The surface signature of a canopy-generated shear instability

We present results from a laboratory study on the free-surface signal generated over an array of submerged circular cylinders, representative of submerged vegetation. We aim to understand whether aquatic ecosystems generate a surface signature that is indicative of both what is beneath the water surface as well as how it is altering the flow. A shear layer forms over the canopy, generating coherent vortex structures which eventually manifest in the free-surface slope field.

Experimental measurements at the surface suggest a Strouhal number that is twice the most amplified mode predicted by linear stability theory. This implies that vortices may evolve between generation at the canopy height and their manifestation at the water surface. Additionally, it appears the flow never becomes fully-developed, likely due to interaction of flow structures with the free surface, as well as due to gradual changes in water depth from the driving barotropic pressure gradient.

We connect the vortex properties measured at the surface with measurements of the bulk flow, and show that correlations between these quantities are adequate to create a parameterized model in which the interior velocity profile can be predicted solely from measurements taken at the free surface.