Detailed analysis of overwash on a gravel barrier

Matias, A., Masselink, G., Turner, I., Williams, J.J. and Ferreira, O., 2011. Detailed analysis of overwash on gravel barriers. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), ���– ��. Szczecin, Poland, ISSN 0749-0208 Overwash is an important process for the evolution of gravel barrier islands inducing rollover, breaching, barrier breakdown, outlet formation and closure. This paper uses results obtained from the large-scale BARDEX experiments undertaken in the Delta Flume (The Netherlands) to investigate the morphological response of a proto type gravel barrier to wave and tidal forcing during overwash conditions. The overwash experiments were divided in Test Series (E1 to E10), and for this study only E9 and E10 were used because they recorded the most significant morphological changes as a result of overwash. The barrier morphology was surveyed before and after each run, and also continuously monitored at 4 Hz using acoustic bed-level sensors. During E9 overwash occurred 15% of time but during E10 owing to crest lowering overwash flows passed over the crest 60% of time, which ultimately led to barrier inundation. Throughout the experiments, overwash promoted erosion of the barrier crest and deposition on the back-barrier. Erosion at the crest was induced by overwash flows that reached more than 70 cm depth. The fast flows (up to 4 ms -1 ) reached the back-barrier as shallow flows with declining velocities that cause the deposition of the gravel grains. Back-barrier flow depths were always smaller than 20 cm. No relation between overwash water depth and sediment deposition/erosion was derived from this study because either bed changes were measured for shallow flows (particularly around 10 cm water depths). This is because there is complex sediment dynamics that result from the feedback processes taking place constantly throughout the barrier profile. Even without changes in hydrodynamic forcing, as overwash occurs the barrier crest lowers and moves inland, all sub-aerial morphologies flatten, hydraulic gradient decreases (as a consequence of barrier geometric changes), and the submerged lagoon barrier margin moves onshore. The constant variations in flow characteristics due to wave spectra interactions with a mobile bed make the understanding of overwash sediment transport difficult and still a research under progress.