VOLUMETRIC CONTRIBUTION OF DRY RAVEL TO LARGE-SCALE DEBRIS FLOWS

Dry ravel is a hillslope sediment transport process by which individual particles of sediment move down a slope by rolling, bouncing, or sliding solely under the influence of gravity. This process leads to dry sediment loading of channels and has been recognized as a contributor to the initiation of post-fire debris flows in bedrock dominated landscapes in southern California. To test this, we approximated the volumetric contribution of dry ravel in a soil-mantled landscape to the Montecito debris flows that occurred approximately three weeks following the 2018 Thomas Fire in the Santa Ynez Range. We analyzed satellite imagery of the burned hillslopes after the fire but before the debris flow to investigate post-fire dry sediment transport. Maps and LiDAR scans of dry ravel deposits were used to develop a statistical model of the relationship between slope and dry ravel occurrence and to predict where raveling was likely to occur. These statistical analyses are being used to estimate the volumetric contribution of dry sediment transport processes to debris flows. Preliminary estimates suggest a maximum dry ravel volume to be 9% of the total debris flow volume. The contrast between these results from a colluvial landscape and those in bedrock dominated landscapes suggests the importance of dry ravel to the development of debris flows may depend on the extent, continuity, and texture of the soil mantle.