The geology, soils, topography and climate in some parts of Marlborough combine to create the potential for land instability. The two most obvious examples are land in the Marlborough Sounds and hill country along the southern margin of the Wairau Plain. In the Sounds, much of the geology is fractured schist with limited topsoil. This type of land is naturally unstable where it occurs on steep slopes and especially in times of intense rainfall.
Past slips and areas susceptible to instability have already been mapped and are included in the Marlborough Sounds Resource Management Plan. Similar to flood hazard areas a cautious approach to development in areas of instability is also taken: a resource consent is required to construct buildings to assess the level of risk. Council also requires that all engineering work related to the construction of a building has to be carried out by a geo-technical engineer accredited by the Council.
One of the difficulties with this approach has been determining what is an acceptable level of risk. There are also probably areas that have a similar geology and topography to those that are mapped, and to which arguably a similar level of risk could apply, but development in these areas is not subject to the same level of scrutiny.
The soils on the hill country to the south of the Wairau Plain also create a land instability hazard, but for quite different reasons. The dominant material in these hills is loessial soils, which are susceptible to erosion. When wet they tend to dissolve away through extensive sub-surface tunnels. In severe cases, the tunnels cave in to form gully erosion. Unlike the Sounds, no specific provision is given to the potential for instability in the Wairau/Awatere Resource Management Plan when considering development on these hills.