Distribution of landslips in the Wairarapa hill country

Abstract

Data gathered from airphoto and field surveys are statistically analysed to establish the distribution of landslips which occurred in the Wairarapa area, North Island, New Zealand, during the winter of 1977. The pattern of landslip occurrence is closely related to slope aspect and position on the hillslope. Slipping was concentrated on the upper parts of northerly facing slopes. The preference for northerly aspects is evident in each of the distributions of slips drawn separately from the upper, middle, and lower parts of the hillslopes. The 1350 km² area studied yielded a mean density of 0.98 slips/ha. For 1 catchment field measurements of 4.78 slips/ha and a surface lowering of 69 mm were recorded. In comparison with other recorded events the degree of erosion experienced was severe. Slope angle, slope hydrology, and rainfall distribution appear to have little influence on the distribution of landslips. However, much of the hillslope regolith had been affected by mass movement in the past and it was found that the amount of “undisturbed” regolith remaining prior to the landslip episode showed a positive correlation with the occurrence of mass movement during the event. It is concluded that the mantle of undisturbed regolith is less stable (particularly along its downslope edge) than slope material which has been subject to previous slipping. In the past, landslip-ping has been more effective on shady slopes and in this respect the distribution of landslips during the 1977 event (predominantely on sunny slopes) is a result of the unusual climatic conditions which prevailed and the distribution of slope material produced by previous events.