Flow pattern adjustment subjected to effects of aquatic vegetation in a rectangular shallow basin from a numerical perspective

To further understand the impact of aquatic vegetation on hydraulics and water quality in the context of shallow basin restoration, this study explores the influence of varying vegetation density and height on flow patterns within a rectangular shallow basin with the depth-averaged hydraulic modeling. The reliability of the model was assessed with existing experimental flow velocity data in a non-vegetated rectangular shallow basin, accurately replicating characteristic asymmetrical flow circulations stemming from flow instability. Subsequently, a series of scenario models are constructed, incorporating various combinations of vegetation density and height. Our findings reveal that aquatic vegetation significantly stabilizes flow motion in the shallow basin by introducing flow resistance, thereby facilitating the transition from asymmetrical to symmetrical flow patterns. Moreover, the synergistic effect of increased vegetation density and height further promotes the diminishment of flow circulations and dead zones, ultimately leading to plug flow characterized by minimal back mixing. This phenomenon is attributed to rapid longitudinal flow adjustment and the homogenization of the flow velocity field by aquatic vegetation, which effectively mitigates transverse flow shearing. To quantify the relationship between vegetation resistance parameters and flow patterns, we analyzed the area proportion of living water zones (excluding dead zones) within the basin. Our results indicate that a dimensionless parameter, incorporating both vegetation density and height, serves as a robust indicator of flow pattern. Based on this insight, we develop a piecewise function capable of predicting the area proportion of living water zones.