Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

Coastal Erosion Hazard: Perceptions and Collective Decision Making Thesis

Sinnathamby, S.
Journal / Source
MA Thesis
Publisher / Organisation
Department of Geography, University of Canterbury
The purpose of this report is to: Present the findings of a GIS analysis of shoreline positions from historical surveys and aerial photographs. Analyse the changes in relation to long term trends and specific process events and human activities. Discuss the findings in relation to possible future beach management options. The report addresses the plan shape change of the shoreline for the whole length of the beach between the St Clair headland and Lawyer Head. The assessment is made from plotting information from historical surveys and aerial photographs onto a geo-referenced base photograph. The information ranges from surveys of sections of the shore at various dates from 1888 through to 1952, and aerial photographs at various dates from1942 through to 2007. The shoreline position at Ocean Beach has moved seaward since the 1880s. The dunes have become stable, in that they now form a single line along the backshore, and are fully vegetated except for the seaward face and embryo (newly formed or forming) dunes. The beach position moves landward and seaward by about 10 to 15 m in response to storms and periods of swell respectively. There can also be slight changes to the orientation of the beach in response to the direction of wave approach. The shoreline position appears to have a seaward limit, at about the position of the 1957 High Water line in the western half of the beach, and the 1990 line in the eastern half of the beach. Infilling and levelling of the beach hinterland prevents landward migration of dune sands, and effectively holds the landward limit of the dune position. Dynamic dune processes whereby sand is released from the dune to the beach in times of storm, and new dunes form, grow and become vegetated during long periods of quiescent wave energy and abundant sand supply, are limited in across-shore extent due to the relatively high, steep face of the main dune. Dune development seaward is limited by the episodic occurrence of high-energy wave events. Hazards to beach users arise due to the steepness and instability of the eroded dune face during and after storms, and potential loss of hinterland resource due to retreat of the dune during storms. However there is also likely to be less landward retreat of the dune as a response to storm wave attack because the high steep dunes provide a larger volume of sand to be released to the beach per metre of horizontal retreat. A significant result of storm erosion of the dunes is that it disrupts access to the beach from the hinterland at the eastern end of the St Clair seawall, at Moana Rua Road and at St Kilda. It is unlikely that the line of the dunes or beach could be artificially 'encouraged' to move seaward without construction of some feature offshore that provides for development of a tombolo or artificial headlands for a new curvature to the embayment. Shore Processes and Management Ltd Page 3 3 It would be possible to create a dynamic dune buffer, of a lower elevation than at present along the beach between the Marae and Kettle Park by providing a more gradual slope to the hinterland. However, there would need to be provision of a width of land to allow for potential landward retreat of the dune toe in response to storms, to at least the position of the 1942 dune toe. This is a position about 30 m landward of the 2007 dune toe position, and may require sacrifice of existing hinterland resource use. It appears that the beach position is relatively stable as long as there is a supply of sand to the nearshore. This sand can come from offshore, from the dunes or move from the beach to the nearshore and back. Beach sands can also be blown onshore to form new dunes as part of the recovery of the beach after storms.