An eye on coastal change: Characterising spatio-temporal coastal sensitivity at high-resolution - Okia Reserve, Otago Peninsula, New Zealand

Abstract

Natural coastal systems exhibit dynamic behaviour and are sensitive to a range of forces. This sensitivity is projected to change in the context of climate change. Advanced coastal monitoring techniques can detect and characterise modifications in the coastal environment in unprecedented spatial detail, across a relatively wide spatial area. Repeated use of high spatial resolution remote sensing technologies such as Remotely Piloted Aircraft Systems (RPAS) and Light Detection and Ranging (LiDAR) allowed a spatio-temporal signal of the evolution of both the vegetation and geomorphology at the 231 ha Okia Reserve (Otago Peninsula, Dunedin) to be measured across short (14 months) and long (12.5 years) temporal scales. The Okia Reserve is an important ecological site with rare flora and fauna, including breeding habitats for endangered Yellow-eyed Penguins (Megadyptes antipodes) and New Zealand Sea-lions (Phocarctos hookeri). It also contains a nationally significant archaeological site along the southern edge of the site. This research mapped changes between September 2004 and March 2017, and between March 2017 and May 2018. Three-dimensional change detection techniques are used to quantify significant spatiotemporal changes over each period. These changes supported a fine-scale assessment of the sensitivity of both the vegetation and geomorphology at Okia Reserve, as well as a projection of potential effects of sea-level rise. The short-term sensitivity assessment showed significant inlet/vegetation edge and volumetric erosion around the southern edge of the reserve at Papanui Inlet, much of it focused on a nationally significant archaeological site. Temporal comparison with the long-term results confirmed significant unabated inlet/vegetation edge and volumetric erosion at Papanui Inlet, in the order of 0.8 – 1 m/yr. Analysis of the projected impact of coastal inundation and flooding further emphasised the impending exposure of the nationally significant archaeological site, with the southern end of the reserve likely to be impacted by direct coastal inundation as early as 2050. Results in other areas such as Papanui Inlet Spit and Victory Beach suggest a state of dynamic equilibrium with accumulation, erosion and likely redistribution of sediment. Victory Beach itself displayed a similar magnitude of nett volumetric change (approx. -10000 m3) over both the 14-month and 12.5-year time periods. Both periods also displayed a sustained growth rate in vegetated areas. Observed wetlands and flooding indicated further dynamic behaviour in the coastal system as a response to seasonal variability and storm events. High-resolution quantification of anthropogenic interactions within the reserve accurately highlighted the magnitude and extent of localised changes to vegetation and topography as a result of some of these activities (e.g. vehicle access track maintenance and new tracks). The combination of accurate high-resolution topography, analysis of tidal levels and the effect of four sea-level rise scenarios showed the southern end of the reserve is particularly at risk to direct coastal inundation and indirect flooding with the potential of significant modification to natural habitats and endangered wildlife breeding sites, as well as impedance to vehicle and pedestrian access.