THE ENCYCLOPEDIA OF IRELAND

Transcription

1 THE ENCYCLOPEDIA OF IRELAND For publication by Gill & Macmillan, Dublin COASTAL & MARINE THEME ARTICLES: - (By Robert J. N. Devoy, Coastal Resources Centre, Department of Geography, University College Cork (NUIC), r.devoy@ucc.ie) 1. Coastal Erosion (including Global Warming) 2. Irish Sea 3. Gulf Stream 4. Continental Shelf

2 1. Coastal Erosion (including Global Warming) Coastal erosion results from a combination of wave and storm action, longer term sealevel rises, the strength of coastal rocks and sediments, intertidal zone width and human impacts. Ireland's coasts (c.7800km total length) are exposed to high wave energy (max J/m/yr), with breaking swell waves on Atlantic coasts reaching heights of up to 3m. The many types of coast that occur are variably susceptible to erosion. Rockdominated coasts (total c.3000km) are the least vulnerable (Photograph) (erosion rates are <c.0.01m/century). Erosion episodes though may occur here as sudden rock-falls and landslides. Coastal features associated with rock-erosion include steep cliffs (e.g., > 20m high Torr Head, Co. Antrim; Cliffs of Moher, Co. Clare), headlands and Bays (e.g., Galley Head, Co. Cork) (Photograph), sea-stacks, arches and caves (e.g., Co. Waterford). Coasts most vulnerable to erosion are those composed of unconsolidated (soft) sediments (total c.3500km), comprising glaciated debris, sandy beaches (c.1000km), sand and gravel barriers, dunes and machair. These coastal sediments are most common on Ireland's eastern and southern coasts, but occur as more isolated areas on western and northern coasts (e.g., Inch, Clew Bay and Lough Foyle). Erosion rates for these 'soft' coasts average m/yr, but often reach values of 1-2m/yr (Figure, Map). The effects of this erosion are visible as 'rapid' cliff retreat; onland movements of dunes and other barriers (rates of 0.6-6m/yr) and on beaches, the lowering of levels with the loss of the finer sediments. Rock and sediments eroded generally move alongshore, providing an essential source of materials for the development of beaches, dunes and estuaries at other coastal locations. Coastal erosion is often noticed most because of its impacts upon people (e.g., Dublin Bay). This erosion, whilst part of a natural pattern of environmental changes, may have human causes. In the future increased coastal erosion from global warming will be one consequence of human impacts on the environment. An approach to the treatment of such erosion needs careful management to ensure environmental sustainability. Assessments of global warming (warming caused by increased emissions of greenhouse gasses), predict increases of temperatures globally of 1.4 o -5.8 o C by c An outcome of this will be an acceleration of relative sea-level (RSL) rise and changing storminess patterns. Ireland's RSL rise at present is c.1.0mm/yr. Under climate warming this will accelerate to c.4-6mm/yr by It is predicted that Ireland will experience fewer but more powerful storms, with the northwest and southeast regions becoming more stormy. Under climate change scenarios c.1700km of the coastline is particularly vulnerable to change: currently c.350km of coasts have some type of defence structures. Literature: - Journal of Coastal Research, 17 (2002), R. J. N. Devoy, Coastal Vulnerability and the Implications of Sea-level Rise for Ireland; National Climate Data Centre: - Intergovernmental Panel on Climate Change, Shanghai (2001), A Report of Working Group 1: A Summary for Policymakers. RJND

3 [Coast erosion- two graphics: - 1 photograph & 1 map.] Coastal Erosion, Figure-Map. Zones in Ireland likely to be vulnerable to relative sealevel rise and coastal erosion under climate warming to 2100 (After Devoy (1990), Technology Ireland, 22).

