Objective detection and quantitative analyses of a Shoreward Propagating Accretionary Wave in a double sandbar system

Abstract

A shoreward propagating accretionary wave, or SPAW, is a bar like feature that is shed off from the landward side of a bar and subsequently migrates through the trough and eventually merges with the beach or the more shoreward (inner) sandbar. A SPAW event between 09-07-2001 and 07-09-2001 near the beach of Egmond aan Zee, The Netherlands, was studied. During the event the SPAW migrated from the outer towards the inner sandbar. On the basis of time-exposure images and wave data, the roller dissipation maps were computed and used together with an initial bathymetric map in an assimilation model to estimate the bathymetry during the study period. To include the SPAW in the assimilation process, existing methods developed by Aarninkhof and Ruessink (2004) were adapted. Previous studies only included the intensity peaks over the sandbars; the adaption involved the intensity peak between the inner and the outer sandbar, caused by the SPAW, to be included in the calculation of the roller dissipation maps. The estimated bathymetric maps show the presence of the SPAW between the inner and the outer sandbar. The average SPAW width, length, height and volume are 70 m, 290 m and 0.7 m, respectively. The width and length found by the assimilation model are higher than the width and length found in time-exposure images, which are 31 m and 218 m, respectively. Not the whole lifecycle of the SPAW was captured: once the SPAW could be detected in the bathymetric maps it was already closer to the inner sandbar than to the outer bar. During the time the SPAW could be detected in the bathymetric maps the SPAW width and length increased, this is in contrast with previous research on SPAWs that stated that the SPAW maintained its shape. A comparison between the SPAW event in the bathymetric maps and event in the timeexposure images showed the SPAW dimensions and dynamics for the two sources did not correspond directly.