Examination of event magnitude, contributing drainage basin area, channel gradient, and rainfall influences on channel yield rates of post-fire debris flows

Development of methods for estimating volumes of post-fire debris-flow material along drainage networks requires a better understanding of the factors that control channel erosion and deposition within recently-burned drainage basins. The amount of material eroded and deposited by debris flows at locations along a channel can be quantified using the channel yield rate; the change in debris-flow volume per unit length of channel caused by passage of a debris flow. Here, we use channel yield rates measured in basins recently burned the 2009 Station fire in the San Gabriel Mountains of southern California to examine relationships between these rates and event magnitude, contributing drainage basin area, channel gradient, and rainfall characteristics. Following the Station fire and prior to any significant rainstorms, two to nine cross section surveys were established along the entire lengths of the main channels of three steep, rugged drainage basins. Surveys of the channel cross sections were made both before and after two distinct debris-flow triggering storms. These data were used to calculate post-fire debris-flow channel yield rates at 40 locations. Tipping-bucket rain gages installed near the surveyed channels provide rainfall amounts and intensities. Measurements of the amount of material removed from debris-retention basins located at the drainage basin outlets provide information on debris-flow volumes deposited at drainage basin outlets. High-resolution LiDAR data (1 meter) provide accurate elevation data for defining contributing drainage basin areas and channel gradients. The measured channel yield rates varied from 1 to 19 m³/m, with a mean of 4 m³/m and standard deviation of 5 m³/m. The greatest yield rates coincide with locations immediately downstream of field-mapped bedrock steps or channel junctions. The coincidence with bedrock steps suggests that in-channel "fire-hose" entrainment is a major contributor to debris-flow volume. High channel yield rates measured below channel junctions indicate the influence of increasing volume from tributary flow on entrainment rates, suggesting the influence of channel network densities on debris-flow volumes. Channel yield rates were greatest for the largest-volume debris flows and triggering storm rainfall amounts and occurred at the same locations for both debris-flow events. The lowest channel yield rates were measured along field-mapped bedrock-lined channels or channels with little stored material available for erosion. A correlation between channel yield rates normalized by contributing drainage basin area and channel slope indicates that debris-flow entrainment rates will be greatest at locations with large contributing areas and steep slopes. This preliminary work highlights the importance of considering sediment availability, drainage network form, contributing drainage basin area, and channel slope when developing models for predicting post-fire debris-flows volumes along drainage networks. Future investigations will examine the influence of burn severity on channel yield rates and work towards developing a predictive model for channel yield rates within recently burned drainage basins.