Longshore variations in nearshore wave processes at magilligan point, northern ireland

Abstract

SUMMARYMagilligan Point is a recurved cuspate foreland at the mouth of Lough Foyle. Two wave regimes intersect in the estuary mouth and the manner of their interplay controls shoreline changes. Ocean swell waves from the N and NE are refracted around the recurve, losing both height and energy longshore. Width of the surf zone decreases and waves tend to steepen, although both these changes and wave refraction owe something to nearshore geometry. Angle of wave approach becomes more acute and a westerly flowing longshore current moves sand S and SW along the beach. Estuary waves from the S and SW are wind-driven with high-frequencies and steepnesses. They generate a northeasterly current which returns material N, but dies out as the waves become obliterated by nearshore attenuation and breaking of swell. It is possible to identify a time-averaged null-point where shoreline wave power is balanced, although this tends to shift over short periods causing rapid morphological changes. The existence of two independent, but counteractive cells ensures the long-term maintenance of the foreland, without requiring major or continuous supplies of fresh sediment.

Mechanisms associated with the erosion of sand dune cliffs, Magilligan, Northern Ireland

“…Although Sporn (1976) points out that the effect of roots may be either advantageous or detrimental to slope stability, in the case of sand dunes it is clearly the former, with the main effect being to increase cohesion. Carter (1980b) calculated that root development on seaward dune slopes might increase the sediment retendon time by a factor of 9 or 10. The management implication is that artificial strengthening of dune slopes by planting (Carter, 1988) may only have the effect of altering the mode of failure, and not in reducing the long-term erosion rate.…”

“…These two wave climates interact at Magilligan Point, and are partly responsible for controlling the shoreline morphology (Carter, 1979(Carter, , 1980a Tidal range at Magilligan varies from 3-1 m at springs to 1-8 m at neaps, with highest astronomical tides reaching 2.25 m OD Belfast.…”

A morphological classification of coastal forelands, with examples from South Africa

“…The role of wave energy in shaping the lateral margins of forelands as well as supplying longshore sediment to the foreland apex is well established (Carter, 1980;Heron et al, 1984;Ashton et al, 2001). Seasonal changes in wave direction and height can lead to beach rotation (Dolphin et al, 2011), asymmetric foreland shapes (Sanderson et al, 2000) and formation of 'travelling forelands' where the entire foreland shape migrates alongshore (Escoffier, 1954;Burningham and French, 2014;Hesp et al, 2016).…”

“…This develops where shoreline progradation takes place in the form of nested gravel beach ridges that have a consistent alignment at an oblique angle to the shore (e.g., Semeniuk et al, 1988;McNinch and Leuttich, 2000;Alcántara-Carrió and Fontán, 2009). Cuspate forelands thus have a regular progradational character controlled by the linear and nested nature of these beach ridges (Carter, 1980;Fontolan and Simeoni, 1999;Roberts and Plater, 2007;Lampe and Lampe, 2018) and driven by differential wave action 3 (Falqués et al, 2018). These forelands therefore develop a distinctive triangular-shaped morphology with generally symmetrical lateral margins, and may have backbarrier lagoons or wetlands behind the foreland or between individual beach ridges within the foreland itself.…”