Morphodynamics of intermittently open–closed coastal lagoon entrances: New insights and a conceptual model

Abstract

Ocean entrances to coastal lakes and lagoons are a complex, sensitive and important part of the coastal environment. They provide estuarine water exchange between inland waterways and the open ocean, and are increasingly the focus of intense urban and industrial development. Coastal lagoons that are only intermittently connected to the ocean are particularly sensitive. During periods of closure, water quality issues are exacerbated, along with the increased risk of flooding. Mechanical clearance was undertaken in 2006 at the ocean entrance to Narrabeen Lagoon, Sydney, Australia. Historically, this site has intermittently opened and closed (both naturally and mechanically), with the ingress of marine sand into the entrance restricting water exchange between the estuary and the ocean. This ‘instantaneous reset’ of the system provided a unique opportunity to monitor and quantify generic inlet sedimentation processes and mechanisms. An intensive data collection campaign was undertaken for one entire year following the clearance. This dataset includes current and water level measurements, calculation of the alongshore sediment supply, and regular, highly detailed morphological surveys of the infilling lagoon entrance system. This contribution utilises this unique dataset to quantify the sediment ingress volumes, rates and pathways within the accreting flood-tide delta of the lagoon entrance. It was found that ingress of sand into the system proceeds via a rapid landward infilling process (as opposed to backfilling toward the ocean) with the evolution of the flood-tide delta occurring in distinct phases. The location of the divergence point of the flood tidal current, the amount of available accommodation space within the entrance system and the influence of storm events, were found to be key factors in the evolving morphodynamics of this intermittently open–closed coastal lagoon.