Relation between the design of creeks and the morphology of river bypasses in estuarine environments

Abstract

Framework Within the framework of the 'Room for the River' policy in the Netherlands several river bypasses are studied. One of these projects is the Noordwaard, which is a special project, because a large polder (about 20km2) will change into a tidal influenced area. The primary water defences in the North of the polder (Nieuwe Merwede) are to be lowered to a treshold. The water defences in the South (Biesbosch) will even be removed completely. In this way water can be bypassed from the Nieuwe Merwede to the Biesbosch, this will result in a substantial decrease of normative water levels on the Nieuwe Merwede. The hydraulic and morphologic consequences for the Noordwaard will however be severe. Focus In this thesis the relation between creek design and morphology of the 'new' Noordwaard is studied. This is achieved by researching the morphologic and hydraulic consequences of the 'new' Noordwaard. After which the (possibly alleviating) morphological effects of a couple of design features are studied. Modelling In order to do this several 1D models are used. Model runs were performed with a 'small' model, for the Biesbosch, and with a 'large' model, including the whole of the Dutch delta basin (Noordelijk deltabekken). For which two types of boundary conditions were applied: - Historical 1998 (survey data for a representative year); - Probabilistic (discharge fractions of the Rhine). Morphology From the results of both calculations a couple of conclusions could be drawn. The major conclusion is that there will be predominantly moderate sedimentation, up to 6 mm/year for the creeks and up to 2 mm/year for the floodplains. Some aspects with a strong effect on morphology are: - Yearly river discharge variations; - The height of the threshold (small dyke) at the Nieuwe Merwede; - Local geometry; It also showed that the historical boundary conditions offer a more realistic approach to the morphology of estuarine river bypasses, then the probabilistic boundary conditions. The probabilistic approach was unable to deal with some essential aspects for modelling estuarine river bypasses, like time dependent processes (i.e. flood waves) and realistic sea water levels. Alleviating measures A detailed analysis on possible measures to reduce the sedimentation in the Noordwaard, showed that a simple meandering creek system best suites the morphological demands. Other alternatives, like divers under the threshold, even increased the sedimentation in the polder. This is caused by local circumstances, i.e. relatively large sediment concentrations in the discharge of the Nieuwe Merwede.