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The youngest uplifted marine terrace at Kahutara Point on Māhia Peninsula, New Zealand, is undergoing rapid retreat (backwear) despite being fronted by a wide contemporary shore platform that attenuates wave energy. In this paper, wave energy and water level were recorded across the shore platform, and these data were used to model (a) how frequently waves reach the terrace riser and potentially erode it and (b) how frequently waves overtop the terrace. Analyses of wave data across shore normal transects show that under quiescent and storm conditions >90% of the energy delivered to the back beach and terrace riser is at infragravity frequencies (i.e., <0.05 Hz). Significant wave heights are reduced in a landward direction for both gravity (Hm0H) and infragravity (Hm0L) wave frequencies, with 50%–80% of Hm0H and 20%–50% of Hm0L reduced between seaward and landward sensors. Wave energy during quiescent conditions is strongly attenuated at the seaward margin whereas under storm conditions, proportionally more energy is delivered to the marine terrace riser. The development of a simple inundation model at Māhia reveals that the northern flank of Kahutara Point is more vulnerable to wave inundation, with 17 storms overtopping the youngest marine terrace between 1980 and 2020. Furthermore, despite there being no evidence of a 1‐in‐100 year storm event occurring at Māhia since 1980, there has been an increase in storminess since 2013. Changes in storm frequency may have offset decreases in wave energy (from energy attenuation) with increased platform width associated with marine terrace retreat. [ABSTRACT FROM AUTHOR]

Flights of Holocene marine terraces are useful for reconstructing past earthquakes, but coastal erosion can remove terraces from the landscape, potentially leading to incorrect estimates of earthquake magnitude and frequency. Relatively little effort has been afforded to studying terrace erosion processes, and this paper presents the first field evidence that we are aware of documenting terrace erosion rates. Two case studies from New Zealand provide a unique opportunity to observe the beginning and end phases of terrace development. We present downwear and backwear erosion measurements, showing that both sets of processes are important. Micro‐erosion meter measurements from Kaikōura Peninsula, South Island, confirm that downwear processes are modifying new marine terraces that were created when the peninsula was uplifted about 1 m during the 2016 earthquake. Erosion rates were high immediately following uplift as the relatively barren intertidal rock shore platform rapidly transformed into an incipient marine terrace with cover deposits. However, the Kaikōura earthquake uplifted shore platforms only a small distance above the upper tidal limit and ongoing downwear and backwear erosion may begin to remove parts of this terrace in future decades. We explored this prospect with a case study at Māhia Peninsula, North Island, where 100–300 years have elapsed since the last terrace‐forming earthquake. Historical photographs were used to document about 80 years of backwear erosion. Terrace erosion rates have been nearly constant through this period, and extrapolation implies that the terrace will be removed in places by 2030. The erosion data in this paper provide new insights into how terraces can be removed from the landscape, but there are many complicating factors. To help understand these factors we present a new conceptual model of marine terrace creation and destruction for soft‐rock coasts. [ABSTRACT FROM AUTHOR]

There is currently little understanding regarding storm surge in New Zealand, particularly in the southern region. To date, there has been no attempt to establish the frequency and magnitude of storm surge using observed sea level data on the open coast. The aim of this study is to examine the nature of storm surge in southern New Zealand and explore the influence of the Southern Annular Mode (SAM) and the El Nino Southern Oscillation (ENSO) on the frequency and magnitude of storm surge. Bayesian estimation is used through the 'extRemes' package in R to determine the return period for storm surge at three locations around southern New Zealand. The 100-year return level varied between 0.91 and 1.31 m. In addition, a statistically significant increase (2.88%) in storm surge intensity (SSI) on the south-east coastline was found. Applying cross-correlation to assess the response of SSI to ENSO and SAM, returned no detectable relationship, except a weak relationship between SAM and SSI on the south-east coast where a two-three month lagged response was present. [ABSTRACT FROM AUTHOR]

