Abstract

Storm sedimentation on a fringing. sandy gravel barrier system in southeast Ireland is examined. Recent wave overtopping and overwashing during severe onshore storms have resulted in a distinctive pattern of crest and backbarrier sedimentation. This is not easily matched with available models of washover sedimentation drawn from the barrier islands of the eastern United States. Beach crest overtopping occurs at heights up to +6 m MHWS and is related to the development of upper beach swash ramps that are plastered against storm-wave-eroded dune scarps located at the barrier crest. Ramp sediments comprise coarse basal beach gravels overlain by coarsening-upward units and finally topped by distinctive seawarddipping fine grit/coarse sand couplets. In places where ramps reach the dune crest, overtopping swash has deposited a fine marine gravel veneer (<0.5 m), thus raising the overall barrier height during severe storms. At lower (+2 m to +4 m MHWS) and irregular longshore positions, overwash breaching the crestal dune line has led to the deposition of small-scale backbarrier washover fans (relative to east coast United States examples). Depositional washover activity is observed, however, at both the beach crest in the form of breach or throat plug sedimentation, as well as on the backbarrier in the form of washover fans and splays. As the swash limit rises during the storm, initial scouring in the throat is followed by emplacement of a plug-shaped conglomeratic unit, distinguished by a dominant vertical clast fabric and an overall coarsening-upward particle site sequence. Both features are generated by patterns of swash turbulence within and on the flanks of the throat. Survival of the plug through subsequent phases of overwashing causes a volume reduction and size filtering of later washover sedimentation. Longshore positioning of ramps and breaches depends on the pre-storm relief of the primary barrier crest.

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