By Andrew Cooper, University of Ulster
The islands of the Outer Hebrides of Scotland comprise the Uists, Harris and Lewis.
The isolated, rocky islands are fringed with beautiful sandy beaches that are often compared to those of the Caribbean, in appearance, but not climate! They are also famous for the unusual coastal features known as “machair” that occur there.
Machair (a Gallic word for a flat plain) is a flat sandy plain made up of wind-blown sand that is covered in grasses and is therefore a favoured site for grazing by local communal farmers known as crofters. It only occurs in western Scotland and Ireland where winds are very strong and persistent. The machair along the Uists is highest at the back of the beaches from which their sand is blown. Sometimes there is even a dune ride on the seaward edge of the machair, but the machair slopes landward and large areas are below present high tide level. They therefore need the protection of the dune which acts like a natural dyke. Both the beaches and dune/machair are made of predominantly of carbonate sand (the shells of dead marine creatures). The coastline has been retreating along most of this coast from many years due to rising sea level and a sediment deficit. There has been a long history of removal of beach sand for agricultural purposes.
A severe storm in 2005 caused widespread damage and loss of life along the island chain. It also caused substantial coastal erosion, particularly of the coastal dune and machair. At many locations, the erosion of the dune has left the machair behind it at risk of flooding from future high tides and there is much concern about the potential loss of farmland behind the dune. The erosion has also revealed some important archaeological sites- Iron Age ‘wheel houses’ (so called because they resemble a wheel in plan) have been exposed at the coast by the erosion. They are being carefully recorded by local archaeologists as successive high tides cause more erosion and reveal more parts of the structures.
Concern among the crofters (communal farmers) about potential flooding of low-lying areas behind the frontal dune has led to calls for coastal defences to be constructed and low-cost works involving tyres are being considered, despite the environmental damage they are likely to cause.
The photos show the spectacular beach at Seilibost, and traditional farming on the machair (the flat plain of wind-blown sand, which is the main area of arable land). Erosion of the seaward edge of the dunes, and machair, exposed archaeological sites and peat exposed on the foreshore, are all indications of shoreline retreat, which is promoted by low sediment supply and sea level rise.
By Andrew Cooper, University of Ulster
Anegada, the most northeasterly of the British Virgin Islands is a sandy island that sits on top of a Pleistocene reef that is now exposed above sea level. The island’s northern shore has a wide modern reef terrace that supplies broken down shell materials for a sandy beach that runs along the entire northern shore of the island for almost 20km. The fine white sand of the beach, palm trees and the aquamarine colour of the sea create a classic tropical beach landscape. The sparsely-inhabited island has a few tiny beach resorts (comprising a bar and/or a few holiday cottages) such as Cow Wreck Bay (named after the wreck of a ship carrying a cargo of cows), and Loblolly Bay.
Although the island is exposed to Atlantic waves, most of them break on the wide reef and lose most of their energy before reaching the beach. The beach is therefore well protected and historically the shoreline has been quite stable. Despite this, there are local areas of erosion that are probably created by local sediment scarcity as sediment moves along the shoreline. At one such area, west of Cow Wreck Bay, the coastline has been retreating for a few years. Unfortunately, this was one area in which a few holiday cottages had been built. One cottage has now (October 2009) been undermined by shoreline recession and is falling into the sea. Another will soon go the same way. Not far away along the coast to the east, is an undeveloped area where, ironically, the beach is advancing. That area, however, has a well-defined dune scarp that indicates that until recently, it had been eroding. The situation of alternating areas of erosion and accretion suggests that this stretch of coast, while protected from high wave energy, advances and retreats locally as the available sand moves along the coast creating local deficits and surpluses. Fortunately no shoreline stabilization has taken place on the north side of the island and the coast remains free to fluctuate in response to waves and sediment variability and so retain its natural beauty.
