Observations of Intertidal Bars Welding to the Shoreline: Examining the Mechanisms of Onshore Sediment Transport and Beach Recovery

Abstract:

Many coastlines throughout the world are in a net erosional state due to factors such as climate change and anthropogenic activities. While most coastal erosion occurs episodically during major storms, beaches recover during extended periods of low wave energy. Despite the importance of beach recovery on limiting coastal vulnerability, the mechanisms driving onshore sediment transport are much less well understood than those of storm-driven offshore transport. Intertidal bar (i.e., swash bar) welding to the shoreline is one proposed mechanism of sediment delivery from the nearshore to the backshore. However, studies of swash bars and their contribution to beach building have been scarce because of the sporadic nature of these events and difficulty measuring sediment fluxes in the intertidal zone.

Several beaches in the US Pacific Northwest are prograding rapidly in part due to highly dissipative conditions and an abundant sediment supply. For example, at South Beach State Park (SBSP) in Newport, OR the shoreline accreted at an average of 6 m/yr from 1960 to 2002. To explore the role of intertidal bar welding on supplying sediment to this dynamic backshore, we recently completed a boutique field experiment at SBSP. Topographic and bathymetric surveys carried out over 9 months document the short term (<weekly) through seasonal responses to the nearshore planform (-15m to +10m). A mooring was deployed offshore SBSP to measure waves, currents, suspended sediment concentrations, and the density structure of the water column to characterize the environmental conditions driving the observed morphologic changes. A co-located X-band marine radar and meteorological station provide information regarding spatially complex wave transformations and the forces driving aeolian sediment transport, respectively.

Our observations at SBSP indicate a dynamic coastal landscape including multiple nearshore bars. During the experiment we observed onshore migration of the subtidal bars, significant changes to the swash bar system, and progradation of the subaerial beach. With this new dataset we are exploring the importance of intertidal bar welding on supplying sediment to the backshore as well as the environmental conditions and physical mechanisms that promote onshore delivery of sediment.