From patch to channel scale: The evolution of emergent vegetation in a channel

Computational fluid dynamics was used to study the evolution of small patches of vegetation into a vegetated landscape in a channel. The growth of new vegetation occurred in regions where the flow velocity was reduced below a threshold value defined as a fraction of the channel-average velocity (U-0). Two threshold values, or limiting velocity values (LV), were used: LV = 0.5 and 0.7. Two initial blockage factors (percentage vegetation coverage of the channel) were considered, 3% and 0.3%, chosen to represent cases with and without, respectively, hydrodynamic interaction between the initial patches. The simulation illustrated both positive feedbacks between flow and vegetation, which enhanced vegetation expansion, and negative feedbacks, which led to patch erosion and limited patch growth. The most rapid expansion of the vegetated area occurred during the initial simulation steps, when the flow blockage due to vegetation was small. A higher velocity threshold (higher LV) produced more rapid initial growth and a higher final coverage of vegetation. The patches evolved to one or a few elongated islands extending along the channel.