Coastal Overwash: Processes and Modelling

Sammanfattning: Overwash is the flow of water and sediment over the crest of a beach system when the runup level of waves or the water level, often enhanced by storm surge, exceeds the local beach or dune crest height. The impacts of overwash on coastal barriers or low lying mainland coasts are striking. Overwash may cause deposition of sand on and landward of the beach crest, large fan shaped deposits on back barriers, large sheet like deposits over an entire barrier, sand deposition into back barrier waterways, or may even lead to breaching of coastal barriers. It would therefore be highly useful to be able to predict the occurrence of overwash events and the magnitude and shape of the washover deposited during them. Although a number of studies describing overwash and washover deposits have been published, there remains a large scope to describe overwash processes, overwash hydrodynamics and to develop models for predicting the magnitude and shape of washover deposits on the back barrier. The objective of this study was to improve the capability to predict sediment transport caused by overwash, and hence the resulting topographic changes. The sediment transported by overwash is a function of the overwash hydrodynamics, and the overwash hydrodynamics are affected by a number of different, interacting processes. One of the main tasks for this study was therefore to identify and describe both the forcing and back barrier processes that affect overwashing flow. Overwash was shown to occur due to both wave runup overtopping the beach crest and surge levels exceeding the beach crest height. On the back barrier, overwash hydrodynamics and sediment transport were shown to be affected by the back barrier water level, friction, infiltration, lateral spreading and anthropogenic influences. New, mid-scale laboratory experiments of runup overwash were conducted to gain an understanding of back barrier flow hydrodynamics. The laboratory data were supplemented with field data to derive relationships to estimate overtopping depths and wave front velocities on the beach crest and back barrier. Additionally, three different types of overwash model were developed. The first, a parametric model, uses simple, readily available data to predict the type of cross-shore morphodynamic change expected for a given incipient barrier profile and maximum storm characteristics. Secondly, an analytical model was derived to calculate order-of-magnitude beach face retreat and overwash volumes for schematised incipient beach profiles. Finally, a numerical model was developed to calculate in more detail the barrier profile change resulting from an overwash event. This model uses incipient beach profiles and a time-series of storm characteristics to calculate the beach profile change. All three models were calibrated, validated and verified against a large, new data set of pre- and post-storm beach profiles measured where overwash had occurred and show promising results for predicting beach profile change.