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Deltares | R&D Highlights 2015 Flood Risk
Gravel beaches and barriers are found on many high-latitude, wave-dominated coasts across the world. Due to their natural ability to dissipate large amounts of wave energy, gravel coasts are widely regarded as an effective and sustainable form of coastal defence. However, during extreme events, waves may overtop, overwash, and even lower, the crest of the gravel beach, flooding the hinterland. Despite the obvious public importance of gravel beaches and barriers, beach managers currently have few operational management tools to predict how gravel beaches will respond to storms. Deltares worked in partnership with lead partner Plymouth University, the Channel Coastal Observatory, the Environment Agency, HR Wallingford and UNESCO-IHE to address this shortfall by developing a predictive modelling capability for storm impact on gravel coasts.
The numerical model developed for this purpose is based on the open-source XBeach model for sandy coasts, and is called XBeach-G. The model simulates the morphological response of gravel beaches and barriers to storms by solving the hydrodynamics of individual waves in the nearshore and swash zone, overtopping flows, groundwater processes, and gravel sediment transport. The model was developed using insights gained from a unique dataset of storm processes on gravel beaches measured at Chesil Beach (Dorset, England) by Plymouth University, as well as from detailed data collected in physical model experiments. The coupled approach of data collection and model development allowed for the rapid and focused development of the numerical model, and a robust analysis of the model’s strengths and weaknesses.
Model validation was carried out using hydrodynamic and morphodynamic data collected at five gravel beaches in the UK during multiple storm events over a three-year period.
Validation simulations showed that the model can simulate the hydrodynamics in the coastal zone (wave transformation, wave run-up and wave overtopping) well, and predict the storm-driven morphological change on a wide range of gravel beaches with universal model calibration coefficients. The XBeach-G model can also predict the key indicator of coastal flooding - the potential for waves to overtop the beach crest - much more accurately than the empirical models currently used for this purpose. Crucially, this analysis identified the possibility of flood risks on many gravel coasts being significantly under-estimated using existing empirical relations, and highlighted the value of process-based models in improving existing methods for flood risk analysis. In order to ensure the easy uptake of the XBeach-G model by coastal managers and local engineering firms, the project has developed a simple graphical user interface (GUI) for the XBeach-G model and has organised a training workshop on the application of the model for over twenty public and private parties.
Further reading:
McCall et al (2015). Modelling the morphodynamics of gravel beaches during storms with XBeach-G, Coastal Engineering, 103, 52-66, ISSN 0378-3839, http://dx.doi.org/10.1016/ j.coastaleng.2015.06.002 robert.mccall@deltares.nl T +31(0)88 335 8106
Storm impacts on
gravel beaches
< Example of the XBeach-G GUI showing storm wave run-up and groundwater response in a gravel barrier
Storm waves hit Chesil Beach (Dorset, England). Photograph courtesy of T. Poate (Plymouth University)
Comparison of measured beach change and beach change predicted by the XBeach-G model in front of the sea wall at Chesil Beach
