Diffusion in floating particles in flow through emergent vegetation: Further experimental investigations

In this paper we present the results of a new laboratory investigation aimed at providing a better understanding of the transport and diffusion processes of floating particles (e.g., buoyant seeds) in open channel flow with emergent vegetation. The experiments are designed primarily to study the influence of vegetation density and flow velocity on the relevant interaction mechanisms between particles and vegetation. The aim is also to ascertain the validity of a stochastic model recently proposed by Defina and Peruzzo [2010]. We find that i) the proper definition of plant spacing is given as 1/np dp, dp being the plant diameter, np the number of plants per unit area; ii) the particle retention time distribution can be satisfactorily approximated by a weighted combination of two exponential distributions; iii) flow velocity has a significant influence on the retention time and on the efficiency of the different trapping mechanisms, and iv) vegetation pattern and density have a minor influence on the probability of capture and on the retention time of particles}. Indeed, the comparison between model predictions and experimental results is satisfactory and suggests that the observed relevant aspects of the particle-vegetation interaction processes are properly described by the model.