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Earth Science Conservation Review
| Lough Foyle Intertidal Flats | Londonderry |
| Site number: | 1132 | ||
| ESCC: | IA | ||
| Locality Type: | Coastal section | Status: | ASSI |
| Grid Reference: | C65853915,C60102265 End(s) of linear site, Not Entered | ||
| Google maps: | 55.19506,-6.96675 | ||
| County: | Londonderry | Council area: | Causeway Coast & Glens Borough Council |
| Period: | Quaternary | ||
| Stages: | Holocene | ||
| Site Description | |
|---|---|
| Highlights: | |
The tidal flats of Lough Foyle have been little studied but several localised sites of interest from a morphodynamic perspective are known. These include the inner margin of Magilligan foreland where a series of transverse bars extends between the Roe estuary and Magilligan Point and an estuarine washover barrier at Carrickhue whose dynamics were assessed by Lowry (1983). Broadly the tidal flats may be divided into two sections separated by the Roe Estuary mouth. North of this the tidal flats are sandy and are periodically covered by shore-oblique sand waves backed by a narrow, high intertidal beach. South of the Roe wide intertidal flats of muddy sand are backed by a semi-continuous washover barrier composed largely of reworked and transported shell material. The intertidal flats themselves cover an extensive area along the eastern shore of Lough Foyle and have undergone volumetric changes during the past 150 years as a result of extensive reclamation during the mid-19th century. | |
| Introduction: | |
Lough Foyle is a shallow, semi-enclosed coastal body of water which covers an area of about 160km2. It is separated from direct ocean wave influences by Magilligan Foreland which forms an effective sandy barrier. Its back-barrier area extends landward as far as Culmore Point where it narrows markedly and merges with the estuary of the River Foyle. Its tidal prism was estimated at about 1.75 x 1,000,000m2. In the lough several distinct depositional subenvironments exist which include subtidal channels, intertidal flats, upper intertidal beaches, washover barriers and localised salt marsh areas. The intertidal flats are most extensive along the Northern Ireland shore and are noted for their ornithological interest. They also contain extensive populations of mussels and oysters. Although the intertidal flats are thus of importance from a habitat perspective, they are also of interest from an earth science view point, in that the opportunity exists to demonstrate a range of intertidal flat features and processes and to study the origin and evolution of the intertidal flat sediments themselves. The shores of Lough Foyle have been extensively modified through reclamation works, mainly in the mid-19th century. This has effectively destroyed much of the former salt marsh and replaced formerly mobile shorelines with stablising structures. It is highly likely that changes in intertidal flat sedimentation patterns have accompanied this reclamation. | |
| Description: | |
Lough Foyle is an extensive marine water body (c.160km2) confined between the mainly rocky Inishowen Peninsula (County Donegal) on which coastal sediment bodies are restricted to pocket beaches, and an essentially sedimentary shoreline on the eastern (County Londonderry) side. The entrance to the Lough is constricted and the tidal prism enters via a 1.6km wide channel. Tidal range at the mouth of the lough is 2.3m (Carter, 1991) and strong tidal currents are generated as the tidal prism enters and leaves the Lough. Subtidal channels are mainly confined to the western (Inishowen) side of the Lough where a series of bifurcating channels exists. These are routinely dredged to maintain navigability in the shipping lanes. Intertidal flats are best developed along the eastern (Northern Ireland) shore of Lough Foyle. They are virtually unstudied (as is the entire internal section of the lough) from a contemporary morphodynamic perspective save for a few comments in general texts (McDonald & McMillan, 1955) and detailed studies on specific features (Lowry, 1982). Ocean wave influences on Lough Foyle are severely limited through the presence of a constricted inlet and most wave action is generated within the lough by winds blowing across its surface. Consequently most waves are of short period and wavelength (Carter, 1980; Lowry, 1982) and are driven by S and SW winds which generate a northeasterly current. Ocean swell does penetrate the mouth on occasion but is rapidly dissipated across the intertidal flats in the vicinity of the inlet (Carter, 1980). The intertidal flats of Lough Foyle are generally sandy although finer grain sizes become volumetrically more important toward the upper reaches. Such sandy sediments are poorly cohesive and subject to movement under the contemporary hydrodynamic regime. Consequently a hierarchy of bedforms is produced which range from linear bars (few m wavelength) to small scale ripples (few cm wavelength). These bedforms, particularly the small scale ones display many of the characteristics of late stage emergence runoff which typify intertidal flats (flat-topped ripples, ladderback ripples, rill marks, microdeltas). The large scale bedforms may be indicative of alongshore sediment transport