Modelling the influence of vegetation on channel patern and flow dynamics

Riparian vegetation can significantly influence the morphology of the river, affectingchannel geometry and flow dynamics.In order to get more insight on this phenomenon, two types of numerical tests were carriedout, the first one aiming at reproducing the flume tests carried out by Kurabayashi andShimizu [2003] to study the effects of vegetation that were observed during a flumeexperiment, the second aiming at reproducing the effects of vegetation on the large scalepattern of the River Allier. This is a dynamic river and the presence of riparian vegetationappears to affect flow and planform. This river is well studied river and there are many dataabout it.In this research the DELFT3D software was used to simulate the laboratory experiment ofKurabayashi and Shimizu [2003] and the planform changes of the river Allier, with andwithout the presence of floodplain vegetation to show the role of vegetation in decidingwhether the channel will adopt a meandering or braided pattern.The 2D depth-averaged mathematical model proved to be suitable for simulation of thechannel pattern formation of the River Allier but not for the flume experiment. The initialriver planform was that of the straight channel with a constant width. Flow andmorphological parameters used were based on measured data.For the river Allier, two models were made, one without vegetation and one with vegetation.For the model without vegetation, two flow conditions were considered: a varying discharge(including floods) and a constant discharge. The results show that for the case of varyingdischarge the channel splits in many branches divided by mid bars, producing a braidedpattern. This remains active throughout the simulation period (10 years). The results of thesimulation with a constant discharge showed a much lesser braiding degree, in other words,braiding appeared essentially dependent of discharge fluctuations. However, a lesser degreeof braiding was obtained also with a constant discharge.The second model was created to study the influence of vegetation on the channel pattern.To simulate the influence of the vegetation on flow and morphodynamics, the ReductionFactor Approach of Baptist, [2005] was used.The results of the model with vegetation showed a much more stable channel, less lateralmigration with a meandering pattern. The high roughness provided by floodplain vegetationdiverts the flow into the main channel causing increased velocity in the main channel anddecreased velocity on the floodplains. This decreased the width-to-depth ratio.Colonization by vegetation was simulated by introducing vegetation on growing point barsduring low flow stages.The effects of vegetation density on the channel pattern and flow characteristics were alsostudied by means of an extra simulation with less vegetation density. The results showedthat the braided intensity increased as vegetation density decreased, the total number ofbranches increased and the width-to-depth ratio of the channel increased