Wednesday, 8 January 2014

Shingle - great coastal defence and good for wildlife

Shingle beaches are fascinating places with their own unique flora and fauna. They also perform a great service, absorbing wave energy and protecting the coast from erosion. We should make full use of the ecosystem services they provide, argues Ivor Rees,a marine biologist, and formerly a senor lecturer in Ocean Sciences at Bangor University.

Morfa Abererch shingle beach
Shingle beaches may not have the immediate charisma of some other coastal habitats but such apparently harsh environments and the plants and animals colonising them have much intrinsic interest. Even without the biological interest, the shingle features around Wales have long been recognised for their geomorphological importance. Recognition of the dynamic functioning of such features is vital for understanding their ecology, but it is even more important given the interplay of rising sea levels and the role shingle plays in coast protection.

Wales has a legacy of settlements, roads and railways placed in the lee of, or even on top of shingle banks. Recent estimates are that sea level around Wales is rising at 3.1mm per year and revised predictions by the International Panel on Climate Change are of rises of 0.6–1.0m by 2100. Striking a balance between allowing the sea to adjust such natural features and imposing 'hard' engineering is one of the most challenging marine conservation issues of our times. In spite of the extent to which coastal geomorphology has featured on the educational syllabus, shingle ridges and storm beaches are perhaps the most misused coastal habitats.

Shingle formation
Shingle banks and ridges are formed by the swash of storm waves throwing pebbles onto the crest well above the reach of normal waves. Owing to rapid percolation through the interstices, backwash is reduced, so wave action tends to be constructive. In storms shingle can be thrown to considerable elevations, thus the ridge generated by the sea can be several metres above the land behind it. In the extreme example at the Portland end of Chesil Beach, the crest is 13.3m above normal high tide and towers above a street of houses behind it.

Shingle stability and the availability of humus are key factors which permit succession from bare shingle to full vegetation. The first signs of relative stability come with the colonisation of the pebbles by lichens. Accumulation of humus is more erratic, as much of it ultimately comes from the strandlines of seaweed and other flotsam. Some plants that can cope with some instability thrive along strandlines, giving rise to lines of vegetation formed mainly of sea-beet Beta maritima, several oraches Atriplex spp. and curled dock Rumex crispus. Another factor is the availability of fresh water, allowing less salt-tolerant species to colonise. In addition to rainfall, day–night heating and cooling causes dew to form within the interstices between the pebbles.

Vegetated shingle is quite a scarce Biodiversity Action Plan habitat around Wales. The total area in the whole Principality, as estimated by Natural Resources Wales, is only about 810 hectares. Put in context, sand dunes cover over 70 times as much. A particularly fine example occurs at Morfa Abererch on the south side of the Llŷn Peninsula. Here, as a result of long-shore drift, shingle has accumulated against a rock headland as a series of parallel ridges. Even the oldest ridges are still very sparsely vegetated. Up close the stable pebbles are seen to have been colonised by lichens, with English Stonecrop Sedum anglicum in the crevices between. Further out, spectacular clumps of Yellow-horned Poppy Glaucium flavum occur, with Sea Campion Silene maritima growing in patches. The sequence of colonisation can also be clearly seen on the shingle spit at Abermenai Point on Anglesey, where a veneer of sandy turf has developed over the stable shingle. Although such places remain relatively undisturbed, a disproportionate and unexplained number of shoreline specialist species have been lost or contracted their range in Wales since being noted in the diaries of 18th century botanists visiting Wales. These include Cotton Weed Otanthus maritimus, last seen in the 1890s, and Oyster Plant Mertensia maritima, the last of which was destroyed by coast protection works.

The last site where Little Terns nest in Wales is at Gronant on a shingle feature. They nested on the great shingle bank between Criccieth and Craig Ddu and on the Cemlyn ridge into the 1930s and clung on north of Towyn into the 1970s. Buglife also lists a series of Biodiversity Action Plan (BAP) insects as exploiting shingle habitats.

Ecosystem services
Behind the legends of Cantref Gwelod and other drowned lands probably lie folk memories of storm surge events, when major changes happened to shingle ridges or they were breached. These events may have caused low-lying land to flood for long periods as stream outlets were blocked, or maybe river mouths diverted by shingle spits opened in different places. Even in modern times some shingle banks intermittently block streams, so water levels behind them vary erratically. This type of fluctuation used to occur at Ystumllyn near Criccieth until the outlet was diverted through a tunnel in the Craig Ddu rock headland.

It has to be remembered that the origins of many coastal features around Wales, and much of the gross morphology of the offshore seabed, can be traced to the glacial and post-glacial history of this part of north-west Europe. Indeed, most Welsh shingle spits, bars and bay head beaches came from reworking glacial till and outwash deposits. In only a few places has the shingle come directly from erosion of adjacent rock strata or, in even fewer places, quarrying of them. Paleo-oceanographic studies of relative sea levels during the last 12,000 years show the likely sequence of inundation, taking account of the melt of the major ice sheets and isostatic rebound. These imply that the genesis of the major shingle structures we see today may have been much further out to sea than the present coastline, and that they gradually migrated landward as sea level rose.

As such features are constructed by wave action, unless they are starved of further material their height generally keeps pace with sea level rise and they continue to protect areas behind them from flooding by waves. Because percolation takes up much of the swash passing the front of the crest, wash-over is limited. It is noticeable that it is more often where shingle is covered by hard impervious surfaces, such as promenades and car parks, that water from waves floods on to property. Making the fullest practicable use of the ecosystem services provided by shingle banks in their natural, unimpeded state has potential for long-term cost savings, as well as being best for safeguarding their ecological interest.

[This article was first published in the winter 2010 edition of Natur Cymru.]
The following film explains the dynamics of Ynyslas Spit and the role of shingle.