The impact of bare gaps on meadow-scale flows within fragmented emergent aquatic vegetation

Aquatic vegetation has been shown to shape landscape morphology by interacting with flows and influencing sedimentological processes. These interactions are modulated by the spatial distribution and density of the vegetation. We conducted idealised numerical simulations using a depth-averaged hydrodynamic model to explore how circular gaps (of diameters 0.4 to 9 m) within otherwise continuous beds of emergent aquatic vegetation alter in-canopy horizontal flow patterns. Our findings indicate that gaps reroute and concentrate horizontal flow, creating areas of accelerated flows within the canopy. As gap size increases, horizontal flow speeds also increase, though the rate of increase diminishes substantially for gap diameters larger than 5 m. In cases with paired gaps, two distinct interaction modes were identified, one occurring when the gaps were aligned with the flow, leading to mutually reinforced flow intensification within the gaps, and another when they were positioned side by side, resulting in diminished flow acceleration associated with each bare gap. The maximum flow change within the canopy occurred at streamwise gap separations of three gap diameters, with minimal changes for gap separations of greater than four diameters. The increase in horizontal flow speeds was directly proportional to the percentage of bare bed cover. For a given percentage of bare bed cover ranging from 8 to 40 %, flow changes were also modulated by the fragmentation level, which is determined by the number and size of gaps. The results reveal that fragmentation levels control in-canopy flow speeds, whereby fewer, larger gaps exerted a more pronounced impact on in-canopy horizontal flow speeds compared to numerous smaller gaps. The interconnected nature of vegetation coverage and gap distribution underscores the role of fragmentation levels in ecohydrological dynamics.