\chapter{Introduction}
\section{Introduction to the XBeach model}

The devastating effects of hurricanes on low-lying sandy coasts, especially during the 2004 and 2005 seasons have pointed at an urgent need to be able to assess the vulnerability of coastal areas and (re-)design coastal protection for future events, and also to evaluate the performance of existing coastal protection projects compared to 'do-nothing' scenarios. In view of this the Morphos-3D project was initiated by USACE-ERDC, bringing together models, modelers and data on hurricane winds, storm surges, wave generation and nearshore processes. As part of this initiative an open-source program, XBeach for eXtreme Beach behaviour, has been developed to model the nearshore response to hurricane impacts. The model includes wave breaking, surf and swash zone processes, dune erosion, overwashing and breaching \citep{Roelvink2009}.

Existing tools to assess dune erosion under extreme storm conditions assume alongshore uniform conditions and have been applied successfully along relatively undisturbed coasts \citep{Vellinga1986,Steetzel1993,NishiKraus1996,Larson2004}, but are inadequate to assess the more complex situation where the coast has significant alongshore variability. This variability may result from anthropogenic causes, such as the presence of artificial inlets, sea walls, and revetments, but also from natural causes, such as the variation in dune height along the coast or the presence of rip channels and shoals on the shoreface \citep{Thornton2007}. A particularly complex situation is found when barrier islands protect storm impact on the main land coast. In that case the elevation, width and length of the barrier island, as well as the hydrodynamic conditions (surge level) of the back bay should be taken into account to assess the coastal response. Therefore, the assessment of storm impact in these more complex situations requires a two-dimensional process-based prediction tool, which contains the essential physics of dune erosion and overwash, avalanching, swash motions, infragravity waves and wave groups.

With regard to dune erosion, the development of a scarp andepisodic slumping after undercutting is a dominant process \citep{VanGent2008}. This supplies sand to the swash and surf zone that is transported seaward by the backwash motion and by the undertow; without it the upper beach scours down and the dune erosion process slows down considerably. One-dimensional (cross-shore) models such as DUROSTA \citep{Steetzel1993} focus on the underwater offshore transport and obtain the supply of sand by extrapolating these transports to the dry dune. \citet{OvertonFisher1988}, \citet{NishiKraus1996} focus on the supply of sand by the dune based on the concept of wave impact. Both approaches rely on heuristic estimates of the runup and are well suited for 1D application but difficult to apply in a horizontally 2D setting. Hence, a more comprehensive modelling of the swash motions is called for.

Swash motions are up to a large degree a result from wave-group forcing of infragravity waves \citep{Tucker1954}. Depending on the beach configuration and directional properties of the incident wave spectrum both leaky and trapped infragravity waves contribute to the swash spectrum \citep{Huntley1981}. \citet{RaubenheimerGuza1996} show that incident band swash is saturated, infragravity swash is not, therefore infragravity swash is dominant in storm conditions. Models range from empirical formulations \citep[e.g.][]{Stockdon2006} through analytical approaches \citep{Schaeffer1994,Erikson2005} to numerical models in 1D \citep[e.g.][]{List1992,Roelvink1993b} and 2DH \citep[e.g.][]{VanDongeren2003,Reniers2004a,Reniers2006}. 2DH wavegroup resolving models are well capable of describing low-frequency motions. However, for such a model to be applied for swash, a robust drying/flooding formulation is required.

\section{Model approach and innovations}

Our aim is to model processes in different regimes as described by \citet{Sallenger2000}. He defines an Impact Level to denote different regimes of impact on barrier islands by hurricanes, which are the 1) swash regime, 2) collision regime, 3) overwash regime and 4) inundation regime. The approach we follow to model the processes in these regimes is described below.

To resolve the swash dynamics the model employs a novel 2DH description of the wave groups and accompanying infragravity waves over an arbitrary bathymetry (thus including bound, free and refractively trapped infragravity waves). The wave-group forcing is derived from the time-varying wave-action balance \citep[e.g.][]{Phillips1977} with a dissipation model for use in combination with wave groups \citep{Roelvink1993a}. A roller model \citep{Svendsen1984,Nairn1990,StiveDeVriend1994} is used to represent momentum stored in surface rollers which leads to a shoreward shift in wave forcing.

The wave-group forcing drives infragravity motions and both longshore and cross-shore currents. Wave-current interaction within the wave boundary layer results in an increased wave-averaged bed shear stress acting on the infragravity waves and currents \citep[e.g.][and references therein]{Soulsby1993}. To account for the randomness of the incident waves the description by \citet{Feddersen2000} is applied which showed good skill for longshore current predictions using a constant drag coefficient \citep{Ruessink2001}.

During the swash and collision regime the mass flux carried by the waves and rollers returns offshore as a return flow or a rip-current. These offshore directed flows keep the erosion process going by removing sand from the slumping dune face. Various models have been proposed for the vertical profile of these currents \citep[see][for a review]{Reniers2004b}. However, the vertical variation is not very strong during extreme conditions and has been neglected for the moment.

Surf and swash zone sediment transport processes are very complex, with sediment stirring by a combination of short-wave and long-wave orbital motion, currents and breaker-induced turbulence. However, intra-wave sediment transports due to wave asymmetry and wave skewness are expected to be relatively minor compared to long-wave and mean current contributions \citep{VanThielDeVries2008}. This allows for a relatively simple and transparent formulation according to Soulsby \& Van Rijn \citep{Soulsby1997} in a shortwave averaged but wave-group resolving model of surf zone processes. This formulation has been applied successfully in describing the generation of rip channels \citep{Damgaard2002,Reniers2004a} and barrier breaching \citep{Roelvink2003}.

In the collision regime, the transport of sediment from the dry dune face to the wet swash, i.e. slumping or avalanching, is modeled with an avalanching model accounting for the fact that saturated sand moves more easily than dry sand, by introducing both a critical wet slope and dry slope. As a result slumping is predominantly triggered by a combination of infragravity swash runup on the previously dry dune face and the (smaller) critical wet slope.

During the overwash regime the flow is dominated by lowfrequency motions on the time scale of wave groups, carrying water over the dunes. This onshore flux of water is an important landward transport process where dune sand is being deposited on the island and within the shallow inshore bay as overwash fans \citep[e.g.][]{Leatherman1977,WangHorwitz2007}. To account for this landward transport some heuristic approaches exist in 1D, e.g. in the SBeach overwash module \citep{Larson2004} which cannot be readily applied in 2D. Here, the overwash morphodynamics are taken into account with the wave-group forcing of low-frequency motions in combination with a robust momentum-conserving drying/flooding formulation \citep{StellingDuinmeijer2003} and concurrent sediment transport and bed-elevation changes.

Breaching of barrier islands occurs during the inundation regime, where a new channel is formed cutting through the island. \citet{Visser1998} presents a semi-empirical approach for breach evolution based on a schematic uniform cross-section. Here a generic description is used where the evolution of the channel is calculated from the sediment transports induced by the dynamic channel flow in combination with avalanche-triggered bank erosion.

\section{XBeach skillbed}

The XBeach code and related functionalities develop fast. As a result there is a need from modelers and code developers to develop a tool that gives insight in the effect of code developments on model performnace. The XBeach skillbed tries to fulfill this need by running a range of tests including analytical solutions, laboratory tests and practical field cases.

\newpage