Bottom Drag Coefficient on a Shallow Barrier Reef

The present paper reports of a field experiment over a shallow, roughness‐varying barrier reef at Maupiti island, French Polynesia. The depth‐averaged momentum balance is used to estimate the bottom drag coefficient Cd, which varies from 0.01 to 0.3, with dependence on both depth and reef structure. The depth effect on Cd is well predicted by a log dependency, as used in previous laboratory and fields observations. The present results extend the approach to a system with more wave exposure and higher roughness‐to‐depth ratio. Additionally, the statistical relationship between high‐resolution reef topography and hydrodynamical parameters is discussed. Plain Language Summary Coral reefs are essential for human societies and ecosystems in many tropical coastlines. Having reliable hydrodynamic models of these systems is important for proper management and engineering applications. Reefs play an important role in sheltering shorelines by attenuating wave energy and slowing currents. Thus, they must be accounted for in the models. However, due to the complexity of coral geometry, there is still no reliable manner to represent or translate the physical observation of corals' geometrical structure to a friction term in the governing equations of the models. Additional representative challenges exist due to the varying importance of friction, depending on the water depth. This paper demonstrates good alignment between a metric of the physical geometry of coral to a frictional parameter used in the governing equation, the momentum balance. Additionally, the depth dependence of our results is described well by a classical equation meant to describe turbulence near a wall, log‐layer theory. In total, the results yields good agreement between terms in the momentum balance and aid in the fundamental connection of the physically‐observed and the numerically‐representation reefs. Key Points Field estimates of bottom drag coefficient over a shallow rough coral barrier reef using a bulk momentum balance approach Drag coefficients are dependent on the local depth and colony structure, and well predicted by a log‐dependency The recovered data allows for a discussion of the connection between hydrodynamic roughness scale and high resolution seabed topography