Vegetation in floodplains and seasonal channel beds plays an important role in the fate of sediments, acting as a trap as well as reducing the channel's sediment carrying capacity. While sediment transport in natural channels without any intervention is already a stochastic process, it becomes more nondeterministic in the presence of vegetation in the channel bed. Not much research has been conducted so far into determining the sediment transport rate within vegetated channel flows. On the basis of flume experiments carried out at WL / Delft Hydraulics in the year 2002, this study focused on adding new insight using a data-driven modelling technique.
Previous studies showed that turbulence could effectively pick up sediment as sand bursts. Although bursting is not a new concept in sediment transport, when it comes to flow through vegetation, these bursts seem to have a different source of energy, since it happens before the bed-shear stress exceeds the threshold value for initiation of motion. To trace out the actual mechanism of sand bursts and to quantify whether bursts were dominating the transport volume, the problem was approached by integrating data mining techniques into available theory and recorded data.
When plotted in polar coordinates, the available data showed distinct orientations of the combined force of instantaneous horizontal and vertical velocity components, all pointing towards a particular direction along the entire plant section, most prominently in the immediate vicinity of the plant top layer. The progression of a large amplitude wave (monami) over the submerged plants was seen inducing periodical pumping effects on the bed. Sand bursts are apparently the result of these pumping actions. The existence of coherent structures discovered in this study is not only useful for sediment transport modelling but also will provide new insight into modelling hydrodynamics within vegetation.
This project was executed in cooperation with UNESCO IHE Delft (MSc study). |
