Monday, February 02, 2009

On Building Dunes

click the pic to enlarge

For the record, I am not a coastal biologist, scientist, or engineer although all three interest me, so be careful about using my words against anybody else. But I think folks may have lost a bit of common sense about "how to build a sand dune." And to learn how to grow dunes, you have to learn how the winds affect the shifting piles of sand.

So that ugly picture is a wind rose from Corpus Christi - the closest one I could find - that show wind direction, wind speed, and wind speed frequency. This one was for a 30 year average of days in January. What you see are two long barbs going to the northeast and the southeast. Other months such as July only has one barb to the southeast because there are no cold fronts with north winds in the summer.

But this means something, chiefly that building a "continuous dune line" will be frustrated from winds coming both directions, not at once of course but that significant dune shifting will result in the winter months, always eroding the face and creating a long slope behind the lee side of the dune.

To me this has two implications: that a "herringbone" type construction is needed instead of a linear one, and second that the idea is to build onto already existing humps, coppice dunes, and mature, vegetated dunes rather than piling sand and seaweed. In other words, any new dunes must grow naturally.

The herringbone construction is essentially a "vee" that slows down the wind from the northeast and southeast during the critical winter dune building months. Either wind direction, the project design works. Other designs, either laterally (north-south) or perpendicular (east-west) will both fail from either wind direction!

But human nature being what it is, folks want to "fill the gaps" with artificial dunes that to me, just won't work. All dunes have a certain wind flow around them and we will never have a true "continuous dune line" no matter what we do - and the air effects around tall buildings makes it even worse in the urbanized part of South Padre Island. We will always have dune gaps and wash-overs - and as long as condo roof drains are allowed to drain onto the Gulf side (a violation of the Clean Water Act), we'll have a horrible problem.

Not many people see things the way I do. I'm not going to get into what did and didn't work so hot. But using some common sense, one can build onto robust, mature dune and make them move up and down the beach. I'd use a ton of railroad vine roots and clippings, is my idea.

Finally, I wrote this because of an article in the Island Breeze which mentioned something like "if we don't vegetate the dunes, we'll get horrible hurricane damage." What a crock. Vegetated dunes were cut down like a butter knife by Hurricane Ike waves. The idea is just to slow down the damage a little bit. All I want is to bring some common sense to the table, no rocket science.


Rob Nixon said...

That Island Breeze article was full of inaccuracies as Carl often does in his pieces.

You're right, the idea of the dune line is to add another level of protection for the properties by dampening the wave energy, albeit temporary.

The damage in Hurricane Ike was exacerbated by gaps in the dunes Sam like I have said before. Water got behind the dunes and along the retaining walls and washed them out from behind as well as from the front.

As far as naturally building dunes, it is being done as naturally as it can be. The projects so far, where available used plantings to accumulate sand along with sand fencing to "grow a dune". In some cases these were done on sand that had to be cleared from retaining walls that had blocked roof drains.

What is being done is essentially a soft seawall. In lieu of that we could always ask the ACOE and GLO to allow one huge concrete seawall be built and then there would be no public beach or any beach for that matter.

If a storm of the magnitude og Ike hit south of SPI, it would be a moot point anyway, because the island would suffer the fate of Bolivar and Galveston anyway and there would not be any beach front property to protect anymore.

Sam said...

Good post Rob and no way I was criticizing Dr. Heise, maybe more to share some thoughts about what has worked in my experience - which was all blown out by Ike such as the dunes north of Surfside and SPI.

You mentioned one time about roof drains draining into the outside of the sea wall - is that correct? That and the notion that many condos like to scoop out in front on their buildings seems strange to me.