Building with Nature

Case studies: Side channel Dutch Rhine branches

Room for the river

Side channels are secondary channels that are connected to the main channel of a river, but are in general much smaller and convey much less discharge than the main channel. Side channels used to be part of natural river systems but have disappeared in the recent decades due to engineering measures. By reconstructing side channels, the dynamic environment of the river is partly restored. They have been constructed for ecological reasons (e.g. as part of projects of the Water Framework Directive) or for flood management reasons (e.g. as part of the Room for the River program). Side channels increase the discharge capacity of the river, and hence, reduce flood water levels. They have added value with respect to biodiversity and recreational possibilities. They also add to the well-being of people, as floodplains with side channels contribute to the image that people generally have of natural rivers.

Method

After making an inventory of the side channels in the Netherlands, measurements with respect to bed decomposition and discharge distribution were carried out in a selected side channel system. Also, for a selection of side channels, the morphological development over time as function of the discharge was studied. Parts of these findings were used in a PhD project on Side Channel Dynamics.

Results

The measured data was used to analyse the morphological behavior and get information about the rate of sedimentation. It was found that -depending on the location of the side channels- mostly fine material (that is transported as suspended bed material load in the main channel) is diverged into the side channel. The role of the discharge regime is crucial: in times of extremely high discharges, the side channel is flushed. In times of low discharges, there is little activity in the side channel. Vegetation growth may increase in those conditions and the side channel becomes part of the floodplain.

The BwN-program also leads to innovation opportunities. Experiments have been carried out with so-called fish finders (a small device used by fishermen to find fish) to study the possibilities to use these devices for bed topography measurements. Results are promising, but the uncertainties are rather large.

With respect to the application of an assessment framework for Building with Nature measures, the findings of the side channels have been used in a general setting to define indicators to compare BwN. The framework turned out to be useful, but needs to be improved for actual use.

Local context

Side channels have been taken as a typical example of river restoration measures in larger rivers. The aim of those measures is to increase the discharge capacity and hence lower flood levels in the downstream area. In smaller catchments, or in the headwaters, one typically would like to slow down the flow by retaining water. This requires different BwN-measures (e.g. leaky dams, retention ponds). One of the conclusions of this project is therefore that scale and location determine the nature of the BwN-measure.

Future research directions

  • The role of connectivity of floodplains in reducing flood risk and improving natural values.
  • Improvement of the Assessment framework for BwN measures, and coupling to the UN Sustainable Development Goals.
  • Morphology of floodplains: how fast is the sedimentation of floodplains and how does this relate to the composition of the bed and the hydrograph?
  • Historical and future floodplain landscapes: the effect of restructuring floodplains on a timescale of decades.

Documents

All documents of the BwN project can be found at publications, below you find the documents belonging to this specific case study:

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