INUNDATION-AREA STATISTICS AND MOBILITY EQUATIONS FOR DEBRIS FLOWS AND ROCK AVALANCHES

Inundation-area statistics have been used previously to establish equations that predict the maximum valley cross-sectional areas (A) and the total planimetric areas (B) inundated by lahars with various lahar volumes (V). The predictive equations obey 2/3 power laws and have the scale-invariant forms A=a₁V^2/3 and B=a₂V^2/3, where the a coefficients are calibrated using a database of lahar geometries around the world ¹. Here, a new database is used to show that similar 2/3 power law equations apply to non-volcanic debris flows and rock avalanches. With recalibration of a coefficients, two new sets of equations are established to predict the cross-sectional and planimetric areas inundated by these types of rapid mass movements. These new equations provide an objective methodology for discriminating the mobilities of lahars, non-volcanic debris flows, and rock avalanches on the basis of a coefficients. Moreover, the mobility equations can be embedded in a GIS-based computer program, LAHARZ ¹ , to generate hazard maps that depict inundation limits for debris flows and avalanches with a range of hypothetical volumes. In depicting hazards from comonplace flows or avalanches with volumes less than about one million cubic meters, satisfactory implemetation of LAHARZ requires high-accuracy, high-resolution digital topographic data. Such data are not widely available at present but will likely become more available in the future.

¹ Iverson, R.M., Schilling, S.P., Vallance, J.W., 1998, Objective delineation of lahar-inundation hazard zones, GSA Bulletin, v.110, no.8, 972-984.