Experiment of the sediment transport pattern and resistance mechanism for sand-bearing flow in vegetation patches

Erosion and sediment transport can vary greatly with the change of spatial and temporal scales in the continuous system from slope water flow to the river channel. The vegetation can be the main driver of the spatial and temporal evolution in the water flow structure, sediment erosion transport, and landscape patterns, as the landscape gradient decreases. It is worth noting that vegetation morphology and canopy structure are important factors to determine the impacts of vegetation on the fluid structure and sediment transport. In addition, the properties of sediment are also the key to clarifying the turbidity currents transport pattern in the natural environment. However, there is a lack of attention to the above two points in the available studies. In this study, the simulated plants were utilized using Acorus calamus L. and Abutilon theophrasti Medicus to deploy the vegetation patches of different densities in a randomized distribution, in order to configure with the natural sediment in different grain size gradations of turbidity currents. A lock-exchange flume experiment was carried out to clarify the influence of vegetation patches with the natural plant canopy morphological characteristics on the turbidity currents transport pattern and resistance. The feedback mechanism was also studied between the sediment grain size gradation factor and the turbidity currents movement pattern. The plant leaf length-width ratio was proposed as a measure of the change in the canopy morphological characteristics. The influence of canopy morphological characteristics and sediment grain size class on turbidity currents flow was expressed, in terms of the non-dimensional canopy dragforce at the plant stem scale C_Daad, the non-dimensional canopy dragforce at each position C_Daax, and the non-dimensional depositional flux D_F. The results in the test conditions were as follows: 1) The transport and deposition processes of turbidity currents were influenced by the morphology of the plant canopy and the content of coarse-grained suspended material (weakly cohesive particles of 10-100 μm). There was an inverse ratio between vegetation canopy morphology size and non-dimensional depositional flux DF. Furthermore, the sediment was easily deposited in the vegetation patches with large canopy morphology. The coarse-grained suspension content was positively proportional to the non-dimensional depositional flux D_F, whereas, there was no deposition of turbidity currents with the high coarse-grained suspension content. The trend of bed sand refinement along the water movement analysis was more significant in the turbidity currents streams with the larger vegetation canopy morphology or high coarse-grained suspended mass content. 2) The vegetation patches with the greater density and canopy morphology produced the greater canopy resistance. There was a greater effect on the water flow structure, the stronger water and sand blocking effect in the random distribution state. There was a low degree of influence of sediment grain size class on the vegetation canopy resistance, compared with the vegetation characteristics. 3) The turbidity currents movement state was zoned using the dynamic mechanism. There was also a strong feedback mechanism between the vegetation canopy morphology and sediment grain size gradation, together with the non-dimensional depositional flux D_F in the resistance-dominated zone (C_Daax>4.5). Both plant canopy morphology and vegetation density shared the positive feedback mechanisms with the canopy resistance. There were vegetation canopy morphology and sediment grain size on the turbidity currents. The finding can also provide some references to deepen the erosion mechanism in small watersheds. In practice, the transport and deposition processes of turbidity currents in small watersheds often need to consider the dynamic changes in water flow resistance and sediment characteristics that are affected by vegetation characteristics. The transport and deposition of sand-bearing water can be more complex in further research.