Estimating seepage loss during water replenishment to a floodplain oxbow: a case study from the Drava Plain

Abstract

The water regulation and channelization works of the past centuries did not only alter the drainage pattern and individual stream channels, but also the water budget of the adjacent floodplains and wetland areas, particularly oxbow lakes. For the maintenance of the wetland habitat of the Cun-Szaporca oxbow in the Hungarian section of the Drava River floodplain lakewater/groundwater interchanges are of utmost importance. The implementation of a large-scale landscape rehabilitation scheme is under way. To estimate water budget for the oxbow at present and after water replenishment, field studies (pumping tests) and laboratory analyses (conductivity, effective porosity) were carried out. Our research shows that the critical factor in water retention is the transmissivity of lakebed deposits – not only the present ones, but also those which become lakebed after future upfilling. Based on the laboratory hydraulic analyses of the undisturbed sediment samples highly different conductivity values were found for the middle and offshore parts of the oxbow lakes – a pattern just the opposite expected for active river channels. Relatively coarse fraction (~80 μm) dominates the shoreline zone and allows higher seepage rate from the oxbow lake. With considerable losses to groundwater (and indirectly to the Drava River) expected, the success of the replenishment scheme is questionable.