Palaeo-Environmental Study Area P19 Shannon Estuary, West Coast, Republic of Ireland
Total Page：16
File Type：pdf, Size：1020Kb
Palaeo-environmental Study Area P19 Shannon Estuary, west coast, Republic of Ireland PALAEO-ENVIRONMENTAL STUDY AREA P19 SHANNON ESTUARY, WEST COAST, REPUBLIC OF IRELAND A SURVEY OF ARCHAEOLOGICAL AND PALAEO-ENVIRONMENTAL SITES ATTESTING CHANGES IN THE MIDDLE AND LATE HOLOCENE COASTAL ENVIRONMENT OF THE SHANNON ESTUARY ON THE WESTERN COAST OF EIRE. Plate P19 The shore of the Shannon Estuary, Eire, at Carrickdirty Rocks comprises prehistoric land surfaces which are now submerged at high tide. The construction of earthen embankments in the post-medieval period reclaimed much of the coastal saltmarshes but only thorough understanding of past coastal trends can reveal the future prospects of these defences. Fergus Estuary, Co. Clare. 1. LOCATION The Shannon Estuary, situated on the Atlantic coast of Ireland, is the largest estuary on the Irish coast (Figure P19.1). The Rivers Shannon, Fergus (Plate P19), Inny and Suck, which together drain an area of c.15,700 km2 or about one-third of Ireland, feed it. The drainage basin associated with these rivers includes most of the Irish central lowlands and some major inland lakes, including Lough Derg and Lough Ree. The greater Shannon estuary area comprises the lower reaches of the River Shannon (c. 96 km between Limerick City and the sea) and incorporates the Fergus estuary south of Clarecastle. It includes parts of north Co. Kerry, north Co. Limerick and west Co. Clare. The lower estuary comprises the area to the west of Pallaskenry on the south bank and west of Kildysart on the north bank, while the upper estuary comprises the remainder. Water depths 1 Palaeo-environmental Study Area P19 Shannon Estuary, west coast, Republic of Ireland vary from c. 37 m at the estuary mouth to c. 19 m at the confluence with the Fergus, shallowing eastward to less than 5m in the channel at Limerick City. 2. MODERN GEOGRAPHY 2.1 The setting of the Shannon Estuary The topography of the land adjacent to the Shannon and Fergus estuaries is generally low-lying and level, with surface elevations seldom exceeding 60 m O.D. The majority of lands lie between the tidal High Water Mark (HWM) and 30 m O.D. (Ordnance Survey of Ireland 1974). Exceptions to this may be found on the southern shore of the Shannon Estuary between Tarbert and Foynes and on the northern shore between Labasheeda and Clarecastle, where the relatively narrow coastal lowlands rapidly give way to higher ground. In this area, the estuary is at its most constricted being typically c. 1.25 km wide, whereas the estuary’s widest dimension can be found at the confluence with the Fergus estuary where it measures c. 14.5 km across. Much of the lowland area is subject to intermittent flooding in periods of high rainfall, and is dependent on embankments for protection from marine flooding. Drainage of the lowlands is through a dense network of rivers, streams, creeks, artificial drainage channels and sluices. 2.2 Climate, soils and modern land-use Ireland has a typical west maritime climate, with cool cloudy summers and mild, moist winters. The Shannon Estuary region, compared with the country as a whole, has a higher annual average humidity and rainfall, lower mean annual temperature and shorter frost-free and sunshine periods. Annual average rainfall (1951 - 1980) for the upper Shannon estuary is between 800 and 1000 mm and for the lower estuary is between 1000 and 1200m (Rohan 1986). Mean daily temperatures (1951 - 1980) vary from 5.0 - 5.5_ C _(January) to 15.0 – 15.5 _ C (July) (Rohan 1986) with relative humidity readings show a range of 69% to 92 % (An Foras Talúntais 1966). The dates for 50% probability of first and last frosts (1951 - 1980) are November 15th and April 1st, respectively (Rohan 1986) and average wind speed (1951-1980 at Shannon Airport) is 5.6 m s-1 dominantly from westerly and southerly directions (Rohan 1986). Soils within the lowlands adjacent to the Shannon Estuary have been classified as gley / brown earth / peaty gley (Association 43) by An Foras Talúntais (Gardiner & Radford 1980). These soils have a poor drainage capability with a clay/loam texture and a sticky consistency when wet. An organic-rich surface horizon over a grey and mottled sub-horizon of a massive structure is a primary characteristic. The silt content of these soil profiles is high (c. 50%) and the clay content decreases from c. 28% near the surface to less than 10% near the soil base. Soils in this area are frequently waterlogged, with the water table rarely occurring more than 0.40 m below the ground surface. These soils typify what has been termed the ‘wet mineral lowland’ (Culleton & Gardiner 1985), which in the absence of drainage and nitrogen enhancement, are only suitable for grazing or forestry. Much of the lowland consists of alluvial deposits on flat relief at or below sea level. The flats comprise a mixture of partially drained pasture and marsh/salt-marsh. Some of the lowland has been reclaimed. The primary land use is agricultural, with rough, seasonally grazed pasture