Man-induced regime shifts in small estuaries—II: A comparison of rivers

This is Part II of two papers on man-induced regime shifts in small, narrow and converging estuaries, with focus on the interaction between effective hydraulic drag, fine sediment import and tidal amplification, induced by river engineering works, e.g. narrowing and deepening. Paper I describes a simple
linear analytical model for the tidal movement in narrow, converging estuaries, and a conceptual model on the response of tidal rivers to river engineering works. It is argued that such engineering works may set in motion a
snowball effect bringing the river into an alternative steady state.
Part II analyses the historic development in tidal range in four rivers, e.g. the Elbe, Ems, Loire and Scheldt, all in North-West Europe; data are available for many decades, up to a century. We use the analytical model derived in part I, showing that the effective hydraulic drag in the Ems and Loire have decreased considerably over time, as anticipated in Part I. We did not find evidence that the Upper Sea Scheldt is close to its tipping point towards hyper-turbid conditions, but risks have been identified. In the Elbe, resonant conditions seem to have affected the tidal evolution in the last decades.
It is emphasized that the conceptual picture sketched in these papers is still hypothetical, and needs to be validated, for instance through hind-cast modelling of the evolution of these rivers. This will not be an easy task,
as historical data for a proper calibration are scarce.