HR: 0830h
AN: V51F-0344    [PDF]
TI: Simulations of Lahar Propagation Over Variable Topography
AU: * Fagents, S A
EM: fagents@hawaii.edu
AF: University of Hawaii, HIGP/SOEST, 2525 Correa Rd, Honolulu, HI 96822 United States
AU: Baloga, S M
EM: steve@proxemy.com
AF: Proxemy Research, 14300 Gallant Fox Ln, Ste 225, Bowie, MD 20716 United States
AB: Volcanic debris flows, or lahars, form readily through combination of friable volcanic material with water derived from rainfall, snowmelt, or crater lake release. Lahars are capable of traveling tens of km at velocities of tens of m/s, with depths of 1-10 m. Their highly destructive nature thus demands accurate assessment of flow propagation for hazard mitigation purposes. We have developed a method of applying lahar flow models to topography in digital format (e.g., DEMs), with the goal of improving predictive models. We treat changes in elevation between pairs of adjacent points as a series of N inclined planes having different slopes. Changes in slope from one inclined plane to the next produce discontinuities in flow depth and velocity, such that flow rate boundary conditions must be established repeatedly at the interface between each interval for all timesteps. To demonstrate our approach, we use a time-dependent flow model based on global and local volume conservation equations for a flow whose bulk properties are encapsulated by the dimensionless coefficient, C, the resistance to flow. Flow depth and velocity are solved in each interval, while simultaneously requiring that the sum of each incremental volume matches the total flow volume. We thus compute the time required for a flow front to travel from its source to a given point along the flow path. We derive mathematical conditions on topography and flow parameters that differentiate between gravity- and momentum-driven regimes. Using long-runout lahars from Mt. Ruapehu as examples, we varied the number of increments into which an analog representation of Ruapehu's concave-upward topography was divided, in order to assess the influence of topographic resolution on transit time predictions. We find that as N increases (i.e. as the topography is represented more accurately), predicted transit time decreases asymptotically. This is a result of the nonlinearity of the problem: high velocities over steep upper slopes more than compensate for lower velocities on low, distal slopes. We find that 250 m horizontal resolution is required for computed transit times to converge to an accurate solution. Use of coarser topography will lead to significant overestimates in transit times, leading to erroneous hazard assessments for downstream communities.
DE: 3210 Modeling
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting