A comparison of berm accretion in the swash zone on sand and gravel beaches at the timescale of individual waves

Understanding of shoreline erosion and accretion has been inhibited by the lack of data on bed-level changes at the fundamental time scale of individual waves. A new ultrasonic bed-level technique was used to measure shoreline erosion and accretion on a sand (Truc Vert, Bordeaux, France) and gravel (Slapton Sands, Devon, UK) beach in March and May 2008, respectively. The ultrasonic sensors were deployed in a linear array across the high-tide shoreline and measured bed-level changes on a swash-by-swash basis with an accuracy of 0.001 m (1 mm). A characteristic accretionary (berm-building) tide was selected from each dataset. Both the sand and gravel berms built up close to the high-tide swash limit, accreting around 0.07 m, with the sand berm extending 10 m cross-shore and the gravel berm being half as wide. The net morphological change integrated across the swash zone reached over 900 kg of accretion per metre width on the sand beach and 150 kg m-1 on the gravel beach. Swash-by-swash bed-level changes at the location of berm deposition were < 0.005 m for the sand beach and < 0.01 m for the gravel beach. At both sites during this period of berm building there were almost as many swash-by-swash erosion events as accretion events suggesting that a form of equilibrium acts to limit net bed-level changes. The swash-by-swash accretion events were slightly larger and slightly more numerous than the erosive events, resulting in overall accretion and berm formation.