AN EMPIRICAL-METHOD FOR ESTIMATING TRAVEL-TIMES FOR WET VOLCANIC MASSFLOWS

Travel times for wet volcanic mass flows (debris avalanches and lahars ) can be forecast as a function of distance from source when the appro ximate flow rate (peak discharge near the source) can be estimated bef orehand. The near-source flow rate is primarily a function of initial flow volume, which should be possible to estimate to an order of magni tude on the basis of geologic, geomorphic, and hydrologic factors at a particular volcano. Least-squares best fits to plots of flow-front tr avel time as a function of distance from source provide predictive sec ond-degree polynomial equations with high coefficients of determinatio n for four broad size classes of flow based on near-source flow rate: extremely large flows (>1 000 000 m(3)/s), very large flows (10 000-1 000 000 m(3)/s), large flows (1000-10 000 m(3)/s), and moderate flows (100-1000 m(3)/s). A strong nonlinear correlation that exists between initial total flow volume and flow rate for ''instantaneously'' genera ted debris flows can be used to estimate near-source flow rates in adv ance. Differences in geomorphic controlling factors among different fl ows in the data sets have relatively little effect on the strong nonli near correlations between travel time and distance from source. Differ ences in flow type may be important, especially for extremely large fl ows, but this could not be evaluated here. At a given distance away fr om a volcano, travel times can vary by approximately an order of magni tude depending on flow rate. The method can provide emergency-manageme nt officials a means for estimating time windows for evacuation of com munities located in hazard zones downstream from potentially hazardous volcanoes.