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Complexity of a debris-flow system at Forest Falls, California
Cato, Kerry Goforth, Brett
Cato, Kerry
Goforth, Brett
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2019
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Abstract
Historical patterns of debris flows have been reconstructed at the town of Forest Falls in the San Bernardino Mountains using a variety of field methods (mapping flow events after occurrence, dendrochronology evidence, soil chrono sequences). Large flow events occur when summer thunderstorms produce brief high-intensity rainfall to mobilize debris, however the geomorphic system exhibits properties of non-linear response rather than being a single-event precipitation-driven process. Previous studies contrasted the relative water content of flows generated by varying intensity summer thunderstorms to model factors controlling their velocity and pathway of deposition. We hypothesize that variation in sediment discharge also results from complexity in this geomorphic system, and this paper presents ongoing empirical field studies focused on sources of complexity in three formative components of recently monitored debris-flows at Snow Creek Canyon: 1) thresholds of sediment delivery from sources at the higher reaches of bedrock canyons; 2) storage effects in sediment transport down the bedrock canyons; and 3) feedbacks in deposition and transport of sediment as flows from the bedrock canyon collect into a single active channel on a fan landform downslope. An example of the first component occurred in March 2017, when snow melt generated a rapid translational landslide/debris slide of about 80,000 m3; this sediment was deposited in the bedrock canyon, but moved no further down-gradient. A second component has been observed when accumulation of meta-stable sediments in the active channel remain in place until fluvial erosion or subsequent debris flow created dynamic instability to mobilize the mass downslope. The third component occurred in the active channel where low-water content debris flows deposited sediments that filled the channel, raising the channel grade level to levy elevation, allowing for subsequent spread of non-channelized flows onto the fan and new scouring channel pathways down fan. Assessment of spatial and temporal complexities in a debris-flow system can improve risk prediction.
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