Experimental investigation of phase lag in compound channels under repeated unsteady flow cycles

Compound channels, characterized by a main channel and adjacent floodplains, are prevalent in lowland rivers. This study experimentally investigates the physical mechanisms governing unsteady flow dynamics in compound channels, focusing on the effects of key flow parameters: peak discharge (Qm), flow period (T), and peaking time (tp). Experiments were conducted in a laboratory compound channel under continuous cyclic operation with relative depth ratios ranging from 0.43 to 0.55, which represents conditions near the transition between strong and weak channel-floodplain interaction. The water level fluctuations had counterclockwise hysteresis loops in the stage-discharge relationship. Wave crest and trough water levels increased linearly with Qm, reflecting cumulative water storage effects during successive unsteady cycles. Increasing T resulted in elevated maximum and reduced minimum water levels, while increasing tp accelerated flood passage with corresponding water level reductions. Cross-sectional velocity distributions consistently showed a distinctive bulge extending from the floodplain edge into the main channel, with minimum velocity at the interface region. The maximum cross-sectional velocity increased with Qm but decreased with tp, showing notably smoother temporal variations when tp ≤ 1.5 min. A significant phase lag was observed in discharge dynamics, with unit width discharge peaking in the main channel earlier than in the floodplain. For large Qm, this produced a characteristic three-stage rise-and-fall pattern. Using simplified sinusoidal models, we identified that the depth difference between channel and floodplain, velocity differential, and phase lag between water depth and velocity collectively drive the observed discharge phase lag phenomenon. This study provides new experimental evidence that phase lag between main channel and floodplain discharge persists even at relatively high relative depth ratios (Dr = 0.43–0.55), advancing the current understanding of compound channel hydraulics under repeated unsteady cycles relevant for tidal rivers, regulated channels, and flood sequences. This comprehensive characterization of unsteady flow physics in compound channels provides crucial insights into the mechanisms controlling temporal evolution of hydraulic parameters, with important implications for an improved flood modeling and management.