A geomorphic classification of intermittently open/closed estuaries (IOCE) derived from estuaries in Victoria, Australia

Abstract

A large proportion of estuaries along microtidal wave-dominated coastlines worldwide have entrances that intermittently close to the ocean when tidal currents and fluvial discharge are insufficient to erode sediment delivered onshore by waves. In this study, these systems are termed “intermittently open/closed estuaries” (IOCE) in order to include all estuaries which intermittently close to the ocean. IOCE do not fit neatly into existing generalized estuary classification models and have been traditionally recognized as a single estuary type that constitute a rare subset of wave-dominated estuaries. In this study, 111 estuaries in Victoria, Australia, are used to develop a classification model that delineates between different IOCE types. This was undertaken using historic aerial imagery and quantification of the estuary channel width, catchment area, lagoon dimensions and tidal prism derived from remotely sensed data. Field surveying of entrance morphology was undertaken for a subsample of 35 IOCE characteristic of each section of the coast and which had detailed entrance condition records. Using this subset, IOCE were classified into three distinct types using multiple methods of statistical delineation (non-metric multidimensional scaling, hierarchical cluster analysis and distribution analysis). These three types are: (1) Type A, the largest IOCE which both close and open infrequently but for the longest durations; (2) Type B, medium sized IOCE which open and close several times per year for weekly to monthly durations; and (3) Type C (tidal creeks), the smallest IOCE located specifically in high rainfall, mountainous catchments and which exist in a predominantly open state. The three types of IOCE showed an order of magnitude difference in entrance closure duration as controlled by variations in the catchment area, tidal prism volume, dimensions of the estuarine lagoon and the entrance channel at the mouth. The classification is also applicable to wave-dominated coastlines internationally where IOCE are present.