Print Email Facebook Twitter Planform changes and overbank flow in meandering rivers: The river Allier Title Planform changes and overbank flow in meandering rivers: The river Allier Author Blom, A. Contributor Wang, Z.B. (mentor) Crosato, A. (mentor) De Vriend, H.J. (mentor) Faculty Civil Engineering and Geosciences Department Hydraulic Engineering Date 1997-08-01 Abstract In this study the river migration model MIANDRAS has been applied to simulations of the planform changes of the river Allier, a tributary of the river Loire in France. The Allier is a very dynamic and natural river system with braided and meandering river sections. Two meandering reaches of the Allier, located between Varennes-sur-Allier and Moulins, have been examined in the present study. MIANDRAS is a numerical model that simulates meandering processes in shallow alluvial rivers with erodible banks. In MIANDRAS the river is schematised as a single meandering channel, as the floodplains are assumed non-conveying. For the prediction of river planform changes, the rate and direction of bank erosion are computed by coupling a steady-state flow and bed topography model with a time-dependent bank erosion model. Sediment gradation, timedependent variations of the bed topography (i.e. migrating bars) and flow exchanges between the main channel and the floodplains are not taken into account. MIANDRAS did not perform satisfactorily in predicting the planform changes of the Allier. The 2(and 3-) dimensional flow processes in flood periods, caused by floodplain constrictions and flow exchange between the main channel and the floodplains, appeared to be an important feature influencing the bank erosion. Therefore, the influence of overbank flow was studied in more detail. The 3-dimensional flow model DELFT3D-FLOW (TRISULA) was used to simulate overbank flow processes in one of the examined reaches of the Allier. The results of the MIANDRAS computations can be explained reasonably well from the results of the overbank flow computations. Near the upstream floodplain constriction, where high bank erosion rates were observed in the prototype river, high velocities occurred in the overbank flow computations. At this location high bank erosion coefficients had to be adopted in the MIANDRAS computations. The relatively low erosion rates at the downstream boundary can be explained by the low velocities occurring in overbank flow. These are due to backwater effects near the downstream floodplain constriction. The small erosion rates at this location resulted in small erodibilities in the MIANDRAS computations. Floodplain bed roughness appears to have a significant influence on velocity directions and magnitudes. The velocity field is correlated to the flow exchange between the main channel and the floodplains. This flow exchange strongly decreases in the case of a higher floodplain bed roughness. These results agree with the results of scale model experiments performed in the Flood Channel Facility at HR Wallingford, England. The results of this study give new indications for the range of applicability of MIANDRAS. Besides the possibility to use MIANDRAS in case of rivers with mainly inbank flow, the model can also be applied to rivers with overbank flow periods, provided that the flow exchange between the main channel and the floodplains is small. A small flow exchange occurs in case of a high floodplain bed roughness, a straight floodplain and main channel, or relatively small flood discharges. Subject river bendsmeandersAlliersedimentation To reference this document use: http://resolver.tudelft.nl/uuid:4cf2e5b2-491e-4026-a150-f46621074333 Related item https://doi.org/10.4121/uuid:f26fed82-5af5-4dc7-924f-d7ebfd06fddf Part of collection Student theses Document type master thesis Rights (c) 1997 Blom, A. Files PDF Blom1997.pdf 32.18 MB Close viewer /islandora/object/uuid:4cf2e5b2-491e-4026-a150-f46621074333/datastream/OBJ/view