Analysis of the vertical changes of the Lower Drava River morphology due to natural and anthropogenic impacts

Abstract

Based on the revised River Basin Management Plan („VGT2”) of the Hungarian part of the Danube River Basin, significant bed erosion can be observed in most Hungarian rivers. The incision has been the most intensive in case of the Drava River (approx. 3-4 cm/year), caused by natural and anthropogenic (river regulation interventions, dams and hydropower plants, sand and gravel mining). However, the effects of individual factors tend to overlap, making it difficult to distinguish the hydromorphological response of the river to them. An additional difficulty in identifying the impacts of bed incision is that little information is available to investigate the various factors. In the present study, the value of the incision was estimated based on the analysis of the available hydrological (water level time series), morphological (sediment data, bed surveys, river regulations) and dredging data. Before the 18th century, human interventions were not significant, and the morphology of the river was natural. Taking anthropogenic interventions as the triggering effect, the incision process of the Lower Drava riverbed can be divided into three periods. In the first period (from the end of the 18th century to 1974), only river regulation interventions took place, but their impact was significant (1,3 m deepening in ca. 120 years). In the second period (from 1975 to 2002/2011), three Croatian HPPs were built, and simultaneously, intensive sand and gravel extraction took place in the Lower Drava River. The effects of the HPPs was most intensive in the nearest station at Őrtilos (the rate of incision exceeded 7,7 cm/year), while sand and gravel mining along the full reach caused ca. 0,80 cm/year incision. In the third period (from 2003/2012 to the present), based on the analysis of low water levels and bed surveys, it can be concluded that a new state of equilibrium appears to be formed almost at the entire section of the Lower Drava River after the cessation of anthropogenic effects. It is important to note that the 10-20-year period since the end of dredging is not necessarily considered representative for trend analysis.