corridors. Reconnaissance field visits suggest that the intertidal flats may be broadly divided into two zones separated by the mouth of the Roe Estuary. North of the Roe Estuary, on the western margin of Magilligan foreland the intertidal flats have a smooth planar profile backed by a steep, upper intertidal beach and an eroding cliffline in Holocene beachridge deposits. The upper intertidal morphology commonly comprises a series of evenly spaced bars with a wavelength of 20-30m which extend obliquely offshore from the coast. These features extend in an unbroken "corridor" from the Roe Estuary to Magilligan Point. Although not studied in detail this morphology corresponds closely to the "transverse bar and rip" beach state identified in the Australian model of beach morphology (Wright & Short, 1984), thus suggesting a wave-induced origin for the features. An alternative interpretation is that these features may be indicative of current-driven bedload transport in a longshore sediment transport corridor in which sediment is transported northward toward Magilligan Point. Occasionally aerial photography reveals a second set of bars offshore of this set and orientated about 90 deg. to it. This second set of bars is most likely related to refracted swell penetrating the entrance of the lough. Such an interpretation is based on the orientation of the sandbars and their periodic occurrence. South of the Roe Estuary the intertidal flats become increasingly muddy and a number of low relief shell banks occur on them. These banks are composed of the disarticulated shells of the tidal flat molluscan fauna. They migrate onshore at variable rates and contribute to the formation of washover barriers in nearshore locations throughout most of the southern section of the lough. They are commonly banked against salt marsh deposits, although along much of the shoreline salt marsh has been replaced by seawalls fronting reclaimed land. At Carrickhue the dynamics of a low-amplitude barrier composed mainly of disarticulated estuarine shells was documented by Lowry (1983). This barrier migrated steadily onshore between 1953 and 1983 at spatially variable rates which decreased from 20-50m per year soon after initiation to 2-3m per year as they reach the shoreline and begin to transgress marsh deposits. Until about 1945 the shell banks thus formed were exploited for agricultural purposes and thus the long term evolution of such features is not fully understood. | |
| Importance: | |
The intertidal flats of Lough Foyle form an important habitat, particularly for birdlife. From an earth science perspective, the intertidal flats are of national importance inasmuch as they are associated with micro/mesotidal conditions in a wind-wave and tidal current-dominated environment and afford excellent opportunity for demonstrating a range of sedimentary processes in both the short and medium term. Taken collectively with Magilligan Foreland and the ebb-tidal delta, part of which is exposed as the Tunns Bank, the intertidal flats and subtidal channels form a textbook example of a tidal inlet and associated depositional subenvironments. Within Lough Foyle the combination of wind-wave dominance and sand grain size produces a modal transverse bar and rip beach state near Magilligan Point and thus affords the regular opportunity to demonstrate such a situation. An apparently unique feature to Northern Ireland is the estuarine washover barrier at Carrickhue. It is, however, possible that similar features may exist in Strangford Lough but under different tidal and wave conditions. Finally, the possibility that post-19th century reclamation morphological readjustment may be continuing, cannot be discounted. This situation offers the possibility of examining coastal response to reclamation. | |
| Interpretation: | |
The location and setting of the intertidal flats in Lough Foyle is strongly suggestive of a marine origin, whereby the present intertidal flats accumulated via flood-tidal deposition landward of the constricted inlet produced by the growth of Magilligan foreland. Such constricted tidal inlets are commonly flood-dominant and give rise to accumulation of marine sand in the back barrier as flood tidal deltas. Tidal records from Lisahally indicate a slight flood dominance in the lough under contemporary conditions. On the seaward side of the inlet the intermittently intertidal Tunns Bank and its submerged extension together comprise the ebb-tidal delta. The probable lack of additional sediment supply to the modern system suggests that contemporary morphological changes in Lough Foyle occur through redistribution of an already existing sediment volume. In this regard it is likely that the lough comprises a series of sedimentary cells within which sediment is recycled. A link between 19th century reclamation and recent morphological change in Lough Foyle has been suggested by Lowry (1983). Under such a scenario, reduction in tidal prism is the primary forcing agent for hydrodynamic change which is reflected in morphological adjustment through alteration of tidal and wave regimes until the system adjusts it shape to achieve dynamic equilibrium under the altered boundary conditions. Limited observations suggest that transverse bars commonly between