The first two years of an inlet: sedimentary dynamics

A monitoring program, including the acquisition of a series of topo-bathymetric surveys and oblique aerial photos, has been carried out at Ancão Inlet since its artificial opening in June 1997. This paper presents morphological and volumetric results, including inlet channel evolution (in terms of width, depth and cross-sectional area) and tidal delta formation. The results are analysed in relation to the prevailing oceanographic conditions. This allows the development of a conceptual evolutionary model for the natural evolution of artificially opened inlets with four stages: Stage 1 is the artificially opened inlet; Stage 2 represents the inlet after reaching dynamic equilibrium for the channel while ebb- and flood-tidal deltas are still developing; Stage 3 is the “mature” migrating inlet; and Stage PS (stage post-storm) is the inlet situation after high-energy conditions. The transition from Stage 1 to Stage 3 is a continuous process with intermediate sub-stages. Transition to Stage PS is a discrete phenomenon, caused by high-energy events, after which the inlet gradually changes back to the previous stage. Ancão Inlet was in Stage 1 in June 1997. In April 1998 the inlet channel reached dynamic equilibrium, thus beginning Stage 2. One year after the opening (July 1998) the inlet showed fully developed ebb and flood deltas, reaching the end of Stage 2. Transition to Stage 3, with the beginning of the downdrift migration and bypassing processes, took place during the second year. Stage PS characteristics were observed in January 1999 after high-energy storm conditions, with strong evidence of landward sediment transport. Although the conceptual model was developed for Ancão Inlet, it is envisaged that it could be applicable to other mixed-energy migrating inlets opened in an artificial way.