During the last decade there has been a growing interest in multipurpose reefs as a solution to provide coastal protection while also creating favorable surfing conditions at the same time. Assessing the surfing quality of a submerged reef requires highly detailed information of the wave breaking characteristics such as the shape of the overturning wave and the propagation velocity of the moving breakpoint. In addition the powerful and highly non-uniform wave breaking conditions across the reef induces strong return currents and areas of high local velocities both of which strongly affects the wave breaking characteristics and represents crucial reef design parameters, when assessing structural stability, induced erosion issues and adjacent beach safety.

Recent advances in CFD modelling and the utilization of code parallelization (allowing running simulations across multiple workstations) have now made it possible to carry out detailed studies of the complex and highly non-linear wave transformation and current interaction processes occurring over a submerged reef structure.

Moving wave breakpoint velocity comparison Test 1 (H = 8 cm, T = 2.26 sec)

Moving wave breakpoint velocity comparison Test 2 (H = 8 cm, T = 1.70 sec)

The CFD model NS3 developed by DHI consists of a fully non-linear 3D Navier-Stokes solver with a Volume of Fluid treatment of the free surface. The model has previously been used and validated in Mayer et al. (1998), Nielsen & Mayer (2001), Christensen (2006) and Bredmose et. al. (2006) and Nielsen et al. (2008).  NS3 can be coupled with DHI’s MIKE 21 BW to provide the 3D offshore boundary condition hence effectively decreasing the necessary NS3 model domain size and allowing the representation of regional effects such as non-uniform refraction around adjacent headlands and offshore focusing generated by offshore bathymetrical features.

Breaking wave height comparison along the reef (H = 8 cm, T = 2.26 sec)

Breaking wave height comparison along the reef (H = 8 cm, T = 1.70 sec)

Visual comparison between model results and camcorder recordings of the wave breaking characteristics at three different locations along the reef

The NS3 model was used to reproduce the wave transformation processes occurring in a large scale physical model of an artificial surfing reef presented in (Henriquez 2004). Calculations of the wave breaking height and wave pealing velocity along the reef were in excellent agreement with physical model measurements. A visual comparison between the numerical model predictions and physical model video observations of the surfing wave shape and breaking characteristics also demonstrated excellent agreement.

A full result description will be presented in a paper on the International Multi-purpose Reef Conference taking place in Jeffery’s Bay, South Africa, May 2009.

References
Nielsen, A.W, Mortensen, S. B., Christensen, E.D (2008) “Numerical Modeling of Wave Run-up on a Wind Turbine Foundation”, Proc. OMAE 2007, Lisbon, Portugal.

Bredmose, H., Skourup, J., Hansen, E.A., Christensen, E.D., Pedersen, L.M. and Mitzlaff, A. (2006) “Numerical Reproduction of Extreme Wave Loads on a Gravity Wind Turbine Foundation”, Proc. OMAE 2006, Hamburg, Germany.

Christensen, ED. (2006) “Large eddy simulation of spilling and plunging breakers”, Coastal Engineering Vol. 53, Issues 5-6, 463-485.

Henriquez, M (2004) “Artificial Surf Reefs”, Delft University of Technology

Mayer, S., Garapon, A. & Sørensen, L.S. (1998) ”A fractional step method for unsteady free-surface flow with applications to non-linear wave dynamics. Int. J. Numer. Meth. Fluids, 28, pp 292-315.