4 Coastal Erosion, Figure-Photograph. Headland and Bay coastline, Galley Head, Co. Cork, showing the influence of rock structure in coastal erosion

5 2. The Irish Sea This is a semi-enclosed shelf sea, separating Ireland and Britain (Figure-Map), positioned between Lat. 51 o 40' - 55 o 0' N and Long. 3 o 0' - 6 o 15' W, c.360km in length from north - south and with a surface area of c.47,000km 2. Entry of ocean water and waves from the North Atlantic occurs through the North Channel (c.40km wide) and southward via the St. George's Channel (c.80km wide). Due to Ireland's protecting effect wave-energy in the Irish Sea is only c.20% of that on Atlantic coasts. Deep-water significant wave heights (H sig ) decrease from south - north (median values (H sig ) m), though maximum storm waves (H max ) reach >9m in height. Bathymetry in the Sea deepens along a northeast to southwest trend, reaching depths of >100m in the St. George's Channel. This Channel forms part of a series of semi-connected hollows (deeps), some reaching to -200m, aligned north - south in a trough-like feature along the eastern side of the Irish Sea. The 'trough' is probably of multiple origins and ages, incorporating deep geological structures (graben), palaeovalleys and glacifluvial scouring from the last c.100, 000yr. Most of the Sea region is shallow (<60m). The eastern margin of Cardigan Bay and the Cumbrian-Lancashire embayments (c.40% of the Sea area) is <40m deep. Geologically rocks of the Carboniferous - Devonian Periods form a basement to the region. Six main depositional basins further structure the Sea. These contain variable thicknesses of sedimentary rocks (3-6km thick) dating from the Upper Palaeozoic to the present. Igneous rocks of a wide age range also occur extensively. Former thick ice cover of the Irish Sea has led to continuing land uplift of northern areas (rates of mm/yr). Southwards marginal crustal subsidence occurs (e.g., Cardigan Bay <0.1mm/yr mm/yr in the Severn Estuary), though dispute exists about the accuracy of these trends.] Seafloor sediments comprise a relatively thin (3-60m) and very varied cover of former glacial and also marine sands, gravels and muds. The Sea has a patchy 'rim' of fine-muddy sediments with sands and gravel areas developed in deeper water, especially south of the Isle of Man. Concentrations of sands and gravels can also occur closer inshore (e.g., the Kish Bank, Ireland). These deposits are of significant resource value as building aggregates. Due to the complexity of coastal shape and bathymetry, then tidal patterns and current directions in the Sea are equally complex. Tidal ranges along the coasts vary widely, from microtidal (<2m) to macrotidal >4m (e.g., Spring Tidal range- south-east Ireland, 0.8m; Dundalk Bay, 4.9m; Morecambe Bay. >8.3m). The affect of the Sea on regional climates is significant; it's existence leading to increased autumn-winter precipitation over the neighbouring land areas. In summer the relatively cool seawater (July average 9-12 o C) encourages stratification of the overlying air. This may intensify the effects of air pollution from surrounding industrial regions. In the mid-late 20 th Century economic, resource and political matters concerning use of the Sea have assumed an increasing importance. Issues include those of marine animal and fish conservation (e.g., depletion of herring and cod stocks), emissions of radionuclides and other chemical pollutants, seabed resources, wetland conservation and coastal protection. Literature/ Further Information: - Marine Institute: Geographical Society of Ireland, Special Publication No.3 (1989), J. C. Sweeney (ed.), The Irish Sea: A Resource at Risk; Coastal Resources Centre

