@article {bnh-2587, title = {Predicting continental shelf waves in Australia}, number = {160}, year = {2016}, month = {02/2016}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

A range of processes can cause extreme water levels that endanger human life and cause property damage. Storm surges caused by tropical cyclones are well known to pose a risk for coastal inhabitants.\  However, a lesser-known effect of landfalling cyclones is the generation of coastally trapped waves that can propagate along the coast and influence water levels thousands of kilometres away. Examples of such waves include continental shelf waves (CSWs) that travel to the left along the coastline in the southern hemisphere and are common along the Australian coastline (Church et al., 1986; Eliot and Pattiaratchi, 2010).

This study investigated the generation and propagation characteristics of continental shelf waves (CSWs) in Western Australia, a hotspot of continental shelf wave activity in Australia, utilising the Regional Ocean Modelling System (ROMS).\  An idealised model of the North West Shelf (NWS) with ~ 4 km spatial resolution and 30 vertical sigma layers was forced by simulated tropical cyclones that varied in speed, direction (parallel, perpendicular or at 45{\textdegree} to the coastline) and category.\  Realistic cyclone simulations were performed for Tropical Cyclone Bianca in 2011 using an Australia-wide model and comparisons made to observations.

Results from the idealised numerical model showed that a cyclone{\textquoteright}s path, speed and category affect continental shelf wave generation and propagation.\  Greater cyclone category resulted in higher amplitude continental shelf waves.\  Cyclones that travelled parallel to the coastline generated continental shelf waves that were faster than the cyclone forward speed and produced higher amplitude waves than those generated by different trajectories.\  Increased parallel cyclone forward speed corresponded to an increase in the continental shelf wave height. If the parallel cyclone forward speed was too fast (i.e. greater than 14 ms-1), the continental shelf wave height decreased.\  Parallel moving cyclones travelling near to the continental shelf break with a forward speed of ~ 7.5 ms-1 generated the highest amplitude waves. If the cyclone discontinued travelling parallel, the continental shelf wave gained similar properties to continental shelf waves generated by perpendicular or 45{\textdegree} travelling cyclones.\  These type of cyclones generated continental shelf waves that had a lower wave height, which were even lower for faster moving cyclones.\  The propagation speed of these waves scaled to the Coriolis parameter and characteristic length scale of the continental shelf.\  The lower wave height of these waves can be explained by the continental shelf wave travelling as a "free wave".

Propagation characteristics of continental shelf waves generated by Cyclone Bianca and Carlos from realistic cyclone simulations showed cyclone path and continental shelf bathymetry had an influence on CSW height and propagation speed. The calculated approximate travel speed from lag correlation analysis was 5.8 ms-1 for both continental shelf waves.\  Tropical Cyclone Bianca travelled parallel to the Western Australian coastline which reinforced the continental shelf wave height. Tropical Cyclone Carlos continued offshore from Exmouth so the continental shelf wave had little energy to propagate around the curved coastline in the Capes Region.\  Both continental shelf waves decreased in propagation speed between Exmouth and Carnarvon where the continental shelf width decreased rapidly.\  The continental shelf wave generated by Tropical Cyclone Bianca rapidly decreased at Albany which may be explained by the curvature of the coastline which resulted in wave scattering.\ \ \ 

Results from this study demonstrate how particular cyclone properties influence the generation of continental shelf waves and through this improves our ability to predict the influence of CSWs on extreme water levels along the Australian coast. .\  It is recommended that design criteria, modelling studies and inundation risk assessments for coastal regions in Western Australia need to consider the effects of continental shelf waves.\ \ 

}, issn = {160}, author = {Charitha Pattiaratchi and E.M.S Wijeratne and Yasha Hetzel and Ivica Janekovic} }