Contribution of nonlinear mechanisms in the persistence of multiple tidal inlet systems

Multiple tidal inlet stability has been modeled following either a single-inlet stability approach or a simple linear analytical approach, and the results suggest that these systems are not stable. However, there is evidence that shallow multiple tidal inlet systems are strongly nonlinear and that they can be stable on a historical time scale. Based on numerical simulations of the hydrodynamics of Ría Formosa, a shallow lagoon in the south of Portugal, in which multiple tidal inlets have persisted in a historical time scale, the effects of tidal distortion and residual circulation on the persistence of multiple inlets in the system were analyzed. Numerical simulations using a 2-D vertically averaged finite element model served (1) to identify the existence of three hydrodynamically independent sub-embayments in the system, (2) to create realistic physical disturbances (change in inlet cross-sectional area) and analyze the hydrodynamic response of the system to such disturbances, (3) to identify and quantify the existing nonlinear hydrodynamic processes (tidal distortion, ebb/flood dominance, residual flow patterns), and (4) to analyze the overall stability of the western sub-embayment using the closure curve approach. The results show the importance of the nonlinear processes in the hydrodynamics of multiple inlet systems, and the need to consider them when analyzing inlet stability in such systems, and suggest that, specifically for Ría Formosa, if the existing jetties at one of the inlets in the western sub-embayment were removed, this area of the system would reach natural stability and equilibrium with three inlets.