The monitoring and modelling of the impacts of storms under sea-level rise on a breached coastal dune-barrier system

Little is known about the impacts of storms on breached barriers, and virtually nothing is known about the impacts of storms under a rising sea-level on these systems. This PhD research aims to help fill this gap. In 2008, barrier breaching at Rossbehy, Co. Kerry resulted in the establishment of a new tidal inlet. Semidiurnal tidal exchange through the new channel has been on going since this event. Rossbehy provides an excellent opportunity to study the influence of storms on barrier evolution post-breaching. A two-year monitoring campaign was undertaken to assess the morphological impacts of storms on Rossbehy and a neighbouring barrier, Inch. Multiple topographic surveys were conducted using terrestrial laser scanning (TLS) technology. The logistics of collecting, storing, processing, and analyzing this data were addressed in this research project. Major volume losses were recorded over the duration of the monitoring period at Rossbehy, but not at Inch. This difference was likely due to the orientation of the sites in relation to the main inlet channel. Meteorological data and numerically simulated nearshore wave data were used to identify and characterize storm events that occurred during the monitoring period. Strong negative statistically significant correlations were observed between rates of dune volume change and storm duration for events that occurred during the monitoring period. Additional statistical analyses revealed that event duration in combination with maximum significant wave height were the best predictors of dune volume change at Rossbehy. A novel experiment was set up to assess the impacts of storms under future sealevel rise (SLR) on Rossbehy using numerical modelling and TLS data. Numerical modelling was performed in MIKE21. TLS data was used to evaluate the effectiveness of the model in simulating dune volume changes near the breach. The results of the experiment indicate that under future SLR, storms will contribute to a net offshore movement of sediment in the near shore zone of Rossbehy. This will inevitably lead to shoreline retreat and could result in the possible drowning of the barrier if back barrier saltmarsh sediments cannot accumulate fast enough to keep up with rising sea-level. Based on the results of the monitoring campaign and modelling experiments, a conceptual model of the evolution of the system was developed – the S-SLR model. The model integrates the influence of storms under a rising sea-level into a previously developed conceptual model put forth by O’Shea (2015). The new model accounts for sediment deficits in the near shore zone caused by storms under a rising sea-level. This is the first assessment of the potential impacts of storms under sea-level rise on a breached barrier system in Ireland. It is envisaged that this study will serve as baseline from which to compare future process studies of similar systems