a more than five-foot variation in water level, these tides make a huge difference in the storm surge extent and damage. New Yorkers, especially those in Manhattan, were rudely reminded just how close they live to sea level as water flowed into subway systems and tunnels.

Figure 1 reminds us that tides vary. In this case the second tide was not quite as high and did not contribute as much to the damage. These variations are predictable results of the gravitational influence of the sun and moon. I say predictable in the sense that NOAA uses the record of how tidal elevation varies with time to make the predictions. Few attempt the torturous task of actually calculating the tidal influence of the sun and moon (let alone the other planets) because it is so very complicated.

Scientists give names (tidal terminology is nicely explained here) to the different tidal extremes such as the neap tides and spring tides. The latter give larger tidal ranges (and higher high tides) since the sun and moon are aligned and their gravitational effect is additive. So we can talk about ‘higher high tides’ such as spring tides.

And tidal range is complicated by another factor, the configuration of the basin where the tides occur. Everyone is familiar with this effect if you’ve ever seen pictures of the tidal range in the Bay of Fundy. There the tidal range is the largest in the world and can reach 50 feet. New York is fortunate in comparison!

But that brings me to the interesting effect of climate change and what things might be like when we get the next ‘hundred year storm.’

Figure 2. Change in mean tide (cm) in the North Sea from a sea level rise of 50 centimeters.

Because tidal range varies with the configuration of the tidal basin (whether it’s the North Atlantic, the Pacific, or the Bay of Fundy) and with the depth of water, it makes sense that anything that changes the water depth or configuration of that basin might change the tides. What could do that? A rising sea level will have a measurable and potentially damaging effect on tidal extremes.

Science has not yet studied this very much so we are a long way from knowing what the effect will look like in the case of New York City but one study that was done in the North Sea (Figure 2) suggests that a 50 cm rise in sea level (much less than that now considered by the US National Research Council) could change the range of the high tide by as much at 10cm.This is a big deal when your house is within a foot of mean sea level.

Scientists thought that planning for the worst case of coastal flooding required you to consider the worst-case storm surge, occurring at highest high tide. And if you wanted to get an idea of the worst case in the future you needed to add one more component: sea level rise. Now we must add another unknown that is far more difficult to understand; how will the extreme tides change as the sea level rises and changes the configuration of basins?

All of this science is little consolation to those who have lost loved ones or had to abandon their homes in Manhattan or elsewhere along the East Coast of the US this week. But let’s hope our preparations for what is to come will help prevent even worse losses in future.