Thursday, November 22, 2012

Think storm surge

Hurricane Sandy reminded mariners once again that it is very often the rise in water, called the "storm surge," that causes the worst destruction, especially to boats. We have all seen the photos of piles of boats washed out of marinas, boats sitting in places where they were never meant to go, and entire marinas just gone. And, most of that destruction was due to the tremendous surge of water brought ashore by the storm. At New York's Battery (the southern tip of Manhattan), the record-setting storm surge was more than 14 feet above mean high water. Boats hauled out of the water on nearby City Island were no longer safe as waves rolled ashore.

My boat was hauled about a week before Sandy, by pure luck, and the surge was only three or four feet  above high water at the boatyard well up Narragansett Bay. If it had been 14 feet of extra water I have no doubt I would probably still be trying to figure out how to retrieve my boat from well inland.

In retrospect, I think being on a mooring is probably the best place for a boat during a hurricane, as long as the boats are spaced widely enough to allow for extra scope. In the past, I have added extra long lines to my mooring, doubled them up, and then also put out anchors on very long scope. My theory being that in the worst case scenario the mooring would act like a giant kellet, or anchor weight, which is what some boaters use to increase holding power when at anchor. This scheme worked well for me during Hurricane Bob and Tropical Storm Irene. Despite being on a mooring, the pull on the anchors was so great that it took the better part of a day to retrieve the anchors, indicating they had done their work.

I use some big Fortress anchors for this purpose as they have enormous holding power for their weight and are relatively easy to handle in the dinghy. I use long lengths of nylon rode--the more the better--with only a six-foot or so length of chain near the anchor. Scopes of 10- or 20-to-one allow for plenty of storm tide rise.

Is there a huge tangle of anchor lines and mooring painters after the storm? Yes! But, I prefer the tangle at the mooring to the problem of untangling my boat from powerlines and trees ashore.

If you don't keep your boat on a mooring, I highly suggest searching for a spot where you can anchor her before a hurricane. Keep around and handy some big Fortress and/or Danforth anchors for this purpose. I know that not everyone can do this, but as Sandy demonstrated many marinas are not designed for hurricane storm surges. I can remember being in the downtown Waterside marina in Norfolk, Virginia, during one fall gale with the fixed docks completely under water and the tops of the pilings only a few feet above that waterline. That would not be a place to stay during a direct hit by a hurricane.

Assuming your boat and marina was not destroyed by Sandy, now might be a good time to go down and measure how much piling is showing at the next moon tide, and then compare that to historic storm surges that have hit your area. I see and visit way too many marinas that would fail this test. In the future I'm also going to pay more attention to how far above high tide my boat is hauled and stored.


Anonymous said...

Storm surge seems still poorly understood and unpredictable.
Imagine Sandy making landfall 150 miles away from western Lomg Island Sound but causing historical surge over such a tremendous area.
My marina in Glen Cove has pilings which dealt well with the 7' surge having 6' in reserve but farther west marinas were destroyed when the rise floated docks or rose above breakwaters.
Bryon Norcross of TWC has a blog where he detailed Sandy. Like...why did Sandy stall near Cape May for 2 hours before making landfall at Atlantic City?

John J. Kettlewell said...

It is helpful to speak to "old-timers" in your area, and to read about previous storms that hit. NOAA has some pretty good storm surge prediction tools that can help too. The problem is that because these big storms are such infrequent events a lot of building and coastal change goes on between each one, and there is a huge financial incentive to build for ordinary bad weather while ignoring the threat from the once-in-a-lifetime storm.

Unknown said...

Norcross takes issue with the initial surge prediction from the local NWS being on the low side while the NHC had a more accurate surge prediction. The mayor of NYC was also slow to react to the surge potential, according to BN.

I remember the storm surge of Isabel in 2003 wasn't accurately predicted by local forecasters. It caused 7' damaging flooding in many low-lying communities in the Chesapeake Bay.


John J. Kettlewell said...

Nobody should be basing their storm strategy on a precise number predicting the likely storm surge, but rather on the highest potential surge. I can never understand these marinas with pilings extending only a few feet above normal high water, and now many of them are just gone down in New Jersey. The reports I was reading right before Sandy hit were predicting about 14 feet or so on lower Manhattan, which I think is about what they got.

Rachel Matteson said...

I saw some of the most dangerous surges that you've been talking about and it really is very destructive. Lucky for those whose boats have not been dragged along but if the surge is too high, it is almost impossible to escape it.