Development of shore platforms on Kaikoura Peninsula, South Island, New Zealand (1997)
AuthorsStephenson, Wayne J.show all
Shore platforms on the Kaikoura Peninsula have been examined to determine the roles of marine and sub aerial weathering processes in platform evolution. Erosion was measured to assess rates of development and processes of erosion. Lowering rates on platforms are presented from two years of monitoring using a traversing micro-erosion meter. Cliff retreats were calculated using aerial photographic interpretation. Marine processes were investigated by using deep water wave data, by measuring waves on shore platforms and by analysing measured tidal data. Weathering processes were investigated using tidal data, climate data, the Schmidt Hammer test, and a laboratory experiment on wetting and drying. Lowering rates over two years ranged from 0.07 to19.80mm, and annual rates ranged from 0.154 to 9.194mm/yr. Rates of erosion varied with lithology and the type of platform. Erosion on Type A mudstone platforms was 1.98mm/yr; on Type B mudstone platforms erosion was 0.733mm/yr; and on limestone platforms it was 0.88mm/yr. The grand mean lowering rate for all shore platforms was 1.13mm/yr. These rates fall in the middle of the range of published rates from previous studies at Kaikoura and at locations around the world. For the first time, erosion data from a traversing micro-erosion meter were presented as volumes of material eroded. The total volume of rock eroded from study sites having, each with an area of 45.4cm2 , ranged from 1.20 to 92.50cm3. A significant finding was that rock surfaces swell up as indicated by a rise in surface level rather than lowering from erosion. The maximum measured swelling was 8.90mm. At some measurement sites as much as 90 per cent of measurements showed swelling over a period of 98 days. Values for erosion and swelling were higher during summer months. Both erosion and swelling were shown to be statistically related to season, suggesting that weathering is the group of processes causing both erosion and swelling. Summer provides better conditions for wetting and drying, which is thought to be the most important weathering process on shore platforms. Horizontal retreat rates were calculated over 52 years for cliffs, beaches and lagoon deposits backing shore platforms at Kaikoura, these ranged from 0.05 to 0.91m/yr. Investigation of marine processes showed that the deep water wave environment off the Kaikoura Peninsula is very energetic, but the amount of wave energy delivered to platforms is very low. A comparison of deep water wave energy flux with wave energy flux at the landward cliff of platforms, showed that there was a reduction by as much as five orders of magnitude. An analysis of the role of breaking waves revealed that these were ineffective as an erosional agent because the depth of water offshore causes breaking well before waves arrive on platform surfaces. Shear stresses and dynamic forces under waves were calculated from waves measured on shore platforms. This showed that these forces never exceeded the compressive strength the platform rocks at Kaikoura. It was concluded that wave forces are not directly capable of causing erosion. Evidence of weathering on shore platforms came from a number of distinctive surface morphologies on platforms: honeycombs, salt crystal growths, water layer weathering; and slaking. Schmidt Hammer test data showed: firstly, that weathering had occurred; and secondly, that rock strength was reduced through weathering by as much as 50 per cent. Weathering processes on shore platforms rely on repeated wetting and drying, and for this reason the number of wetting and drying cycles was estimated. The number of cycles ranged from 104 to 379 per year, the variation was due to tidal influences and the growth of algae during winter months. At elevations low in the tide range fewer cycles occurred; the greatest number occurring between the peaks of spring and neap tides, where rainfall adds to the number. Most cycles were estimated to occur between 0.6 and 0.9m above mean sea level on the more landward margins of platforms. It was at these elevations and locations that the highest rates of erosion were measured. Laboratory experiments on wetting and drying showed that only one cycle was needed to cause erosion. Waves were shown not to cause erosion, while sub aerial weathering does. Statistical analysis showed significant relationships between erosion, and wetting and drying and elevation. Based on these results it was concluded that the development of shore platforms at Kaikoura relies on weathering resulting from repeated wetting and drying. This is contrary to recent work which proposed that shore platforms result from marine erosion. Published mathematical models of shore platform development were found to be invalid at Kaikoura, because they were designed on the assumption that platforms are indeed wave cut features. This assumption is incorrect for shore platform development at Kaikoura. An empirical model is presented to explain platform evolution and the differences in platform morphology. A separation between platform types is presented based on the ability of weathering to cause erosion and on compressive strength. This is contrary to a published demarcation between types based on the erosive force of waves and on compressive strength. The type of equilibrium that platforms tend towards is considered. It is proposed that there are two ways to consider equilibrium. First, platforms may be lowered to an as-yet-unidentified elevation; this was viewed as being a static form of equilibrium. Secondly, platforms may continuously widen because weathering is an ongoing process. It was proposed that there is no equilibrium width for shore platforms.