Short-term morphological changes of multiple intertidal bars on macrotidal beaches: from seasonal to storm-scales.

Abstract

<p>DGPS surveys were undertaken on two beaches of Dundrum Bay (east coast of Northern Ireland, Co.Down, U.K.) and analysed to investigate the short-term morphodynamics of a multiple intertidal bar (‘ridge and runnel’) system, as part of the INTERREG MarPAMM project. Ballykinler (east) and Murlough beach (west) are medium to coarse sand environments subjected to short waves and macrotidal conditions. Since April 2019 (ongoing), monthly surveys consisting of 14 cross-shore profiles and 2 intensive sections (in total) were carried out. Hydrodynamic conditions were extracted from the model WaveWatch3 (WW3) run by Ifremer (France) at a node located offshore of the bay. Intertidal bars were well-developed along Murlough beach (profiles 3 to 11) at the beginning of the experiment, with an increase in the complexity of ridges and runnels morphology toward the inlet (profile 12). In contrast, intertidal bars were only well developed in the western end of Ballykinler beach (profiles 13 and 14) and gradually disappeared toward the eastern end (profiles 15 and 16).</p><p> </p><p>Preliminary results from the summer season show no measurable morphological change to significant accretion and onshore migration of the bar crest with low to moderate hydrodynamic conditions. However, there is a strong alongshore variability in the bay, with response to the summer season and is recorded not only between Ballykinler and Murlough beach, but also along Murlough beach. By contrast, the winter season is characterised by a decrease of bar amplitude due to an expansion of the bars wavelength, and in some cases, an onshore migration of the bars crest (mostly noticeably for Ballykinler). The winter season is, however, highly dependent on storm conditions. During the period of November to mid-December 2019 there were 4 storms including Storm Atiyah, and a signature of those highly energetic conditions was recorded in the beach morphology.</p><p> </p><p>Depending on the location along the bay, ridges and runnels underwent bar crest erosion and sediment deposition into the runnels leading to a flattering of the profile, or cross-shore bar migrations.  Calm summer conditions, therefore, appear favourable for ridge accretion and onshore migrations, while energetic winter conditions seem to actively drive bar erosion and profile flattening. An alongshore variability in Dundrum Bay is a response to both seasonal and event conditions and is demonstrated by the results to date. This variability is probably due to wave orientation, wave energy dissipation and wave reflection linked to both offshore and nearshore bathymetry. The shape, position and number of the ridges and runnels should therefore play a key role in the energy dissipation depending on the tidal phase.</p>