Investigation of Post-Fire Debris Flows in Montecito

Abstract

Debris flows in a burned area, post-fire debris flows, are considered as one of the most dangerous geo-hazards due to their high velocity, long run-out distance, and huge destruction to infrastructures. The rainfall threshold to trigger such hazards is often reduced compared with normal debris flow because ashes generated by mountain fires reduce the permeability of the top soil layer, thus increasing surface runoff. At the same time, burnt material and residual debris have very poor geo-mechanical characteristics, e.g., their internal friction angle and cohesion are typically low, and thus an intense rainfall can easily trigger some debris flows. Studying post-fire debris flow enables us to get a deeper understanding of disaster management. In this paper, the debris flow that occurred in Montecito, California, USA, and was affected by the Thomas Fire was used as a case study. Five major watersheds were extracted based on the digital elevation model (DEM). Remote sensing images were used to analyze the wildfire process, the extent of the burned areas, and the burn severity. The hypsometric integral (HI) and short-duration rainfall records of the watersheds around Montecito when the post-fire debris flows occurred were analyzed. Steep terrain, loose and abundant deposits, and sufficient water supply are the important conditions affecting the formation of debris flows. Taking watersheds as the research objects, HI was used to describe the geomorphic and topographic features, open-access rainfall data was used to represent the water supply, and burn severity represented the abundance of material sources. An occurrence probability model of post-fire debris flow based on HI, short-duration heavy rainfall, and burn severity was developed by using a logistic regression model in post-fire areas. By using this model, the occurrence probability of the post-fire debris flow in different watersheds around Montecito was analyzed based on the precipitation with time. Especially, the change characteristics of occurrence probability of debris flows over time based on the model bring a new perspective to observe the obvious change of the danger of post-fire debris flows and it is very useful for early warning of post-fire debris flows.

The cost of rapid and haphazard urbanization: lessons learned from the Freetown landslide disaster

“…Ng et al (2016a), who was the first one to investigate both the mechanical and hydrological effects on slope stability in centrifuge systematically, investigated innovatively the influence of planting density on tree growth and induced soil suction, which showed the role of vegetation in soil and water conservation from a more subtle point of view. Mass-wasting hazards such as landslides (Douglass et al 2005;Li et al 2017), floods (Fedeski and Gwilliam 2007), and debris flows (Cannon and DeGraff 2009) can be initiated when large areas of vegetation are destroyed or removed (Ng et al 2016b;Cui et al 2019). In Sierra Leone, deforestation is a major issue, and in the case of the study event, it may have been the crucial problem.…”

Size distribution and size of loess slides in response to slope height and slope gradient based on field survey data

“…A landslide inventory can be prepared by using different techniques depending on the purpose of the investigation, the scale of the maps, the resolution of the available imagery and the experience of the investigators (van Westen et al 2006;Guzzetti et al 2012). Despite the great advances in and convenience of remote sensing technology (Cheng et al 2018;Wang et al 2018;Cui et al 2019), its application is limited by the presence of vegetation canopies, erosion and plowing (Brardinoni and Church 2004). Moreover, some important parameters, such as the slope gradient and slope height, cannot be measured accurately due to many errors.…”

Characterizing Consecutive Flooding Events after the 2017 Mt. Salto Wildfires (Southern Italy): Hazard and Emergency Management Implications

“…As reported by Parise and Cannon [28], the runoff-dominated erosion by surface overland flow is the principal mechanism controlling the initiation of debris flow processes in burned watersheds. This was also ascertained in the major Montecito debris flow event that occurred in the winter of 2018 in California (USA), causing 23 fatalities, at least 167 injuries, and 408 damaged homes [29,30]. Besides high-magnitude events like this, it is worth underlining that the spectrum of post-wildfire hydrologic and sedimentologic responses also includes less-severe events or no response, as documented in different locations across the globe [31].…”