Vegetation patch effects on flow resistance at channel scale.

Jean-Stephane Bailly, Gilles Belaud, Fabrice Vinatier, David Combemale

Tuesday 30 june 2015

11:30 - 11:45h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 5F. Engineering - Instrumentation

Vegetation of open-channels in cultivated catchments (rivers, irrigation channels, ditches) provide various ecosystem services by increasing flow resistance, nitrogen and phosphorus removal, favoring water infiltration, etc. Past studies focused on the characterization of flow resistance from vegetation characteristics. They investigated aquatic vegetation effect on mainly flow velocity vertical profiles, from which flow resistance parameters, e.g. the Manning coefficient, can be computed. In these studies, explored vegetation characteristics were plant species, plant height and plant densities in homogeneous spatial distribution of plants within channel. In a more multi-scale approach, Nepf (2012) noticed that vegetation characteristics providing spatial heterogeneity in the canopy-scale can produce complex flow patterns and the relationship between plant patterns morphology and flow resistance is still an open question. Keeping a strictly hydraulic perspective and thanks to a specific experimental design in a controlled channel, the objective of this study was to measure how patches of four different ditches plant species affect integrated flow resistance parameters. Theses plants, frequently encountered in the farmland ditches of the Languedoc area, south of France, were selected according to a large range of hydrophilic behaviour, flexibility and branching complexity related to the plant blockage factor. Four different spatial patches (regular, random, lateral or central patches) of each plant with same plant densities were implanted in the channel bottom and the water levels were measured for two different discharges, providing different vegetation/water heights ratio. Resistance parameters were thus estimated from the total head-loss in the channel. These experiments allow us to test the significance effect of channel vegetation patches on flow resistance parameters at the reach scale.