Hart, D.E., 2007. River mouth lagoon dynamics on mixed sand and gravel barrier coasts. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 927 – 931. Gold Coast, Australia, ISSN 0749.0208

This paper examines the dynamics of river-mouth lagoons on mixed sand and gravel coasts receiving high wave energy. The role played by rivers in these non-estuarine systems is recognised as essential for carving out space for the coast-parallel waterbody, defining its physical scale, and for driving cycles of lagoon behaviour from closure to breaching. The role played by waves, by contrast, is not fully recognised in that it is limited to barrier building across river mouths and gradual lagoon-outlet offsetting along the shore. Field observations and water-level records were used in combination with river-flow, wave, and hourly-camera records at the Ashburton river mouth, New Zealand, to establish the importance of wave versus river forcings of lagoon dynamics. Several new behaviours were identified, including storm-induced lagoon breaching, rapid longshore migrations in outlet position and outlet channel truncations in response to storm-wave overtopping. Similar dynamics forced by combined river-flood and storm events were also common. Despite the observed complexity of river, wave and lagoon processes, characteristic morphologies and equilibrium states were apparent, with the lagoon appearing to be dominated by wave processes at all but the highest river-flow stages. Analysis of the frequency of the observed events is used as the basis of a new understanding of lagoon behaviour, from which management implications are drawn.