Print Email Facebook Twitter Stability of the IJsselkop bifurcation at design discharge Title Stability of the IJsselkop bifurcation at design discharge Author Vereijken, K.H. Contributor Uijttewaal, W.S.J. (mentor) Blom, A. (mentor) Mosselman, E. (mentor) Heineke, D. (mentor) Faculty Civil Engineering and Geosciences Department Hydraulic Engineering Date 2016-03-07 Abstract A hydrodynamic study shows high flow velocities at the IJsselkop bifurcation during the design discharge of 16,000 m3/s. The study, performed in 2005, concludes that these velocities might affect 1) the stability of IJssel river bed and 2) the stability of the grass cover layer of the flood plain at the bifurcation. This report provides a better insight in the behaviour of the river branches at the IJsselkop bifurcation during the design discharge. Furthermore, the likeliness has been studied that these flow velocities influence the stability of both the grass cover layer of the flood plain and the bed of the river IJssel. Additionally, it shows the effect on the discharge distribution at this bifurcation if large scale erosion of the grass cover or IJssel bed occurs. The route of the Rhine river towards the sea has changed several times the last few hundred thousand years. It deposited its sediment on various locations with a variation in sediment size, depending on its river type. The top layer of coarse sediment is located close to the surface in the Eastern part of the Netherlands. Additionally, the supply of coarse material results in a coarse top layer of the river bed around the bifurcation at the IJsselkop. The IJssel river bed top layer consists of coarser material than the Lower Rhine bed as a results of the geometry of the rivers. The IJssel bed consists of a coarse layer of approximately 1 m thick on top of a layer with finer sediment. The subsoil of the flood plain consists of ‘sandy clay’, which has been determined from stratification data. The level of the IJssel bed decreases with 1 – 2 cm/yr between 2002 and 2013, which is in the same order of magnitude as the decrease in water level. The stability of the IJssel river bed during design discharge has been assessed based on detailed drilling data. Three situations has been analysed for an insight in the behaviour of the river bed. The first situation is initiation of movement based on uniform sediment, which is based on the Shields mobility parameter. The second situation is initiation of movement based on mixed sediment. This is based on the hiding and exposure mechanism, which reduces the stability of the bed material. The third situation includes instantaneous shear stresses that are imposed by the presence of dunes. The analysis showed that a large area of the considered IJssel river bed can be assumed to be unstable during design discharge conditions. The stability of the flood plain has been assessed with a quantitative and qualitative analysis of the stability of the grass cover. A flow at the flood plain directed from West to East, the lateral discharge, is the result of the water level differences between the two branches just downstream of the IJsselkop bifurcation point, caused by the less gentle bed slope of the IJssel river than that of the Lower Rhine. High flow velocities are present at the entire flood plain, except for the Northwest part. The quality of the flood plain depends on the characteristics of the grass cover and clayey subsoil. The critical flow velocity is 1.3 m/s and 2.0 m/s respectively, based on a `poor’ to `normal’ grass cover quality and a flow duration of 20 hours for these velocities. The presence of discontinuities in the grass layer can initiate local erosion by the presence of turbulent flow and a weakening of the top layer. The lateral discharge may increase significantly as a result of large scale scour of the flood plain. The influence on the IJssel discharge has been estimated with a rough and conservative approach. The consequences of a break-up of the top layer in the IJssel river bed are assessed with a numerical model, Delft3D. Three scenarios are assessed: a hydrodynamic computation, morphodynamic computation and imposing an artificial deepening. The discharge of the IJssel varies from 43% to 47% of the Pannerden Canal discharge during the peak discharge. The velocity profile of the Delft3D model shows that the flow velocity in the IJssel is large compared to the flow velocity in the Lower Rhine and Pannerden Canal, which is the result of the small wet cross-section and the relatively steep bed slope of the IJssel. No significant changes are observed of the hydrodynamics when a morphodynamic update is applied. Only a small amount of sedimentation/erosion at the end of the flood wave has been observed. An artificial deepening of the IJssel has been imposed in the third simulation, to simulate the break-up. The deepening does not lead to other significant changes in water levels, flow velocity, discharge and bed level. Based on these computations it has been concluded that in the case of a break-up no large scale changes are expected to occur. Subject IJsselkopbifurcationriversbed stability To reference this document use: http://resolver.tudelft.nl/uuid:3295421f-bba2-4f32-b08c-9a9dbe0d2824 Embargo date 2016-03-07 Part of collection Student theses Document type master thesis Rights (c) 2016 Vereijken, K.H. Files PDF MasterThesis_KHVereijken_ ... charge.pdf 7.18 MB Close viewer /islandora/object/uuid:3295421f-bba2-4f32-b08c-9a9dbe0d2824/datastream/OBJ/view