We describe the immediate impact of the 14 November 2016 Kaikōura magnitude 7.8 (Mw) earthquake on shore platforms and cliffs around Kaikōura Peninsula. The earthquake caused an instantaneous uplift of ~1.01 m of the peninsula. We resurveyed seven profiles previously used for erosion monitoring and observed changes in the configuration of the shoreline. The coseismic uplift has fundamentally changed the process regime operating on the platforms and altered the future trajectory of shore platform and cliff development. Our observations highlight the interplay of waves, weathering, biology and tectonics. At this location tectonism strongly modulates the process regime, driving instantaneous changes in morphology and altering rates and patterns of erosion. Finally, the uplift of the Kaikōura coast has implications for changing resilience to climate change and sea level rise. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Investigations comparing surface downwearing rates derived from photogrammetry data and the micro-erosion meters (MEM) are limited due to the former's coarse spatial resolution and the temporal scale of data collection. These space and time limitations were revisited with the possibility of simultaneously using the Structure-from-Motion (SfM) photogrammetric workflow and the MEM. Using a cross-scale nested survey approach over one year to cover micro (10−2 m2) and meso (100 m2)-spatial scales, this paper presents erosion rates on uplifted intertidal shore platforms and marine terraces at Kaikōura Peninsula, New Zealand. The survey represents one of the first investigations combining the MEM and the SfM technique to estimate erosion rates on horizontal coastal bedrock. In a unique way, we applied established methods which allowed comparative accuracy between models via similar point matching to provide a detailed representation and visualisation of the changing rock surfaces at similar spatial and temporal scales. Over one year, across a microscale area (~0.01 m2) of rock, the MEM and SfM point clouds (SfM PCs) measured the lowest and highest erosion rates on a similar rock type and erosion site. The mean annual erosion rate of 2.244 mm/yr from the MEM was lower than the rates of 2.608 mm/yr and 10.299 mm/yr estimated using the SfM micro- and mesoscale PCs, respectively. A lower range of erosion rates (0.131 to 4.750 mm/yr) was measured using the MEM compared to erosion rates estimated from the SfM microscale PCs (0.194 to 10.106 mm/yr). Across the mesoscale (1 m2) areas, higher erosion rates were recorded using the SfM mesoscale PCs on all but one erosion site showing that a significant amount of erosion was undetected by the MEM. Using the SfM-orthomosaics, we link the erosion rates measured across the micro and mesoscale areas to intense granular disintegration, flaking, micro- and polygonal cracking and biological activities. These results provide statistical evidence to argue against extrapolating MEM erosion data obtained over small areas to larger areas on shore platforms affected by recent tectonic uplift. The implications of these findings are considered within the context of techniques for measuring shore platform erosion across spatial and temporal scales. • Erosion rates were acquired at similar spatial and temporal scales using MEM and SfM • A significant amount of shore platform erosion was undetected by the MEM • SfM PC models provided higher mean erosion rates for the micro (~0.01 m2) and mesoscale areas (1 m2) • Statistically similar mean erosion rates across microscale areas support substituting techniques. • MEM efficacy at capturing maximum erosion across microscale areas on the Kaikōura shore platforms has diminished [ABSTRACT FROM AUTHOR]

Shore platforms fronting cliffs have highly irregular planform configurations that could modify the alongshore distribution of wave energy, thereby influencing cliff erosion. However, research aiming to understand the geomorphological control of platform morphology on two-dimensional wave transformation remains scarce. We conducted detailed wave measurements within the intertidal zone of three commonly observed mesoscale planform geometries (straight, concave and convex seaward edges). Waves were measured using a phased array of pressure transducers near the seaward edge of three near-horizontal shore platforms in New Zealand. Waves were analysed at infragravity (IG: 0.002–0.05 Hz) and swell frequencies (SW: 0.05–0.125 Hz), with wave energy derived from the zeroth moment wave height, directional wave spectra derived from the Extension of the Maximum Entropy Method and phase lag derived from the cross-spectrum characteristics. Results showed that wave translation occurred on the straight edge platform, divergence on the concave platform, and convergence on the convex platform. These patterns generated areas of wave ray convergence and divergence over the platforms. Wave energy convergence coincided with increased IG energy and reduced SW energy decay, whereas divergence had the opposite effects. Wave reflection generated cross-shore standing IG waves, for which nodal states varied alongshore with platform width. These phenomena resulted in alongshore variations in normalised wave energy E ̂ over the three platforms, which were more pronounced for IG than for SW. Compared to the straight platform, alongshore variations of IG were twice as large on the concave platform and three times larger on the convex platform. This study shows that planform morphology impacts along-platform wave energy distribution. These effects likely influence planform rock coast evolution and should be considered in future numerical modelling studies. • Directional wave patterns over platforms are controlled by planform morphology. • Refraction/reflection influence wave energy patterns of near-horizontal platforms. • SW and IG energy patterns are affected by wave ray convergence and divergence. • Wave energy patterns are affected by variations in the nodal state of standing IG. [ABSTRACT FROM AUTHOR]

