Idealised modelling of storm surges in large-scale coastal basins

Coastal areas around the world are frequently attacked by various types of storms, threatening human life and property. This study aims to understand storm surge processes in large-scale coastal basins, particularly focusing on the influences of geometry, topography and storm characteristics on the water levels along the coast. To this end, an idealised process-based hydrodynamic model is developed. For arbitrary closed or semi-enclosed basins, it solves the linearised three-dimensional shallow water equations, including the Coriolis effect, forced by time- and space-dependent wind and pressure fields. The model’s linearity allows us to analyse the problem in terms of the basin's frequency response (which reflects its resonance properties). The model is fast because it decouples the vertical calculations (analytical) from the horizontal calculations. First, the spectral response of a closed rectangular basin of uniform depth, subject to periodic wind forcing, is found to depend on the spatial characteristics of the wind field and the basin dimensions. Wind with different spatial patterns (uniform, divergent and curl) produce different resonance peaks which are further modified by bottom friction and the Coriolis effect. The latter introduces new peaks associated with cross-wind basin dynamics. Then, in Chapter 3, investigation shows that adding topographic elements (e.g., a topographic step, a linearly sloping bed or a parabolic cross-basin profile) in semi-enclosed basins causes the resonance frequencies to shift, through their effect on local wave speed. In Chapter 4, the model first reproduce the surge caused by Hurricane Katrina, with a schematised domain and forced by the so-called Holland-B model. Next, the storm parameters are varied around the values resembling Hurricane Katrina to study storm characteristics’ influence. The storm direction and point of landfall are found to be the most important parameters determining the surge height. In particular, a storm approaching from south-east making landfall at the Mississippi dike produces the highest surge levels. Due to its flexibility regarding geometry, topography and forcing, and the short calculation times, the new model bridges the theoretical model and complex computationally expensive numerical model, and can thus be used as a quick estimation tool for extensive sensitivity studies.