Influence of vegetation on the long term morphological processes in the Common Meuse River

The Common Meuse River is one of the most natural rivers in TheNetherlands. Restoration works have been implemented to increase thesafety against floods, combined with the development of riparian biodiversity.Space for the river is needed to maintain the safety under extreme conditions,which is obtained by increasing the conveyance capacity of the river bylowering the floodplains and widening of the main channel. These measureshave been applied in the Pilot Project Meers at the inner bend of theCommon Meuse River near Meers. This has produced changes in themorphodynamics processes, along with the development of softwood forestand new vegetation types in the rehabilitated zones. Vegetation grows on thefloodplains decreasing conveyance capacity of the river and therefore regularmaintenance of the restored areas is needed.Cyclic Floodplain Rejuvenation is one of the strategies that have beenapplied in The Netherlands to cope with the increased risk of flooding that isexpected due to climate changes, since predictions indicate an increase in thepeak discharges and of frequency of occurrence.It is necessary to study the effects of works such as lowering of thefloodplains and widening of the main channel on the morphology of the river inthe short and long term. These studies should take into account the influenceof vegetation on the floodplains, since this affects water levels and flowvelocities during high flows. The scope of this research is to investigate theinfluence of floodplain vegetation on the hydrodynamics and morphologicalchanges of the Common Meuse River on the short and long term.The study is carried out using a 2D numerical model, constructed using theDelft3D software package. For the analysis of the effects of floodplainvegetation on the long term main channel morphology the model included theReduction Factor Approach developed by Baptist (2005) to represent thevegetation roughness.Two scenarios were analyzed, one with floodplain vegetation and one withoutvegetation. The case without vegetation used constant values of theroughness coefficient for the main channel and the floodplain. The case withvegetation included different types of plants and trees along the both banks ofthe river. Additionally, one case was built using only one type of vegetationwhich remained constant with time. The simulations demonstrated thatvegetation produces extra elevation of the water levels and reduction of theflow velocities in the vegetated area, increasing the flow velocities in the mainchannel.For the study of the morphological developments, the simulations were donefor different periods of time, the short term study for 5 years and the long-termstudy for 25 and 30 years, respectively. For the short-term study the initial bed topography was the one measured in 1998 and for the long-term study theone measured in 2002. Hydraulic calculations demonstrated that therestorations works are effective in reducing the water levels.The morphological study demonstrated the importance of floodplainvegetation on main channel geometry and river bank stability. For the casewithout floodplain vegetation the simulations showed that the width of themain channel tends to increase due to bank erosion. For the case withspatially-varying vegetation the river banks appear more stable, since themain channel did not show any widening trends, which was mainly due to theeffect of concentrating the flow in the main channel.During the first 5 years a large amount of sediment is deposited in the areaexcavated by the restoration works, as well as along the outer bend at theother side of the river, whereas erosion takes place downstream of theintervention, where the river reaches its original width.The long-term simulation with vegetation showed that the river restores itsoriginal state in about 15 to 20 years; after this time it is necessary to reexecutethe rehabilitation works at the same place to maintain this strategyagainst the flooding risk. Without vegetation the river restoration time appearslonger, i.e. 25-30 years; this means that modelling the morphological evolutionof the Common Meuse River without floodplain vegetation leads to anunderestimation of the maintenance works.Finally, it has been demonstrated that floodplain vegetation has an importanteffect on the morphodynamics processes to safeguard the risk of flood and toprotect the riparian biodiversity; it is therefore recommended to includefloodplain vegetation in models for the study of the short and long term effectof the restoration works and climate changes