Hydraulic drag in vegetated channels — A campaign investigation

The aim of the study is the evaluation of the drag in a vegetated channel and its variation during the growing season. Another aim is the individuation of the characteristic parameters of the vegetation which are mostly representative of the effect of resistence on the hydraulic section. The preliminary analysis of the literature data (Bakly et al. 1992; Chen 1976; Coon 1995; Darby et al. 1996; Klaassen et al. 1985; Petryk et al. 1975; Selling et al. 1997) shows in fact that the drag does not simply depend on a single geometrical parameter but on a whole of factors, characteristic of the vegetation system. Among these the geometry of the plant in relation to the local hydrodynamical characteristics of the flow, the leafage density and the flexibility of the stems (Kouwen et al. 1973, 1980, 1992; Armanini et al. 1998). Because of infinite possibilities of combination of species in natural streams, it appears in fact difficult to reduce to few parameters the drag characteristics.