OBJECTIVE DELINEATION OF LAHAR-INUNDATION HAZARD ZONES

A new method of delineating lahar hazard zones in valleys that head on volcano flanks provides a rapid, objective, reproducible alternative to traditional methods. The rationale for the method derives from scal ing analyses of generic lahar paths and statistical analyses of 27 lah ar paths documented at nine volcanoes. Together these analyses yield s emiempirical equations that predict inundated valley cross-sectional a reas (A) and planimetric areas (B) as functions of lahar volume (V). T he predictive equations (A = 0.05V(2/3) and B = 200V(2/3)) provide all information necessary to calculate and plot inundation limits on topo graphic maps. By using a range of prospective lahar volumes to evaluat e A and B, a range of inundation limits can be plotted for lahars of i ncreasing volume and decreasing probability. Resulting hazard maps sho w graphically that lahar-inundation potentials are highest near volcan oes and along valley thalwegs, and diminish gradually as distances fro m volcanoes and elevations above valley Boors increase. We automate ha zard-zone delineation by embedding the predictive equations in a geogr aphic information system (GIS) computer program that uses digital elev ation models of topography. Lahar hazard zones computed for Mount Rain ier, Washington, mimic: those constructed on the basis of intensive fi eld investigations. The computed hazard zones illustrate the potential ly widespread impact of large lahars, which on average inundate planim etric areas 20 times larger than those inundated by rock avalanches of comparable volume.