[1] Debris flows often occur in burned steeplands of southern California, sometimes causing property damage and loss life. In an effort to better understand the hydrologic controls on post-fire debris-flow initiation, timing magnitude, we measured flow stage, rainfall, channel bed pore fluid pressure hillslope soil-moisture accompanying 24 debris recorded five different watersheds 2009 Station Jesusita Fires (San Gabriel Santa Ynez Mountains). The measurements show substantial differences...

[1] Runoff during intense rainstorms plays a major role in generating debris flows many alpine areas and burned steeplands. Yet compared to flow initiation from shallow landslides, the mechanics by which runoff generates are less understood. To better understand surface water runoff, we monitored stage rainfall associated with headwaters of two small catchments: bedrock-dominated basin central Colorado (0.06 km2) recently area southern California (0.01 km2). We also obtained video footage...

Abstract Shortly before the beginning of 2017–2018 winter rainy season, one largest fires in California (USA) history (Thomas fire) substantially increased susceptibility steep slopes Santa Barbara and Ventura Counties to debris flows. On 9 January 2018, fire was fully contained, an intense burst rain fell on portion burn area above Montecito, California. The rainfall associated runoff triggered a series flows that mobilized ∼680,000 m3 sediment (including boulders >6 m diameter) at...

Debris flows can dramatically increase their volume, and hence destructive potential, by entraining sediment. Yet quantitative constraints on rates mechanics of sediment entrainment debris are limited. Using an in situ sensor network the headwaters a natural catchment we measured flow bed properties during six erosive debris‐flow events. Despite similar thicknesses entrained across all events, time‐averaged were significantly faster for that was saturated prior to arrival compared with dry....

Many theoretical and laboratory studies have been undertaken to understand debris-flow processes their associated hazards. However, complete quantitative data sets from natural debris flows needed for confirmation of these results are limited. We used a novel combination in situ measurements dynamics, video imagery, pre- postflow 2-cm-resolution digital terrain models study event. Our field constrain the initial final reach morphology key flow dynamics. The observed event consisted multiple...

Abstract In the semiarid Southwestern USA, wildfires are commonly followed by runoff-generated debris flows because remove vegetation and ground cover, which reduces soil infiltration capacity increases erodibility. At a study site in Southern California, we initially observed first year following fire. However, at same three years after fire, mass-wasting response to long-duration rainstorm with high rainfall intensity peaks was shallow landsliding rather than flows. Moreover, storm caused...

Abstract Debris flows are a typical hazard on steep slopes after wildfire, but unlike debris that mobilize from landslides, most postwildfire generated water runoff. The majority of existing flow modeling has focused landslide‐triggered flows. In this study we explore the potential for using process‐based rainfall‐runoff models to simulate timing and runoff‐generated in recently burned areas. Two different spatially distributed hydrologic with differing levels complexity were used: full...

It has been proposed that debris flows cut bedrock valleys in steeplands worldwide, but field measurements needed to constrain mechanistic models of this process remain sparse due the difficulty instrumenting natural flows. Here we present and analyze made using an automated sensor network, erosion bolts, a 15.24 cm by force plate installed channel floor steep catchment. These allow us quantify distribution basal forces from debris‐flow events incised bedrock. Over 4 year monitoring period,...

Abstract Postwildfire debris flows are frequently triggered by runoff following high‐intensity rainfall, but the physical mechanisms which water‐dominated transition to poorly understood relative flow initiation from shallow landslides. In this study, we combined a numerical model with high‐resolution hydrologic and geomorphic data sets test two different hypotheses for during rainfall event that produced numerous within recently burned drainage basin. Based on simulations, large volumes of...

Abstract Debris flows generate seismic waves as they travel downslope and can become more dangerous entrain sediment along their path. We present field observations that show a systematic relation between the magnitude of amount erodible beneath flow. Specifically, we observe debris flow traveling channel filled initially with 0.34 m thick generates about 2 orders less spectral power than similar‐sized over same without fill. adapt model from fluvial seismology to explain this observation...

