[Coral-List] impact of beach rock on erosion
rbourke at OCEANIT.COM
Mon Aug 11 17:00:10 EDT 2014
Barbara & List;
The Maldives, among other atoll communities, will be the first to experience the effects of any change in ocean dynamics. These islands are essentially very small sand dunes, somewhat stabilized by vegetation resting on top of very large coral reef systems. Their natural condition is not to be stable, but to move and shift in accordance with long term changes in physical ocean processes. When these resorts were established there was an incorrect assumption made by the developer that the location of the land would remain stable. As a biologist, I've come to realize that many of the biological ecosystems in the tropics are heavily influenced by the physical processes surrounding them, that have little to do with biology. I've been fortunate enough to work with some very talented coastal engineers who understand these phenomena and we are able to bring this information into consideration when faced with problems such as those you are experiencing.
One of our engineers is originally from Sri Lanka and visited a Maledives resort (not Baa) in 1994 at the request of their government. By analyzing grain size distribution around one of the islands and conducting simple underwater observations, it became clear that this particular island had been migrating in towards the lagoon for many decades. To previous inhabitants of the islands (who rebuilt their homes on a regular basis) this slow shift was neither apparent nor important. But as soon as the concrete was poured for the hotel, it provided a baseline from which the annual movement could be measured. The presence of a "beach rock" reef on the ocean side of your island is further evidence that Baa has been shifting in towards the center of the lagoon for a long time - likely many hundreds of years. It is quite likely that if you look into the fossilized remains of invertebrates contained in the beach rock on the ocean side of the island, that these invertebrates will actually be from taxa that are more prominent within the atoll lagoon. Controlling, or at least minimizing, the rate of shift of these islands can be done, but needs an understanding of the physical oceanographic processes impacting each specific island. Each solution for each island on each atoll will be different and you should be cautious about those who will offer solutions without first understanding the dynamics of your specific setting.
From: coral-list-bounces at coral.aoml.noaa.gov [mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of Barbara Gratzer
Sent: Friday, August 08, 2014 11:06 PM
Cc: Ahmed Nazim
Subject: [Coral-List] impact of beach rock on erosion
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!
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