Topographic controls on evolution of shallow landslides in pastoral Wairarapa, New Zealand, 1979-2003.

Abstract This study aims to identify topographic influence on shallow landslides and their evolution during 1979–2003 in the pastoral lower North Island, New Zealand. Landslide-affected sites were mapped from aerial photographs and an ASTER satellite image. The two landslide maps were overlaid in a geographic information system (ArcGIS) to detect the changes that have taken place in landslide-affected areas. The change map and the landslide distribution maps were overlaid with three topographic layers of slope gradient, slope position, and slope aspect that had been derived from a digital elevation model in ArcGIS. The overlay analysis revealed that landslides-affected area increased from 23.75 ha in 1979 to 110.97 ha in 2003. The distribution of landslide-affected sites is inversely correlated with slope gradient, but positively with slope position. Northern aspects are the most vulnerable to landsliding, containing about 78% of the total affected sites. Moderate slope gradients suffered the most severe landsliding in the lower to middle slopes. The overall magnitude and frequency of landsliding is also related to northern and eastern octants on the moderate slope gradients in the low to moderate slope positions. Although the same topographic setting is conducive to landsliding in both 1979 and 2003, the rate of landsliding varies with topographic setting. Hence, topography is critical not only to landsliding but also to the evolution of sites already affected by landslides. Such an understanding over large spatial scales is possible only with the use of multitemporal remote sensing data and GIS that are able to speed up data collection and analysis.