Shoreward Propagating Accretionary Waves (SPAWs): Observations from a multiple sandbar system

Recent observations have shown that the shallow parts of subtidal crescentic bars may separate from the bar and migrate onshore as spatially coherent features, termed Shoreward Propagating Accretionary Waves (SPAWs). It is thought that this onshore migration of SPAWs plays a role in the sand exchange within the bar-beach-dune system. In this contribution, we explore the wave conditions related to the emergence and onshore migration of SPAWs, and to estimate the volume of sand contained within a SPAW. Based on a 14-year data set of video images, we show that SPAWs emerge from crescentic bars during more energetic conditions with obliquely incident waves. Under low energetic conditions SPAWs may migrate onshore, but they disappear under larger wave heights and angles of wave incidence. Using model-data assimilation, we estimated the volume of sand contained within a SPAW to be 14700 m3.