Improving predictions of nearshore processes and shoreline dynamics for beaches along Australia s rocky and coral reef coasts

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1 Improving predictions of nearshore processes and shoreline dynamics for beaches along Australia s rocky and coral reef coasts Ryan Lowe Jeff Hansen, Graham Symonds, Mark Buckley, Andrew Pomeroy, Gundula Winter, Laura Elena Segura, Mike Cuttler, Edwin Drost

2 Australia s diverse coastline Knowledge foundation of nearshore processes has been historically based on open coast sandy beaches Coral reefs Up to ~80% of the world s coastline is rocky (Emery and Kuhn 1982); ~30-50% of Australia s coast is rocky (Short 2009) Large portion of tropical coastline contains coral reefs and mangroves Rocky reefs

3 Challenges with nearshore predictions along these complex coasts Steep slopes (~1:20 to 1:1) Complex 3D morphologies Large bottom roughness Properties very different from beaches

4 Our study sites along Western Australia Ningaloo Reef (coral reefs) Garden Island (rocky reefs)

5 Background on wave forcing of the nearshore zone Key processes and energy transfers Incident energy spectrum Incident sea-swell waves (~5-25 sec period) wave-driven mean currents wave setup swell transmission infragravity waves (25+ sec) + dissipation

6 Wave-driven circulation of the nearshore zone Dissipation of wind waves via breaking is the primary driver of nearshore circulation along most coasts Gradients in wave energy fluxes Wave set-up Mean currents Requires coupled wave-circulation models

7 Wave-driven circulation (Example from Garden Island, WA) Rocky section of coast in SW Australia (ideal case study) Persistent strong wave-driven flows Patterns fixed by bathymetry Analogies to rip currents Mean drifters m/s Moored instruments Channel North Reef Platforms Channel South

8 Influence of shelf circulation on crossshelf exchange Examples from single drifter deployments Drifter Retention Weak along-shelf currents Drifter Expulsion Stronger southward along-shelf currents m/s

9 Dynamics of long (infragravity) waves in the nearshore Offshore: Propagation of waves in groups Group amplitude depresses the mean surface (e.g., Longuet-Higgins and Stewart 1964) Propagation of this depression is referred to as bound wave Nearshore: Bound wave release: Release of bound wave as free IG waves due to short wave breaking (Battjes et al., 2004) Breakpoint forcing: Periodic forcing by wave groups, steeper slopes (Symonds et al, 1984)

10 IG waves often dominate nearshore wave energy Garden Island Fringing reef (Ningaloo) C1 (forereef) C1 C3 C5 C3 (reef flat) C5 (lagoon) (Pomeroy et al., JGR 2012) (Winter et al., JGR, submitted)

11 Implications: sediment transport and beach erosion along reef coasts Beach behaviour along reef fringed coasts can be entirely different than sandy beaches Seasonal erosion/accretion out of phase between reeffronted and adjacent embayed beaches (no net sub-aerial volume change) This behaviour not reproducible by any conventional coastal morphodynamic model! (Segura et al., 2016)

12 Implications: coastal landforms and evolution No existing morphodynamic model would reproduce the landforms observed onshore of many fringing reefs onshore migration Diverging mean flows but accreted shoreline Onshore migration of sand ripples provide sediment to the beach

13 Summary, key knowledge gaps and next steps Despite considerable gains in our capacity to predict conditions in the nearshore, these have been focused primarily on open coast sandy beaches. Such coastal morphologies are not representative of a large portion of Australia s coastline Growth in process-understanding of hydromorphodynamics of complex coastlines in recent years (e.g. large field programs) Long-term nearshore observations (reference sites) are needed for different classes of coastlines -> major gap in WA Need for enhanced process-based models with validation -> operational models are within reach Acknowledgements ARC Future Fellowship ARC Discovery Project (DP ) HMAS Stirling (Fleet Base West), Carnegie Wave Energy