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

Abstract. The Weser estuary has been subject to profound changes in
topography in the past 100 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. We calibrated and validated each model individually to
account for differences in sediments, bedforms, and the resolution of
underlying bathymetric data between historical and recent system states. In
simulations of 1 hydrological year, each with realistic forcing
(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 topography
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 and 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
saltwater intrusion can vary by more than 20 km. An analysis of the salt
flux through a characteristic cross section showed that saltwater
intrusion in the Weser estuary is primarily driven by tidal pumping and only
to a lesser degree due to estuarine circulation. However, results indicate
that the contribution of individual processes has changed in response to
anthropogenic measures.
