By Masters Ocean Engineering Student Jeff Coogan and Dr Robert Weaver
To better capture the characteristics of beach erosion and sand bar formation, a time lapse image series was set up in the Florida Tech wave tank. The cross-shore sediment transport seen in the video occurred over 2 hours and was speeded up to show the process in 1 minute 38 seconds. The primary drivers of the shoreline erosion are a combination of wave breaking and sediment suspension times lasting longer half the wave period. When the waves break the nearby sediment is kicked up and suspended. Once the sediment is suspended the various sized sand grains will take a certain amount of time to settle, based on their size and height in the water column. Depending on the steepness of the waves and the time it takes to the suspended sand to settle, the sand grains will either be transported shoreward, or as in the case of the video, the suspended sediment is carried offshore before it is able to settle. Dean (1973) first described this fall velocity and wave breaking criteria. Away from the region of breaking, the near bottom wave orbital velocity under the crest initiates particle movement suspending the sediment at the bottom, as the trough passes the velocity changes direction and the suspended sediment is transported offshore. The waves modeled in this video are representative of large steep waves that typically occur during winter storms. The final beach profile is therefore characteristic of winter beach profiles, with the formation of beach scarps (the small cliff located on the upper portion of the beach) and an offshore sand bar. The wave parameters in this video are: wave height of 11.5 cm, period of 1.15 seconds, and wave length of 155 cm.
Dean, RG. “Heuristic Models of Sand Transport in the Surf Zone,” First Australian Conference on Coastal Engineering, 1973: Engineering Dynamics of the Coastal Zone. Sydney, N.S.W.: Institution of Engineers, Australia, 1973: -.