Observations of Waves and Currents on an Inundated Barrier Island: the Role of the Back-Barrier Basin

Vertical accretion of sediment on barrier islands is essential in mitigating the effects of sea-level rise and island subsidence. Overwash and the associated onshore sediment transport might ensure aggradation of barrier islands, especially in mesotidal, mixed energy systems. However, artificial sand drift dykes on many barrier islands in the Dutch Wadden Sea effectively prevent overwash. At present the reopening of dunes is considered, but not much field data on overwash is available to substantiate the effects of these measures. Therefore an extensive field campaign was set-up to quantify the hydrodynamic processes during overwash and island inundation across a wide and gently sloping Dutch barrier island spit.

Here, we present much-needed field data collected in the course of several storms in the winter of 2014/2015. Waves and currents were observed using Acoustic Doppler Velocimeters (ADVs) and stand-alone pressure sensors along an 1.3 km instrument array spanning across the spit from the North Sea to the Wadden Sea. Most instruments were installed subaerial at locations ranging from -0.2m to 1.6m with respect to the Dutch Ordnance Datum (NAP), and inundation depths measured at the highest location ranged from a few centimeters to 1.3m. The data shows that flooding of the barrier can occur from two sides. During rising tides, water mainly flows from the North Sea across the barrier to the Wadden Sea. During falling tides, water level gradients can be reversed and water can flow towards the sea. Since a flow reversal was not observed during all events, this suggests that a seaward directed water level gradient can be counteracted by wave-induced set-up. The measured wave field showed that infragravity- and swell-waves typically entered the field site from the North Sea side, while locally generated wind waves frequently entered from the Wadden Sea. The results clearly highlight the role of the back-barrier basin.