The morphodynamics of beach-ridge formation: Magilligan, Northern Ireland

Abstract

Magilligan foreland is a Holocene beach-ridge plain comprising three principal sets of sand ridges. Sets I and II are related to the mid-Holocene progradation of the plain, while Set III represents recent deposition at the distal margin. At present, two types of beach ridge are being formed, in both cases as part of the general recovery sequence following storm erosion of the dune cliffs. Mode I beach-ridge development is associated with shore-normal sediment transport under dissipative wave conditions. These ridges form around the High Water Mark of Spring Tides through the migration of coalescence of swash bars within 30 to 55 days of a storm. Mode II formation is more complex; sediment initially stored in nearshore bars is moved alongshore, causing bar elongation. The bar moves into the intermediate and even reflective wave domains. Eventually narrowing of the bar leads to severance of a downdrift portion, usually at or near the major drift pulse to the north west of the foreland. Normally this sediment-starved remnant welds progressively onto the beach face in the direction of the dominant process gradient (east to west) but occasionally locally generated, counteractive seas move sediment onshore employing the bar as a traverse conduit. Mode II beach ridges “emerge” onto the upper beach around 90–150 days after a storm. The beach ridges have distinctive structures, depending on their mode of formation, and both constitute important sources for dune-building sand.