Emergent and Submerged States of a Bank‐Attached Vegetation Patch: Wake Recovery and Near‐Bank Stress

Abstract

Wakes from finite, bank‐attached vegetation patches are common along channel margins, yet comparisons between canopy states of the same patch remain limited. Bank vegetation is not a static roughness element: the same patch can shift between emergent and submerged hydraulic states through biomass change, flow‐induced bending, or stage rise. This study examined a controlled representation of that transition by holding depth, discharge, and patch planform fixed while reducing effective canopy height through stem truncation, a configuration most analogous to dieback, biomass loss, or bending rather than pure stage rise. A rigid, fixed‐bed bank‐attached patch was tested as an emergent canopy and as a stem‐truncated, overtopped canopy with a submergence ratio of 2.1 and a lower frontal area index. Velocimetry across 12 cross‐sections showed that the emergent wake retained a laterally organized, depth‐spanning structure through 12.5 patch widths; extrapolated threshold crossings placed recovery at 16–19 patch widths. In the stem‐truncated case, velocity and Reynolds stress fields in the measured lower flow showed concurrent lateral and canopy‐top exchange signatures and faster cross‐sectional homogenization. The inferred near‐bed shear stress field indicated broader near‐bank sheltering and a more persistent edge‐stress band in the emergent case, with integrated excess shear about 4.5 times that of the stem‐truncated case. In this tested configuration, the depth‐spanning state maintained a longer downstream zone of altered momentum and stress, whereas the stem‐truncated, overtopped state compressed the low‐stress region and shortened the edge‐stress band.