Slope Erosion Due to Extreme Rainfall: A Case Study from New Zealand

Abstract

The characteristics of three high intensity rainstorms, and the landslides they induced, in the greywacke ranges of the South Auckland district, New Zealand, are described. The return period of storms of such magnitude that they cause landsliding in grass-land areas is about 30 years, and similar erosion in forested catchments is caused by storms with a return period of about 100 years. The landslides are in deeply weathered sandstones and siltstones: most landslides are of the debris avalanche type. Landslides occur preferentially on upper slope segments and as a result slope evolution produces an upper slope convexity. Denudation in areas of greatest storm damage produces an average of 40 to 80 mm of ground lowering, but 10 mm is more common: this gives an average denudation rate of about 0.25 to 1 mm/year from landsliding alone. In areas in which landsliding is the dominant hillslope process valley floors suffer considerable aggradation and the deposits produced by one storm may remain in the valley until the next one. Under such conditions the contention of Wolman and Miller (1960) that the dominant geomorphic events are those with a return period of 1 to 5 years are not applicable, and more catastrophic events of 30 years return period for grassland and 100 years for forest appear to be geomorphically dominant.