Tidal amplification and shallow water tides in a mixed semidiurnal coastal lagoon

Abstract

Tidal amplification of the semidiurnal M₂ component and the generation of shallow water tides were studied along a mixed semidiurnal system. Month-long water level and current velocity data were collected using three acoustic Doppler current profilers. These profilers were moored at depths of around 15 m in three locations: 10 km inland from the mouth of the lagoon, within the central region, and near the head of the lagoon. Harmonic and spectral analysis applied to observations showed landward amplification of a semidiurnal tidal wave and the generation of high frequency tides. A wavelet analysis also depicted the temporal variability of each frequency band showing the generation of shallow water tides that coincided with the spring tidal cycle. The observed tidal amplification and generation of shallow water tides were reproduced using a numerical model to explore the mechanisms involved in the generation of high frequency harmonics and to determine their spatial distribution. An analytical model replicated the amplification of M₂ in an idealized elongated channel under high frictional forces. The four nonlinear mechanisms that led to shallow water tides generation were computed and a relationship with velocities was established using wavelet coherence. The even quadratic frictional term showed the highest coherence with the third diurnal (i.e. MK3), and seventh diurnal (i.e. 4MO₇) frequency band. The fourth diurnal (i.e. M₄) band was consistently related with advection and the odd frictional term. The contribution of this research is twofold: firstly, it provides new observational evidence on tidal amplification due to friction and channel geometry; and it explores the mechanisms that can generate shallow water tides. These results enhance our understanding of tidal dynamics in coastal semi-enclosed bodies, such as coastal lagoons or estuaries.