Rainfall on 9–13 September 2013 triggered at least 1,138 debris flows in a 3430 km area of the Colorado Front Range. The historical record reveals that occurrence these over such large interior North America is highly unusual. began after ~75 mm antecedent rain had fallen, relatively low amount compared to other parts United States. Most were response two intense rainfall periods, one 12.5-hour-long period 11–12 September, and 8-hour-long 12 September. maximum 10 min. intensities during...

Abstract More than 1100 debris flows were mobilized from shallow landslides during a rainstorm 9 to 13 September 2013 in the Colorado Front Range, with vast majority initiating on sparsely vegetated, south facing terrain. To investigate physical processes responsible for observed aspect control, we made measurements of soil properties densely forested north hillslope and grassland‐dominated Range performed numerical modeling transient changes pore water pressure throughout rainstorm. Using...

Hurricane Maria hit the island of Puerto Rico on 20 September 2017 and triggered more than 40,000 landslides in at least three-fourths Rico's 78 municipalities.The number that occurred during this event was two orders magnitude greater those reported from previous hurricanes.Landslide source areas were commonly limited to surficial soils but also extended into underlying saprolite bedrock.Slope failures before, during, after flooding, many transitioned long runout debris flows.Steep slopes...

Wildfire alters vegetation cover and soil hydrologic properties, substantially increasing the likelihood of debris flows in steep watersheds. Our understanding initiation mechanisms postwildfire is limited, part, by a lack direct observations measurements. In particular, there need to understand temporal variations debris‐flow following wildfire how those relate wildfire‐induced geomorphic changes. this study, we use combination situ measurements, monitoring equipment, numerical modeling...

Abstract Hazard assessment for post‐wildfire debris flows, which are common in the steep terrain of western United States, has focused on susceptibility upstream basins to generate flows. However, reducing public exposure this hazard also requires an hazards downstream areas that might be inundated during flow runout. Debris runout models widely available, but their application flows not been extensively tested. Necessary inputs these include total volume mobilized flow, properties (either...

Abstract Post‐wildfire changes to hydrologic and geomorphic systems can lead widespread sediment redistribution. Understanding how moves through a watershed is crucial for assessing hazards, developing debris flow inundation models, engineering retention solutions, quantifying the role that disturbances play in landscape evolution. In this study, we used terrestrial airborne lidar measure redistribution 2016 Fish Fire, San Gabriel Mountains southern California, USA. The areas are two...

Southern California has a long history of damaging debris flows after wildfire. Despite recurrent loss, forecasts the frequency and magnitude postfire are not available for region like they earthquakes. Instead, flow hazards typically assessed in reactive manner wildfires. Such assessments crucial evaluating risk by emergency response teams; however, time between fire first rainstorm is often insufficient to fully develop implement effective plans those place Here, we use both historical...

Deadly and destructive debris flows often follow wildfire, but understanding of changes in the hazard potential with time since fire is poor. We develop a simulation-based framework to quantify hydrologic triggering conditions for as postwildfire infiltration properties evolve through time. Our approach produces time-varying rainfall intensity-duration thresholds runoff- infiltration-generated physics-based simulations that are parameterized widely available hydroclimatic, vegetation...

Abstract Increased wildfire activity in the western United States has exposed regional gaps our understanding of postfire debris‐flow generation. To address this problem, we characterized flows an unstudied area to test rainfall intensity‐duration control hazard. Our measurements and field observations from northern Sierra Nevada (California, USA) show that debris resulted a short burst during low‐accumulation storm. In contrast, much higher accumulation storm (∼10 times more rainfall) with...

Abstract Post-wildfire debris flows (PFDF) threaten life and property in western North America. They are triggered by short-duration, high-intensity rainfall. Following a wildfire, rainfall thresholds developed that, if exceeded, indicate high likelihood of PFDF. Existing weather forecast products allow forecasters to identify favorable atmospheric conditions for intensities that may exceed established at lead times needed decision-making (e.g., ≥24 h). However, these times, considerable...