Estimating the risk of landsliding using historical extreme river flood data

The East Coast region in the North Island of New Zealand has a history of extreme river floods, often resulting from large rainfall of high intensity. Shallow translational landsliding on steep slopes underlain by Tertiary bedrock, and deeper earthflows and slumps on gentler slopes underlain by Cretaceous bedrock, have been associated with the most severe storms, particularly where indigenous forest was cleared for pasture. We propose that extreme river floods provide a surrogate, with direct causal linkage, for severe rain storms associated with landsliding. We employ a river's catchment area as a large areal rain gauge. Historical flood data can be analysed statistically to estimate likely frequencies of occurrence (eg Pearson, 1992). For land-use planning, we seek a probability statement to estimate the risk of slope instability in the river's catchment.