The devastating effects of hurricanes on low-lying sandy coasts, especially during the 2004 and 2005 seasons in the USA, have pointed at an urgent need to be able to assess the vulnerability of coastal areas and (re-)design coastal protection for future events, but also to evaluate the performance of existing coastal protection projects compared to ‘do-nothing' scenarios.
In order to address such questions the Morphos-3D project was initiated by the US Army Corps of Engineers. This project brings together models, modelers and data on hurricane winds, storm surges, wave generation and nearshore processes (wave breaking, surf and swash zone processes, dune erosion, overwashing and breaching). A group of experts in nearshore morphological modelling, led by UNESCO-IHE's Prof. Dano Roelvink, and including researchers from Deltares (formerly Delft Hydraulics), Delft University and the University of Miami, was asked to contribute by developing a new public domain model, ‘XBeach', that can predict nearshore waves and currents, dune erosion (scarping), overwashing and eventually breaching of barrier islands. Based on their long experience with modelling such processes in physical models and in computer models such as Delft3D, the group has developed a prototype model and validated it against a number of large-scale flume tests in the first year of the project, which will continue for two more years.
The XBeach model can be used as stand-alone model for small-scale (project-scale) coastal applications, but will also be used within the Morphos model system, where it will be driven by boundary conditions provided by the wind, wave and surge models and its main output to be transferred back will be the time-varying bathymetry and possibly discharges over breached barrier island sections. The model solves coupled 2D horizontal equations for wave propagation, flow, sediment transport and bottom changes, for varying (spectral) wave and flow boundary conditions. Because the model takes into account the variation in wave height in time (long known to surfers) it resolves the special long wave motions created by this variation. This so-called ‘surf beat' is responsible for most of the swash waves that actually hit the dune front or overtop it. Because of this innovation the XBeach model is better able to model the development of the dune erosion profile and to predict when a dune or barrier island will start overwashing and breaching.
The model has already been validated against extensive large-scale flume data sets including short and long wave distributions, return flow, orbital velocities, concentrations and profile change during dune erosion events. An essential part is an avalanching mechanism which allows a surprisingly accurate description of the evolution of the upper profile and dune face.