Overwash threshold for gravel barriers

This paper uses results obtained from the large-scale BARDEX experiments undertaken in the Delta flume to investigate the morphological response of a prototype gravel barrier to wave and tidal forcing during overwash conditions. Gravel barrier behaviour depends upon a number of factors such as sediment properties (porosity, permeability, grainsize), geological setting and wave climate. Since overwash processes are known to control short-term gravel barrier dynamics and long-term barrier migration, the development of a robust quantitative method to define the critical conditions leading to gravel barrier overwashing is important both for scientific and practical management purposes. It is known that when wave runup exceeds the barrier crest elevation, three outcomes are possible: 1) insignificant morphological change, when wave runup just overtops the barrier crest and flow velocities are very weak; 2) overtopping, resulting in accretion on the barrier crest region and barrier stabilisation; and 3) overwashing, resulting in erosion, lowering of the crest region and ultimately to barrier inundation. This study provides an insight into the critical conditions that distinguish these two possible outcomes and to the different mechanisms that provide the required positive and negative feedbacks to the sediment dynamics. In order to define the overwash threshold condition, and to predict the morphological outcome of particular overwash events, use is made here of the Overwash Potential (OP), defined as the difference between the wave runup and the barrier crest elevation. To make effective use of OP it is necessary to identify a reliable runup predictor. Following tests using 12 runup equations the Stockdon et al. [Stockdon, H.F., Holman, R.A., Howd, P.A., Sallenger, A.H., 2006. Empirical parameterization of setup, swash, and runup. Coast. Eng., 53, 573–588] approach has been identified as the best predictor of runup conditions necessary to generate positive values of OP, with overtopping and overwashing predicted to occur for average OP values of 0.2 m and 0.5 m, respectively. The use of OP values provides a practical means by which to identify potential coastal hazards associated with gravel barrier overwash processes and is considered to have a range of practical coastal management applications.

► Results from BARDEX overwash experiments are presented.
► Critical conditions for overtopping and overwash were provided.
► Overwash Potential was defined based on wave runup and barrier elevation.
► The most accurate and precise runup predictor was identified.