Developing a simplified geographical information system approach to dilute lahar modelling for rapid hazard assessment

Abstract

In this study, we present a geographical information system (GIS)-based approach to enable the estimation of lahar features important to rapid hazard assessment (including flow routes, velocities and travel times). Our method represents a simplified first stage in extending the utility of widely used existing GIS-based inundation models, such as LAHARZ, to provide estimates of flow speeds. LAHARZ is used to determine the spatial distribution of a lahar of constant volume, and for a given cell in a GIS grid, a single-direction flow routing technique incorporating the effect of surface roughness directs the flow according to steepest descent. The speed of flow passing through a cell is determined from coupling the flow depth, change in elevation and roughness using Manning’s formula, and in areas where there is little elevation difference, flow is routed to locally maximum increase in velocity. Application of this methodology to lahars on Montserrat, West Indies, yielded support for this GIS-based approach as a hazard assessment tool through tests on small volume (5,000–125,000 m3) dilute lahars (consistent with application of Manning’s law). Dominant flow paths were mapped, and for the first time in this study area, velocities (magnitudes and spatial distribution) and average travel times were estimated for a range of lahar volumes. Flow depth approximations were also made using (modified) LAHARZ, and these refined the input to Manning’s formula. Flow depths were verified within an order of magnitude by field observations, and velocity predictions were broadly consistent with proxy measurements and published data. Forecasts from this coupled method can operate on short to mid-term timescales for hazard management. The methodology has potential to provide a rapid preliminary hazard assessment in similar systems where data acquisition may be difficult.