Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

The role of iceplant in the north beach dune system Technical Report

Bradly, N.
coastal lagoons, classification, lakes, hapua, river mouth lagoons, erosion, sedimentation, winds, management, Waituna, Waihora/Lake Ellesmere, Wairau, Washdyke, Wainono Lagoon, South Island, New Zealand
Two distinctly different kinds of coastal lagoons are identified on the east and south coasts of the South Island, New Zealand. So-called 'river mouth lagoons' (the mouths of the Rakaia and Waiau Rivers) are one type, referred to here as 'hapua'. Published research on the physical evolution and processes of hapua is summarised. The second type is the 'coastal lake', for which the term 'Waitunatype' lagoon is used. Waituna Lagoon, Southland, is a quintessential example, others are: Waihora/Lake Ellesmere, Wairau Lagoon, Washdyke Lagoon, and Wainono Lagoon. These lagoons develop landward of barrier beaches formed from sands and gravels mainly derived from greywacke terrains subjected to Quaternary glaciation. Lagoons occur in interfan depressions or at the extremities of major outwash fan complexes, on microtidal coasts with very high wave energies and strong longshore transport of sediments. The coasts are either in erosion, or adjacent to 'hinge-points' around which entire coastlines are rotating to face dominant swell directions. Long-term erosion has greatly reduced present areas of Waituna-type lagoons over the last few thousand years. Entire lagoon systems and interconnections between coastal water bodies have been lost. Waituna-type lagoons are normally closed to the sea. Accumulated head, and scour by the water in the lagoons opens them. Wave processes, particularly longshore transport in storms, close them. Artificial opening of Waihora/Lake Ellesmere and Waituna has increased the frequency and duration of openings and lowered water levels. This has greatly reduced areas, water volumes, and wind-driven processes (waves, seiches, and currents) in the lagoons. US research on mid-latitude coastal lagoons shows that relative sedimentation rate is critical. Where rates are faster than sea level rise, lagoons will infill and be short-lived. Where sedimentation rates and sea level rise are roughly equal, a lagoon will maintain a constant water volume while sediments accumulate. Where sea level rise is faster than sedimentation, relative deepening will occur and the water volume will increase. Obtaining adequate data on sedimentation rates in Waituna-type lagoons is a high priority for their management.