From Our Very Own: Kent State University’s Department of Geography Chair Scott Sheridan
We’re all familiar with oceanic tides, which cycle roughly every 12 1/2 hours based on the relative positions of the earth and the moon. Some places have greater ranges than others, based on the local configuration of the coastline. Every couple of weeks, when the sun, earth and moon all line up, we have “spring tides,” in which the tidal fluctuations are stronger than normal.
We also know that sometimes, the weather works to further enhance this tidal cycle, and coastal communities can be vulnerable. When we think of flooding that happens along coastlines, we typically conjure up images of intense storms, such as hurricanes or nor’easters, that not only bring flooding, but heavy winds and rain as well. Events such as these can be very deadly—we can look at last year’s Hurricanes Harvey and Maria, as well as the recent nor’easter—and cause billions of dollars in damage.
Beyond these events, however, floods can also happen under less extreme conditions. The ocean responds to atmospheric pressure—when pressure is high, the ocean height is lower, under the weight of the atmosphere, and when air pressure is low, ocean levels rise.
Similarly, if the winds blow onshore with sufficient speed, they can “pile up” water along the coast and temporarily raise sea levels. Low pressure and onshore winds typically combine most frequently during stormy conditions, but there’s another factor we haven’t discussed yet: sea-level rise.
Global mean sea level has risen by around 8 inches in the past century. While this doesn’t sound like a lot, remember that any temporary increase in sea levels is going to be superimposed upon this increase, and some places have seen bigger rises than others. This can make big storms deadlier—for instance, it’s estimated that the one foot of mean sea-level rise that New York City has seen added over 10 percent, or $2 billion, to the damage total from Hurricane Sandy in 2012.
With mean sea-level rise, it’s also easier for atmospheric conditions to yield what we call blue sky floods, sometimes also called “nuisance” floods, along coastlines. These floods don’t capture the media attention of, say, a Hurricane Sandy, but they can cause substantial problems for coastal communities’ infrastructure.
And they are becoming increasingly likely. In places with relatively shallow continental shelves, such as along the North Carolina coast, nuisance floods have increased several-fold in the past couple of decades, with cities like Beaufort, North Carolina seeing 30-40 flood days a year this decade, compared with fewer than 10 in the 1980s. Many of these happen in the absence of stormy conditions, hence their name.
Here at Kent State, my colleague Cameron Lee and I have worked to understand how the atmosphere affects the likelihood of these coastal flooding events, and how they’ve changed over time. We use a technique called synoptic climatology, which basically involves categorizing the atmosphere into a number of different patterns, and we’ve uncovered a range of patterns that can be associated with elevated water levels along the mid-Atlantic coast.
With some funding from the National Oceanic Atmospheric Administration, we’re currently working on mapping this relationship across the Pacific and Atlantic coasts of the lower 48 states. We’re hoping to develop models that can simulate the likelihood of blue sky floods weeks out, to help give communities a chance to prepare.
Dealing with the implications of sea level rise is going to be one of the most costly endeavors in the U.S., especially in low-lying communities along the Atlantic and Gulf coasts. Helping communities understand the scale and timeframe of blue sky floods can be a starting point in mitigating their vulnerability.