the Roe and Magilligan Point. The combination of locally generated wind waves and the grain size of sediments near Magilligan Point could potentially give rise to a modal transverse bar and rip beach state in the sense of Wright & Short (1984) if the morphology were envisaged as a result of wave-sediment interactions. Alternatively the transverse bars may be viewed as a series of bedforms indicative of a longshore sediment transport corridor driven by tidal or wave-induced currents. This question requires further research. Elsewhere in the Lough, cohesive sediments in the upper intertidal zone give rise to marshes and associated scarp morphology. Locally the dominance of wave over current action produces washover barriers which, although morphologically similar, differ genetically from chenier ridges. Lowry (1983) considered the major difference to be in the fact that cheniers are located within a regressive facies framework and owe their morphology to variations in fluvial sediment discharge whereas the washover barriers in Lough Foyle occur within a transgressive framework and are morphodynamically controlled by tidal water level variations. The mode of origin of these shell rich overwash barriers appears to be via landward migration of shell accumulations from the outer sections of the intertidal flats as low, near linear features. As they approach the shoreline these features increase in amplitude and begin to be dominated by wave-induced overwash processes. They migrate onshore and eventually stabilise against the mainland shore, although present stabilisation is against the railway embankment. Under natural conditions these barriers provided sheltered back-barrier environments within which salt marsh developed. Reference to widespread exploitation of the shell resource in the past for agricultural purposes (Evans, 1955; McMillan, 1957) points to high rates of shell production within the Lough. Human exploitation may have limited the development of such landforms and the likely landforms which may develop under non-exploitative conditions can only be speculative. Similarly, the role of shell production in the overall sediment budget is likely to be enhanced at present compared to earlier times. | |
| Conclusions: | |
The intertidal flats of Lough Foyle have been little studied. As a whole, they comprise an important element in a tidal inlet-associated sedimentary cell that also includes Magilligan Foreland, an ebb-tidal delta and subtidal channels. The intertidal flats have probably arisen through flood-tidal deposition. At present they afford an opportunity to demonstrate the surface sedimentary structures of intertidal flats. Localised sites within the intertidal flats are of particular interest. These include the beach near Magilligan Point and the washover barrier at Carrickhue. | |
| Notes: | |
Ongoing studies in Lough Foyle are aimed at the elucidation of its sediment budget and the role of reclamation in forcing morphological adjustment. These should contribute to the direction of future research in the area. In addition, investigations of the stratigraphy of the intertidal flats in the context of a study of the Holocene evolution of Lough Foyle should be undertaken since the area offers the opportunity to elucidate the postglacial history of the region. | |
| Keywords | |
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| Relations: | Beach deposit, Coastal processes |
| Geomorph: | bedform, sediment cell, tidal flat, tidal inlet, washover barrier |
| NonGeol: | Important habitat for birdlife. |
| Access | |
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| Management: | |
Past and ongoing shoreline defence through embankment construction and informal dumping constitute a threat to the intertidal flats. The erosion may be driven by ongoing morphological adjustment but is not understood at present. Uncoordinated approaches to shoreline protection are likely to induce further changes. | |
| Threats: | Dumping; increased erosion due to shoreline defence construction. |
| Uses | |
| Educ. Level: | Undergraduate / Postgraduate |
| References (6) | |||
|---|---|---|---|
| Carter, R.W.G., 1980. Longshore variations in nearshore wave processes at Magilligan Point, Northern Ireland. Earth Surface Processes and Landforms, vol. 5, pp.81-89. | |||
| Evans, E.E., 1955. The shell industry in Co. Londonderry. Ulster Folklife, vol. 4, pp.55-59. | |||
| Lowry, P., 1982. An estuarine washover barrier in Lough Foyle, Northern Ireland (M. Phil. thesis). (Thesis) University of Ulster, Coleraine. | |||
| McDonald, R. & McMillan, N.F., 1951. The natural history of Lough Foyle, Northern Ireland. Proceedings of the Royal Irish Academy, vol. 54B. | |||
| McMillan, N.F., 1957. Quaternary deposits around Lough Foyle, Northern Ireland. Proceedings of the Royal Irish Academy, vol. 58B, pp.185-205. | |||
| Wright, L.D. & Short, A.D., 1984. Morphodynamic variability of surf zones and beaches: a synthesis. Marine Geology, vol. 56, pp.93-118. | |||
| Map(s): | See the 1:50,000 O.S. Sheet 4 (Coleraine). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Lough Foyle Intertidal Flats, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1132. Accessed on 2026-11-06. |
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