Coastal Cliff Erosion Rates and Cliff Instability Mechanism at Cape Kidnappers, Hawke's Bay, New Zealand

Abstract

This study explored whether there is a relationship between annual cliff retreat rates and the occurrence of various cliff failure modes (slide, wedge and toppling failure) at Cape Kidnappers, Hawke's Bay. It used Digital Shoreline Analysis System (DSAS) to measure the decadal cliff retreat rates from historical aerial photographs (1988, 1996, 2011, 2015) and calculated the volumetric sediment loss that is supplied to the littoral beach sediment budget. Digital discontinuities mapping from unmanned aerial vehicle (UAV) imagery was used to identify and export the orientation of discontinuities, and kinematics analysis was used to analyse the tendency of occurrence of different types of failure modes based on variance of lithology. DSAS analyses revealed that the average spatial and temporal cliff retreat rates between 1988 and 2015 across the entire study area were 1.19 0.27m/y and 1.21 0.23 m/yr respectively. The spatial and temporal mean volumetric sediment loss between 1988 and 2015 were 67.9 32.1 m3/m/yr and 61.0 5.3 m3/m/yr respectively. An additional nding was that the standard deviation of cliff retreat rates was higher than in the cliff sections where massive conglomerate units and appeared (0.22 0.49), implying that the presence of conglomerates results in greater variability in cliff erosion. The results of kinematic analysis revealed that planar failure and wedge failure are more common failure mechanisms than toppling failure along the cliffs at Cape Kidnappers. Linear correlation analysis was used to investigate how the tendency of occurrence of different types of slope failures modes correlate with cliff retreat rates, which provides more indications of how much these different cliff failures modes affect the cliff recession. Potential relationship between the cliff retreat rates and different modes of failure were assessed for three main lithologic units. In massive conglomerates, planar sliding and wedge sliding are negatively associated with cliff retreat rates (r=-0.74, r=-0.91, respectively). In sandstone and mudstone units, planar sliding and wedge sliding are positively associated with cliff retreat rates (r=0.39, r=0.51, respectively). For the entire cliff, all lithologies, the occurrence of planar sliding and wedge sliding is positively related to increased cliff retreat rates (r=0.54 r=0.38, respectively). Overall, the occurrence of toppling failure are not associated with cliff retreat rates in any lithologic unit.