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You are here : eLibrary : IAHR World Congress Proceedings : 36th Congress - The Hague (2015) ALL CONTENT : Untitled : Open-channel flow over longitudinal roughness transition from highlysubmerged to emergent vegetation
Open-channel flow over longitudinal roughness transition from highlysubmerged to emergent vegetation
The understanding of vegetated flows is of primary importance since a new trend in river management consists in
restoring rivers and floodplains to their natural form. The present laboratory study focuses on the particular case of a
floodplain with a longitudinal roughness step change from highly-submerged dense meadow to emergent rigid stems set
on the dense meadow (representing a wood), and vice versa. First, it was found that the flow depth solely varies upstream
from the roughness change (but the flume is too short to observe the uniform flow depth at the upstream end of the flume).
In this region, the vertical profiles of mean velocity, Reynolds stress and turbulent intensities are self-similar, when the
mean streamwise velocity is normalized by the bulk velocity and the turbulent quantities by the shear velocity.
Downstream from the roughness step change, the mean flow and turbulent quantities are spatially evolving over
longitudinal distances that are about 35 to 50 times the water depth. Second, a simple 1D model is found to accurately
predict the longitudinal profile of water depth through the two types of roughness transitions (from meadow to rigid stems
and vice versa). Bed-roughness is modelled by Manning¡¯s formula and the drag caused by the stems is modelled by a
volume force with constant drag coefficient and constant frontal surface per unit area. Results of this model show that the
convergence length upstream of the roughness step change, that is necessary to reach the uniform water depth of the
upstream roughness, scales with the uniform water depth of the upstream roughness Hup and can be estimated in the
present case as 2000 Hup.
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Chapter : IAHR World Congress Proceedings
Category : 36th Congress - The Hague (2015) ALL CONTENT
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Date Published : 20/08/2015
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