Author(s): Fazeleh Kabiri; Mohammad Reza Majdzadeh Tabatabai; Mohammad Shayannejad
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Keywords: Hyporheic zone; Phosphorus removal; Pool-riffle sequence; Vegetation cover; Residence time
Abstract: Rivers are one of the essential sources of development in human societies, their optimum protection and management are the most important responsibilities to be made by human beings. Natural habitats in rivers are created and developed through pool-riffles sequences and vegetation cover has a great effect on river water and sediment quantities as well as water quality variables. The presence of vegetation in the riverbed and banks of increases the roughness of the mobile boundaries and reduces the average flow velocity. Hyporheic zone is the area of interaction between surface water and subsurface water that exists in the riverbed and banks with the potential to reduce scattered pollutions. Water moves inwards and outwards of the hyporheic zone, leading to interactions that are vital in biological and physiochemical processes. The morphology of the river bed affects the residence time in the hyporheic zone due to the pressure difference. The hyporheic zone controls important physicochemical processes such as filtration of pollutants and the cycle of phosphorus and creates a sustainable ecosystem that includes species live there temporarily and permanently. In this study, the effect of vegetation cover on the reduction of pollutants such as phosphorus in the hyporheic zone in a pool-riffle sequence is investigated to compare with non-vegetated cover. Data collected in a laboratory flume at Isfahan University of Technology with a length of 15 meters, width of 90 cm, height of 60 cm and a maximum flow of 50 liters per second. Completely submerged, dense and flexible artificial grass with a height of 30 mm is established to cover the entire bed on the top of the gravel surface. To trace the flow in the hyporheic zone, edible color and "GrapherTM from Golden Software, LLC" are used. It is observed that the vegetation cover increases the residence time, which in general, reduces the hyporheic velocity. As phosphorus removal is occurred by through flow in sediments which may reduce the hyporheic velocity and increase phosphorus removal. At a flow rate of 50 liters per second with the vegetation cover, the rate of phosphorus removal in the period of 12 hours is 93.7%, which is almost 30% higher than that of the none-vegetated bedform. . The amount of phosphorus removal reached 100% in a flow discharge of 30 liters per second and in the presence of vegetation cover in a period of 11 hours. The results clearly show that the vegetation cover in the hyporheic zone increases the phosphorus removal efficiency, in other words, the process of phosphorus absorption in the hyporheic region could be improved.
Year: 2022