Rainfall data analysis at Newport and the relationship to landsliding in Pittwater

Bruce F. Walker

It is well recognised that the most common natural triggering factor for landslides is rainfall. A general relationship of more landslide events in wetter than average years was apparent from initial examination of the 195 landslide events on the database gathered as part of the National Disaster Mitigation Programme project to study the likelihood of landsliding in the Pittwater area. This paper reports the results of a more detailed analysis of rainfall data using daily rainfall and cumulative rolling totals from 2-day to 90-day periods. The resulting rainfall data was related to the landslide events on known dates which comprises only about 40% of the landslide database. No single pattern of results was available from the data. The chance of landslides occurring in Pittwater increases with higher 1-day rainfall. There is probably almost 100% chance of one or more landslides in the Pittwater area when the 1-day rainfall is 125mm or more. Days on which multiple landslides are likely to occur are often related to a maximum return period associated with 30 to 60 day antecedent rainfall. All the multiple landslide days are related to relatively long recurrence period rainfalls of about 20 years.