Mechanisms to account for the longshore spacing of overwash throats on a coarse clastic barrier in southeast Ireland

Abstract

The longshore spacing of overwash throats on two contrasting sections of a coarse clastic reflective barrier beach in southeast Ireland is examined for evidence of spatial periodicity. A fixed barrier section (Carnsore) with an irregular barrier-top dune field exhibits random longshore throat positions related to pre-existing irregularities in the seaward dune boundary. A free barrier section (Tacumshin) without a barrier-top dune field shows regular longshore throat positions with a peak throat-to-throat periodicity of between 30–40 m. Similar longshore periodicity is found in persistent high-level beach cusps (above MHWS), though not always spatially in phase. There is no consistent near-shore rhythmic morphology that can account for this spacing. The storm event which generated the Tacumshin overwash appears to have dominant, longer incident wave periods than normal in Celtic Sea storms. On weight of circumstantial evidence, it is reasoned that this dominant wave period range (9 –11 s) can be related to a zero-mode, sub-harmonic edge wave whose longshore wavelength matches the peak periodicity of overwash spacing. Overwash sites on this reflective barrier may be determined by an edge wave controlled extension of run-up which generates high level cusps and thereby creates washover routeways. The presence or absence of a barrier-top dune field is a secondary control on whether washover is successful in reaching the back barrier. Usually only where surge penetration and pre-existing dune irregularities coincide, does washover occur.