COMPENSATIONAL BEHAVIOR OF DEBRIS FLOW FANS IN SOUTHERN COLORADO

Cross-fan sections were mapped in detail for three debris fans in Colorado to quantify sediment transport and avulsion tendencies. Specifically, the tendency of debris flows to avulse out of an active channel and endanger structures located elsewhere on the fan is a critical aspect of hazard assessment and mitigation. Mapped exposures are typically located along fan margins and are mapped and sampled to identify the various depositional facies present, their extent, and their stacking relative to one another in order to quantify the level of compensation present in each debris-flow system. Compensation refers to the tendency of episodic flow events to preferentially fill topographic lows rather than frequently avulsing into new channels. A quantitative system developed by Straub and Pyles (2012) allows for the calculation of a modified compensation index which ranges from 0.0, which reflects anticompensational tendencies, to 0.5, which corresponds to random behavior, to 1.0, which represents perfectly compensational stacking patterns. Analysis of several sites will show how various inherent fan characteristics, such as its geologic setting, ranges in grain size, flow size, and fan topography, interact to impact compensation. Sites were chosen on the basis of their unique geographic locations and geologic settings (Woodland Park, Poncha Springs and Cedaredge, Colorado), clarity of stratigraphy, relative portion of exposed strata compared to the fan as a whole, number of events visible in the exposure, ease of access, and general safety. Each site displays numerous discernible periods of deposition mapped in several unique facies categories including cross-bedded stream flows; loose, clast-supported outwash flows; tightly interbedded sands, gravels, and small debris flows; and matrix-heavy debris flows. The interrelationships between these facies, coupled with the compensation calculation, provide a historical record of debris-fan-building processes that has not been available previously. This type of analysis could eventually allow developers and homeowners to identify areas at greatest risk from debris flows in the event of channel avulsion.