Waves from more than one direction

In answer to a comment I received yesterday about the sand ripple pattern photographs I posted, I do think that sometimes the complex sand ripple patterns on the beach are created when the water coming ashore hits the beach or recedes from the shore in different directions, at varying angles. These videos show very shallow water waves gently breaking on the sand in that way, and then ebbing, flowing over the sand ripples they are shaping, on the beach at Rhossili in the Gower Peninsula of South Wales.

 

Erosion of the tombolo at Dogs Bay

View of Dogs Bay, Connemara, Ireland

Looking northeast from the offshore island towards the white calcareous sandy beach and dune system, known as a tombolo, that connects the island to the mainland at Dogs Bay in Connemara on the west coast of Ireland.

Dogs Bay is a famously beautiful sandy beach in Connemara on the west coast of Ireland. The beach is actually part of a geomorphological feature known as the Dogs Bay/Gorteen Bay tombolo. A tombolo is a spit or bar of sand or gravel connecting an island to the mainland or another island. In general terms, persistent winds from the southwest have meant that waves meeting the island and wrapping around it, slow down as they converge on the northeast and landward side of the island, where force and speed of the waves decreases and they deposit their load of sediment. Over time the sediments gradually accumulate to such an extent that they rise as a bar above the water. The sediment bank eventually stretches all the way from the island to the mainland, and the connecting bar is termed a tombolo.

The sand has become stabilised by the growth of vegetation; and at the present time is a very special and rare type of habitat known as machair. Machair only forms on calcareous soils. At Dogs Bay the sand is  composed mainly of minute fragments of the carbonate skeletons of marine animals such as sea urchins and their spines, sponge spicules, bryozoa, seashells, snails, and most remarkable of all, the intricate microscopic skeletons of one-celled creatures called Foraminifera. Dogs Bay is one of only a few beaches in the world with predominantly Foraminiferan sand.

Machair, and the surface of the Dogs Bay tombolo, is unlike many of the coastal dune systems that I have visited in England, where the dunes are full of peaks and troughs, and where marram grass dominates. Marram is often a major initial factor in the stabilisation of the loose grains. Here, however, a grassland vegetation of low species diversity is encouraged to grow in a moist, cool, windy, oceanic climate on the fairly level and compacted alkaline soil of a mature sand dune system, and grazing by animals is vital to the maintenance of the habitat. Useful information about the features in this area is available from an on-line field guide produced by the Irish Quaternary Association. Click here for details of tombolo formation and machair habitat (pages 13-17).

Arriving at Dogs Bay, it was clear to see the impact of the earlier winter waves. Storms in the first few months of the year had ripped up and washed away the road and the car park at the entrance to the beach. A sign post now lying on the shore showed where it had been. On scrambling down to the beach, a close inspection of the wonderful curve of the dunes at the top of the shore revealed that the leading edges had been sliced away leaving hanging sheets of machair turf and huge clumps of vegetated dune material on the shore. Wooden posts, perhaps fencing from the top of the dunes delineating boundaries and preventing grazing animals from falling over the edge, were lying loose on the beach below, sometimes apparently supporting  the dune-top hanging mats of vegetation.

The tombolo is a vulnerable feature of the landscape. It seems that there is a history of natural damage to it in this location. In pictures 4 and 5 of the gallery below, the cross-section through the eroding dune shows a narrow horizontal dark brown band about half way down the vertical surface. This is a richly organic ancient soil level (palaeosol) that is associated with archaeological remains such as shell middens and the remains of a settlement, showing that people in the past used the site and exploited its marine resources to augment their diet. The palaeosol  is present at each end of the line of dunes but is absent in the central part. Its absence in this part could indicate that either the dunes at that time in that place were not stabilised by vegetation, or that the centre of the tombolo has been severely eroded in the past and recovered from the damage.

Apparently, about ten years ago, local people feared that the tombolo would be totally breached. Maybe it was at that time that steps were taken to prevent destruction of the spit. It is clear from the presence of rock-filled metal gabion cages on the beach that conservation measures were in place prior to last winter’s storms that battered much of the coastline around Ireland and Britain. However, at the time of my visit on a cold, wet, mostly dull day at the end of March 2014, it was evident that more of these measures might be required in future to prevent further damage.