Hydrodynamics of spur and groove formations on a coral reef

TLDR

Spur and groove formations are common on coral reef fore‑reefs, yet their influence on wave‑driven hydrodynamics remains poorly understood. The study employed a two‑dimensional, depth‑averaged, phase‑resolving nonlinear Boussinesq model (funwaveC) to simulate SAG hydrodynamics, revealing that alongshore pressure‑gradient and nonlinear‑wave imbalances drive the flow. Modeling shows that SAG formations and shoaling waves generate nearshore counter‑rotating Lagrangian circulation cells, with the strongest effects from spur‑normal waves, higher wave height, weak alongshore currents, taller spurs, and lower bottom drag; the circulation pattern aligns with a scaling analysis of nonlinear‑wave, pressure‑gradient, and bottom‑stress balances, and is most efficient when the alongshore SAG wavelength permits diffraction‑induced wave‑height differences without altering the mean wave angle.

Abstract

Spur and groove (SAG) formations are found on the fore reefs of many coral reefs worldwide. Although these formations are primarily present in wave‐dominated environments, their effect on wave‐driven hydrodynamics is not well understood. A two‐dimensional, depth‐averaged, phase‐resolving nonlinear Boussinesq model ( funwaveC ) was used to model hydrodynamics on a simplified SAG system. The modeling results show that the SAG formations together with shoaling waves induce a nearshore Lagrangian circulation pattern of counter‐rotating circulation cells. The mechanism driving the modeled flow is an alongshore imbalance between the pressure gradient (PG) and nonlinear wave (NLW) terms in the momentum balance. Variations in model parameters suggest the strongest factors affecting circulation include spur‐normal waves, increased wave height, weak alongshore currents, increased spur height, and decreased bottom drag. The modeled circulation is consistent with a simple scaling analysis based on the dynamical balance of NLW, PG, and bottom stress terms. Model results indicate that the SAG formations efficiently drive circulation cells when the alongshore SAG wavelength allows for the effects of diffraction to create alongshore differences in wave height without changing the mean wave angle.