6 RJND [The Irish Sea- Graphic: - Map (Irish Sea Bathymetry)] Irish Sea, Figure- Irish Sea Bathymetry.

7 3. Gulf Stream This forms part of the main circulation cycle (gyre) of surface water in the North Atlantic Ocean, moving heat from the equator to the Arctic. Solar heated water from equatorial areas, collects around the Caribbean and the Gulf of Mexico. This is forced northwards as a 'jet' of water (geostrophic current), by wind action and the earth's rotation (Coriolis and horizontal pressure gradient forces), along the offshore zone of eastern North America. The Gulf Stream moves as a fast 'river' of warm water, reaching depths >600m (maximum velocities of c.2.0m/sec, with surface temperatures off Florida of >24 o C). Peak values of water transport of c.150mm 3 /sec occur at c.65 o W. Under the influence of dominant westerly winds the Gulf Stream fans-out progressively and diffuses heat eastwards. In this process the changing warmer water mass becomes the North Atlantic Drift (NAD). At c.30 o W the NAD splits. Cooled water returns southwards (the Canaries Current); remaining relatively warm water continuing on to European coasts. Further dividing of the NAD takes place, moving this water around Ireland and Britain, northwards to Scandinavia and north-westwards to Iceland and southwest Greenland. The impact of the NAD on Ireland's Shelf waters and atmosphere is to maintain much warmer conditions that would be expected for it's northerly position (i.e. at c.51 o 30' N-55 o 30' N, on a parallel with southern arctic Canada). It increases the biological productivity and biodiversity of Irish marine environments and helps reduce atmospheric temperature - climate extremes over land (winter-summer temperature differences of only c.10 o C). A possible result of increased global warming would be to reduce ocean circulation, effectively shutting-down the warming effects of the Gulf Stream/NAD upon Ireland and Europe. This would allow cold polar water to move southward, as happened during earlier glacial times. Literature: - National Climate Data Centre: RJND

8 4. Continental Shelf This forms a zone of low angle sea floor (c.1 o ) above the 200m isobath: oceanwards occurs the steeper continental slope (3 o -6 o ) and the ocean basins (>3000m deep) (Figure- Map). Ireland has a wide shelf area of c.110, 500km2 (exclusive of the Irish Sea); forming a portion of Ireland's even larger seabed territory of c.900, 000km2. Ireland's continental shelf is part of the seismically quiet (passive) Northwest European shelf. This results from the processes of ocean spreading, sediment accumulation, relative sea-level changes and glaciation. The shelf formation began c.100m yr ago with the development of the North Atlantic under the action of continental drift. Over time the igneous crustal rocks underlying the shelf have been overlain by thick sequences of sedimentary rocks, accumulating to form the low angle "bench" of the shelf. Structural troughs (graben) linked to the ocean spreading also underlie the shelf. These structures are often coincident with palaeovalley systems. These have formed at times of low sea level (sea levels periodically falling by >100m), particularly during the Ice Age (the Pleistocene), cutting deep canyons across the shelf. Seismic surveys show that river channels (e.g., from Dingle Bay, Lee, Blackwater, Barrow and Suir rivers in Ireland) probably connect into the palaeovalleys. The Shamrock and Whittard major canyons lie to the south of Ireland, with smaller canyons, channels and notches found northwards along the Irish shelf margin (e.g., the Gollum Channel, Porcupine Sea Bight). Sediment fans (>1-2km thick) accumulate at the ocean ends of these canyons, as well as independently along the shelf edge (e.g., the Donegal and Barra Fans). Surface sediments form a thin veneer (average 1m-30m) of mainly fine materials (sand sizes) over the shelf. Large areas of bare rock also occur, swept clear by wind-wave and tidal current action. These water flows also produce seabed-forms of sand sheets, waves and ribbons. Some of these features may be inherited from times of lower sea levels and glaciation. Present Atlantic wave action produces high-energy conditions on the Irish shelf with maximum significant wave heights (H max ) of 15-20m (50 yr return period). Thick sequences of glacigenic sediments also occur offshore from the northern and southern coasts of Ireland, with relict glacial ridges (moraines) present on the seabed (e.g., off Galway Bay and Clew Bay). The shelf forms a zone of potentially rich resources: of minerals, hydrocarbons (the Porcupine Sea Bight), wave-energy and of fish and mariculture. Literature/ Further Information: - Marine Institute: & Coastal Resources Centre RJND [Continental Shelf- Graphic: - Map (Irish continental shelf geomorphology.)]

9 Continental Shelf, Figure- Irish continental shelf geomorphology (From Commission of European Communities, European North Atlantic Margin (ENAM I) Project).