Cross-sectional stability of double inlet systems

Abstract

Barrier coasts and their associated tidal inlet systems are a common feature in many parts of the world. They constitute dynamic environments that are in a continuous stage of adapting to the prevailing tide and wave conditions. Commonly, these coastal areas are densely populated and (partly) as a result there often exists a strong conflict of interests between issues related to coastal safety, economic activities and ecology. To manage these different interests, it is important to gain more understanding of the long-term morphological evolution of tidal inlet systems and their adaptation to natural changes and human intervention. In this thesis the focus is on double inlet systems, where two tidal inlets connect a back-barrier basin to an ocean or a coastal sea. To investigate the morphological evolution of double inlet systems and their adaptation to internal or external change, the equilibrium configuration and stability properties of the cross-sectional areas of the two tidal inlets are studied in detail. To that extent, a widely used empirical relationship for cross-sectional inlet stability is combined with (i) a lumped-parameter (L-P) model (Chapters 2 and 3) and (ii) a two-dimensional, depth-averaged hydrodynamic (2DH) model for the water motion (Chapter 4). The Marsdiep-Vlie inlet system in the western Dutch Wadden Sea and the Faro-Armona inlet system in the Portuguese Ria Formosa serve as case studies throughout this thesis. With the assumptions of a cross-sectionally averaged, uniform inlet flow velocity and a uniformly fluctuating basin surface elevation, model results of the L-P model show that stable equilibrium configurations where both inlets are open exist. It is necessary, however, to account for the important processes either explicitly, e.g. including a topographic high in the back-barrier basin as observed in the Wadden Sea (Chapter 2), or parametrically, e.g. allowing for inlet entrance/exit losses for relatively short inlets such as in the Ria Formosa (Chapter 3). By solving the depth-averaged, linear shallow water equations on the f-plane with linearised bottom friction, the 2DH model explicitly accounts for spatial variations in surface elevation in the ocean, inlets and basin. Model results show that these spatial variations, induced by e.g. basin bottom friction, radiation damping, and Coriolis effects, are crucial to simulate and explain the long-term evolution of double inlet systems. This approach further allows the identification of a stabilising and destabilising mechanism associated with the persistence or closure of one (or both) of the inlets in a double inlet system and hence with its long-term evolution.