Hydrodynamics of Coral Forereef Spurs and Grooves

Abstract

Hydrodynamics govern key ecological and geological processes on a coral reef. On the forereef, high energy and wave dissipation are greatest, and typically produce spur and groove (SaG) morphology: spurs extending seawards from the reef crest, separated by grooves. This thesis quantifies the hydrodynamic processes of SaG morphology.

Using numerical wave modelling with forereef morphologies derived from light detection and ranging (LiDAR) from One Tree Island in the southern Great Barrier Reef (GBR), this thesis tested SaG controls on wave energy dissipation and transmission at the reef crest. Realistic morphologies provided an extra 40% wave dissipation than when SaG were removed. Under RCP2.6, increased wave heights (+0.8 m) and water levels (+0.3 m) doubled dissipation rates across the forereef. The worst case (RCP8.5) and Total Disaster (TD) scenarios resulted in a 4-fold reduction in dissipation and increased wave transmission at the crest by up to 2.7 m relative to present conditions.

Two field experiments were conducted at One Tree Reef (OTR; 23°30′S, 152°06′E). The first campaign (6–8 October 2022) used high frequency (8 Hz) current observations in a low-energy SaG system; fair weather yielded mean significant wave height (Hs) = 0.25 m and currents up to 2 m/s. The second campaign (25 November 2022–6 April 2023) captured a long-term dataset across a high-energy SaG system. Alongshore tidal currents outpaced wave-driven cross-shore flows for 31.6% of the record, yet their geomorphic impact appeared limited. By contrast, near-bed orbital velocities and bed shear stresses on the upper forereef exceeded sediment mobility guides for 94% of the record, and mobility thresholds were surpassed during storm events, supporting a model of SaG maintenance driven primarily by wave-induced abrasion.

Overall, SaG enhance wave energy dissipation and regulate sediment dynamics, underpinning reef resilience under present and projected future conditions.