Extreme low submergence in deep-canopy flows

TL;DR

This study explores how extreme low submergence affects flow structures in deep-canopy flows, revealing significant modifications like seiching and unique velocity profiles that influence momentum transfer.

Contribution

It provides new insights into flow regime transitions and flow modifications under very low submergence conditions in deep-canopy flows.

Findings

Classical three-region flow structure at moderate submergence

Significant flow modifications including seiching at extreme low submergence

Enhanced momentum transfer near the canopy base at low submergence

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.