A Nonlinear State Shift: Morphodynamic Thresholds During Progressive Vegetation Uprooting on Alternate Bars

Vegetation fundamentally regulates river-bar morphology, yet how bars respond when vegetation is progressively removed remains poorly understood. Flume experiments under two steady flows using a two-stage protocol were conducted: Stage 1 trimmed about 30% from the leading edge of an apex patch; Stage 2 cleared the remainder. Bed evolution was analyzed using depth-normalized relief, areal aggradation-degradation fractions, lateral mass-balance indices metrics, and thalweg-based wavelength. Partial removal (Stage 1) triggered an abrupt morphodynamic transition: bed aggradation surged from 20–25% to 56–77%, marking a threshold shift from scour-dominated to deposition-dominated conditions and reorganizing the entire bar–wake system. Complete removal (Stage 2) stabilized this configuration, with deposition remaining dominant and the flow wake lengthening and reattaching farther downstream. At higher discharge, bar wavelength expanded by ~ 10–59%, reflecting longer wakes and reduced roughness, while the lower discharge mainly deepened local relief without major re-spacing. Morphodynamically, Stage 1 acts as the trigger, converting a forced, asymmetric deflector bar into a diffusively depositional form; Stage 2 acts as the stabilizer, allowing the reach to relax toward a free-bar template governed by intrinsic flow–sediment dynamics. Practically, these findings highlight that partial vegetation loss can induce threshold instability, creating scour hotspots. In contrast, complete clearing tends to redistribute sediment more evenly and stabilize bar spacing—offering direct guidance for river restoration and vegetation-management design.