Natur Cymru is a quarterly magazine about the wildlife and environment of Wales. As of 1st April 2017 Natur Cymru has ceased publishing. We hope this is a temporary situation. Back copies are still available for sale, please see our website for full details and to find out more about Natur Cymru and its role in reporting on Welsh wildlife www.naturcymru.org.uk
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 BangorUniversity.
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 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 ChesilBeach,
the crest is 13.3m above normal high tide and towers above a street of houses
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 LlŷnPeninsula. 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.
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.