Hydrodynamics and equilibrium of a multiple-inlet system

Although it is generally acknowledged that most multiple inlets are unstable and cannot coexist, there is evidence to suggest that such inlets can indeed be stable over decadal time scales. Multiple-inlet systems servicing a single embayment, in contrast to single-inlet systems, confer particular hydrodynamic characteristics such as the potential existence of residual discharges and currents between the inlets. This paper presents detailed measurements of hydrodynamic variables obtained over complete spring and neap tidal cycles in a multiple-inlet system in Southern Portugal (Ria Formosa). The results indicate that the two main interconnected inlets servicing an embayment can coexist, at least over a time scale of several decades. However, their coexistence cannot be explained simply using empirical equilibrium relations or inlet hydraulics. Although residual flow between the inlets appears to play an important role enhancing their stability, it is the availability of sediment stored in the ebb-tidal deltas (and its capacity to be carried into the inlets during storm events) that ultimately dictates the overall equilibrium, independently of the flushing capability provided by the hydraulics. Such movement of sediment leads to the obstruction of the inlet channels, thereby affecting the hydraulic efficiency and eventually leading to inlet closure in the long term. The results augment existing knowledge concerning multiple-inlet systems and improve understanding of their short- to medium-term stability.