Characterizing free-surface expressions of flow instabilities by tracking submerged features

Under unidirectional flow, complex bottom roughness such as seagrass canopies can induce Kelvin–Helmholtz (KH) shear instabilities, and these vortices can impact the free surface and leave a signature with an inherent frequency. Therefore, one approach to inferring the presence and properties of submerged ecosystems may be to look at the behavior of the water surface. We present an imaging-based laboratory method developed to characterize this canopy-induced shear instability. Much like a lens, a curving free surface refracts light at the interface (Moisy et al., Exp Fluids 46:1021–1036, 2009). Using cameras placed above the length of a flume, the water surface slope is measured by tracking the apparent distortion of submerged model vegetation in a series of images, manifested as a slight shimmering over time. We demonstrate that the synthetic Schlieren technique can: (1) measure the spectral signature of the canopy-induced shear instability on the free surface, (2) provide high-resolution spatial information on the development of the instability over the entire canopy length, and (3) measure the propagation speed and length scale of the coherent KH rollers at the surface and detect distinguishable differences in these properties for varying canopy geometry.