Solute transport characteristics in the streambed due to rigid non-submerged plants: Experiment and simulations

The presence of aquatic plants in natural and artificial rivers can cause substantial changes in the flow pattern. In this study, laboratory experiments and numerical simulations are conducted to explore the influences of rigid non-submerged plant densities on the transport of solutes in the hyporheic zone. The laboratory experiments and numerical modelling results show that solute enters into the streambed around plant, and solute in the overlying water experienced a sharp decrease at the beginning, followed by a gradual decline phase. The concentration in the shallow pore water increases rapidly, and increases at a decreasing rate with the increase of depth. The presence of a plant causes pressure gradients at the sediment–water interface before and after the plant. Specifically, the presence of plants induces the convection exchange of surface water and pore water, enables the solute to transport in the streambed through three circulation flows. Solute tends to transport to the root of the plant, or bypass the plant and return to the overlying water over time. The rate and quantity of solute transport in the streambed are related to the streamwise and spanwise distances of the plant. When the plant densities reach the distance that affect the flow structures near each other, smaller streamwise distances lead to the transport of fewer solutes into the streambed and prevent them from penetrating to the deeper part of the streambed, while larger spanwise distances have the same effect on solute transport in the streambed. The solute effective exchange depth for streamwise distance of 20 cm is 4.3 times that of 3 cm, while which for spanwise distance of 3 cm is 5.05 times that of 15 cm at time of 28 h. This study shows that the presence of aquatic plants leads to the circulation of solutes in the hyporheic zone, which promotes the transport of pollutants from overlying water to streambed through hyporheic exchange after the occurrence of a water pollution event. Further, plants densely in the streamwise direction and sparsely in the spanwise direction will reduce the range of pollutants transport in the streambed. This study is of great significance for water ecological restoration through rational arrangement density of aquatic plants.