Modelling the impact of anthropogenic measures on saltwater intrusion in the Weser estuary

Abstract

Abstract. The Weser estuary has been subject to profound changes in topography in the last hundred years through natural variations and river engineering measures, leading to strong changes in hydrodynamics. These changes are also expected to have affected the dynamics of saltwater intrusion. Using numerical modelling, we examined saltwater intrusion in the Weser estuary in four different system states (1966, 1972, 1981, 2012). Models of each system state were set up with the respective topography and boundary values. The resolution of historical and recent topographical data is usually not comparable, which needs to be compensated, e.g., by calibration of roughness parameters. Therefore, each model was individually calibrated and validated. In simulations of one hydrological year for each system state (hindcasting study), the influence of topography is overshadowed by the effects of other factors, particularly river discharge. At times of identical discharge, results indicate a landward shift of the salinity front between 1966 and 2012. Subsequent simulations with different topographies but identical boundary conditions (scenario study) confirm that topographic changes in the Weser estuary affected saltwater intrusion. Solely through the topography changes, at a discharge of 300 m3 s-1, the position of the tidally averaged and depth-averaged salinity front shifted landwards by about 2.5 km between 1972 and 1981 due to deepening measures in the Lower Weser between these years. It shifted by another 1 km between 1981 and 2012. These changes are significant but comparatively small, since due to seasonal variations in run-off, the tidally averaged salinity intrusion can vary by more than 20 km.