Nov 15, 2010

Why It Didn't Work

From disappointment comes anger and from anger comes ignorance.  Most of us here at Folly Beach and the surrounding area were blown away when we arrived at the beach Saturday morning to find barely rideable waves.  But what happened?

Since Hurricane Season I've been really researching how waves are formed.  Initially I was curious how close a storm needed to get in order for us to receive waves.  I found out that it really didn't matter how close the storm got it mattered how intense it was and whether local and surrounding wind conditions would favor the swell.  Take this for example: California can receive a huge South swell from storms raging off the coast of New Zealand thousands of miles away.

Late last week the mega low pressure system that produced waves for beaches from the Mid Atlantic states to Southern Florida really began to strengthen.  Winds were blowing from the NE at a sustained 40mph plus for hours and hours on end.  That doesn't seem like much but the distance in which the winds were blowing in the same direction (called the 'fetch') was immense; nearly the whole Eastern Seaboard of the United States. The longer the winds are blowing in the same direction the longer the period the swell.  So with a fetch nearly the size of the Eastern Seaboard buoys were showing 15 second plus intervals with sea heights running into the 20ft range give or take.

We all love those longer period swells because we know they pack all the power to do your Jordy Smith double grab airs.  But if you call Folly Beach your home break you may want to rethink how long is too long of a swell period.  If you've been surfing around this region of the United States you've probably heard about the continental shelf, how big it is, and how it detrimentally effects those big swells.  I always took it a step further and asked why is the shelf was such a big deal.  I mean it's still pretty deep out there and it's just a wave on the top of the sea.  Right?

Yes it's deep out there but not deep enough for this past swell.  The continental shelf off the coast of South Carolina and Georgia can extend as far out as 100 miles plus.  This sets up a bad situation for long period swells.  I always thought most of a waves energy traveled near the surface of the ocean.  I couldn't have been anymore wrong.  Most of a swells energy travels well below the surface of the sea.  The formula below dictates how far down a particular swell period will travel below the surface.  And it really has nothing to do with the height of the wave it all has to do with swell period.

Depth of Wave = (Swell Period²)(2.56)

The buoys from this last big swell were showing an average of about 15 seconds.  If you plug 15 seconds into the above formula you get 576ft.  The deepest part of the continental shelf off of our coast doesn't even come close to that depth.  200 feet is the maximum depth of the shelf; and that is pushing it.  When that 15 second swell came barreling into the shelf more than half of the energy of the swell was getting eaten up by the ocean floor. 

Now that half the energy is gone from the swell it still has to travel through shallower and shallower water when it finally reaches the Washout.  Have you ever noticed waves doubling up during the long period swell events?  Doubling up is the result the continental shelf and shallow near shore waters refracting the swell making it a mess.  Making things even worse was the angle of the swell.  Folly Beach more or less is a Southeast facing beach.  A Northeast swell will not only be heavily angled when it reaches our shores it has even more time to decay over the continental shelf.

Hurricane Earl worked in our favor because he was so close to our shoreline.  The swell period didn't have enough time to formulate into a mega long period swell.  Earl actually produced waves so close to the continental shelf the swell just formed in the presence of the shelf instead of being impacted by it.  This is usually the case with most of our hurricane swells.

Hopefully we've learned from this past weekend what our coastline and beaches can and can't handle.  Of course I am no scientist I am just curious.  I've done a little research and from that research, I've compiled this
post.  Some of the information may not be totally accurate but I've tried to make it as accurate as possible.  Please leave me a comment by scrolling to the top of this post and clicking on the comments link if you have any questions, corrections, or general comments.

Thanks for making it all the way down here and taking the time to read everything.

Mike C

sources: surfline, stormsurf, noaa


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  1. I think it has alot to do with Cape Hatteras blocking the NE swell. Looking at the NOAA models, there was a definite shadow. If it had been a little more of ENE than E, it would've made it in here. Jacksonville has a pretty wide shelf, but it was good down there this weekend. The shelf slows it, but the Cape blocks it, for whatever my opinion is worth.

    Anyway, FL south of Cape Canaveral was still overhead yesterday.

  2. Hmmm, if this were totally true, wouldn't Florida suck the same as us. I need to go back and look, but I'm pretty sure Florida got a killer swell here as predicted. Looks like they have more shelf than us between a NE swell, so not sure how much the shelf did here to kill our swell. Obviously the swell angle comes into play too, but I'm not sure the shelf theory holds up when Florida ( even further away from the source ) rocked..

    BTW, I don't understand it myself, so I'm just another PO'ed surfer trying to figure it out.

  3. Yes Cape Hatteras and the steep NE angle made it nearly impossible for the swell to creep in here. I tried to show that in the map.

    I'm not entirely sure but I think when I was looking at the depths of the continental shelf off the coast of here as opposed to off the coast of Florida, the shelf is deeper the further south say from Georgia.

    But yeah I got to thinking last night if the swell had some more east in it I don't think I'd be writing this. All in all though, the angle of the swell and the prolonged period of it being refracted off the shelf made for a pretty terrible situation for us

  4. Well according to my mathematical calculations it seems to appear that the fetch hit catch and the vertical wind swell models dont seem to line up with my cross references. Blah Blah Blah you can't predict Folly Beach. Everyone knows that!

  5. Folly is very predictable yo... it's not some island in the Carribbean... -Dig the blog, btw

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