Near-horizontal shore platforms display highly irregular plan shapes, but little is known about the way in which these irregularities influence the significant wave height (Hs) on the platforms and the frequency components of the nearshore wave field. We use a nonlinear Boussinesq wave model to conduct harmonic and bispectral mode decomposition analyses, studying the control of concave and convex platform edges over wind (WW: 0.125 - 0.33 Hz), swell (SW: 0.05 - 0.125 Hz) and infragravity (IG: 0.008 - 0.05 Hz) frequencies. For breaking and non-breaking waves, increasing the platform edge concavity intensified wave divergence and subsequent attenuation of SW and IG across the outer platforms, reducing by up to 25%. Increasing the platform edge convexity intensified focusing and amplification of SW and WW over the outer platforms, increasing by up to 18% and 55% for breaking and non-breaking waves. In the presence of breaking, IG amplification depended on the generation of wave divergence across the inner platform, a condition determined by a critical convex curvature threshold (Κ=1.8) balancing wave focusing from refraction and defocusing from breaking. We find that convex curvature can determine the relative dominance of WW, SW and IG across platforms. Alongshore, coherent wave interactions governed IG stationary patterns defined by a node near the platform centreline and two antinodes on either side of concave edges. A node was generated at the platform centreline, and two antinodes were observed on either side of the convex edges for Κ>1.8, with the opposite pattern observed for Κ<1.8.

Since 1973 micro-erosion meters (MEM) have been used at Kaikōura Peninsula to determine lowering rates on inter-tidal shore platforms. Rates measured over two, two year periods (1973-1975 and 1994-1996) and at decadal scales (20-30 years) demonstrate that platform surface lowering is on average 1.1 mm/yr. The 14 November 2016 Kaikōura magnitude 7.8 (Mw) earthquake caused an instantaneous uplift of 0.8-1.0 m of the peninsula. The uplift offers the rare opportunity to examine how such an event alters processes and rates of erosion on these shore platforms, since these are now partially marine terraces as the inner margins of some platforms are now above high tidal levels (but perhaps not storm surge). Since the earthquake, 42 MEM sites have been measured seven times at 3 monthly intervals. Most recently in September 2018. MEM sites show widely varying responses to the uplift. Erosion rates are at some MEM sites three times the previous annual rate while other sites show significant amounts of rock swelling (3-4 mm in 6 months), or aggradation as weathered rock fragments are no longer removed by wave action. The coseismic uplift has fundamentally changed the process regime operating on these platforms. Zones of maximum wetting and drying have migrated seaward causing previously slow eroding (< 1 mm/yr) MEM sites to accelerate to twice the pre-earthquake rates. Erosion rates demonstrate rapid adjustment of the platform surface to this disturbance and illustrate how uplifted marine terraces can be rapidly eroded despite being above sea level. The preservation of the new marine terrace is probably dependent on further uplift within the next 300-400 years, otherwise erosion by lowering and backwear will likely remove the new surface. This scenario has significant implications for marine terrace preservation and the recording of coseismic events in the landscape.