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Ph.D. Research Profile: Anna Taylor

Weathering and hydrological processes on shore platforms

This thesis examines morphological change and processes causing change on shore platforms formed in five different lithologies around New Zealand. Study platforms located at Kaikoura, Akaroa Harbour and Lake Waikaremoana are variously eroded into limestone, greywacke, basalt and two types of mudstone.

Rates and patterns of surface change are presented from three years of monitoring using a micro-erosion meter. Shore platform rock characteristics are described from observations of lithological features and with expressions of rock strength calculated by point load testing, Schmidt hammer testing, and a rock mass strength index. Marine processes were investigated by using deepwater wave data and direct measurement of wave induced flows on a shore platform. Weathering processes were assessed from observation of morphology and laboratory tests.

Annual rates of surface change ranged from 0.01mm.yr-1 to 12.82mm.yr-1. All marine rock surfaces measured showed dynamic changes with both surface swelling and lowering measured. Mean rates of surface lowering on the basalt were 0.29mm.yr-1, greywacke 0.78mm.yr-1, limestone 1.19mm.yr-1, Kaikoura mudstone 1.41mm.yr-1 and Lake Waikaremoana mudstone 9.13mm.yr-1. These rates are similar to published rates from previous shore platform studies and are of sufficiently great magnitudes that they can have formed the current shore platforms over the period that water levels have been at present relative levels.

Rock characteristics differed between platforms yet profiles displayed similar form and dimensions. Platform orientation is across lines of weakness, both bedrock dip and strike, on all rock types examined. Platforms are therefore wholly eroded and not related to lithology.

The three tests conducted to assess rock strength reflected strength of bedrock, surface rock and the body of rock as a whole. Rocks were classified as moderately strong (greywacke) through to very weak (Lake Waikaremoana mudstone). Comparison of elements of platform morphology and rate of surface change with rock characteristics showed no correlations with bedrock strength. Surface strength showed positive correlations to platform width and surface strength and rock mass strength showed negative correlations to gradient and elevation. There were no strong trends between rock characteristics and surface level change. These observations contradict concepts of rock strength control of morphology given in the literature.

Investigation of marine processes showed that all but the smallest waves break before reaching the seaward edge of the shore platforms studied. Simultaneous measures of wave parameters in deepwater and onshore showed reductions of up to 67% in wave height and over 90% in wave energy and wave energy flux. It is an important finding of this study that the highest energy deepwater waves do not necessarily deliver the highest energy to the shore platform. Therefore, it is not applicable to use deepwater wave parameters or calculated breaking wave height as indicators of onshore wave assailing force to characterise the wave environment at shore platforms.

Direct measurements of velocity fields across a shore platform are reported for the first time in the geomorphic literature. Flows of up to 2.54m.s-1 were recorded. Flow velocity was highest in the centre of the platform showing that wave force did not dissipate in a consistent way as waves flowed onto and across the platform. There were strong lateral components to flow and high levels of turbulence. Direct measurement of flow has enabled quantitative estimation of Clapotis, shock pressures, water hammer, hydrostatic pressure, shear stress, air compression, cavitation and abrasion for the first time on shore platforms. Air compression in rock cavities and abrasion are capable of erosion as independent mechanisms. Other mechanisms in combination may cause erosion at the micro scale.

Quantification of the competence and capacity of wave induced flow in sediment transport on shore platforms was undertaken. Removal of sediment from platform surfaces is an integral component of processes forming shore platforms and has not previously been quantified. It was shown that flow was competent to transport large sand at all locations across the platform while the flow measured had potential to move boulder-sized sediment. Net potential sediment transport was offshore and large blocks could be removed from the platform over the period of 2 to 3 tidal cycles. This differs from other coastal environments where net potential sediment movement can be in both on and offshore directions.

Morphological evidence of weathering processes included: honeycombs, salt crystallisation, rock disintegration, solution patterns and water layer weathering. Tests for susceptibility of each rock type to weathering mechanisms of wetting and drying and saturation showed that Lake Waikaremoana mudstone, Kaikoura mudstone and parts of the basalt were most susceptible to weathering by these mechanisms. Greywacke was least susceptible. Given the importance of wetting and drying suggested in the literature as a mode of shore platform development, it is an important finding that not all rocks in which shore platforms are formed are susceptible to this form of weathering. It was shown that susceptibility of rocks to weathering was not a control on shore platform morphology or erosion rates.

Through investigation of processes it has been shown that there are complex controls on the morphological development of shore platforms. These controls could not be explained using models of shore platform development currently available therefore a model of shore platform development has been presented in this thesis. It is based on a process - response model and provides a more universal framework within which to view shore platform development.