pre-dominant. Reclaimed areas are drained by a system of channels leading to flap sluices and other outlets, while the marshland is poorly served by a combination of artificial drainage channels and dense meandering creeks and their tributaries. Outside of the lowlands, soils around the Shannon Estuary catchment vary considerably according to the pattern of geologic and glacigenic parent materials, as well as the influence of soil-forming factors (climate, living organisms, topography, time). Soil survey data from County Limerick and County Clare (An Foras Talúntais 1966) demonstrate the presence of soils of the Brown Earth Group (good, arable soils), Brown Podzolic Group (good, arable soils with management), Grey-Brown Podzolic Group (good grassland soils, also cultivable), Gley Group (suitable for pasture), Podzol Group (rough grazing, forestry), Regosol Group (lowland, 2 Palaeo-environmental Study Area P19 Shannon Estuary, west coast, Republic of Ireland immature, variable productivity), Lithosol Group (rough grazing, forestry) and Organic Soil (Peats) Group. These main groups are subdivided into local Soil Series according to site- specific criteria. Human settlement within the lowlands is generally sparse, though several small villages occur on the flats (e.g. Pallaskenry, Clarina, Bunratty, Hurler's Cross). Larger towns such as Foynes, Newmarket-on-Fergus, Ennis and Clarecastle support substantial populations. Additionally, important industrial and transport infrastructure lies adjacent to the estuary, including the Shannon Industrial Complex and Shannon Airport. Limerick, the largest urban centre in the region, lies at the head of the Shannon estuary and acts as the main industrial, commercial and transport hub for the Irish mid-west region. 3. CONTEMPORARY COASTAL PROBLEMS Human activities appear to have played a significant role in the morphological development of the Shannon Estuary. It is likely that human settlements were present on the margins of the estuary from at least Neolithic times onwards. Neolithic culture, based on rudimentary agricultural activities, may have involved the first significant deliberate manipulation of the local land resource. If so, it is almost certain that local wetland succession patterns were affected, which in turn would probably have caused morphological responses within the estuary system. Technological advances through time were likely to have increased the potential for human intervention in the coastal wetlands of the Shannon Estuary catchment, though the scale and significance of human-induced environmental change still remains speculative. One of the most important ways in which human activity has influenced coastal morphology in the Shannon Estuary has been through enclosing coastal lands for agriculture. Commonly called reclamation (or land-claim), such activities have important consequences for sedimentary and morphodynamic processes within estuaries, through the drainage and alteration of estuarine environments. These may include changes in water and sediment transport patterns and changes in the tidal prism, which may in turn lead to localised changes in hydrology and relative sea level. Reclamation can lead to a reduction in the area over which wave energy can be dissipated, leading to the arrival of higher energy and hence a more erosive regime at the coast (Brampton 1992). Drainage and reclamation can also lead to subsidence and compaction of sedimentary sequences, affecting the preservation of archaeological material (Van de Noort & Ellis 1995), as well as the value of sedimentary sequences for environmental reconstruction (Tooley 1978; Shennan 1994). In contrast to elsewhere in Northern Europe there has been little historical or geographical research on the processes of coastal reclamation (but see Rowe and Wilson (1996) and Firling (1996) for studies of the evolution and reclamation of Wexford Harbour) in Ireland. Exactly how much land has been reclaimed within the Shannon Estuary is unknown. On the Co. Clare coastline (including the Fergus Estuary) there is c. 5,900 ha of identifiable reclaimed estuarine land. Large tracts also occur on the south bank (mainly Co. Limerick) of the estuary, stretching inland around the River Maigue and surrounding Limerick City. This suggests that the Shannon is a highly modified estuary, and that in its original form, the river channel probably meandered through large tracts of wetland and marsh Most of the sea-defence embankments were built and maintained in the post-medieval period by the local landed gentry until the passage of the Land Purchase Acts, when responsibility passed to local trustees set up by the Land Commission. These embankments were not always maintained properly and breaches certainly occurred. Most of these embankments were transferred to the Commissioners of Public Works under the terms of the Arterial Drainage Act, 1945. During the fifties, a number of important schemes were carried out by the Commissioners to enhance the flood protection along the Owenagarney River, near Bunratty; from Bunratty to Rineanna on the north side of the Shannon and at the mouth of the Fergus.