A numerical study of the effects of vegetation patterns on wetland performance.

Andrea Bottacin-Busolin, Luca Fabris, Mattia Zaramella, Andrea Marion

Friday 3 july 2015

13:45 - 14:00h at South America (level 0)

Themes: (T) Managing deltas, (ST) Wetland protection and shore restoration

Parallel session: 16J. Environment - Wetlands

Conceptual wetlands of simple geometry and randomly distributed vegetation fields with imposed statistical properties (stem density average, variance and correlation length) were generated to assess the effect of vegetation patterns on wetland performance. The hydrodynamics was solved using a two-dimensional depth-averaged model where flow resistance is generated by bed friction and stem drag. Mass transport was solved using an advection-dispersion model with a decay rate dependent on the local properties of the vegetation field. Wetland performance was found to depend primarily on average vegetation density, whilst stem density variance and correlation length have minor effects on it. The results show that the reduction of concentration can be significantly less than calculated assuming the wetland to be a continuous stirred-tank reactor (CSTR), a consequence of the fact that the actual flow patterns produce concentration fields that are far from well mixed. The maximum concentration reduction is shown not to coincide with the condition of maximum mass removal. Mass removal is a combined effect of concentration reduction and flow rate, and the results clearly show that, for a constant head loss between inlet and outlet, there is an optimum value of mean stem density producing maximum mass removal. This is a result of the increased resistance produced by high stem density on the fluid motion leading to a significant reduction of the flow rate passing through the wetland. The average stem density remains the dominant factor determining total mass removal. However, a minor effect is associated also to the vegetation distribution represented by stem density variance and correlation length. In particular, the ensemble average of the total mass removal is shown to decrease when variance and correlation length of the vegetated field increase. This effect finds a sound physical explanation in the evidence that high stem density variance and correlation length represent fields with highly concentrated vegetation patches where the presence of channels among patches produce preferential flow paths for fluid and transported matter.