Wave‐breaking hydrodynamics within coral reef systems and the effect of changing relative sea level

Abstract

Abstract. This paper considers the hydrodynamics of wave-driven flow across a coral reef and the resultant flushing of its lagoon. Current depth coefficients are introduced that describe the fractional change of across-reef and lagoonal current with change in the relative local sea level, i.e., the depth of water over the reef flat. The coefficients are derived from an analytical hydrodynamic model and compared with data from two reefs. The first is Ningaloo Reef in northwest Australia, which is a typical barrier reef with a narrow coastal lagoon, and the second is Kaneohe Bay in Oahu, Hawaii, where the reef is both unusually deep and wide. The current depth coefficients are shown to be sensitive to the form of the frictional law on the reef flat and can be deduced from measurements of the astronomical tidal currents in the lagoon. Both quadratic and linear friction laws are considered for the reef flat and good agreement obtained with a linear law based on the high-frequency motion due to surface gravity waves.

Numerical modeling of the impact of sea-level rise on fringing coral reef hydrodynamics and sediment transport

“…The increased mixing and flushing of the reef flat with sea-level rise may help to dilute material delivered to the inner reef flat from the adjacent land, but it might also result in greater transport of terrestrial sediment onto the fore reef. The model results presented here, along with the observations made by Ogston et al (2004), Storlazzi and Jaffe (2008), and Lowe et al (2009) and modeled by Gourlay (1996), Hearn (1999), and Hearn and Atkinson (2000), show the effect of sea level on the magnitude of currents, driven both by wind and by wave-breaking, on coral reef flats.…”

“…Greater water depths over a fringing reef would reduce bottom friction and increase water depth relative to the wave height, resulting in larger and more energetic waves that could propagate over the reef crest and reef flat without breaking and larger wind-waves develop in situ on the reef flat, similar to the model results presented by Hearn (1999) and Hearn and Atkinson (2000). These findings are supported by data from Storlazzi et al (2004), who showed that while wave heights offshore of the reef crest on the fore reef (depth *10 m) are independent of sea level (r 2 = 0.003, n = 961, P not significant), both wave height and wave period on the reef flat (depth *1 m) are significantly correlated with sea level (r 2 = 0.791 and 0.797, respectively; both n = 961 and P \ 0.001), suggesting that waves on the reef flat are depth-limited.…”

“…tide) current speeds were on the same order (0-5 and 5-20 cm s -1 , respectively) as the measured currents and also showed the same proportion of greater (*2-8 times) alongshore current speeds on the reef flat and over the fore reef compared to the cross-shore current speeds. This is in contrast, however, to most observations and models of primarily cross-shore flow and sediment transport on atolls and barrier reefs (e.g., Hearn 1999;Hearn and Atkinson 2000;Lowe et al 2005) where vigorous wave-driven onshore flow over the reef flat can occur because it is balanced by strong return flow out of channels in the reef. In fringing reefs without a nearshore gully, wave-driven setup along the shoreline offsets this cross-reef flow and results in primarily alongshore flow and transport.…”

“…Sediment fractions are solved individually in the transport and bed-update modules and therefore were tracked separately. Hydraulic roughness length scales were varied between approximately 0.01 and 0.10 m, with the higher value used for coral surfaces based on previous observations on Hawaiian reefs and numerical modeling results (e.g., Hearn 1999;Lowe et al 2005), and the lower range (*0.01 m) set by the seabed grain size. Complete overviews of the formulations, testing, and validation of Delft3D Online Morphology have been reported in Lesser et al (2004).…”

Topographic complexity and roughness of a tropical benthic seascape

“…Modeling the hydrodynamics of coral reef systems is one example. Traditionally, such models empirically estimate reef roughness by parameterizing conventional friction models based on a few in situ current measurements [Hearn, 1999]. The accuracy of such parameterizations decreases as roughness increases, and also as it exhibits spatial heterogeneity [Nunes and Pawlak, 2008].…”