|Response of tidal rivers to deepening and narrowing|
Winterwerp, J.C. (2013). Response of tidal rivers to deepening and narrowing. International Marine & Dredging Consultants/Deltares/Svašek Hydraulics BV/ARCADIS Nederland BV: Antwerp. 98 + annexes pp.
tidal amplification, hydraulic drag, dispersion equation, regime shift, Elbe, Ems, Loire, Scheldt
This report discusses 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. In this report, a simple linear analytical model is derived, solving the linearized shallow water equations in exponentially converging tidal rivers. Distinguishing reflecting and non-reflecting conditions, a non-dimensional dispersion equation is derived which yields the real and imaginary wave numbers as a function of the estuarine convergence number and effective hydraulic drag. The estuarine convergence number describes the major geometrical features of a tidal river, e.g. intertidal area, convergence length and water depth. We present a conceptual model on the response of tidal rivers to narrowing and deepening. It is argued that, upon the loss of intertidal area, flood-dominant conditions prevail, upon which fine sediments are pumped into the river, reducing its effective hydraulic drag. Then a snowball effect may be initiated, bringing the river into a hyper-turbid state. This state is self-maintaining because of entrainment processes, and favourable from an energetic point of view, and therefore highly stable. We may refer to an alternative steady state.Next, the historic development in tidal range in four rivers is analysed, 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, showing that the effective hydraulic drag in the Ems and Loire have decreased considerably over time, as anticipated. 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, tidal reflections against 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 here 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 of the models required are scarce.