The following blog post was written by Dr. Thomas Herrington, UCI Associate Director, who generously let me share his thoughts with WaterLog readers:
Back-to-back nor’easters between March 1 and 8 had many residents along the Mid-Atlantic seaboard dreading the next high tide cycle. High astronomical tides combined with the powerful cyclone that developed off the coast on March 2 generated minor to moderate coastal flooding and was quickly followed by a rapid-moving coastal low pressure system on March 7 that generated gusty onshore winds, significant inland snowfall and continued tidal flooding. In total, 12 consecutive high tides exceeded minor or moderate flood levels over the seven-day period. (A pair of late-month storms brought heavy snow and travel headaches, but did not cause significant erosion or flooding.)
Nor’easters are notorious for elevating water levels and causing tidal flooding. There was the infamous “Five-High” Nor’easter of March 1962 that devastated coastal communities from New Jersey to North Carolina. The December 1992 Nor’easter generated 11 consecutive high-tides, one of which held the water level record along the New Jersey coast until Sandy obliterated it and everything else in its way. The December 1992 storm was followed closely by the March 11-12 Nor’easter of 1993, also known as the Great Blizzard of 1993. That storm only hung around for two high tides but did do considerable damage to beaches and coastal infrastructure. The list goes on and on.
So where do the back-to-back nor’easters of early March stack up to those of the past? Well for starters, they were not very significant in terms of coastal erosion and storm damage. Strong north-northwest winds along the Mid-Atlantic during the first storm spared the coast from the pummeling Boston and most of coastal New England received from high waves and large storm surges. The second nor’easter generated strong onshore winds but moved fast enough to only affect the coast for a day.
Coastal residents in the Mid-Atlantic, however, did experience significant “nuisance” tidal flooding for a week, trapping coastal residents in their homes and forcing motorists to detour around flooded areas. In Atlantic City, minor flood levels were exceeded on nine high tides for a total of 32 hours and moderate flood levels were exceeded on three high tides for a total of 3 hours.
Accounting for Sea Level Rise
To assess the March storms’ ranking among those past, let’s take a look at how they compare to the March 1962 and the December 1992 nor’easters. To do that, we first need to realize that sea level was slightly lower back then. In fact, sea level has been rising along the New Jersey coast at an average rate of 4.07 millimeters per year for over 100 years. In 1962, sea level was 9 inches lower than it is today and in 1992, sea level was 4¼ inches lower. If we reduce the observed water levels at Atlantic City during the back-to-back nor’easters by 4¼ inches, we will basically create the water levels that would have been observed for the same back-to-back storms in 1992.
Figure 1 below presents a comparison of the observed water elevations at Atlantic City in March and the 1992 adjusted water levels. Here I have used elevations in feet relative to the North American Vertical Datum of 1988 (or NAVD88, which is roughly equivalent to what people presently consider the mean waterline) for the comparison since NAVD88 is a geodetic datum unaffected by changes in mean sea level. Reducing the observed water levels by 4¼ inches eliminates all of the moderate flooding experienced during the March nor’easters and reduces the minor flooding from 32 hours to 18 hours spread over eight high tides.
The impact of sea level on the tidal flooding is even more pronounced if we adjust the observed water elevations to the 1962 mean sea level (Figure 2). Reducing sea level by 9 inches (0.75 ft), the back-to-back nor’easters would have only generated minor flooding on five of the 12 consecutive high tides for a total duration of 6.9 hours. None of the high tide cycles would have generated moderate flooding and the impact would have been limited to the March 2-4 time -period.
In terms of magnitude, the March 1962 Five-High storm reached a maximum water elevation of 5.83 feet above NAVD88, exceeding the major flood level of 5.38 feet above NAVD88 by half a foot. In comparison the back-to-back March 2018 nor’easters would have only generated a maximum water elevation of 3.92 feet above NAVD88, well within minor flood levels. Similarly, the December 1992 Nor’easter generated a maximum water elevation of 6.27 feet above NAVD88, exceeding the major flood level by just under a foot. The 2018 nor’easters would have only generated a peak water elevation of 4.32 feet, still within the minor flood limits.
Minor Storms to Become More Serious Events
In conclusion, the back-to-back March nor’easters were insignificant compared to the historic storms of the past. But why was the impact perceived as so significant within our coastal communities? The answer lies within the persistent slow rise in sea level that has occurred over the past 100 years.
The addition of 9 inches of sea level between 1962 and 2018 has transformed a storm that would have been insignificant in 1962 into a storm that generated moderate flood impacts along the Mid-Atlantic seaboard in 2018. If we assume sea level will rise another 9 inches over the next 56 years — a rate many climate scientists will tell you is significantly lower than expected — then the back-to-back March 2018 nor’easters would become a very significant coastal event (Figure 3).
In 2074, water levels will be expected to exceed minor flood levels on three high tides, moderate flood levels on 10 high tides, and reach the major flood level limit at the peak of the storm. In total, water levels would exceed moderate flood levels for 21.7 hours and minor flood levels for 57.5 hours.
Figure 1. Comparison of observed water levels at Atlantic City in March 2018 and predicted water levels for the same period adjusted for 26 years of past sea level rise. The blue line represents the observed water elevations in 2018, the red line is the water levels adjusted to 1992 mean sea level, the horizontal green line is the minor flood level threshold, and the horizontal red line is the moderate flood level threshold.
Figure 2. Comparison of observed water levels at Atlantic City in March 2018 and predicted water levels for the same period adjusted for 56 years of past sea level rise. The blue line represents the observed water elevations in 2018, the red line is the water levels adjusted to 1962 mean sea level, the horizontal green line is the minor flood level threshold, and the horizontal red line is the moderate flood level threshold.
Figure 3. Comparison of observed water levels at Atlantic City in March 2018 and predicted water levels for the same period conservatively adjusted for 56 years of future sea level rise. The blue line represents the observed water elevations in 2018, the red line is the water levels adjusted to 2074 mean sea level, the horizontal green line is the minor flood level threshold, the horizontal red line is the moderate flood level threshold, and the horizontal blue line is the major flood level threshold.