A fluid dynamics approach to modelling the 18th March 2007 lahar at Mt. Ruapehu, New Zealand

Lahars are water-sediment mass flows from a volcanic source. They can be triggered by a variety of mechanisms and span a continuum of flow rheology and hydraulic properties, even within the same event. Lahars are extremely powerful landscaping agents and represent a considerable hazard potential. However, this highly dynamic character and a lack of direct measurements has made modelling lahars difficult. This study therefore applies a fluid dynamics model; Delft3D, to analyse the 18th March 2007 dam break lahar at Mount Ruapehu, New Zealand. The modelled lahar routed through the Whangaehu gorge in similar to 30 min, crossed the Whangaehu fan in similar to 60 min, and then over a further 3 h travelled an additional similar to 22 km distance along the Whangaehu River to the Tangiwai bridge. The modelled mean frontal velocity was 6.5 m s(-1) along the gorge although peak velocity reached up to 19.6 m s(-1). The modelled lahar flow front progressively slowed across the fan but along the River it accelerated from 2.1-3.3 m s(-1). Calculated peak velocity along the River was < 4.5 m s(-1). These results generally compare well with gauged records, with historical records, and with other modelling approaches. However, discrepancies in frontal velocity and time to peak stage arise due to (1) specifying roughness, which arises from slope variations between adjacent computational nodes, and which is stage-dependant, and (2) due to rapid topographic changes that produce frequent hydraulic jumps, which are inadequately accommodated in the numerical scheme. The overall pattern of discharge attenuation, and of relationships between topographic and hydraulic variables, is similar to that calculated for lahars on other volcanoes. This modelling method could be applied at other similar sites where a likely source hydrograph and high-resolution topographic data are available. These results have important implications for hazard management at Ruapehu and for examining geomorphic and sedimentary impacts of this lahar.