Vegetation Hydrodynamics Applied to Vegetation Patch Growth, Transient Storage Zone and Water Treatment

Abstract

The thesis is organized as follows.

Chapter 1 is the Introduction, where the motivation, research questions and specific objectives of the thesis are described.

In Chapter 2, laboratory experiments were used to simulate the growth of aquatic vegetation patches. Previous studies have shown that patches can hydrodynamically interact with one another, causing flow patterns that differ from a single patch. Here, patch growth was associated with regions of enhanced sediment deposition. Two side-by-side patches were built with arrays of rigid cylinders in a hydraulic flume, and the flow characteristics were defined to mimic natural conditions found in lowland streams. Each experimental run represented a cycle of growth, in which new cylinders were placed in the regions of enhanced deposition. The hydrodynamic interaction between the patches promoted three new pathways for potential vegetation growth: (1) enhanced sediment deposition in the centerline of the patch pair, downstream of the steady wake of the patches, which resulted in lateral patch merger; (2) enhanced deposition over a longer streamwise distance behind each patch of the pair, when compared with deposition behind a single patch; and (3) enhanced deposition on the outer edge of the patches, indicating that they can expand laterally.

In Chapter 3, we move from the basic research to an engineering application of aquatic vegetation. We provided new design recommendations for floating treatment wetlands (FTWs), demonstrating how FTW arrangement can be used in design to increase the FTWs ability in removing pollutants, thus improving the water quality of a channel reach. In addition, we explored how root length impacts removal efficiency. Given that FTWs can be configured in different shapes, we focused on FTWs spanning the channel width, arranging them in series. We explored the tradeoff between increasing the gap distance between FTWs, which improves flow mixing, elevating the pollutant concentration and thus potential removal for the next FTW in series, but decreases the number of FTWs in the channel; and vice-versa (i.e., reducing the gap distance to accommodate more FTWs in the channel, with the cost of having a poorer flow mixing and pollutant availability between FTWs). The numerical simulations showed that higher gap distances reduced the total mass removal in the system, but increased removal per FTW. We also explored the case of a continuous FTW with the same volume of several FTWs in series, in order to assess the importance of the gap in providing mixing that improves mass removal. Without gaps, the continuous FTW removed less mass due to lower mixing.

Finally, in Chapter 4 we conclude the thesis and discuss ideas for potential studies that could derive from it.