Beach changes in the Hauraki Gulf, 1965–68: Effect of wind, sea-level change, and off-shore dredging

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Abstract
Beach changes in an area where the sea floor is in equilibrium with sea level are not only a function of the local energy input and time, but also of sea-level change. The magnitude of beach change, erosional and progradational, is related to maximum depth of the local sea-floor profile of equilibrium and both are a function of local wind strengths and directions. Removal of sand by man from shallow depths and from beaches within the essentially closed Ocean Beach-Mangatawhiri Sand System exceeds natural input by an average of about 1750 m3 per kilometre of coast per year. Starvation of the local beaches is tentatively shown by the difference in behaviour of Mangatawhiri Spit and beaches of Great Barrier Island; the latter are part of another system where sand is not being removed by man. Mangatawhiri Spit beach is apparently being starved at a rate of about 4750 m3 per kilometre per year; this rate may include the continued effect of a more localised removal of sand. Starvation of beaches in the Ocean Beach-Mangatawhiri Sand System may accelerate their erosion now and for some time in the future, particularly during periods of rising sea level.

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