Extreme low submergence in deep-canopy flows

Abstract: This letter investigates converged statistics in three-dimensional deep-canopy-dominated flows under two low relative submergence conditions: $h/k=1.5$ and $h/k=1.2$. Using a multi-plane telecentric PIV setup, time-averaged velocity fields were obtained across nine planes. For $h/k=1.5$, the flow structure exhibited a classical three-region behavior: a uniform emergent zone, a mixing zone and a logarithmic free-flow zone. In contrast, the extreme low submergence case ($h/k=1.2$) revealed significant flow modifications, including seiching - a periodic free-surface oscillation typically associated with emergent flows. Local double-averaged velocity profiles showed a unique undershoot near $z/k=0.8$, reflecting enhanced momentum transfer from cavity flows to alleys of the canopy. These findings highlight the transition mechanisms from submerged to emergent flow regimes, providing insights into canopy hydrodynamics and the influence of submergence on momentum transport.