Quantifying lahar hazard and vulnerability

Lahars are a class of gravity driven mass flows containing a mixture of volcanic debris and water which can cause severe damage to exposed populations in their path. They are generated through a range of initiation mechanisms during both eruptive and quiescent phases of volcanic activity. Lahars also vary in composition from hyper-concentrated streamflowsto large, water saturated debris avalanches. Flow behaviour, volume and erosional processes differ as a consequence of this variability, affecting robust estimates of lahar hazard and destructive effects. Qualitative and semi-quantitative hazard estimates often rely on geologic and historic records to prescribe lahar volume, type and frequency. This is an important limitation on hazard estimates, as historic data is often incomplete and can be irrelevant under different environmental conditions or when eruptions alter topography and hydrology of volcanic slopes. Lahar risk assessment, which relies on a quantification of lahar hazard and damage, is therefore difficult to estimate. In this thesis, new approaches to quantify the hazard and vulnerability components of lahar risk are developed. For lahar hazard assessment, change points in volcanic eruption record completeness are systematically calculated, along with estimations of uncertainty in the date. The frequency and composition of rain-triggered lahars estimated through a lahar susceptibility model that accounts for the mechanics of lahar initiation. This susceptibility is combined with lahar flow models in a new methodology to determine lahar hazard. The primary cause of building losses is identified using detailed numerical modelling in urban areas. This quantification of vulnerability identified that exposure may have a larger effect on lahar risk than vulnerability. The approaches presented here, while limited at times by scarce input data, can be applied to better understand and quantify lahar risk.