[Coral-List] impact of beach rock on erosion
Rob Hilliard, imco
rhilliard at imco.com.au
Wed Aug 13 08:14:12 EDT 2014
I can only underline the advice posted by Dennis, Jacqui, Robert etc -
please do not consider removing beach rock as a simple or effective
solution to beach erosion, for all the reasons they note. As mentioned
to you in my 15th May email, many Maldivian resort companies try to
'over-fatten' or 'over-replenish' the sand beaches that lie in front of
their beach units and/or water bungalows that they built on both sides
of their 20 year leased sand cay - in the face of the substantial
semi-annual wind shift that occurs each year. The seasonal change to
the local wind / wave regime causes most sand cay beaches to alter,
sometimes quite dramatically, with the waveward beach
temporarilyreducing markedly to expose much beach rock and sometimes
exposing/eroding the roots of littoral palms, mangroves and other
vegetation - much to the annoyance of tourists who have been allocated a
unit that borders that beach during its eroded season (hence the sand
bagging and/or sand pumping operations that are often undertaken to
'preserve' the beach for that season).
The most rapid beach changes typically happen in the late May-June
period - when the beach sands can get rapidly shifted from one side of
a cay to either a particular spit, or get 'lost' to a nearby cay (or
dredging pit), or else eventually get swirled around the cay to end up
'fattening' the opposite beach . The exact movements of the sand grains
depend on each particular island's position, aspect and seasonal wave
exposure, including its location and distance from the atoll's 'rim'.
The subsequent 'recovery phase' typically takes place naturally and
gradually following the NW monsoon, between late October and December.
Over decades, the overall net change of the seasonal shifts is often
skewed towards one particular direction - causing the sand cays to
slowly but naturally 'migrate'. For example, some notable historic
structures on Ari Atoll that were well recorded as constructed and/or
used on the west side of certain large sand cays, are now located on
their eastern side, and some have dissappeared completely. 'Migration'
of sand cays - particularly those on atoll rims - is a long term
natural effect, and removing the beach rock will tend to accelerate
rather than slow this process. Replacing or 'bolstering' it with
offshore artificial structures has proved expensive, is usually
unsightly, and requires expensive and often biannual maintenance - much
to the delight of local contractors. The loss of naturalness caused by
the artificial structures also causes a downward pressure on resort
popularity/bed demand and associated revenue.
Good luck with your studies. A very useful study of sand cay beach
movements on south Ari Atoll was commissioned by Villa back in 1997/98.
This study was by Dr Jim Searle (a specialistin beach geomorphology),
and a copy is probably still available if you contact Villa (the study
was for LeProvost Dames & Moore and supported by the IFC).
Robert Hilliard PhDPg.Dip (EMS)
InterMarine Consulting Pty Ltd
19 Burton Road,Darlington
Western Australia 6070
Mob: +61 427 855 485
Office: +61 8 6394 0606
Fax: +61 8 9255 4668
*rhilliard at imco.com.au <mailto:rhilliard at imco.com.au>*
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On 12-Aug-14 6:00 PM, coral-list-request at coral.aoml.noaa.gov wrote:
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> Today's Topics:
> 1. Re: impact of beach rock on erosion (Dennis Hubbard)
> 2. Impact of beach rock on erosion (Jacqui Michel)
> 3. Re: impact of beach rock on erosion (Robert Bourke)
> 4. Coral Restoration Foundation in Key Largo seeking a
> Development Officer (Coral Restoration)
> 5. Funding opportunities for research at Lizard Island, Great
> Barrier Reef (Anne Hoggett)
> Message: 1
> Date: Mon, 11 Aug 2014 10:34:09 -0400
> From: Dennis Hubbard <dennis.hubbard at oberlin.edu>
> Subject: Re: [Coral-List] impact of beach rock on erosion
> To: Barbara Gratzer <barbaragratzer at gmail.com>
> Cc: Coral-List <Coral-List at coral.aoml.noaa.gov>, Ahmed Nazim
> <nazim at cms.li>
> <CAFjCZNYbko0ou8UtUxv6LGg8s+EKo3umdxOqHGGTgu7R8BumDg at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
> 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.
> On Sat, Aug 9, 2014 at 5:06 AM, Barbara Gratzer <barbaragratzer at gmail.com>
>> 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!
>> Coral-List mailing list
>> Coral-List at coral.aoml.noaa.gov
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