Computer simulation of shallow landsliding in New Zealand hill country

Abstract

Much of the North Island hill country in New Zealand was converted from forest to pasture in the late 19th century, leaving slopes susceptible to landsliding during heavy rainstorms. Excessive soil loss has called into question the sustainability of pastoral farming, and increased sedimentation has caused problems downstream, such as increased flood risk and degraded fish habitats. A catchment-wide model of landslide erosion and sediment delivery to streams is required to help plan soil conservation and mitigate offsite effects. We describe a computer simulation model of shallow landslides and associated sediment delivery to streams, based on a high-resolution digital terrain model (DTM). The probability of landsliding during a storm is determined for each DTM pixel from GIS (geographic information system) layers of land systems, rainfall, vegetation/land cover, and slope. Where landslides occur, they are routed down the hillside, leaving a trail of sediment about 20 cm thick, on average, until they either reach a stream or become exhausted. Sediment delivery ratios can be calculated for any area. We demonstrate the model by assessing the effects of two afforestation scenarios on soil loss and sediment delivery for a simulated heavy storm in the Waipaoa catchment (220 x 103 ha), using a DTM of 3200 x 3600 25m pixels.