Berm growth and closure of a coastal lagoon entrance by swash overwash

New field observations of sediment overwash on a natural beach are presented and compared to predictions from a process based hydrodynamic and swash sediment transport model. The observations were made at an intermittently closed and open lake or lagoon (ICOLL) and show rapid vertical growth of an initially low beach berm as a result of swash overtopping and sediment overwash. Total sediment overwash volumes passing the crest of the beach for a series of high tide overtopping events were obtained from survey data over a period of 12 days either side of spring tides. A process based model for monochromatic swash is extended to random wave conditions through robust parameterisations of wave runup, wave runup distributions and sediment transport and is applied to predict the total overwash transport. This approach avoids the requirement for a surf zone transformation model to obtain nearshore bore heights. The sediment transport calculations use independently determined friction factors from both field and laboratory studies. Predicted sediment transport overwash rates are well correlated with the measurements (r2=0.93), and agree with observed tidal and cumulative overwash rates to within about 20%. The model has applications over a wide range of beach and wave conditions and represents a first step in modelling the growth and recovery of natural beach berms.