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

Abstract

AbstractCurrent 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.