R. van Santen1*, R. McCall2, R. Wilmink3, L. Coumou1, E. Quataert2, S. Pluis3, H. Steetzel1, R. de Goede2, N. van Kuik3
1 Arcadis; 2 Deltares; 3 Rijkswaterstaat
Dunes and beaches are an integral part of the primary sea defences along the Dutch coastline. The existing (and legally embedded) methodology for dune safety assessments is built around the empirical 1D model Duros+. The applicability of this model, however, is limited for coastal areas with more complex bathymetry, multiple dune rows and/or the presence of coastal structures. As part of the national program BOI (Assessment and Design Instrument for flood defences) a renewed instrument is being developed, based on the model XBeach, for assessing, designing and maintaining dunes.
Calibration and validation of XBeach
One of the initial steps in developing the new dune assessment instrument is the calibration and the validation of the new core model, XBeach. A new set of model parameter settings has been derived based on both laboratory experiments and field data. To ensure consistency in the setup of XBeach models for the purpose of a dune safety assessment BOI-specific guidelines and procedures have been developed (e.g. for schematisation of coastal profiles). As part of the study also some model improvements have been implemented and tested. Related to the morphodynamic processes, for example, the grain size sensitivity of XBeach has been improved. Based on the new BOI-settings and developed guidelines for model setup, multiple field cases are considered to validate XBeach and to gain insight in the accuracy of the model.
For the validation study a series of Dutch and international field cases was selected based on availability of data and representativeness of the local situation (profile, grain size, storm conditions, erosion volumes). For each of the selected cases one or more coastal transects are considered for which (1D) XBeach models are set up in accordance with the new guidelines and the BOI model settings. The results of the model simulations are used for the validation of the modelled hydrodynamic and morphodynamic processes. In total, nine case studies and more than 60 transects are considered for a successful quantitative comparison between model results and observational data.
We look forward to presenting the results of the morphodynamic field validation at the NCK conference.
Figure 1: Example of input and output data of one of the morphodynamic validation cases: dune erosion along the Flemish Coast during the Sint Nicholas storm (2013).
Towards a renewed instrument for dune safety assessments
The calibration and validation of the BOI-version of XBeach is the first step towards the intended new instrument for assessing the sandy Dutch coasts. The formal safety assessment aims at estimating the probability of flooding, therefore also a semi-probabilistic method is being developed for dunes. The calibrated XBeach model, the semi-probabilistic method and a set of updated guidelines and procedures jointly support the future assessment, design and maintenance of dunes along the Dutch coastline.