Field observations of turbulence, sand suspension and cross-shore transport under spilling and plunging breakers

Measurements of wave orbital velocity, near-bed turbulence levels and sediment suspension were obtained under plunging and spilling breakers in the outer surf zone on the beach at Vejers, Denmark. For the same range of relative wave heights and indicators of wave nonlinearity, we observed significantly larger suspended sediment concentrations and onshore-directed rates of suspended sediment transport under (long-period) plunging breakers, compared to (short-period) spilling breakers. This is consistent with the long-held understanding that, for a given beach slope, onshore transport and beach accretion is associated with longer-period waves and offshore transport and erosion is associated with shorter-period waves. An intra-wave analysis of hydrodynamics and sediment suspension revealed that the main reason for the larger suspended sediment transport rates under plunging waves was i) larger time-averaged suspended sediment loads under plunging waves, and ii) a larger difference in cumulated sediment load under the wave crest phase compared to the wave trough phase for plunging breakers. This latter difference was due to an earlier arrival at the seabed of higher levels of turbulent kinetic energy (TKE) under plunging waves compared to spilling waves. Hence, both magnitude and timing within the wave cycle of TKE production is important in a quantification of sediment transport under breaking waves. For ensemble-averaged intense suspension events, contributing roughly 50% of the total sediment suspension during individual records, we found that instantaneous near-bed sediment load was linearly related to instantaneous levels of Froude-scaled TKE.