The photographs show the natural beauty of the Anegada north shore beaches on a stable section of coast, an area that was once eroding but which is now accreting seaward, and an area of active erosion. The collapse of a holiday cottage is also shown, which, while bad news for its owner, is good news for the beach.
By Andrew Cooper
Fire Island, NY is a barrier island that runs east to west on the southern coast of Long Island in New York. Like much of the coast in the area, the island has been heavily modified in an effort to slow the natural erosion and landward migration of the barrier island.
These efforts to slow the natural erosion involve massive fabric tubes filled with sand called geotubes (photo with truck). These tubes are a temporary effort to protect houses that are in danger of being damaged by seasonal storms. The tubes can not be moved when filled with sand, eventually degrade and fall apart over time, and pose threats to wildlife in the water and on land. Another measure used to slow erosion are groins made of concrete and placed in the water (photo with truck). These structures cause accelerated erosion in areas right next to them, often making the situation worse.
This beach is also being nourished in the additional photos. A large ship pumps sand from the ocean bottom to the beach. There, the water runs off and the sand is spread out on the beach by heavy equipment. This disrupts plant and animal life on the beach and makes is a very unpleasant place for a walk! The sand often does not match the composition of the sand that is naturally deposited on the beach as evidenced by different colors and grain size.
Fire Island, New York is an excellent example of a developed barrier island that is experiencing natural erosion. This makes Fire Island Beachcare’s Beach of the Month for July 2009.
By Andrew Cooper
Assawoman Island is in Virginia, USA on the peninsula of land between the Chesapeake Bay and Delaware Bay. Like many of the world’s barrier islands, Assawoman Island has been migrating landward for decades. Evidence for this can be found in the sea shells on the beach, multiple overwash fans (both pictured), and the occurrence of marsh mud balls on the beach.
This overwash fan formed during a storm when a low point in the dune line allowed storm waves to overtop the island and cover the marsh with beach and offshore sand. The process of overwash allows island to migrate towards the mainland by removing sand from the front of the island and adding it to the back.
Examining the seashells on the beach gives another indication that the island is moving toward the mainland. The dominant shell species on the beach are back-barrier fauna, such as oysters. These organisms lived in the estuary behind the island before they were buried by the island migrating toward the mainland. As the island migrates landward, these shells become exposed in the surf zone (where waves break). With each storm that passes, they are churned up and transported onto the beach. The dark colors of the shells indicate that they have not been exposed to the sun for long periods of time.
The large number of shells also highlights the lack of any public access to this beach. Months go by without much human activity at all making it a prime nesting site for the federally threatened piping plover. The pristine condition of Assawoman Island make it Beachcare.org’s June 2009 Beach of the Month.
By Andrew Cooper
Cedar Island, North Carolina, USA is an east – west trending island along the southern rim of Pamlico Sound. It is a fetch limited barrier island and beach, not subjected to open ocean waves. The breakwaters, visible in the photo, protect the landing for the ferry to Ocracoke Island. For more than 50 years there has been a well maintained fence along the road here to keep cattle and wild horses off the parking lot where cars line up for the ferry. As a result of the fence, grazing has been prevented on the western half of the island (in the lower part of the photo) while the eastern half of the island has been extensively overgrazed.
Where there has been no grazing, vegetated dunes have formed and a dark green rim of trees at the back of the island exists. On the overgrazed part of the island there are no dunes, very little forest and the island is almost featureless and flat. The beach on the ungrazed part is narrow and backed with dunes (see photo). The beach where the vegetation has been removed by grazers is wider and more or less merges with the island (shown in the photo of the Canadian Geese).
The normal yellow brown color of modern beach sands in the southeastern US is due to iron staining on shells and on the surface of quartz grains. Here on Cedar Island the sand is white because the iron staining (and the shells) were removed by long weathering in an ice age sand deposit which furnished the sand for the island. Note the double line of surf indicating that there are two offshore bars.
This photo was taken on a March, 2009 flight by Andrew Cooper.