Estimation of vegetation porosity in vegetated waterways

Vegetation porosity in a vegetated channel has been identified as the main parameter that contributes to the flow behaviour and resistance in the channel. Estimation of vegetation porosity is quite a challenging task due to the diverse characteristics of vegetation. In this study, several porosity measurement methods based on vegetation frontal area and vegetation volume were used to estimate the porosity of vegetation (Lepironia articulata) in a laboratory flume for various flow and vegetation characteristics. Digital image analysis was one of the methods used to estimate porosity. The study also involved measurements of velocity using a three-dimensional acoustic Doppler velocimeter and flow depths at different spatial locations along the flume. The volumetric method for porosity measurement, which considers the fraction of the actual volume of the vegetation to the volume of water, is considered more practical and accurate than the other methods that were studied. The results of this study indicate that, by assuming vegetation to be cylindrical in shape and considering only the frontal area of the most upstream vegetation, porosity could be underestimated by 14%. Digital image analysis gave a difference of only 5%. From the laboratory data, correlations between the mean velocity, water depth and vegetation porosity were established. It was observed that by reducing the vegetation porosity by 8%, the velocity could reduce by between 35–60% depending on flow rate.