Littoral applications include the development of capabilities for the prediction of waves, currents and morphological change in the littoral zone as part of Rapid Environmental Assessment activities for Bluelink partners.
Two forecast systems have been developed: one that is laptop-based, and one that requires high-performance computing (HPC). The Littoral Ocean Modelling System (LOMS) is the laptop-based system, and ROAM-surf the HPC system. These systems allow the user to set up and run a surf zone model at an arbitrary location and involves coupling the nearshore waves and currents, sediment transport and morphology. They are designed to run in a forecast mode (~5 days) using forecast wind and waves from external sources.
Secret Harbour Field Program – 2012 to 2014
Data collected during an extensive field program continues to provide valuable input for validation of the modelling system. During the field program, a nearshore Research Facility (NRF) was developed to observe morphological change on a natural beach to be used in model validation and tuning. Morphodynamic time scales are typically much longer than hydrodynamic time scales, except perhaps through extreme events, and measuring morphological change under breaking waves is a challenge with conventional hydrographic surveying methods. Long-term, continuous monitoring of morphological change and the nearshore wave field is most effectively done with remote techniques such as video and radar. The NRF consists of an XBand radar and a 6 camera video system was deployed at Secret Harbour, south of Perth (Fig. 1) Directional wave data were collected offshore of the NRF and provide incident wave forcing conditions for the model. Monthly surveys of the nearshore bathymetry were obtained using RTK GPS and depth sounder mounted on a jet ski. The same GPS unit mounted on a quad bike was used to map the beach.
The merged time exposure video image in Fig. 2 shows the distribution of wave breaking and, because wave breaking is a depth-dependent process, provides a proxy for the underlying bathymetry.
An archive of video images from Secret Harbour is available.
Littoral Zone Modelling
A numerical model, Called XBeach, is used to simulate depth-averaged wave-driven currents. The wave field is estimated using a simplified time dependent wave action balance equation to give the directional distribution of the frequency integrated wave action density. This time-varying wave action balance includes refraction, shoaling, current refraction, bottom friction and wave breaking. The model forcing can accommodate non-stationary time-varying incident wave energy and the corresponding bound long wave associated with incoming wave groups. The incoming wave groups force low frequency motion in the nearshore that is thought to be important in generating alongshore variations in the bathymetry such as alternating sandbars and rip channels as shown in Fig. 3.
Infra-gravity response to sea breeze forcing
Analysis of field observations of sea-surface elevation reveals the role of time-varying incident forcing and bound wave release mechanisms, on the generation of infragravity waves in the nearshore. Observations of infragravity response were obtained during sea-breeze and swell periods when the wave height varied from 0.2m to 1.2m and wave period from 3s to 16s. The observations show a stronger response from infragravity waves to swell forcing than wind-wave forcing . We found that in the sea breeze case, steep wave conditions involve long wave generation by breakpoint forcing. In our observations, release of bound waves is the dominant mechanism during swell periods, and happens when short waves reach shallow waters before or at the breakpoint.
Contardo, S., G. Symonds, N. Mortimer, 2010: Bluelink II Workpackage P3 Inner Shelf and Nearshore Field Program. CAWCR Technical Report No. 028, 34pp.
Gunson, J., G. Symonds, 2010: 2010. Wave-driven currents on a barred beach during a sea-breeze cycle, in Proc. Aust. Wind Waves Res. Science Symp., 19-20 May 2010, Gold Coast, Qld, Aust., editor Keith Day, Centre for Aust. Weather and Climate Research Tech Rep No. 29, 27-30.
Holman, R. A., G. Symonds, E. B. Thornton, and R. Ranasinghe (2006), Rip spacing and persistence on an embayed beach, J. Geophys. Res., 111, C01006, doi:10.1029/2005JC002965.
Ranasinghe, R., G. Symonds, K. Black, R. Holman, 2004: Morphodynamics of intermediate beaches: a video imaging and numerical modeling study. Coastal Engineering, 51, 629-655.
Symonds, G., L. Zhong, N. A. Mortimer, 2011: Effects of wave exposure on circulation in a temperature reef environment. Journal of Geophysical Research, 116, C09010, doi:10.1029/2010JC006658.