River, Coastal and Estuarine Morphodynamics: RCEM 2005 – Parker & García (eds) © 2006Taylor & Francis Group, London, ISBN 0 415 39270 5 Long term evolution of self-formed estuarine channels Ilaria Todeschini, Marco Toffolon & Marco Tubino Department of Civil and Environmental Engineering, University of Trento, Italy ABSTRACT: The aim of this paper is to investigate the long term configuration attained by a tidal channel, namely its longitudinal bed profile and shape. Channel evolution is simulated through a one-dimensional numer- ical model, where the role of intertidal areas is neglected. The channel has a rectangular cross section and it is closed at one end. The major novelty of the work is that the width of the channel is allowed to change in time: in the present model lateral erosion is computed as a function of bed shear stress, provided it exceeds a threshold value within the cross section.The system, forced by a prescribed free surface oscillation at the seaward boundary, reaches an equilibrium profile of both bottom elevation and channel bank. The equilibrium geometry of the channel is determined for different initial and boundary conditions. Furthermore, the spatial variations of the cross section at equilibrium are analyzed in order to identify those conditions under which the commonly used exponential law of channel width variation is reproduced. Finally, a comparison is pursued between model predictions and bathymetries of real estuaries. 1 INTRODUCTION The definition of long-term equilibrium conditions of tidal channels is an issue of great importance for many aspects related to the management of tidal environ- ments. In spite of its relevance, the problem still awaits for a complete investigation. In previous works (Schuttelaars and de Swart 2000; Lanzoni and Seminara 2002; Todeschini et al. 2003) the attention has been mainly focused on the case of well-mixed, tide-dominated estuaries, characterized by a given funnel shape, negligible river discharge and vanishing bottom slope (Perillo 1995; Savenije and Veling 2005). Furthermore, the influence of tidal flats has been ignored. The above works suggest that, when a reflective boundary is assigned at the landward end, the morphological evolution of a tidal channel, starting from a horizontal bed profile, is characterized by the formation of a sediment wave that migrates slowly landward until it leads to the emergence of a beach. This condition generally inhibits the further development of the channel and determines an asymp- totic intrinsic length of the estuary. The resulting bed profile is characterized by an increasing bottom ele- vation in the landward direction, as confirmed also by the experimental observations of Bolla Pittaluga et al. (2001). It is important to note that channel convergence directly controls the above equilibrium length; in fact, it strongly affects both the hydrodynam- ics (Friedrichs and Aubrey 1994) and the consequent morphodynamical evolution (Lanzoni and Seminara 2002). The interest for the study of such class of estu- aries is motivated by the fact that, in nature, many estuaries display a typical funnel shape. The Thames and the Bristol Channel, whose plan view can be seen in Figure 1, are examples of this estuarine category. Understanding the reason why tidal channels are convergent and defining the conditions under which the exponential law for width variation, which is so often observed in nature, is reproduced are the main objectives of the present work. We then remove the main assumption on which previous models are based, namely that of fixed banks, and let the channel bank vary with time. We note that various attempts to repro- duce the equilibrium cross section of tidal channels have been pursued in previous works (e.g. Gabet 1998, Fagherazzi and Furbish 2001). However, the above analyses mainly focus on the local scale, hence the full coupling with the morphological evolution of the channel is not taken into account. On the other hand, several contributions already exist in the case of rivers. For instance, a numerical model of widen- ing and bed deformation has been proposed by Darby and Thorne (1996) who considered both planar and rotational failures and calculated channel widening by coupling bank stability with flow and sediment transport algorithms. In our work a strongly simplified approach is adopted, whereby only the effects related to flow and sediment transport processes within the tidal channel 161