Modeling the effect of vegetation on river floodplain hydraulics

Vegetation in river floodplains has significant influence on the flood hydraulics and fate of suspended
sediments, nutrients and contaminants. In the present, work preliminary 3-D calculations were
performed to examine the effect of vegetation on the mean flow in open channels using the CFD
model CFX-12.1, employing the RANS k-epsilon turbulence model. Calculated flow velocity
distributions were compared against an experiment of free surface uniform flow in a vegetated
experimental channel, filled with cylindrical submerged elements representing vegetation; these
elements were rigid and arranged in a staggered pattern.
Four unstructured numerical grids were employed, ranging from approximately 9.5 to 27.5 millions of
tetrahedral elements. The main characteristics of the flow were (a) the formation of small recirculation
regions in the wakes of the cylinders and (b) the relative uniform flow conditions
throughout the length of the channel. Low flow velocities were observed in the vegetated region,
implying the resistance due to vegetation, and higher velocities close to the free surface. The best
agreement with experimental data was achieved for the finest grid that also included grid refinement
at the top of the cylinders. Grid independence behaviour using relatively very fine grids was rather
surprising and requires further detailed investigation.