Debris-flow avulsion tendency estimated from boreholes or channel cuts

Abstract

Debris fans are an important landform for human habitation and development in mountainous regions. While much research in debris flow runout has focused on quantifying recurrence intervals and volume predictions, less research has focused on predicting their paths, which is important to reduce hazards to life and property. This project uses a quantitative stratigraphy tool, the compensation index, to evaluate the likelihood of debris path alteration by avulsion. The compensation index is a quantitative measure of the strength of compensational stacking, which measures the tendency of flow events to preferentially fill topographic lows. A significant constraint on the use of the compensation index for debris-flow hazard analysis is the need for large natural exposures of fan stratigraphy and the difficulty of mapping flow boundaries. Therefore, a more readily available proxy for estimating the compensation index of debris fans is needed, such as vertical stratigraphic sections from borehole data or natural channel incisions. Data from four previous studies was used to correlate trends in unit thickness to the compensation index calculated using data generated from simulated boreholes along the measured cross-sections. It was found that the central tendency and coefficient of variation of unit thickness had moderate correlation to the compensation index, and therefore predictive capacity, in both submarine and subaerial debris fans. By contrast, the same data for fluvial channelized floodplains revealed no correlation to compensation index. Therefore, a limited number of vertical stratigraphic sections could provide a reasonable estimate of the compensation index, and therefore avulsion tendency, of a debris-flow deposit.