Impact of streamwise heterogeneity due to gaps and patchiness on canopy flow dynamics

Canopies such as seagrass meadows alter their environment by impacting the flow and turbulent structures. However, unlike well-studied homogenous canopies in which fully developed mixed layers develop, most aquatic canopies display patchiness, e.g. gaps/clearings that alter the flow. We aim to answer the question: how will gaps and patchiness affect the flow characteristics in a canopy system?

We conducted experiments in a recirculating flume with model vegetation. The canopy is interrupted by a spanwise homogeneous gap, and momentum and turbulent statistics were observed in the gap and in the wake. Observations suggest that there is a critical length of canopies over which the canopy will fully develop the flow (mean/turbulent), and downstream characteristics will be identical regardless of the upstream gap. But below this threshold, the turbulence downstream is affected by the upstream gaps. Changing the gap lengths brings about two major phenomena. First, it affects the decay of the mixing layer in the gap before it re-enters the downstream canopy segment and impacts local momentum fluxes. Second, depending on packing density of the fragments, we may observe canopy segments behaving independently of one another, interacting with one another, or essentially uninterrupted flow.