[Coral-List] impact of beach rock on erosion

[Dennis Hubbard]

Hi Barbara:

PLEASE do not follow you instincts. There are probably others who are
better versed on the theoretical mechanics that I am, but I have seen a lot
of coastal erosion related to structures placed for "protection". These do
in fact induce significant turbulence which often results in undercutting
and drawing sand from behind the structures near their ends. It is common
to see the remnants of these structures offshore and. like with beachrock,
waves often break on them. However, they typically have little effect on
the beach behind and, in some cases will help slow erosion.

Think of it this way. Yes, there is turbulence around the beachrock as is
the case with artificial seawalls. However, when the waves break across the
beachrock, two other important things also happen. First significant energy
is either lost or transferred (from potential to kinetic energy) in the
breaking process. The turbulence in combination with the dominantly
landward flow will result in more sediment moving toward the beach than
away from it. Thus, the lower energy at the beach (the rock is effectively
acting like a submerged breakwater) and the concentration of the turbulence
on the beachrock and the area immediately behind it results in less
offshore movement.

So.... do not fall into the trap of thinking that just because you are
making your breakwater/seawall out of naturally derived materials that it
will behave any differently than a concrete structure. Also, consider that
you will have to get that coral rubble from somewhere an that will likely
have unintended consequences on bottom roughness (which robs waves of
energy) as well as micro-habitat. This is not a "close-to-natural" solution.

I know that your intentions are good, but this is not the answer. Small
structures offshore might break up or re-focus wave action. On the
mainland, this could be large quarried rocks. However, the only source of
such material in your case is the shelf out front or other cemented areas
of coastal rock. The posts on Majuro highlight the folly of that approach.
So, the reality is that the island is paying the price for first world
carbon. However, in the case of the resort, they built in the wrong place
based on the resort paradigm that you need to be as close to the beach as
possible...... and I'm guessing that they took out all the stabilizing
beach vegetation and replaced it with open sand and palm trees. I spent a
lot of time as a coastal consultant trying to get resorts to think about
better setbacks and maintaining vegetation (in the Caribbean, that's sea
pea, goats foot and sea grape - all of which are the first things to fall
to the architect's pen.

Dennis


On Sat, Aug 9, 2014 at 5:06 AM, Barbara Gratzer <barbaragratzer at gmail.com>
wrote:

> Dear Coral-Listers,
>
> My question is not directly related to corals, however I hope there are
> some people out there who can share their experience, opinion and expertise.
>
> Maldives are well known to suffer from tremendous erosion problems over
> the past few years. Natural reasons are natural shifts (up to several
> metres per year), mass bleaching events, the tsunami in 2004, who probably
> allocated huge sandmasses, thereby influencing under currents throughout
> the Atolls, and loss of natural vegetation such as sand stabilising trees.
>
> I am working in a resort in Baa Atoll where we are trying to identify
> other mechanisms that influence beach erosion. Our aim is to use as natural
> techniques as possible to keep sand shifts to a minimum. It was suggested
> that beach rock, which has eroded over the past 20 years and now is about
> 15 - 20 metres away from the shoreline, additionally adds to beach erosion.
> We assume: Since a wave brakes when the wave hight is less than half of
> the wave length, the waves are crashing on the beach rock rather than on
> the beach, thereby creating high turbulence in between the rock and the
> actual shoreline where waves would naturally brake. We further assume this
> turbulence creates larger sand shift movements. We are wondering if beach
> rock, once exposed, should be removed and natural walls such as coral walls
> should be enhanced on the crest instead.
>
> Is there any available literature about currents near beaches, turbulences
> on the reef flat or impacts of exposed beach rock on currents?
>
> I am looking forward to receiving your answers and thank you in advance!
>
> Barbara
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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*"