Non-monotonic relationships between return periods of precipitation surface hazard intensity

Hazardous surface processes such as floods and mass movements are often induced by a common trigger such as extreme precipitation. The relationship between the intensity of the trigger and the surface hazard is generally assumed to be monotonically increasing (increasing precipitation never decreases hazard intensity). The validity of this assumption of complex multi-hazard events has not been thoroughly investigated. In this research, the relationship between cumulative precipitation and hazard intensity was investigated by a simulation of 50 return period precipitation events on the Carribean island Dominica. Here, several tropical hurricanes have induced events with (flash) floods, slope failure, debris flows and landslides within the past decades. Results show that complex multi-hazard interactions break the common assumption for the relationship between trigger and hazard intensity. In particular, landslide dam formation and mass movement dilution result in hazard intensities that are not a one-to-one increasing function of trigger intensity. Spatial variability in this behavior is quantified using a rank-order correlation coefficient between trigger return period and hazard return period. Since trigger and hazard return periods are, in the study case, not approximately equal, the hazard for a specific location can not be classified based on trigger return period. This has implications for risk calculation and decision making related to disaster risk reduction.