In situ measurements of post-fire debris flows in southern California: Comparisons of the timing and magnitude of 24 debris-flow events with rainfall and soil moisture conditions

Abstract

[1] Debris flows often occur in burned steeplands of southern California, sometimes causing property damage and loss of life. In an effort to better understand the hydrologic controls on post-fire debris-flow initiation, timing and magnitude, we measured the flow stage, rainfall, channel bed pore fluid pressure and hillslope soil-moisture accompanying 24 debris flows recorded in five different watersheds burned in the 2009 Station and Jesusita Fires (San Gabriel and Santa Ynez Mountains). The measurements show substantial differences in debris-flow dynamics between sites and between sequential events at the same site. Despite these differences, the timing and magnitude of all events were consistently associated with local peaks in short duration (< = 30 min) rainfall intensity. Overall, debris-flow stage was best cross-correlated with time series of 5-min rainfall intensity, and lagged the rainfall by an average of just 5 min. An index of debris-flow volume was also best correlated with short-duration rainfall intensity, but found to be poorly correlated with storm cumulative rainfall and hillslope soil water content. Post-event observations of erosion and slope stability modeling suggest that the debris flows initiated primarily by processes related to surface water runoff, rather than shallow landslides. By identifying the storm characteristics most closely associated with post-fire debris flows, these measurements provide valuable guidance for warning operations and important constraints for developing and testing models of post-fire debris flows.

Comparative analysis of the differences between using LiDAR and contour-based DEMs for hydrological modeling of runoff generating debris flows in the Dolomites

“…Only short duration, high intensity precipitations are able to deliver the high discharge required for moving a sediment bed for the generation of a debris flow (Berti and Simoni, 2005;Dalla Fontana, 2007, 2008;Cannon et al, 2008;Kean et al, 2011;Staley et al, 2013). The rainfall events responsible for debris flows in the two basins are listed in Table 2 (h, precipitation height; D, precipitation duration).…”

Improvement in quantifying debris flow risk for post-wildfire emergency response

“…Expected rainfall conditions affecting the recently burned area is incorporated in the debris flow models as a critical factor Kean et al 2011;Staley et al 2013). …”

“…Cannon et al (2011) documents a number of California wildfires occurring during July through November where debris flows were triggered by storms taking place 1 to 6 months later. The stormrelated uncertainty would be reduced if the risk of postfire debris flows can linked to a particular rainfall intensity -duration threshold Kean et al 2011). Such a threshold would provide a means for predicting when evacuation and similar mitigations might be implemented.…”

Constraining the relative importance of raindrop‐ and flow‐driven sediment transport mechanisms in postwildfire environments and implications for recovery time scales

“…Consequently, knowledge of the true flow depth with respect to the channel bottom is uncertain. Following Kean et al [2011], we estimated channel bottom elevation by linearly interpolating the channel bottom elevation between periods of active flow within the channel and used this approximate elevation to compute stage.…”

“…Swanson [1981] estimated that wildfire-related processes account for approximately 25% of the long-term sediment yield in the western Cascade Range, Oregon, while Orem and Pelletier [2016] attribute more than 90% of denudation over geologic time scales to wildfire-affected erosion in the Valles Caldera, New Mexico. Hillslope sediment has also been identified as an important source of material for postwildfire debris flows [Meyer and Wells, 1997;Cannon et al, 2001aCannon et al, , 2001bKean et al, 2011]. Therefore, understanding hillslope-scale sediment transport following wildfire as well as its sensitivity to vegetation and soil recovery time scales is important for improving our knowledge of the long-term evolution of mountainous landscapes and assessing postwildfire hazards.…”