The Use of Surface Water and Groundwater Flow Models for the Environmental Assessment of the Feasibility of the Establishment of an Alluvial Forest in a Floodplain

Wilko Schweers and Manfred Koch, Department of Geohydraulics and Engineering Hydrology, University of Kassel, D-34109 Kassel, Germany

email: kochm@hrz.uni-kassel.de

The reestablishment of natural, ecologically viable floodplains along previously heavily engineered rivers has been of major environmental concern in many industrial countries in recent years. For one reason, this is for the mitigation of extreme flood events, and for another one for the general ecological improvement of the floodplain habitat itself that, owing primarily to heavy agricultural use in these often very fertile arable fields, frequently has been under tremendous stress. In many instances, the reconversion of an agriculturally used floodplain to a natural alluvial forest has been proposed as an ecologically sound measure for that purpose.

As an application to a floodplain section along the very heavily loaded, saline (due to huge outfalls from the local potash-mining industry) Werra river in Hesse, the feasibility of the establishment of an alluvial forest in sections of that floodplain was investigated through the use of surface water and groundwater flow models. The former employed to calculate the flood-stage levels over the plain for several extreme river flood events under various land use alternatives, namely, a partly and a complete afforestation of the floodplain. The results of the surface water modeling effort show, that for the sake of flood protection requirements, only a parcel situated more remotely from the river should be afforested.

Using the groundwater flow model MODFLOW the interaction of the Werra river and the adjacent alluvial aquifer---which is also in strong connection with surface water through numerous gravel pits---was simulated. Seasonal and annual fluctuations of the groundwater table, using measured river stages as boundary conditions, were calculated in order to establish groundwater level duration exceedence curves under different sections of the floodplain. With this areas of possible water stresses (i.e. due to either groundwater levels too high or too low) under various land use scenarios are delineated. Furthermore, water quality aspects were investigated by means of particle tracking, whereby the possible maximum range of infiltration of heavily saline Werra river water during the existing short periods of influent conditions was computed. To this regard no adverse impact on the groundwater and ergo eventually on the root-zone water quality in the section of the proposed fluvial forest was found.