Development of shore platforms on Kaikoura Peninsula, South Island, New Zealand

Abstract

Shore platforms on the Kaikoura Peninsula have been examined to determine the roles of marine and subaerial 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 to 19.80mm, 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 fITst 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 45.4cm 2 , ranged from 1.20 to 92.50cm 3 • 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 0.91mJyr.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 ii 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. TIus showed that these forces never exceeded the compressive strength the platform rocks at Kaikoura. It was co...

Cool barnacles: Do common biogenic structures enhance or retard rates of deterioration of intertidal rocks and concrete?

“…More broadly, on rocky platforms the thermal and hygric influences of encrusting species may mediate episodic and rapid (block-scale) erosion (e.g., Naylor et al, 2012) and alter rates of topographic evolution via subaerial weathering (e.g., Moura et al, 2012;Stephenson and Kirk, 2000). As well as further work on barnacles, more evidence for the roles of other common encrusting species (such as mussels and …”

Lithological control on the elevation of shore platforms in a microtidal setting

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Late Holocene seacliff retreat recorded by 10Be profiles across a coastal platform: Theory and example from the English Channel

“…On rocky coasts, and particularly on chalky coasts, cliffs may retreat as fast as 20 cm/yr, while the shore platform undergoes downwearing (e.g., Stephenson and Kirk, 2000a;Stephenson and Kirk, 2000b;Trenhaile and Byrne, 1986). The processes responsible include: chemical weathering during tide water level fluctuations (Duperret et al, 2005;Kanyaya and Trenhaile, 2005;Stephenson and Kirk, 2000a;Sunamura, 1992), biological action (e.g., Nesteroff and Mélières, 1967), and the mechanical action of waves (e.g., de Lange and Moon, 2005;Stephenson and Kirk, 2000b).…”

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