Predicting and Monitoring the Evolution of a Coastal Barrier Dune System Postbreaching Michael O’Shea and Jimmy Murphy Hydraulics & Maritime Research Centre University College Cork Youngline Industrial Estate Pouladuff Road Cork, Ireland m.o’shea@ucc.ie ABSTRACT O’Shea, M. and Murphy, J., 2013. Predicting and monitoring the evolution of a coastal barrier dune system postbreaching. Journal of Coastal Research, 29(6A), 38–50. Coconut Creek (Florida), ISSN 0749-0208. A study of the morphodynamic evolution of a midbay barrier beach system of Inner Dingle Bay, County Kerry, Ireland is presented. The system has been under observation by the Hydraulics and Maritime Research Centre (HMRC) of University College Cork since 2008 when the Rossbeigh barrier breached. This event had been the culmination of 10 years of intensive erosion. Since breaching occurred, erosion rates have continued to increase on Rossbeigh, while offshore an ebb tidal delta continues to grow. Similar barrier inlet systems are examined in order to identify trends and features that might provide an insight into the future morphology of Dingle Bay. The study utilises field data collection sediment transport analysis and remote sensing to understand the coastal processes driving the systems evolution. The changing orientation of Rossbeigh, the relative stability of the Inch barrier beach, and the variability in ebb tidal delta were the findings of the long-term morphological part of the study. The influence of the ebb tidal delta on wave refraction, emergence of a vegetated gravel ridge in the breach zone, and the role of tidal currents in sediment transport along Rossbeigh are significant findings of the monitoring campaign. These results provide indicators of future evolution of the system. The study concludes that the realignment of the beach will continue on Rossbeigh beach while alongshore tidal currents dominate sediment transport in the breached area. ADDITIONAL INDEX WORDS: Morphodynamics, coastal barriers, erosion, breaching, dunes, Dingle Bay, Ireland, ebb tidal delta. INTRODUCTION Barrier beaches are dynamic flexible systems usually protect- ing softer vulnerable shorelines from the erosive power of the sea. These systems can also contain one or several tidal inlets that act as a regulator of water flow between open ocean and back barrier area. When these barriers exhibit signs of breakdown such as breaching, the consequences can be serious for the back barrier area. Such signs are evident in the Dingle Bay midbay barrier system. The dune system had been in a state of dynamic equilibrium with shorelines eroding and prograding seasonally; however, a breach in the Rossbeigh dunes occurred in the winter of 2008 and has since widened to over 600 m. The impact this breaching has on the surrounding environment is multifaceted and potentially of high economic cost. The loss of an amenity and habitat is already occurring with the removal of protected dune; the change in sediment transport patterns is increasing sedimentation in the back barrier area and affecting aquaculture. It is the reported (Kerry County Council, 2009) increase in flooding since the breaching event, however, that is potentially the greatest impact of the erosion in Rossbeigh. Approximately 100 homes and businesses are located on the low-lying, ground level, ,10 m above ordinance datum (AOD), coastline directly behind the barrier beach. Anecdotal evidence (KerrysEye, 2010) suggests that recent storm surge flooding corresponds to the emergence of the breach in the barrier beach. If present erosion trends continue, it is feared that flood risk in the back barrier will increase, thus increasing the potential economic loss. A study by the Hydraulics and Maritime Research Centre (HMRC) of University College Cork is being undertaken, aiming to gain an understanding of the morphology within Dingle Bay. The drivers of recent erosion are identified by analysing hydrodynamic and topographic field data. The study also provides an insight into the evolution of both barrier beaches by utilising historical maps, aerial photographs, and recent satellite imagery. BACKGROUND Barrier beaches and islands have been subjected to many field studies. A wealth of knowledge has been gleaned from naturally dynamic barrier systems where cyclical breaching is followed by migration and regeneration. Research on these systems has typically been conducted on a decadal scale with conclusions derived from large databases of information DOI: 10.2112/JCOASTRES-D-12-00176.1 received 6 September 2012; accepted in revision 4 January 2013; corrected proofs received 27 March 2013. Published Pre-print online 10 April 2013. Ó Coastal Education & Research Foundation 2013 Coconut Creek, Florida November 2013 Journal of Coastal Research 29 6A 38–50