Physical Modelling of Wave Overtopping and Coastal Defence Reinforcement Design at the Low-lying Community of East Rhyl, UK

A detailed design of a Coastal Defence Scheme was developed to reduce the risk of coastal flooding to the low-lying community of East Rhyl, North Wales. The Rhyl community is currently protected by an existing coastal dyke and recurve wall. However, wave overtopping has flooded the local community several times in recent history as the beach is narrowing. The Coastal Defence Scheme must reduce wave overtopping for future storm events.

The Denbighshire County Council, aligned with current UK standards, desires flooding immunity to 200-year Annual Recurrence Interval storm event for 2117. For such a long period of time. many variables can affect such targets. Consequently, a multi-variate analysis was followed to deliver a stochastic design of the Rhyl Coastal Defence Scheme. Stochastic overtopping design requires high-accuracy of overtopping estimates. However, measuring, modelling and forecasting wave overtopping accurately is challenging because this wave-generated flow is ephemeral, very dynamic, highly turbulent and multi-phasic.

Two down-scaled models were tested to provide high accuracy overtopping observations. A 1:25 two-dimensional flume model and a 1:40 three-dimensional basin model were constructed and used in tandem, over 5 months. The 2D model allowed rapid prototyping of the structure and the accurate measurement of flow parameters including overtopping rate and volume, pressure loads on the defence structure, green-water elevation, etc. The 3D model was used to validate findings with consideration of the effects of wave direction since broken waves `slide` along the seawall, which is a major contributor to the non-uniform overtopping along the East Rhyl coast. The 2D and 3D physical models were calibrated and validated against numerical models deployed to design the overtopping flow drainage system. Particularly, the physical testing results were adjusted and compared to the estimates calculated using the overtopping calculator, Neural Network. The physical test also measured wave loads on the precast RC wall rock, armouring stability and wave reflections on the structure.

The new Coastal Defence Scheme will consist of over 600m of precast reinforced concrete wall, rock armour revetment and overtopping flow retention basin. This is to ensure an optimised and cost-effective coastal defence to protect the East Rhyl Community for years to come.