Andy Rindsberg wrote:

>David Kirsh wrote,
> >  Andy, what's riprap? What about offshore barriers to attentuate the
>power of the waves and provide habitats for fauna?
>
>Riprap consists of boulders used to armor a riverbank, shoreline, roadcut,
>etc., and is not recommended if you want to preserve the beach as a beach.
>
>
>Offshore barriers are still in the experimental stage and I can't speak
>authoritatively about them. So far, they seem to cause more problems than
>they solve, for reasons that have little to do with any particular design.
>The barriers are hazardous to swimmers and boaters. Aesthetically, they are
>discordant and ugly. Biologically, they add hard substrates to shores that
>had none. But worst, they either don't work as advertised, or they do work
>and make sand accrete onto the protected beach while unprotected beaches
>down-drift are starved of sand and therefore undergo erosion. Now that this
>is a predictable effect, landowners sometimes succeed in suing up-drift
>landowners. The ultimate culprit in such cases is most often engineering
>works that cut off the supply of sand at the source for a whole stretch of
>coastline.
>
>Another culprit is building too close to the shoreline. If a storm wave hits
>a wall, then part of the energy reflects back onto the sand, which is then
>transported seaward. If a condo, for instance, is built too close to the
>sea, then it can increase erosion on its own beach. This is a predictable,
>common-sense effect that you can test for yourself by planting a board
>vertically in the sand parallel to the beach where the waves can hit it.
>Watch what happens to the sand on the seaward side of the board. In your
>imagination, scale that up to the size of a seawall or a condo. You don't
>even have to imagine a storm of the scale of Hurricane Ivan to be convinced
>that building too close to the sea is a bad idea.
>
>Global sea-level rise. Hm. It's been pretty slow so far; on some coasts
>(such as the northern Gulf of Mexico, especially Louisiana), subsidence is
>more rapid and therefore more important, if equally inexorable. Cut off the
>source of sand and a shoreline may be severely eroded in only a few years,
>not decades.
>
>I hate to say it, but I have to agree with Orrin Pilkey: If the decision has
>been made to protect houses on a beach, then the most effective and least
>offensive way to do it is by beach renourishment. But I also agree with him
>that it's better to build farther inland to begin with. In the long run, you
>get either an armored coastline or ruined houses, neither of which is very
>attractive.
>
>Andrew K. Rindsberg
>

Your comments have been spot on. Here in SE Fla the Army Corps of
Engineers has made a career of  dredging sand from the inter-reef areas
to renourish the beaches in front of multi-million dollar houses with
private beaches.

Yer tax dollars at work.

The details of these projects can be obtained on CD, free, from the ACE.
(Shore Protection Project general evaluation report with final
environmental impact statement)

Each has detailed impact studies on the flora and fauna. Of the ones I
have here on my desk, NONE mention mollusks.

After the sand has been pumped on the beaches, it becomes a very popular
spot for shell collection. Most of the shells though, and all the larger
ones, have been smashed.

I have dove the borrow areas, both right after renourishment and years
after. The pits do not fill in anywhere near as quickly as I would have
thought. Right after renourishment, they are good collecting sites,
especially for helmets. Later, the pits collect a lot of trash and are
pretty much barren. The pits last longer than the renourished beaches do.

There is also a lot of sand and sediment thrown up on the reefs during
these projects. Initially, there is some degradation of the reef, but
there does not seem to be any long term effect on otherwise healthy reef.

As you point out, onshore currents (which run south here, just off shore
the currents run north) constantly move the beach. Yet borrow areas seem
to be north of the renourished beaches.

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