Megaripple/Flat Bed Transition in Rip Current Channels

Sherman, D.J., 2018. Megaripple/flat bed transition in rip current channels. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 246–250. Coconut Creek (Florida), ISSN 0749-0208.Megaripple and flat bed conditions are common bed configurations in energetic nearshore environments. Their existence is controlled by shear stresses associated with wave orbital motion, superimposed net flows, and water depth, representing flow regimes. This is based on the premise of a characteristic set of flow conditions associated with particular bed configurations. A simple flow regime sequence for surf zones is ripples, megaripples (or dunes), and flat bed. Each of these bed configurations may include several secondary characteristics. Despite the importance of understanding flow regime ranges and transitions, there have been few detailed studies for characteristic megaripple and flat bed conditions. This paper presents the results of a field experiment designed to acquire flow, sedimentological, and bedform data from rip current feeder channels in an inner surf zone of a barred nearshore. Waves and water depths were measured with pressure transducers, and currents were measured with bidirectional current meters. Sand samples were gathered and bed configurations were measured and photographed by divers. Analysis indicates that the mobility number, as calculated with a number of characteristic velocities, provides first-order bedform discrimination. Observations indicate that bedform transitions can occur over sub-hour time spans, although the response times are slower as combined wave and current velocities are reduced. Under the latter conditions, especially, non-equilibrium configurations may persist.