Waterproofing New York City

When unusual events occur we often get asked to assist on unusual remedial projects. As marine guys we never expected to get called in on 9/11, but we were, and after Sandy we have been asked to assist with waterproofing design of NewYork City subway stations.

This is interesting work, because it allows us to interact with very smart people in other engineering and design specialties and the interaction between different disciplines often results in the most innovative and effective solutions.

Also things that may be second nature for one discipline may not be second nature for another discipline. As maritime engineers we care about wave heights, but not storm surges (since ships are supposed to float on top of storm surges). On the other hand, when considering shore based installations, storm surge becomes at least as significant as wave heights.

This blog looks at how high the water went during Sandy and what to expect in the future.

New York City/New Jersey/Long Island is a rather uncomfortable flood zone, but, fortunately it is not like the Netherlands where a dike breach will result in massive rapid flooding of large areas with few escape opportunities. In the New York Metropolitan area sufficiently high land is almost always close by. However, economic impact potentials can be quite impressive with severely flooded infrastructure and, in the case of hurricane force winds, very high wave impacts in its coastal areas. Therefore it is certainly useful to figure out how high the water can rise in a really bad storm.

Storm surge is the increase in water level above the expected tide for that day. A storm tide is the expected tide at the peak of the storm plus the storm surge.

Storm surge does not take into account wave heights that impact the shore. Therefore, a storm surge of 15 feet and 9 foot high waves will result in wave impacts of 19.5 feet above the expected tide level (Since wave height is measured trough to crest, but even that is fuzzy since there may be breaking waves).

As such: Wave Impact Height = ½ Wave Height plus Storm Surge plus Normal Tide.

There is actually a lot of confusion about this terminology and many numbers are simply incorrectly stated.

The present method for predicting storm surge is to use the SLOSH computer modeling system. In various publications it is now suggested that a storm surge in excess of 32 feet is possible in New York City if a Category 4 hurricane were to strike the city at the most uncomfortable location and at the most uncomfortable angle. That is a lot of water, but this number may be misleading since a 2010 Journal of Geophysical Research paper indicates, using SLOSH modeling, that the maximum surge that can be expected in NYC is about 5 meters (16 feet, but this is true surge, not related to the state of tide). This paper indicates this is a very rare event (1 in 50,000 years; probably less likely to occur than a major NYC earthquake), but add hurricane ocean waves, and we are dealing with a very impressive environmental force, especially along the shores. (This paper also indicates surges in the 3 meter (10 feet) range as a once in 500 year event.)

Nevertheless, the 2009 NYC Natural Hazard Mitigation Plan indicates that a Category 3 Hurricane could put Coney Island under 21 feet of water. This appears to include storm driven waves and therefore we are still not quite reaching these 32 feet heights. Within SLOSH modeling there are other numbers such as Maximum Envelope of Water, and also Maximum of Maximum (MOM). For New York City there are some predictions for MOM’s that are in the 34 foot range, but these are localized and rare events, and will not apply over the entire New York Metropolitan area.

So how much water could we expect in a really rare situation?

For reference, the highest recorded Atlantic/Gulf Coast surge that I could find was during Katrina at Pass Christian at 27.8 feet (this might be storm tide, other sources mention 25 feet). The highest recorded wave impact occurred in Biloxi at 34.1 feet even though the surge was less, at around 22 feet. (This source reported a 22 foot surge, a 1 foot tide and 11 foot waves. This is incorrect, since this should result in a wave impact height of 28.5 feet instead of 34.1 feet and therefore the highest wave might have been 22 feet which is quite high, but may occur occasionally)

The surge during Sandy was reported at 14 feet at Battery Park, and it did strike during high tide. In another source it is reported at 9.23 feet, and this may indicate that the 14 feet number relates to storm tide, since the late October high tide was about 5 feet above baseline.

In 1821 NYC suffered a storm surge of about 13 feet and it suffered a storm surge of about 11 feet from Hurricane Donna in 1960, but it is not clear if these are actually storm surges or, rather, storm tides.

Besides Sandy, a more personal high mark was a surge of about 5 feet on top of a high tide in the December 1992 Noreaster, which set the pre-Sandy high water mark in my boat club house. This storm also resulted in serious flooding in NYC, but paled in comparison to Sandy. The Sandy high water mark in the club house is about 4 feet above the 1992 Monmouth Boat Club high water mark, which would support that, during Sandy, we had a 9 foot storm surge atop a high tide.

So how special was Sandy? Comparison to the SLOSH model would indicate that we had a 100 to 200 year event. The SLOSH model also would indicate that in a worst case (1 in 50,000 years, ignoring sea level rise, which, compared to these numbers, is relatively small at this time) we could have a surge that is 7 feet higher than we encountered during Sandy plus much higher waves. But these waves we would only encounter along the coast and would be much lower at the Battery.

Therefore, for hardening the New York City core against the very worst flooding, we would do well looking at flood protection against storm tides (tide plus surge) of about 22 feet (coincidence of a high tide of 6 feet plus a very rare 16 feet surge), but we have to compare this against the reality that, as far as we know, New York City, since its founding, has never seen more than a 14 feet storm tide.

There are two other considerations. One is added impact of waves, and in this regard in the coastal areas exposed to ocean waves, Sandy did not begin to be a truly impressive storm and is nowhere near a 100 year storm. For this reason (and a number of other really good reasons) a good case can be made that we have no business at all in living in these exposed areas and it leaves one wondering why the government supports such poor risk management.

The other factor is global warming. At this stage it is incredibly difficult to predict how much the ocean will rise and if global warming will increase the frequency and size of storms, but if global warming played a factor in Sandy, due to only limited ocean level rises so far, it only amounted to a few inches, not feet.

Regardless, some planning to reduce the impact of storm surges should be money well spent. Maybe it will result in local waterproofing or maybe New York City will develop a Delta Works approach, but it certainly will be interesting.