Cross-sectional stability of tidal inlets: influence of basin geometry and basin friction

Berends, Koen D. (2012) Cross-sectional stability of tidal inlets: influence of basin geometry and basin friction.

Abstract:An inlet cross-section is considered in equilibrium when there is no net import or export of sediment from the inlet channel, and cross-sectionally stable when small deviations from this state return the inlet to the equilibrium. It is common to assess the cross-sectional stability of tidal inlets using zero dimensional pumping-mode (PM) models in combination with the stability concept of Escoffier (1940). It is currently unknown what the influence of basin geometry and basin friction is on the stability of tidal inlets. The objective of this study is to investigate the effect of basin geometry and friction on the hydrodynamics and cross-sectional stability of tidal inlets and to evaluate the validity of using PM models to assess inlet stability. This is done by formulating an idealised, linear, horizontal depth-averaged two dimensional (2DH) model for single inlet systems. The model is applied on the Frisian and Texel inlet systems. Both systems are part of the Dutch Wadden Sea inlets. Both inlets are schematised as three adjacent compartments: the ocean, inlet and basin. Each compartment is characterised by a width Wj , length Lj , depth Hj and offset to the system centre �j . Derived characteristics are the basin surface area Ab = WbLb, basin aspect ratio Sb = Wb Lb and inlet cross-sectional area Ai = WiHi. The model is forced by an incoming Kelvin wave in the ocean compartment. Formulating the model required choosing representative values for the ocean compartment and the amplitude of the incoming Kelvin wave to ensure a desired tidal range in front of the inlet mouth, amongst others. Bottom friction in the inlet and basin was determined using an iterative method to assess the value of Lorentz’ linear friction coefficient, which involves a velocity scale. A PM-model was formulated accounting damping through friction in the inlet and radiation damping. Regarding hydrodynamics, it was found that the frequency of maximum tidal amplification due to Helmholtz-mode resonance is sensitive to basin geometry. This is attributed to tidal wave propagation through the basin. It was furthermore found that the PM model corrected for radiation damping better predicts the trend of the 2DH model at short inlet channels. This suggest that radiation damping is more important for short inlets. The Texel inlet system is much more dissipative than the Frisian inlet system. Regarding cross-sectional stability it was found that basin geometry has a greater influence on large systems. The aspect ratio of the basin is found to have a profound influence on inlet stability at large basins. In the Texel inlet case, the case-study presented in chapter 6 showed that the aspect ratio of the basin could even result in the absence of a stable root. The influence of two damming projects in theWadden Sea, which altered the basin geometry, are studied in case studies. The case studies showed that the damming of the Lauwerzee in 1969 causes the Frisian inlet channel to diminish in size. Basin depth has a negligible effect on the predicted stable inlet cross-sectional area. The PM- and 2DH model results show similar predictions. The Texel inlet is predicted to increase its cross-sectional area as a response to the closure of the Zuiderzee, assuming that the closure of the Zuiderzee led to a higher average basin depth. Basin depth has a large influence on the predicted cross-sectional area. Retaining the average basin depth before closure, the stable cross-sectional area will even slightly decrease, while a very steep increase in depth could lead to a cross-sectional area which is twice as big as before the closure. It is concluded that in such systems, basin depth is the most uncertain and important parameter for determining the stable cross-sectional area. It is concluded that the PM model is only valid for relatively small or deep basins.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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