Quantifying solute mixing and flow fields in low velocity real emergent vegetation

The treatment efficacy of ponds is dependent on hydraulic residence. In turn, residence times are directly affected by the systems internal hydraulics and mixing characteristics. Aquatic vegetation is known to aid water treatment through habitat generation, promoting bio-chemical degradation and through active-uptake. The physical presence of vegetation can significantly alter the residence times. The heterogeneous distribution of vegetation – in borders or patches – has a strong impact on local mixing via stem and shear scale processes. Lacking is the quantification of the seasonal impact of real, spatially random vegetation on mixing and thus treatment efficacy.

The development of a precise 1D Laser Induced Fluorometry (LIF) and application to a laboratory control of real and artificial vegetation are discussed. Quantifying mixing in low velocity emergent vegetation presents observational challenges when seeking higher precision. Detailed, two-dimensional tracer distributions were visualised. Flow field and mixing characteristics were quantified for partially (shear) vegetated stems for two artificial and two real vegetation densities.

The mixing associated with real vegetation is significantly different to that of artificial vegetation. The spatial heterogeneity of real vegetation presents a number of challenges when attempting to quantify their geometry. A significant range in stem diameter and local stem density was recorded; further, site selection for trace visualisation has a significant impact on results due to the variability in stem distribution. The application of LIF to full and partially vegetated real vegetation provides valuable data for practitioners and CFD modellers.