"Superstorm Sandy battered New York City with high winds and an unprecedented 13-foot storm surge, overwhelming hundreds of miles of coastline and extending its impact far inland. Its effects, from which the City is still suffering, include $19 billion of damage to the City’s parks, roads, water system, schools and buildings."
Yes...the storm surge wreaked havoc to low-lying areas. Yes...it was unprecedented. Absolutely no argument there.
But there is something a bit misleading about the 13-ft storm-surge statement, which is also published in myriad press reports on Sandy. Though it is technically correct, amongst the general population it understandably conjures up an image of a 13-ft tsunami coursing over the low-lying coastal areas. That's not what happened.
The actual surge, measured from street level, in badly flooded Lower Manhattan, for instance, was more like three ft or so above the street, says Langan Engineering. That was enough to do a huge amount of damage. It was, in many ways, disastrous. But though the surge was technically 13.8 ft, it was not literally 13.8 ft. If it had been, disaster would have turned to calamity!
Here's why: The 13.8 ft number was not measured from mean sea level, say geotechnical and civil engineers from Langan. Rather it was measured from "mean low low sea level," at Sandy Hook in New Jersey, at the time of the surge. The National Geodetic Vertical Datum, a U.S. standard, defines sea level as "00" ft at Sandy Hook. That elevation of 00 fluctuates with the tides.
Mean low low is the lowest elevation of the sea. It happens when there is a full moon. Ergo, at about 8:30 pm on Oct. 29, when there was a full moon, the mean low low (00) was at its lowest.
Here's Wikipedia's two cents about datums, much of which is too technical for me to understand! "The Sea Level Datum of 1929 was the vertical control datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. The datum was used to measure elevation (altitude) above, and depression (depth) below, mean sea level (MSL).
Mean sea level was measured at 26 tide gauges: 21 in the United States and 5 in Canada. The datum was defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment.
Since the Sea Level Datum of 1929 was a hybrid model, it was not a pure model of mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed the National Geodetic Vertical Datum of 1929 (NGVD29) in 1973. NGVD29 was superseded by the North American Vertical Datum of 1988 (NAVD 88), based upon an equipotential definition and a readjustment, although many cities and Corps of Engineer projects with established data continued to use the older datum."
In New York City, each borough has its own datum. Brooklyn has two, says Langan. The transit authority has yet another datum, because it did not want the subway elevations measured in numbers below zero. Too confusing for everybody.
At the Battery in Lower Manhattan, the base flood elevation is 11 ft above the NGVD, says Langan (I don't make this stuff up, I just report it!). The base flood elevation is a number, or datum, that describes habitable space at or above the 100-year flood level, as described on FEMA maps, which are based on the NGVD.
Not that a wall of water three or four ft high doesn't make for a disaster! Yesterday, I saw the aftermath of the power of the water, as I toured One New York Plaza on Water Street in the Battery of Lower Manhattan.The surge blew out the garage doors, taking all the cars on the ramps and on the first level below grade to the garage on the second level below grade. Some 30,000 sq ft of retail space was totally trashed on the first level underground, called the concourse. In all, three basement levels were filled to the ceiling with millions and millions of gallons of water.
Brookfield is in the process of drying out the basements and rebuilding. The plan is to move the switch gears to the lobby level.
This blog's clarification is likely and quite understandably lost on the storm's victims. They are too busy trying to rebuild their shattered lives. But for the rest of us, it is important to note that there is a gargantuan difference between 3 ft and 13 ft! If the differential had been 13 ft, there would have been millions more victims!