Morphology and incidence of landslides in the Kaimai Range, North Island, New Zealand

Abstract

Studies of aerial photographs show clear evidence of increased erosion, since 1943, concentrated at differing altitudes in different parts of the Kaimai Range, North Island, New Zealand. In the Te Rere area, the increase was greatest between 1943 and 1960, but in the other 2 areas (Mt Te Aroha and Te Hunga) the increase was greatest between 1960 and 1974. There is also a high normal erosion frequency at Te Hunga due to steep terrain. Landslides do not appear to be concentrated on slopes of any particular aspect, slope, soil type, or basement geology, but may be linked with areas of poor vegetation. A sample of landslides in each area was visited and mapped in detail. Measurements demonstrated a marked difference between upland and lowland landslides. At the upland sites, landslides occurred on slopes as low as 15°, and on slopes of over 35° bedrock was usually exposed, even in the track. Presence of allophane at the failure plane, and unhealthy vegetation at the landslide sites, markedly reduces slope stability and causes a lower natural angle of repose. At lowland sites, most landslides occurred on slopes steeper than 40° and bedrock was less frequently exposed. Examination of scars suggests that the landslides occurred under saturated soil conditions during a storm. In the scar, bedrock is usually exposed, minimising the risk of further erosion; but in the track, deep soils and slow revegetation may increase the risk of further erosion, particularly if browsing mammals are present.