Abstract
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 northwest 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 has 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 hyperturbid conditions, but risks have been identified. In the Elbe, tidal reflections against the profound step in bed level around Hamburg 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 modeling of the evolution of these rivers. This will not be an easy task, as historical data for a proper calibration of the models required are scarce.
Highlights
In many European rivers, tidal ranges have increased dramatically in the course of the twentieth century, and some rivers (e.g., Loire and Ems) have evolved into hyperturbid systems
It is the aim of the present paper to enhance our understanding of these processes. This is done in two papers, analyzing the longterm historic evolution of the tide in four rivers, e.g., the Elbe, Ems, Loire, and Scheldt Rivers
In Part I, we hypothesize how a reduction in river-induced flushing and increase in flood dominancy may pump in large amounts of fine sediments, enhancing the effects of river deepening on tidal amplification
Summary
In many European rivers, tidal ranges have increased dramatically in the course of the twentieth century, and some rivers (e.g., Loire and Ems) have evolved into hyperturbid systems. Though it is recognized that large-scale engineering works, such as reclamations,embankments, and deepening, have caused these tidal amplifications and shifts towards a hyperturbidstate, the underlying mechanisms are not yet fully understood It is the aim of the present paper to enhance our understanding of these processes. This is done in two papers, analyzing the longterm historic evolution of the tide in four rivers, e.g., the Elbe, Ems, Loire, and Scheldt Rivers. This analysis is presented in the current Part II, using the linear analytical model and relevant nondimensional parameters, derived and discussed in Part I of this paper.
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