Defining the rainfall intensity- duration threshold to determine the initiation of shallow landslides in Tajan and Nekaroud rivers catchments (eastern Mazandaran province)

Rainfall induced landslides result in economical losses and casualties all over the world anualy. The main triggering factor of shallow landslides and debris flows in the rainy Province of Mazandaran (northern Iran) is the rainfall events. This study investigates the landslides triggered by rainfall throughout the watersheds of Tajan and Nekarud rivers. Twenty three recorded rain events occurred during the October 2012 to March 2013 in the eastern Mazandaran. Out of the 23 rainfall events during this 6-month period, 15 short to middle rainfall resulted in slope failures. Intensity-duration (I-D) of the rainfall events which induced landslides and debris flows were plotted in logarithmic scale. The threshold of I-D was determined for those rainfall events that induced landslides. This threshold is acquired by statistical analysis of quantile regression via plotting fifth percentile regression line to the lower bound of data of rainfall events that inducing landslides. The 5th percentile equation of the regression line is defined as I=0.51×D-0.60, where I is the intensity (mm/h) and D is the duration (h). Comparing this threshold with the global thresholds obtained through other researches indicates a lower local threshold in this area. The results shows that the rainfall events with shorter duration and higher intensity caused more landslides compare to those with longer duration and thus lower intensity. This might be due to the high drainage potential of the soil mass, and the highly fractured bedrock of marl and marly limestone.