Decoupled morphodynamic responses of dune crests and localized scour across the sand-gravel transition in straight alluvial channels.

This study empirically and statistically investigates the morphodynamic responses of median sediment grain diameter (d50) on local erosion and the morphodynamic regime within an alluvial channel. Experiments conducted across three distinct sediment sizes demonstrated a strong positive correlation between flow rate (Q) and bedform geometry, specifically maximum scour depth (dscour) and dune crest height (hdune). Crucially, median grain size governed bed stability. The finest sediment exhibited high sensitivity, forming dynamic and regular dunes, while the coarsest sediment showed resistance consistent with armor layer formation and yielded a stable bed geometry. The medium sediment defined a transitional regime of localized scour and bars. High-accuracy empirical models were developed to predict maximum scour depth (R2 = 0.97, where R2 is the coefficient of determination) and dune crest height (R2 = 0.93) as functions of flow rate and median grain diameter. Absolute Relative Deviation (ARD) analysis confirmed the models' robust reliability (±10-15% error range). These findings provide essential quantitative equations and enhanced insights for the sustainable design and maintenance of hydraulic structures in alluvial environments.