Modelling of Coastal Overwash : Review and application of state-of-art empirical, numerical and analytical methods

Coastal overwash is the flow of water and sediment over the crest of the beach that does not directly return to the water body, such as the ocean, sea, bay or lake, where it originated. There are two different phases of overwash, runup overwash and inundation overwash. Washover is the sediment transported and deposited inland by overwash process. Coastal overwash may occur either during storm conditions such as hurricanes or severe storm events or non-storm conditions such as extreme tide levels or seiches in lakes. The modelling of coastal overwash and the prediction of overwash occurrence are significant for coastal authorities and local managers to give evacuation warnings to people who live near the coast. Estimates of washover volume are useful for planners and engineers to mobilise equipment for cleaning up sediment after a storm. Profile change estimations are useful for planners creating ?buffer zones? indicating where it is safe to build houses. Modelling overwash is also important for evaluating sediment budgets of barrier islands, especially where these islands are migrating due to overwash. Beach response morphologies can be divided into five types, that is, crest accumulation, dune lowering, dune rollback, dune destruction, and barrier rollback. However, in reality it is not easy to identify what kind of morphologic response the beach has experienced, and in some cases a combination of beach response types are observed. A new empirical formula for overwash volume was derived by taking into account the excess runup height over the beach crest and the duration of the overwash event. This formula yields predictions that are comparable to the measured field data. However, it needs more testing with other data sets before its general applicability has been validated. A number of modelling studies were carried out in order to simulate the overwash processes. This study evaluates different levels of coastal overwash modelling: empirical, analytical, and numerical (SBEACH) models based on a number of high-quality, detailed data sets which were collected in the United States. The latest version of SBEACH with a new algorithm to simulate overwash flow either by runup or inundation overwash was employed. Flow on the backside of barrier was considered in the enhanced model including the effects of lateral spreading and infiltration. It was shown that the simulated beach profile evolution was in satisfactory agreement with measured post-storm profiles. (examensarbetet är utfört vid Avd för Teknisk Vattenresurslära, TVRL)