Spatial patterns of erosion across an intertidal platform: Tarihunga Point, Auckland, New Zealand

Abstract

It is well understood that shore platforms play an important part in defending cliffed coastlines from wave energy. The outer edge of platforms initiates wave breaking, and the depth of water on platforms controls the rate of energy dissipation and ultimately the amount of wave energy available to erode the cliff toe. It follows that it is necessary to understand the processes that contribute to platform downwearing, because they control platform water depth. Recent studies have sought to characterise shore platform downwearing through precise measurements of micro-scale erosion and resistance tests on the platform surface. Many useful contributions have been made through such methods, but a substantial limitation is that measurements are made on the rate of change of pre-weathered rock. This thesis develops a new method of measuring erosive processes across a shore platform using ‘unweathered’ cement-based samples. The samples were installed across a platform profile at Tarihunga Point, Whangaparaoa Peninsula, northeast of Auckland, New Zealand, and were exposed to subaerial weathering and wave action over 5 months from 26/05 – 26/10, 2012. Morphological change before and after the experiment was detected with a FARO laser scanner, and change in the field was monitored at 14-day intervals using vertical photography. These measurements are combined with hydrodynamic data obtained during a storm-wave event, and a detailed morphological description of platform features. Results demonstrate that weathering of the samples, through constant wetting and drying driven by tidal cycles, plays an important role in sample breakdown. Cracks in the samples produced by this process are a pre-requisite for block quarrying through wave attack. Block quarrying was limited to samples at the seaward edge of the platform, where 7-11% of total sample volume was removed, indicating a dominance of marine processes in this region. The central and upper areas of the platform toward the cliff toe are dominated by weathering processes, due to extensive attenuation of wave energy across the platform surface, and longer periods exposed over a tidal cycle. The unique nature of this study presents an important contribution to the respective role of wave and weathering processes in platform downwearing. For the shore platform at Tarihunga Point the complex interplay between these processes is quantified experimentally, with erosion of samples accomplished through a combination of slow acting ongoing weathering and episodic wave events.