SLIDE (Wellington): geomorphological characterisation of the Wellington urban area

The goal of the Stability of Land In Dynamic Environments (SLIDE) research project is to improve the resilience of New Zealand’s buildings and infrastructure through better knowledge of the behaviour of slopes and the development of strategies for more robust remediation approaches. One aspect of the SLIDE research is the development of a geomorphology map for terrain within the Wellington urban area in order to help identify those slopes that have been anthropogenically modified and are subject to failure. Many hundreds of slope failures affect Wellington’s roading network each year, and most of these failures occur on slopes that have been modified by urban development. One of the SLIDE project’s aims is to better understand how anthropogenically modified slopes may perform in future strong earthquakes and in heavy rain events. Recent heavy rain events and earthquakes, especially the M7.8 14 November 2016 Kaikōura earthquake, have underlined how vulnerable some of Wellington’s slopes may be in such events. However, it should be noted that not all anthropogenically modified slopes are potentially unstable and likely to fail in future earthquake and/or rain events. This report presents Version 1.0 of the geomorphology maps and the methodology used to carry out the mapping. We have classified the geomorphology of the Wellington urban area (the study area), at a regional scale of nominally 1:500, into two main layers: 1) surface morphology and 2) near-surface materials. An additional third layer comprising anthropogenically modified ground, subdivided into: a) cut slopes and b) fill bodies, was also created. Multiple datasets were used to complete this work, including historical and recent aerial photographs, LiDAR-derived digital elevation models and photogrammetrically-derived digital surface models, which were subtracted from each other to create surface difference models. The interpretations were selectively ‘ground truthed’ using both field assessments and Google Earth ‘Street View’ imagery. The surface difference models and aerial photographs were successfully used to identify the primary areas of anthropogenically modified ground within the study area. The oldest suitable photo survey available was undertaken in 1938, and this covers the Wellington City to Miramar Peninsula area. The remainder of the study area from the city westwards and northwards is covered by a photographic survey undertaken in 1945. These surveys, along with other historical photographs, form the ‘baseline’ for our interpretations of landscape modification. These areas of modified ground were interpreted as being mainly due to slope cutting (negative surface changes) and the formation of fill bodies (positive surface changes) for the construction of buildings, roads and other infrastructure. The approximate date of construction of the mapped cut slopes and fill bodies has also been estimated, primarily using the multiple epochs of aerial photographs available for the study area. Our mapping is limited in time to the last available aerial photo dataset (2013), and modifications since this time are not captured in Version 1.0. Other anthropogenic features, such as retaining walls, were also identified. Although several thousand retaining walls were mapped, identifying all retaining walls within the study area from remotely sensed data, their type of construction and whether they were engineered or not, was not possible and was outside the scope of this research project. The mapping presented in this Version 1.0 of the geomorphology database is an interpretation that is designed to support general concepts of how anthropogenic landscapes differ from those of the natural environment. The digital geomorphological layers presented here have subsequently been used in the assessment of landslide hazards within the study area (e.g. Massey et al. 2019). (auth)