Experimental study on flow structure and turbulence characteristics of vegetated overbank flow in deep-trough-dominated compound channels with double-layer floodplains

The water-sediment exchange between the main channel and the floodplain of a compound channel is the core driving force of cross-sectional morphological evolution. However, the joint regulation mechanism of two-layer floodplain topography and vegetation arrangement patterns remains unclear. In this study, flume experiments were conducted. Rigid acrylic rods were used to simulate emergent vegetation. Three typical vegetation arrangements were established on the upper floodplain, namely linear, interspersed, and patchy arrangements. The three-dimensional velocity field and turbulence statistics were measured under fixed-bed conditions without sediment. The water depth was kept constant at 19.5 cm, and three discharge conditions of 15, 19, and 23 L/s were tested. The coefficient of variation was introduced as a quantitative indicator to evaluate the spatial uniformity of hydrodynamic parameters. The results indicate that, compared with the non-vegetated condition, the interspersed vegetation arrangement reduces the bed momentum thickness by up to 37.5% and effectively suppresses the peak transverse velocity gradient. The patchy vegetation exhibits the strongest flow-blocking effect in the local floodplain region ( y / B = 0.65-1), where the velocity is reduced by approximately 43.7% compared with the non-vegetated condition under low-flow conditions. This reduction is accompanied by a significant increase in vertical Reynolds stress and the occurrence of a peak in turbulent kinetic energy ( TKE ). Under high-flow conditions, the linear vegetation yields the minimum TKE in the region above the upper floodplain. The coefficient of variation evaluation indicates that the interspersed vegetation exhibits better uniformity in velocity and velocity gradient, which is conducive to bed stability and scour control. The patchy vegetation shows a relatively uniform turbulent kinetic energy distribution but stronger vertical stress fluctuations, making it more favorable for sediment deposition on the floodplain. These findings can provide a theoretical basis for ecological restoration and engineering optimization design of deep-trough-dominated compound channels.