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Lough Foyle Intertidal FlatsLondonderry
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Summary Full report
Site Type: Coastal section
Site Status: PASSI
District: Limavady Borough Council
Grid Reference: C65853915, C60102265
Rocks
Rock Age: Quaternary (Holocene)
Interest
Other interest: No data, Beach deposit, Coastal processes, bedform, sediment cell, tidal flat, tidal inlet, washover barrier, Important habitat for birdlife.

Summary of site:

Lough Foyle covers an area of about 160 square kilometres and is fed by both the Foyle and Roe rivers. Were it not for this flow it is likely that Magilligan Strand would have closed onto the eastern side of Inishowen somewhere around Inishowen Head and even in present circumstances, with about one and three quarter million cubic metres of water ripping in and out on each tidal cycle, the mouth is severely constricted but remains stable at around 1 km. One important effect of this is to isolate the lough from the influence of ocean waves.

This account describes the intertidal flats on the east side of the Lough where the great bulk of sedimentation occurs. It should be emphasised that this is probably a highly artificial environment affected by extensive drainage works in the mid19th century, claiming considerable areas of salt marsh on the east shore for agriculture and regular modern dredging as far as the Foyle estuary to maintain a navigable channel for shipping as far as Londonderry. Little research has been performed on the sediments of the lough which is why there is such uncertainty surrounding the consequences of human activity. Most wave action affecting the area is generated inside the lough by prevailing southerly and south westerly winds and with such a short travel over water, the wavelengths they generate tend to be short; they drive a north westerly current. The flats are predominantly sandy and the entire area is actively influenced by contemporary water dynamics. The flats are conveniently divided into two units by the mouth of the river Roe. North of the mouth the flats are predominantly sandy with a smooth, flat profile extending as far as a steep intertidal beach backed by a small cliff cutting into older beach ridge deposits. The flats are about 2 km wide in the vicinity of the river mouth and narrow progressively to the north to less than half a kilometre south of Magilligan Point. A series of bars spaced at regular wavelengths of 20 to 30 m extend obliquely across the inshore flats and form a continuous belt from the river mouth to Magilligan Point. These have two possible origins, the first by storm waves breaking well offshore dissipating their energy on the edge of the flats, a situation that creates bars in the surf zone with runnels (troughs) between or by northerly currents driving a belt of sediments along the shore. A second set of bars offshore of this belt and at right angles to it, is probably formed by the refraction of oceanic swell penetrating the lough mouth. South of the mouth of the Roe the sediment is muddier and there are low banks of disarticulated mollusc shells derived from the local shelly fauna. These banks migrate shoreward creating discontinuous barriers along the shoreline, much of which is now protected by artificial structures. Waves break over these banks without destroying them. Their rate of migration ranges from 20 to 50 m annually in open water slowing to less than 5 m along the shoreline. In this southerly area wave action predominates over tidal flow. Very little new sediment enters the lough at the present time so it seems likely that what we see now is a cycle of redistribution by wave action and currents within a series of cells (mostly closed areas of sediment activity) that form the bed of the lough. Land take from the lough in the 19th century reduced the volume of water exchange in each tidal cycle (the tidal prism) and it is supposed that this has had a major effect. The removal of shell debris for agricultural purposes in the past appears to have had little impact. There has been little research done on the sediments of the intertidal flats of Lough Foyle which is surprising for a deposit so strategically important to one of Northern Ireland's major ports. The consequences and historic impact of human activity on the lough should be better understood at a time of globally rising sea levels.


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