Rockfall Patterns and Recession of Coastal Cliffs on the North Shore of Auckland, NZ: A Terrestrial Laser Scanning and Terrestrial Digital Photogrammetry Approach

Abstract

Patterns of coastal cliff recession were measured at two sites composed of Waitemata Group rocks on the North Shore of Auckland, New Zealand. High resolution terrestrial laser scanning (TLS) and terrestrial digital photogrammetry (TDP) were used to investigate rock fall patterns and to quantify the geomechanical properties of each field site. Long Bay was scanned using TLS over a seven month period (12th June 2014 - 10th December 2014) while Rothesay Bay was scanned over an eleven month period (13th March 2014 - 7th January 2015). These datasets have been supplemented with data from historical laser surveys enabling medium term (≤ 13 years) recession patterns to be determined. Cliff recession rates in Auckland, ranging between 3.5-180mm y-1, have previously been determined using historical photographs, cliff-edge distances to structures, and using proxy data, such as shore platform widths. However, the research described in this thesis provides a much higher precision understanding of recession rates over scales of months to a decade (26± 7 mm y-1 recession). Change detection maps, cross-sectional profiles and volumetric statistics show a large contrast in rockfall patterns between the two field sites. Structural analyses confirm that these differences result from local geomechanical controls: weighted joint densities (1.31 and 2.92), maximum block volumes (81.7m3 and 13.07m3) and average rockfall volumes (0.1-0.2m3 and 0.0001-0.01 m3) show large differences between Rothesay Bay and Long Bay respectively. Long Bay change detection maps were characterised by sporadic, isolated block falls whereas Rothesay Bay maps showed spatial and temporal patterns, including upward rockfall propagation at the cliff toe. This upward propagation event combined with recent observations of wave transformation across the Rothesay Bay shore platform indicates that waves actively erode the cliff toe at the field site. Joint block fall has been confirmed through kinematic analysis to be the principal mechanism causing medium-term (≤ 13 years) erosion at both field sites. Upward propagation and joint block fall patterns were found to be facilitated by weak siltstone/mudstone units (UCS values of 136 KPa and 132 KPa) which, through their removal, induced cantilever failure of large jointed sandstone blocks. The two TLS datasets described in this study represent a substantial advance in the understanding of spatial and temporal patterns of rockfall for Waitemata Group rocks in Auckland, New Zealand. Both TDP and TLS datasets also highlight the advantage of high resolution datasets and the need to establish long term TLS monitoring datasets to enable accurate quantification and therefore effective management of coastal cliffs.