Free surface expression of shear instability in unidirectional canopy induced flows

Submerged vegetation is a critical component of the nearshore environment. Interactions between nearshore flow and submerged vegetation can have significant effects on coastal protection and estuarine channel evolution. In situ studies of submerged vegetated canopies are often both expensive and difficult due to the reliance on point sensors that must directly access the water column. Remote sensing using whole field imaging offers the potential to overcome the limitations of in situ point sensors. However, the light attenuating properties of water typically restrict remote imaging to the free surface. To take advantage of the opportunities presented by remote imaging for the study of submerged canopies in nearshore environments, we must develop a more robust understanding of the mechanisms by which interactions between flow and vegetation are expressed on the free surface. In this work, we present experimental findings that demonstrate the existence of a high energy shear instability in canopy induced unidirectional flow. By analogy to unbounded mixing layers, the frequency of the instability obeys a Strouhal scaling with respect to the momentum thickness (θ), St = fθ/U̅ = 0.032. We further show that the momentum thickness is proportional to the mixing layer thickness (t_ml), t_ml ≈ 6.7θ. By substituting t_ml for θ in the Strouhal relationship, the scaling is reduced to St = 0.21, the typical scaling for a cylinder wake. Finally, we examine high energy canopy flows with Re_c > 5000, in which the top of the shear layer induced by the canopy is bounded by the free surface. The instability peak as measured near the free surface preserves a significant proportion of the energy produced at that frequency at the canopy top. We present a simplified model for predicting the instability frequency in such flows, and discuss how it can form the foundation of a methodology for detecting the presence of, and exploring the properties of, submerged canopies using surface imaging data.