Backwater effects of wood packages in the Tongelreep for surface and groundwater
For centuries, streams were canalized and used to discharge as much water as possible in a short time period. This led to a reduction of the ecological quality in streams, drought problems in upstream areas and floods downstream. To counteract these problems, water authorities in the Netherlands are starting stream restoration projects. An example of these projects is the placement of dead wood packages in streams. These wood packages lead to differences in stream velocity and in additional variations in habitat, which result in better ecological quality. These differences in stream velocity also influence the hydrological system and result in backwater effects in streams. However, it is still unknown how to implement these effects in the wide-used surface water model SOBEK. The goal of this research is to investigate ways to implement these backwater effects in SOBEK and analyze the influence of these backwater effects on the groundwater for a case study, the Tongelreep.
The Tongelreep stream of Waterboard De Dommel was used as case study. For this stream, the occurrence backwater effects as result of the wood packages were demonstrated from the available data. At the closest measuring point, a water level increase of 0.2 meters was calculated. In considering this increase, a surface water model was build in SOBEK to explore different ways to implement wood packages. In order to simulate an increase in water level in SOBEK, several methods were examined. The first method is changing the stream cross section and the second is changing the stream friction. The results show a larger effect of a change in friction compared to a change in cross section.
With the best implementation of the wood packages, a detailed analyses of the effects on water levels in the stream was executed in SOBEK. This analyses showed a maximum water level increase of 0.5 meters. Besides examining the surface water, the effects on groundwater level were studied with a MODFLOW model for the study area. In this model, the effect of the higher water levels on the groundwater was examined. The model showed higher groundwater levels of 0.25 meters in the vicinity of 300 meter from the stream.
This study quantified the backwater effects for the Tongelreep stream and gives support for hydrologist to determine the effects of wood packages for stream restoration