Geomorphology of a high-energy barrier island on the rocky west coast of Ireland

Barrier islands on bedrock-framed coasts are poorly known. In this paper we report on the only barrier island system on the high-energy, west coast of Ireland. Bartra Island, County Mayo is approximately 4.5 km long and 0.5 km wide. It is topped by a continuous, well-vegetated dune ridge up to 30m high and is surrounded by intertidal sand beaches. The ocean-facing beach is a gently sloping, generally dissipative sand beach while the back-barrier contains extensive intertidal sand flats up to 800m wide, but more typically 300m. A sandy shoreface fronts the island and it is flanked by tidal inlets with well-developed ebb deltas. These extend directly into the adjacent ocean and appear not to be deflected by waves. Flood deltas as distinctive features are less prominent since the back-barrier contains extensive intertidal sands and has less accommodation space for delta development. There are 10m-deep tidal inlet channels that facilitate regular tidal exchange and the island is backed by an area of open water and intertidal flats. Important differences with better known barrier islands relate to the fact that (i) it is a single island confined to a bedrock-bounded embayment (ii) it is anchored on a relict glacial moraine, (iii) it is topped by high, vegetated dunes that reduce the importance of barrier overwash as an important evolutionary process and (iv) longshore drift is unimportant in its behaviour.