Taller waves on low tide?

Discussion in 'Weather and Surf Forecasting' started by civilengsp, Jun 24, 2014.

  1. sisurfdogg

    sisurfdogg Well-Known Member

    Jun 17, 2013
    Many times surfers and other stakeholders offer to work with such projects in advance with various results. It always helps if the powers that be listen to recommendations, rather than go in with a pre set plan and have a public hearing just for the sake of compliance. Are there waves that people surf near the breakwater project. That is my number one concern as a surfer. If not, my second concern is how the project will affect the water quality of the surrounding marine environment. We really don't need all these coastal hardening projects, although the marine contractors and developers would rather us believe it is vital for our economic health and jobs. But how many jobs are lost once a beach and nearby fisheries are too polluted to produce fish and shellfish? Nearby hotels, fishing charters, dive boats, and the recreational interests of surfers, anglers, and beachgoers suffer. That is why I have the attitude. Nothing personal towards you. You came on a surf forum looking for answers, and you should expect a bit of negative feedback. I will give you points for being civilized in your approach, but beware of a wolf in sheep's clothing is an idiom I like to live by.
  2. Mitchell

    Mitchell Well-Known Member

    Jan 5, 2009
    I've played around with SWAN and worked with some actual coastal engineers (not pretend ones like me) design some projects using it. Like all models it's all about the data. Garbage in, Garbage out. If your data is accurate, and you've set up the model properly, the model will give you accurate results. There aren't actually as many variables as you might think, and the model generally has a handle.

    Is your topographic data accurate, and does it extent into deep enough water to capture all of the refraction effects?

    You mentioned the slope is really mild...if by this you mean there is a flat offshore profile, then you know that large long period waves will be affected by the bottom far offshore, potentially miles offshore if depths are shallow enough.

    You mentioned that you aren't seeing any bottom topo irregularities in the vicinity of the breakwater but how about further offshore? For large, long period swell a bottom irregularity in 20, 30, or even 50 feet of water could have a large effect on wave height, although at these depths you wouldn't expect the tidal swings to change things much, but in 10-20 feet of water it could be significant. What is the tidal range in this location?

  3. goosemagoo

    goosemagoo Well-Known Member

    May 20, 2011
    I'm way out of my wheelhouse here but do the questionable results involve average or peak wave heights? Does the model take into account the waves reflecting off the breakwater then running into the incoming swell resulting in doubled up waves?

    Depending on the tide, the resulting wave peaks could be much higher when the tide is at a sweetspot that allows the swell to reflect off the wall with max efficiency which in turn would cause bigger backwash type peaks. But, they would quickly subside and not affect the breakwater very much.

    Again, way out of my wheelhouse but this question had me scratching my head.
  4. sisurfdogg

    sisurfdogg Well-Known Member

    Jun 17, 2013
    The City of Boynton Beach, FL wanted to extend the north jetty at the Boynton Inlet (South lake Worth Inlet). It is one of the most dangerous inlets on the East Coast with the exception of the Oregon Inlet in the Outer Banks. It is very narrow, and on an outgoing tide if there is an east wind swell and an onshore wind, the volume of the outgoing water will stand the incoming waves way way up. The inlet curves at the mouth, so you can't see what you are getting into until you are in it, and if you try to turn back you will be broadsided and capsize. It has happened often, with loss of life on rare occasion. Inexperienced swimmers constantly get sucked out and some drown there.

    There was an outcry to lengthen the north jetty to make the inlet safer, at considerable cost and environmental impact. It was determined after lots of data modeling and back and forth between fishermen, lifeguards and city officials, that it was a fifty fifty chance that the project would do more harm than good. Refraction off the longer jetty was a big factor in the decision to scrap the project. Cooler heads prevailed.