Experimental observations through LDA of a current with almost rigid submerged vegetation

This work belongs to a research trend which studies, in experimental way, the effects of a vegetated bottom on the hydrodynamic features of the current flowing over it. In previous works about this topic the authors experimentally studied, through an LDA system, the effects on the hydrodynamic behaviour of a boundary layer current developing at the inlet of a rectangular channel fed by a big tank, with different types of vegetated bottom, finding a non dimensional “universal” velocity distribution law. The aim of this work is to extend the experimental study of this current to its whole developing in the channel, in the case of almost rigid submerged vegetation, made by a synthetic carpet, yet using the same model proposed in previous works. The main device of the experimental plant is a channel 4m long and 15cm large, ending through a weir, with variable slope. The experimental surveys have been carried out in logical connection with the previous ones, keeping some fundamental hydraulic parameters unchanged. Local mean velocity measurements in numerous points of verticals located along the axis of the current, in test sections suitably chosen, have been performed. The velocity distributions show that the whole current is characterized by four types of flow: boundary layer, transition flow, uniform flow, and accelerated flow towards the weir. In order to produce a peculiar non dimensional velocity distribution for each test section, the collected experimental data are processed, starting from two characteristic velocity values: ue, relative to the top of the vegetation carpet, and umax, relative to a real or virtual maximum, on the outside of the vegetation. The data processing provided 26 non dimensional velocity distributions. It was perceived that it is possible to unify the distributions relative to boundary layer in a first specific law, and the distributions relative to either uniform flow or accelerated flow in a second specific law. The distributions relative to the transition flow show a trend from the first law to the second one. The whole processing was peculiar for currents with submerged vegetation and can be considered as the starting point of a more general experimental survey aimed to obtain other possible “universal” distribution laws.