Estimating debris-fan compensation index and avulsion tendency using borehole-type data

Abstract

Debris fans are an important landform to human development in mountainous regions. While much debris-flow research has focused on recurrence intervals and volume predictions, little research has focused on predicting their paths. Understanding the paths of debris flows is important to reduce hazards to life and property. This research uses 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 is the tendency of flow events to preferentially fill topographic lows. A 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 time and difficulty to map the unit boundaries. Therefore a more readily available proxy for estimating the compensation index of debris fans was sought after in this study. Borehole data was chosen, which would also be similar to trenching data or naturally exposed small cross sections from stream erosion. The data from four previous compensation index studies was used to correlate trends in unit thickness to the compensation index based on 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 in both submarine and subaerial debris fans. However, fluvial channelized floodplain data revealed no correlation to compensation index. A limited number of vertical stratigraphic sections could possibly estimate the compensation index for thousands of meters across a debris-flow outcrop within about 0.2 of the actual compensation index. Further research is needed to corroborate these findings