The impact of waves on aquatic canopy flow

Benthic canopies (e.g., seagrass meadows, coral reefs and kelp
forests) are important parts of coastal environments, providing
a range of ecosystem services. The drag exerted by these complex bottom roughnesses profoundly impacts the mean flow and
turbulence structure and, consequently, the physical and biogeochemical processes within these ecosystems. While previous studies have mainly focused on steady flow environments
(e.g., rivers, lakes and tide-dominated estuaries), many submerged canopies in coastal environments are subjected to oscillatory flows driven by surface waves. This study aims to investigate dynamics generated under steady and wave flows over
submerged canopies to understand similarities and differences
between the two environments. Accordingly, flow, turbulence
and mixing were compared within identical model canopies
subjected to comparable steady and wave-dominated flows. Results revealed that despite general similarities (e.g., velocity attenuation and vortex generation at the canopy top), there are significant differences between steady and wave-driven flows. In particular, velocity attenuation and, thus, the strength of the
shear layer vortices are much stronger in steady flows. Trends of velocity attenuation and vertical transport indicate that the Keulegan-Carpenter number (KC) is the key parameter describing the impact of oscillation on the flow structure. When KC reaches O(100), the flow, turbulence and mixing essentially resemble those in unidirectional flow environments.