Simulating Barrier Island Evolution: Coupling Process-Based Models

Barrier islands are important features in the coastal zone, among others because they shelter the mainland from waves and storm surge. Thus, the degradation of barrier islands can pose a threat to coastal safety. Hence, there is a societal demand for understanding and predicting barrier island evolution. High energy events, such as storms and hurricanes, play an important part in this evolution, with hydraulic sediment transport causing large and rapid changes in morphology. In the periods between storms, (partial) barrier island recovery takes place, largely driven by aeolian sediment transport on longer timescales. Hence, when predicting the morphological development of barrier islands, both hydraulic and aeolian transport need to be taken into account. Also, both the event timescale of hours to days, and the recovery timescale of weeks to months need to be resolved. Currently, no single numerical model exists that simulates both hydraulic and aeolian transport, though there are models that resolve either one separately. So, to resolve both simultaneously, two models will have to be coupled. The above leads to the two objectives of this thesis; firstly, constructing a coupling between a hydraulic and an aeolian sediment transport model, and analyze physical and numerical aspects of the model interaction. For this purpose the models XBeach and Dune are selected. The coupling is created using the Earth System Modelling Framework (ESMF). The second objective is confirmation of the predictive skill of the coupled model for the evolution of a real barrier island. Assateague Island (MD, USA) is selected, and a combination of model skill score and bias is used to represent the predictive capabilities of the coupled model. From a process point of view, undertow, long wave flow, and increased turbulence due to wave breaking have a significant effect on the sediment transport during storms, and all are represented within the XBeach model. Aeolian transport is less important during storms, because the sand supply is limited by submergence and moisture content. During recovery, aeolian transport does play an important role in transporting the sediment towards the beach and dunes, where it is often trapped by vegetation. The lower wave height during these periods allows for a relatively larger influence of short wave asymmetry, that can lead to hydraulic transport towards the shoreline, creating a sediment supply for beach and dune recovery. This makes the foreshore zone an interface between hydraulic and aeolian processes. The coupling between XBeach and Dune allows the models to exchange information after every communal timestep, thus facilitating dynamic interaction. This is only necessary when simulating recovery periods, for the influence of aeolian transport during storms is assumed negligible. The structure of the ESMF makes it possible to couple multiple models together, requiring only a few adaptations to the structure of the sub-model codes. This also provides the flexibility to add new or replace old models with relative ease. To confirm the predictive skill of this coupled model, a hindcast of six months of morphological development of Assateague Island will be performed. To this end, storms are distinguished from recovery periods, and are simulated in chronological order, the former with XBeach, the latter with the coupled model. The simulations lead to negative skill scores because of a significant overestimation of storm induced erosion. Even when using an overwash sediment transport limiter to reduce the erosion during storms, the skill scores remain negative. It can be concluded that the first objective has been fulfilled, since a coupling between XBeach and Dune has been constructed. Although a hindcast was performed, this could not confirm the predictive skill of the coupled model, so the second objective was not completed.Hydraulic EngineeringHydraulic EngineeringCivil Engineering and Geoscience

Machine learning improves the modelled wave spectrum in the North Sea

To improve the energy density and directional spectra computed with the SWAN model for the North Sea, a data-driven model is trained to correct the SWAN spectra. After training of the data-driven model on a year of observed and modelled data, the energy density and directional spectrum are corrected for three locations in the North Sea. When this correction is applied, the SWAN results are significantly improved. Both the energy density and the directions show a reduction in RMSE of up to 30% for the directions and 26 % for the energy density. Due to the short computational time of the data-driven model, this approach can easily be implemented in an operational forecast system

MODELLING DUNE EROSION, OVERWASH AND BREACHING AT FIRE ISLAND (NY) DURING HURRICANE SANDY

In 2012, Hurricane Sandy caused a breach at Fire Island (NY, USA), near Pelican Island. This paper aims at modelling dune erosion, overwash and breaching processes that occured during the hurricane event at this stretch of coast with the numerical model XBeach. By using the default settings, the erosion rates are substantially overestimated, which was also concluded in several previous case studies. If the discretization of bed roughness along with wave skewness and asymmetry are improved in the model, XBeach is capable of simulating the various morphological changes within the chosen model domain

Modelling Scour in Front of Dune Revetments in a Surf-beat Model

This paper presents adaptations to the XBeach model aimed at including the relevant processes for the generation of scour holes at the toe of a revetment. Dutch assessment rules for the safety of sea defenses need to be adjusted to cope with a combination of sandy dunes and hard elements. To that end, the XBeach model is prepared to be incorporated in the assessment rules. Until now, XBeach did not model scour hole development in front of dune revetments accurately. We suggest to include the advection of turbulence as well as the effect of backwash of short waves that creates additional turbulence in the model. Verification with three physical model experiments shows that with the suggested adaptations of the model a scour hole with significant depth can be modeled.Hydraulic EngineeringCivil Engineering and Geoscience