STORM IMPACTS ON SMALL BARRIER ISLANDS

Abstract:

The shorelines of the Baltic Sea and the inner coastal waters in Denmark consist of many barrier islands. These sandy barrier islands were mainly formed in the Holocene and are still very dynamic. The present day changes in the morphology are dominantly governed by storm waves and associated high water levels. These storms induce collision, overwash or inundation of the barrier crest and generate wash-over fans and barrier breaching.

In this presentation, we focus on the present-day morphologic evolution of these barrier islands, couple these to extreme events, and we will predict the potential changes in this evolution due to changes in the climate and associated sea levels.

We analyzed the morphologic evolution of a series of barrier islands over the last decades using maps, aerial photographs and satellite images. This decadal morphologic evolution was coupled to the frequency and intensity of the local extreme events. The characterization of the extreme events was based on the joint probability of the extreme water levels and storm waves for the specific sites.

The predicted climate change for the Danish waters will lead to higher water levels and an increase of the overwashes on the barrier islands. We analyzed the vulnerability of the small barrier islands to these predicted changes. We analyzed the potential changes in the barrier island morphology in the future by defining the vulnerability of the barrier islands (volumetric approach using Lidar images and dGPS measurements), and by computing the overwash statistics from deep water wave characteristics (see Stockdon et al., 2006; 2012) and via a wave transformation model in the nearshore (e.g. XBeach). The analysis clearly showed a spatial variability of barrier island response due to the external conditions. Overall, the amount of overwash and inundation events increased and the barriers showed more overwash and inundation regimes. This will probably increase the onshore migration rates of the barriers.