Tides in Coastal Seas. Influence of Topography and Bottom Friction

Tides are important in various ways, e.g., by affecting navigation and coastal safety and by acting as a driver for sediment transport and seabed dynamics. To explain spatial patterns of tidal phase and range, observed in coastal seas around the world, we present an idealised process-based model. It solves the depth-averaged linearised shallow water equations, including the Coriolis effect and bottom friction, on schematised geometries with rectilinear coastlines and stepwise topographic vari- ations. Based on an extended Klein-Gordon equation (accounting for bottom fric- tion), Kelvin and Poincaré modes are identified as the fundamental wave solutions in a channel of uniform width and depth. We analyse their spatial structures and dynamic properties, addressing the roles of bottom friction and transverse topographic steps. The solution for a semi-enclosed basin, including topographic steps, is then obtained as a superposition of these wave modes, by applying a collocation technique. As an example, we present solutions that grossly explain the amphidromic system of the Gulf of California. Finally, we discuss the modelling approach and address the links with morphodynamics and climate change

Effects of Density‐Driven Flows on the Long‐Term Morphodynamic Evolution of Funnel‐Shaped Estuaries

Subtidal flows driven by density gradients affect the tide‐averaged sediment transport in estuaries and, therefore, can influence their long‐term morphodynamic evolution. The three‐dimensional Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport modeling system is applied to numerically analyze the effects of baroclinicity and Earth\u27s rotation on the long‐term morphodynamic evolution of idealized funnel‐shaped estuaries. The morphodynamic evolution in all the analyzed cases reproduced structures identified in many tide‐dominated estuaries: a meandering region in the fluvial‐tidal transition zone, a tidal maximum area close to the head, and a turbidity maxima region in the brackish zone. As the morphology of the estuaries evolved, the tidal propagation (including its asymmetry), the salinity gradient, and the strength of subtidal flows changed, which reflects the strong bathymetric control of these systems. The comparison with barotropic simulations showed that the three‐dimensional structure of the flow (induced by density gradients) has leading order effects on the morphodynamic evolution. Density gradient‐driven subtidal flows (1) promote near‐bed flood dominance and, consequently, the import of sediment into the estuary, (2) accelerate the morphodynamic evolution of the upper/middle estuary, (3) promote a more concave shape of the upper estuary and reduce the ebb‐tidal delta volume, and (4) produce an asymmetric bathymetry and inhibit the formation of alternate bars that would form under barotropic conditions. This latter effect is the consequence of the combined effect of Earth\u27s rotation and baroclinicity