On tectonically active coasts, there is a dearth of erosion data documenting how rock coasts adjust (either fast or slow) in response to marine and subaerial processes immediately after coseismic uplift. Here we report erosion rates and evidence of reshaping of shore platform morphology at Kaikōura Peninsula, South Island, New Zealand, following the November 2016 Kaikōura 7.8 (Mw) earthquake, that uplifted platforms by ~1 m and extended their widths. High-resolution topographic data obtained from quarterly surveys over four years using a micro-erosion meter (MEM) and Structure-from-Motion Multi-View Stereo (SfM-MVS) surveys have provided rates of erosion and visual representation of surface morphologies. MEM data revealed variations in erosion, weathering and deposition rates across lithology, tidal positions, and platform elevation after the uplift. After four years, surface downwearing rates for all platforms were on average 2.25 mm/yr, a 104% increase from the pre-uplift rate of 1.10 mm/yr. Average lowering rates on limestone, hard, and soft mudstone platforms were 1.19 mm/yr, 2.31 mm/yr, and 3.21 mm/yr, respectively. Results show a change in patterns of erosion rates and faster rates on mudstone than limestone lithologies, but both faster when compared to pre-uplift rates. Previously reported seasonal trends in erosion rates have disappeared because post-uplift erosion rates are similar during the summer and winter. In contrast, statistically significant differences in lowering rates now exist across rock lithology and elevation. On one MEM site on the harder mudstone platform, a large increase from a pre-uplift erosion rate of 0.43 mm/yr to 19.23 mm/yr (1-year after uplift) and subsequent decline to 1.54 mm/yr after four years demonstrates the role of block detachment not previously seen in the MEM data from Kaikōura. For the first time, we complement MEM data with SfM-MVS-derived orthomosaics to provide evidence of changing rock morphology and processes such as granular disintegration, flaking and algal growth. Results from this study demonstrate how tectonism can fundamentally alter the way in which rock shore erosion rates are influenced by waves, tides, and weathering processes. • Tectonic uplift caused a resetting of erosion rate. • Erosion rates increased post-uplift. • Faster breakdown recorded on mudstones. [ABSTRACT FROM AUTHOR]

The involvement of surfers in surf break co-management initiatives may assist local authorities in providing for surf breaks and develop community stewardship. This research involved a comparative case study of the Auckland and Otago regions in New Zealand to assess the potential, and identify key ingredients, for successful surf break co-management. An online survey of surf break users (n = 181) and qualitative interviews (n = 16) considered the potential voluntary involvement of surfers, the role of local authorities and types of approach to surf break co-management. Findings indicate a level of support from a proportion of surfers to be involved and stress the need for local authorities to provide a supporting policy response. A local-scale co-management approach, similar to National Surfing Reserves in New South Wales, Australia, is considered more likely to produce positive outcomes compared to the involvement of surf break interests in coastal-wide co-management approaches. [ABSTRACT FROM AUTHOR]

Abstract: The interactions between waves and morphology on shore platforms were investigated at four platforms located at Kaikoura Peninsula, New Zealand and the Otway coast, Victoria, Australia. Waves were measured over six tidal peaks of a spring tidal sequence at each site during a six month period (February 2007 – July 2007). This paper reports the results of an investigation into the research question: how are waves modified by platform morphology in a micro-tidal setting? Through the investigation of wave interactions on shore platforms it was found that complex hydrodynamic patterns formed on the shore platforms studied. Through direct measurement of waves on the platforms it was found that wave attenuation was not dependent on platform width, but rather influenced by platform gradient and depth of water on the platform. Spectral analysis was used to assess how wave energy was transformed across the platforms. At three of the four platforms investigated, energy gradients formed across the platform with wave energy recorded within the infragravity range. [Copyright &y& Elsevier]

Using micro-erosion meters (MEM) and traversing micro-erosion meters (TMEM), surface lowering rates of shore platforms on Kaikoura Peninsula, South Island, New Zealand have been measured over a total of 43 years. This record is the longest monitored network of this type. Since 1973, erosion rates have been calculated over two, two year periods 1973-1975, (n=31) and 1993-1996, (n=52) and at decadal scales; 1973-1993 (n=15), 1973-2003 (n= 12), 1973-2016 (n=7), 1993-2003 (n=50), 1993-2008 (n=34),, 1993-2016, (n=18). After 43 years, surface lowering rates remain similar to previously published rates at an average of 0.525 to 1.181 mm/yr. Statistical analysis shows that erosion rates over all measurement periods are derived from the same population. Thus short term rates derived from two years of monitoring remain indicative of decadal rates of erosion. Variations between measurement periods are best explained by the loss of the more rapidly eroding bolt sites. These losses point to the difficulty of maintaining monitoring over longer time scales. In November 2016 a Mw 7.8 earthquake raised the Kaikoura Peninsula approximately 0.8- 1 m, elevating much of the shore platforms above the tide range. This earthquake has reset the MEM record ending this long running erosion monitoring site of shore platform surface lowering. However the MEM network is now being used to monitor post-earthquake response of the uplift shore platforms and new marine terraces. [ABSTRACT FROM AUTHOR]