Operational storm impact forecast information for the coast : design, implementation in MorphAn and evaluation of an operational coastal storm impact application

Dunes protect a large part of the Dutch coast against flooding from the sea. Although most storms will not cause failure of this sea defense, they may still cause a threat to, for instance, seaside towns because of dune erosion and overtopping. Also, high waves and current velocities near the shore may cause damage to coastal structures or property on the beach. Currently, weather, water level and wave forecasts provide coastal managers with timely information about the severity of an approaching storm, but the damaging effects to be expected of that particular storm are usually unclear. Despite ongoing research, no early warning system which incorporates storm impact assessment is currently available at water authorities. In this study, an operational, quick, flexible and easy to use storm impact application is designed, implemented in MorphAn and evaluated. MorphAn is a computer program that is already used for dune assessment and other coastal related analyses by coastal water authorities and Rijkswaterstaat. There are three main components which are coupled in the storm impact application. These are bathymetry data, water level and wave data as well as an XBeach model. JARKUS transect bathymetry data from MorphAn or alternative 2D grid bathymetry sources like recent bathymetry derived from Argus cameras can be selected and are automatically downloaded. Real-time water level and wave forecast data from Rijkswaterstaat with a 48 hour lead time is retrieved for the location of the chosen bathymetry data. Using this water level and wave data from remote servers eliminates local computational time of large and complicated water level and wave models. Therefore, the storm impact application can be used on a normal laptop and is accessible by water authorities. The bathymetry and water level and wave data is automatically fed into the XBeach model, which calculates the hydrodynamic and morphologic processes that occur during the storm. The storm impact application in MorphAn provides a Graphical User Interface (GUI) where a user takes several steps in order to obtain a result that provides insight in the effects of the approaching storm on the coast. These effects are for instance dune erosion or breaching, damage of property, high current velocities near the shore and overtopping of dunes or structures. Coastal managers can directly use this result for communication purposes, as well as for decisions about beach and hinterland evacuations or protecting measures of sea dikes or dunes. The quality of the storm impact application is evaluated with the criteria Speed, Relevance, Understanding, Robustness and Evolvability. The speed is sufficient to assess approximately 240 1D transects as well as a 2D model with a longshore domain size of 250 to 5000 meter, both within a two hour limit. The relevance of the storm impact application as well as the storm impact information it provides is recognized by coastal water authorities. The understanding is improved by providing a GUI as well as variable output visualization complexities, although there are many improvements possible in the GUI that is currently implemented. The robustness of the storm impact application components is good, because of several different available bathymetry sources, redundant Rijkswaterstaat hardware and an XBeach model as well as a MorphAn GUI that are both locally installed. There are several possibilities to evolve the storm impact application by improving current possibilities as well as implementing international data sets to use the storm impact application elsewhere. The information provided by the storm impact application is evaluated with the criteria Accuracy and Performance. The uncertainties of the different bathymetry sources, water level and wave forecasts as well as the XBeach model are assessed. A sensitivity analysis of the uncertainties of water level and wave data provides information about their effect on the result, to be taken into account in decision making. The confidence interval of the dune retreat that is provided by the storm impact application is approximately 15 meters. This confidence interval can be used by decision makers, to determine whether decisions can be bases on the storm impact application in operational practice. The storm impact application developed in this study is only useful if it will be used in practice. Evaluation of the seven criteria that were defined in this study provide a promising outlook for operational practice, although many improvements are possible for the GUI. A prototype of a storm impact application was developed, implemented in MorphAn and evaluated. This application uses real-time water level and wave forecast data as well as recent bathymetry. The storm impact application from this study can be used to provide relevant, accurate and timely storm impact information for the coast. Concluding, an operational, quick, flexible and easy to use storm impact application is now available for operational practice and ready to be further developed.