The characteristics and dynamics of wave‐driven flow across a platform coral reef in the Red Sea

Abstract

Current dynamics across a platform reef in the Red Sea near Jeddah, Saudi Arabia, are examined using 18 months of current profile, pressure, surface wave, and wind observations. The platform reef is 700 m long, 200 m across with spatial and temporal variations in water depth over the reef ranging from 0.6 to 1.6 m. Surface waves breaking at the seaward edge of the reef cause a 2–10 cm setup of sea level that drives cross‐reef currents of 5–20 cm s−1. Bottom stress is a significant component of the wave setup balance in the surf zone. Over the reef flat, where waves are not breaking, the cross‐reef pressure gradient associated with wave setup is balanced by bottom stress. The quadratic drag coefficient for the depth‐average flow decreases with increasing water depth from Cda = 0.17 in 0.4 m of water to Cda = 0.03 in 1.2 m of water. The observed dependence of the drag coefficient on water depth is consistent with open‐channel flow theory and a hydrodynamic roughness of zo = 0.06 m. A simple one‐dimensional model driven by incident surface waves and wind stress accurately reproduces the observed depth‐averaged cross‐reef currents and a portion of the weaker along‐reef currents over the focus reef and two other Red Sea platform reefs. The model indicates the cross‐reef current is wave forced and the along‐reef current is partially wind forced.

Thermodynamics and hydrodynamics in an atoll reef system and their influence on coral cover

“…Results for bottom drag coefficient C D using Reynolds stress varied from 0.0037 to 0.10 (Supporting Information Table SI 2). These results for z 0 and C D are consistent with the wide range of values observed on other reefs (Rosman and Hench ; Lentz et al ).…”

An Alongshore Momentum Budget Over a Fringing Tropical Fore‐Reef

“…Some of the variability in C D in the literature may be attributed to the depth dependence of C D (Lentz et al, ; McDonald et al, ). Higher drag coefficient are reported in shallower environments, such as in Coronado et al (; C D =0.015 when D ∼ 5 m), Lentz et al (; C D =0.03 when D ∼ 1.2 m), Lowe et al (; 0.01 < C D <0.03in a reef‐lagoon system), and Rogers et al (; C D ∼ O(0.01) − O (0.1) when D ∼ 1–3m). However, if there is a large depth to hydrodynamic roughness ratio ( , where z 0 is the hydrodynamic roughness) and a large depth to coral roughness ratio ( where h is the coral roughness), then (Lentz et al, ; McDonald et al, ).…”

“…The bathymetry inshore of the 20‐m isobath is generally homogeneous on the large‐scale but subject to small‐scale roughness variability (Figure ). The root‐mean‐squared roughness measured by the REMUS autonomous underwater vehicle over a 50m 2 box—root‐mean‐squared calculations explained in Nunes and Pawlak ()—is O(10 −3 m), higher than sandy substrates but lower than the bottom roughness typical of coral reefs (Jaramillo & Pawlak, ; Lentz et al, ; Nunes & Pawlak, ). The seafloor slope in the cross‐shore direction is O (10 −2 ).…”

Internal waves influence the thermal and nutrient environment on a shallow coral reef

“…Barotropic tidal currents on the reef flat are estimated using the depth averaged currents and a unified tidal analysis and prediction method (Codiga ). Wave‐ and wind‐driven flow on the reef flat is estimated following Lentz et al (): where u is the cross‐shore velocity, positive toward the east, S xx is the cross‐reef component of the wave‐radiation stress tensor, Δx is the width of the region of wave breaking and reef flat, which is taken as 3000 m, τ sx is the wind stress, and ρ is the density of seawater. C da is the bulk drag coefficient, which is estimated as follows: where κ = 0.4 is the von Karman constant, h is the time‐variable water depth, z 0 = 3.2 cm is the hydrodynamic roughness estimated for our site in a previous study (Lentz et al ), and Π is Cole's wake strength, which is taken as 0.2 (Lentz et al ).…”

“…The wave‐radiation stress tensor, S xx is estimated as follows: where g = 9.81 ms −2 is gravitational acceleration, H s is significant wave height measured at E1, θ w is wave direction, c g is group velocity, and c is phase velocity (Lentz et al ).…”