ENVIRO FLOWS 2002 4 TH ECOHYDRAULICS MODELLING OF MORPHODYNAMICS, VEGETATION DEVELOPMENT AND FISH HABITAT IN MAN-MADE SECONDARY CHANNELS IN THE RIVER RHINE, THE NETHERLANDS MARTIN J. BAPTIST a,b* , GUDA E.M. VAN DER LEE b , FRANZ KERLE c AND ERIK MOSSELMAN a,b a,* Delft University of Technology, Faculty of Civil Engineering and Geosciences, Hydraulic and Offshore Engineering Section, P.O. Box 5048, 2600 GA Delft, Netherlands; phone: +31 15 2789450, fax: +31 15 2785124, E-mail: m.j.baptist@ct.tudelft.nl b WL | Delft Hydraulics, P.O. Box 177, 2600 MH Delft, Netherlands c Universität Stuttgart, Institut für Wasserbau, Lehrstuhl für Wasserbau und Wasserwirtschaft, Hydro Ecology Research Group, Pfaffenwaldring 61, D-70550 Stuttgart, Germany ABSTRACT To maintain safety against flooding at increased river discharges of the Rhine, raising the dikes is no longer opted for. Instead, measures are considered to increase the discharge capacity of the floodplains. Simultaneously, floodplain nature will be restored to former conditions. One of these measures involves the reconstruction of secondary channels. This paper focuses on the natural developments of floodplain vegetation and fish habitats as a result of the morphodynamic developments in three man-made secondary channels located in a floodplain of the river Rhine. A two-dimensional application of the numerical model Delft3D is used to model the hydrodynamic and morphodynamic developments of the secondary channels. A rule-based ecological model is coupled to the spatial output to predict vegetation types, succession and hydraulic roughness. The changing hydraulic roughness is fed back into the hydro- and morphodynamic modelling, to account for the changes in vegetation composition. The changes in morphology and flow characteristics lead to changes in physical habitat for fish, that is modelled with the fuzzy habitat model Casimir. Ultimately, the natural developments of the secondary channels over a period of 30 years are simulated. From the model simulations it can be concluded that secondary channels may show erosion or sedimentation dependent on the characteristics of the channel. In the eroding channel, floodplain vegetation increases the hydraulic roughness of the banks and therefore converges flow in the channel. In the aggrading channels, the entrances fill up and after about thirty years floodplain vegetation can develop within the channels. Fish habitat suitability changes due to the morphological developments. The restoration of secondary channels and subsequent natural development result in an increase of biodiversity, but simultaneously a conflict arises with flood safety, since the erosion of one channel endangers the river embankment, whereas the sedimentation of the other channels decreases their discharge capacity. KEY WORDS: river morphology, river restoration, secondary channel, vegetation, fish habitat.