Abstract:
The importance of the persistence of multiple inlets in coastal systems is fundamental for
issues such as water quality, navigability, and beach/barrier stability. In long embayments, having
extended residence times, the stability of multiple inlets can be important for more efficient flushing
and water exchange between the embayment and the ocean.
Many approaches have been used to analyze inlet stability, but have focused on single
rather than multiple tidal inlet systems, relying solely on measured data to describe and predict
the behavior of tidal inlets and/or suggesting empirical stability relationships between inlet morphology
and inlet/bay hydrodynamics. At present, the only multiple tidal inlet stability model
available combines a linear analytical model for the flow and an empirical relationship for equilibrium,
suggesting that multiple inlet systems are unstable and ultimately all inlets will close or,
at best, one will remain open.
Focusing on shallow multiple tidal inlet systems and in particular on Ria Formosa, a shallow
coastal lagoon in the south of Portugal known to have maintained persistently multiple inlets
in a historical time scale, the morphodynamic and hydrodynamic response to disturbances in the
physical characteristics of the lagoon and inlets was studied through a) the analysis of historical
data of the region, b) the analysis of tidal data (velocity and water surface elevation) collected in
the study site, and c) through the numerical modeling of the system hydrodynamics under various
inlet scenarios (using RMA-2V, a vertically averaged finite element model), with emphasis on the
contribution of the hydrodynamic response (changes in tidal prism, residual discharge and current,
sediment transport capacity, tidal distortion, and cross-sectional averaged maximum velocity)
to maintain the multiple inlets open.
The model results show that multiple tidal inlet systems can exhibit stable inlet configurations,
and that the strong hydrodynamic interaction between inlets, as well as the non-linear
distortion of the tide, play a major role in multiple inlet persistence. Some of the results and findings
are specific to Ria Formosa, and others can be generalized and used to identify processes
contributing to stability in shallow systems with multiple inlets servicing a single embayment.
