[Coral-List] impact of beachrock on beach erosion

[Dennis Hubbard]

Gene makes an important point that we have largely ignored regarding
beachrock - it's rapid cementation when beach conditions are right. On
St.Croix, either Jeff Hanor or Paul Aronson drilled a test well in
beachrock on the eastern end of the island. He capped the well and let it
equilebrate before installing instrumentation a few months later.
Unfortunately, when they returned, the hole was already cemented in.

The lesson in this enecdote is that, if conditions are right, beachrock
formation is FAST by any temporal perspective. The key here may be what
constitutes "right". Certainly, this involves a sand reservoir in which the
mixing of fresh and marine waters can occur. So, the solution may be the
opposite of what has been proposed. The suggestion has involved doing
something to the beachrock to change nearshore beach dynamics. Perhaps the
answer is restoring a beach environment in which beachrock can form at
present sea level. Clearly, the rapidity with which the needed cementation
can occur will allow this.

By way of another post, I disagree that there are "adequate" engineering
solutions. There are adequate solutions and there are engineering solutions
in this locale. However, they are not mutual. The reality is that the
combination of physical factors described by Mike Risk, sea-level rise
caused by largely first-world carbon and the juxtaposition of a permanent
structure next to a very mobile shoreline have created an unsustainable
situation. Complex engineering models may be able to minimize the impacts
of turbulence around an artificial structure, but they will neither totally
eliminate it nor offset the realities of the anthropogenically driven
changes in the natural system.

Dennis


On Tue, Aug 12, 2014 at 2:46 PM, Eugene Shinn <eugeneshinn at mail.usf.edu>
wrote:

> Barbara,
>
> I have beat on and cored a lot of beach rock over the years. In the
> Persian Gulf it can form in one
>
> or two years and creates natural harbors for fishing villages. And then
> there is Bimini where some "alternative thinkers" speculate the
> submerged beach rock offshore is the remains of Atlantis. See paper below:
>
> /Shinn, Eugene A., 2009, The mystique of beachrock, International
> Association of Sedimentology Special Publication No. 41, 19-28/
>
> True beachrock is especially resistant to erosion, however, in many
> places in the Bahamas
>
> beach dunes cemented by calcite can be mistaken for true beachrock.
> Percolation or rainwater
>
> causes cementation in beach dunes and because of past sea level rise can
> now be found in the
>
> intertidal zone.That rock is often mistaken for true beach rock but is
> quite different. It is softer
>
> and easily broken and eroded. However, true beach rock is very hard. If
> you go to Loggerhead
>
> Key in the Dry Tortugas you can see extensive layers of intertidal beach
> rock still forming along
>
> the west side of the island. The rock there contains water pipes that
> were placed in soft intertidal
>
> beach sand for the Carnegie research institute lab back in 1904-1905.
> This island most likely
>
> would have been washed away long ago if not for the beach rock armor
> that is still
>
> forming. The same can be said for much of the swimming beach at Bimini
> in the Bahamas. If
>
> you have true beach rock it probably should not be removed. Gene
>
> --
>
>
> No Rocks, No Water, No Ecosystem (EAS)
> ------------------------------------ -----------------------------------
> E. A. Shinn, Courtesy Professor
> University of South Florida
> College of Marine Science Room 221A
> 140 Seventh Avenue South
> St. Petersburg, FL 33701
> <eugeneshinn at mail.usf.edu>
> Tel 727 553-1158
> ---------------------------------- -----------------------------------
>
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-- 
Dennis Hubbard
Chair, Dept of Geology-Oberlin College Oberlin OH 44074
(440) 775-8346

* "When you get on the wrong train.... every stop is the wrong stop"*
 Benjamin Stein: "*Ludes, A Ballad of the Drug and the Dream*"