The Strait of Georgia is a large, semi-enclosed body of water between Vancouver Island and the mainland of British Columbia connected to the Pacific Ocean via Juan de Fuca Strait at the south and Johnstone Strait at the north. During the winter months, coastal communities along the Strait of Georgia are at risk of flooding caused by storm surges, a natural hazard that can occur when a strong storm coincides with high tide. This investigation produces storm surge hindcasts using a three-dimensional numerical ocean model for the Strait of Georgia and the surrounding bodies of water (Juan de Fuca Strait, Puget Sound, and Johnstone Strait) collectively known as the Salish Sea. The numerical model employs the Nucleus for European Modelling of the Ocean architecture in a regional configuration. The model is evaluated through comparisons of tidal elevation harmonics and storm surge with observations. Important forcing factors contributing to storm surges are assessed. It is shown that surges entering the domain from the Pacific Ocean make the most significant contribution to surge amplitude within the Strait of Georgia. Comparisons between simulations and high-resolution and low-resolution atmospheric forcing further emphasize that remote forcing is the dominant factor in surge amplitudes in this region. In addition, local wind patterns caused a slight increase in surge amplitude on the mainland side of the Strait of Georgia compared with Vancouver Island coastal areas during a major wind storm on 15 December 2006. Generally, surge amplitudes are found to be greater within the Strait of Georgia than in Juan de Fuca Strait.

It has been known for some time that internal wave-induced currents can drive near bed instabilities in the bottom boundary layer over a flat bottom. When the bottom is not flat, the situation can become quite complicated, with a diverse set of mechanisms responsible for instability and the subsequent transition to turbulence. Using numerical simulations, we demonstrate the existence of a mode of instability due to internal solitary wave propagation over broad topography that is fundamentally different from the two dominant paradigms of flow separation over sharp topography and global instability in the wave footprint that occurs over a flat bottom observed at high Reynolds number. We discuss both the two and three-dimensional evolution of the instability on experimental scales. The instability takes the form of a roll up of vorticity near the crest of the topography. As this region is unstratified in our simulations, little three-dimensionalization is observed. However, the instability-induced currents provide an efficient means to modulate across boundary layer transport. We subsequently extend the results to the field scale and discuss both the aspects of the instability that are consistent across scales and those that are different.

Large amplitude internal waves in naturally occurring stratified fluids induce currents throughout the water column and hence have the potential to drive instability, and turbulent transition within, and hence material exchange across the bottom boundary layer. In the presence of broad, small amplitude topography, waves of depression have been shown to induce a vortex roll-up instability that has the potential for cross-bottom boundary layer transport through the generation of coherent vortices. At the same time, the three-dimensionalization associated with the instability is weak. We demonstrate that the presence of a near-bottom stratification provides a means for an enhanced rate of three-dimensionalization. For solitary waves of elevation, which do not yield a coherent response in the absence of a near-bottom stratification, the presence of a near-bottom stratification leads to a local hydraulic response, or a gravity-current-like intrusion, as the wave passes over the topography. This feature forms on the lee slope of the topography, propagates with the wave for some time, and provides a coherent pathway for material to be transported a distance of 1.5 times the topography amplitude into the water column in laboratory-scale simulations. Evidence of coherent structures in the turbulent flow in this region is presented.

We consider the steady flow of a stratified fluid over topography in a fluid of finite vertical extent, as typified by experimental flumes with a rigid lid or the ocean under the rigid lid approximation. We do not specify a functional form of the upstream stratification or background current and derive a general version of the Dubreil–Jacotin–Long equation appropriate for the problem. This elliptic equation is strongly nonlinear and we develop an efficient, pseudospectral, iterative method for its numerical solution. The method allows us to compute laminar, trapped waves with amplitudes more than 50% of the depth of the fluid. We find that when either a background shear current is present or the topography is narrow enough, multiple steady states are possible and we confirm this finding by using integrations of the full time-dependent Euler equations. We discuss instances of waves with closed streamlines, finding that the presence of shear allows for waves with vortex cores that persist for long times in time-dependent simulations and match well with solutions of the steady theory. In contrast, streamline overturning in the absence of upstream shear only occurs for flows that are stratified near the surface and in this instance, time-dependent simulations yield unsteady cores that do not match steady results very well.

Numerical simulations are used to investigate waves generated by flow over crater topography of diameter 100 km in an idealized atmosphere. The atmosphere is stratified with a constant buoyancy frequency profile and the background wind is constant. This study describes the development of a low-level jet along the upstream crater slope and its interaction with the cooler air within the crater valley. This interaction results in a hydraulic jump–like structure that acts as a modified topography, forcing a beam of secondary waves. The hydraulic jump is formed by a retreating gravity current as the cool air within the crater readjusts after the initial tilting of potential temperature contours. A two-dimensional simulation is used to compare features such as wave overturning in two and three dimensions. Several variations on the atmosphere’s profile are considered, including an atmosphere with reduced constant stratification and an atmosphere that is unstratified within the crater. These results indicate that the stratification within the crater is an important component in the development of the hydraulic jump. Also, several topographic modifications are included, such as a crater with no rims and a crater with reduced diameter. These comparisons reveal that the crater rims have little impact on the general wave pattern and that the crater curvature can influence wave breaking and lateral deflections. In addition, cases with rotation break the symmetry and induce more overturning in one-half of the crater.