Analysis of the formation and failure of Ram Creek landslide dam, South Island, New Zealand

During the May 1968 Inangahua earthquake, a 2.8 x 10(6) m(3) landslide dam was formed in highly weathered granitic material in the headwaters of Ram Creek on the West Coast of the South Island, New Zealand. The landslide dam impounded a lake immediately upstream of the dam for c. 13 yr without breaching. The dam crest was breached on 29 April 1981 during an intense rainstorm, resulting in complete and catastrophic failure of the Ram Creek landslide dam. A dambreak flood caused substantial damage downstream. The longevity of the dam was probably due to its resistance to internal erosion, the small size of catchment, and the lack of high intensity/long duration rainfall in the catchment area. Three internationally derived geomorphic indices (Impoundment Index, Blockage Index, and Dimensionless Blockage Index) used to predict the stability (post-formation dam development) of landslide dams were applied to the Ram Creek landslide dam; all three indices correctly predicted the short term (<12 yr) development of the dam but failed to predict the catastrophic failure some 13 yr after formation. It is impossible to predict when a sufficiently intense event will cause overtopping failure of a landslide dam. It is conceivable that the Ram Creek dam could have remained intact for many more decades if the 1981 storm had been a little less intense, or centred a few kilometres farther away. Hence, the stability of this landslide dam was essentially a statistical concept, because the event against which it is stable is stochastic in nature.