Assessing the perturbations of the hydrogeological regime in sloping fens through roads
Abstract. Roads in sloping fens constitute a hydraulic barrier for surface and subsurface flow. This can lead to a drying out of downslope areas of the sloping fen as well as gully erosion. Different types of road construction have been proposed to limit the negative implications of the roads on flow dynamics. However, so far no systematic analysis of their effectiveness has been carried out. This study presents an assessment of the hydrogeological impact of three types of road structures in semi-alpine, sloping fens in Switzerland. Our analysis is based on a combination of field measurements and fully integrated, physically based modelling. In the field approach, the influence of the road was examined through tracer tests where the upslope of the road was sprinkled with a saline solution. The spatial distribution of electrical conductivity downslope provided a qualitative assessment of the flow paths and thus the implications of the road structures on subsurface flow. A quantitative albeit not site-specific assessment was carried out using numerical models simulating surface and subsurface flow in a fully coupled way. The different road types were implemented in the model and flow dynamics were simulated for a wide range of slopes and hydrogeological conditions such as different hydraulic conductivity of the soil. The results of the field and modelling analysis are coherent. Roads designed with an L-drain collecting water upslope and releasing it in a concentrated manner downslope constitute the largest perturbations. The other investigated road structures were found to have less impact. The developed methodologies and results are useful for the planning of future road projects.