Aligned with the flow. Morphodynamics in a river trained by longitudinal dams

Summary:

Within the frameworks of ‘building with nature’ and ‘room for the river’, several measures have been implemented over the past decade in the Dutch river system to reduce flood risk without raising the embankments. Moreover, an ‘integrated river management’ approach, in which multiple river functions are served with a single river intervention, is increasingly being advocated. Longitudinal training dams (LTDs) form an innovative, relatively new river training structure within these frameworks, securing the transport corridor over water and reducing the flood risk, while on the other hand creating ample room for recreation and enhancing the ecological diversity of the river system. An LTD separates the main channel from a bank-connected side channel in the inner river bend, with a sill present at the bifurcation of the two channels. During high water levels the crest of the LTD is submerged, which is estimated to happen about 100 days per year.

In this doctoral thesis, an LTD pilot project in the Waal River (the Netherlands) is studied using a physical scale model and data from extensive field monitoring campaigns. The aim of this study is to understand the processes governing flow patterns and bed morphodynamics in the region of the inlet towards an LTD side channel, and to develop steering controls. It is concluded that local processes govern the flow field in the bifurcation region of main and side channels, and that the vertical flow structure in the side channel is steered by the geometrical details of the sill. Morphological stability of the side channel is primarily determined by the sill geometry. The bed topography in the main channel is significantly influenced by LTD construction on the scale of the LTD pilot as a whole. Overall, LTDs provide a valuable addition to the river engineer’s toolbox, with a wide range of possibilities to steer water and sediment division over main and side channel.