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

Abstract

Floods and debris flows are recognized post-fire responses to rainfall within burned watersheds. The ability of debris flows to travel rapidly over significant distances from the area of initiation and their destructive force make them a hazard of particular concern. Individuals and organizations responsible for infrastructure, property, and public safety along the potential path of post-fire debris flows must understand the risk posed in order to design and implement suitable mitigating measures. Effective mitigation necessitates a rapid assessment of risk because only weeks or months separate the wildfire incident and the possible occurrence of a debris-flow initiating storm event. In the mountainous western United States, better risk assessment is crucial due to the combination of an expanding wildland-urban interface and more frequent large wildfires. Over the last 30 years, technological improvements in mapping fire effects, advances in our scientific understanding of post-fire debris flow occurrence, and development of empirical models to predict debris flow probability and volume have improved quantification of debris flow risk and facilitated more effective debris flow mitigation. How these advances have improved emergency response assessment efforts is exemplified by comparing assessment of debris flow risk for two large wildfires which occurred 26 years apart and affected much of the same area in the Sierra Nevada of California.

A rationale for effective post-fire debris flow mitigation within forested terrain

“…Debris flows pose a serious threat because they move rapidly, travel significant distances from their point of origin, and exert destructive force along their flow path and within their area of deposition (Hungr, 2005;Giraud and McDonald, 2007;Jordan and Covert, 2009) Post-wildfire debris flows clearly pose a hazard when burned watersheds are adjacent to populated areas (Cannon and DeGraff, 2009) and can even be a hazard in a less populated rural settings (DeGraff et al, 2011). Consequently, post-fire evaluation needs tools to effectively identify, within a burned area, those drainage basins having a greater likelihood of generating debris flows in order to undertake timely and effective mitigation (DeGraff et al 2007;Cannon et al 2011;DeGraff et al, 2013;DeGraff, 2014;Staley et al, 2017).…”

Minimizing economic impacts from post-fire debris flows in the western United States

“…Because debris flows in the first few years following a fire are often triggered when relatively common rainstorms fall on drainage basins (basins) that would have been unlikely to generate debris flows before the fire, they increase hazards to downstream communities (Cannon and Gartner 2005). Furthermore, rising population density in the wildland-urban interface combined with observed increases in wildfire occurrence and intensity in recent decades (Cannon and DeGraff 2009;DeGraff 2014) are expected to exacerbate the problem, resulting in increased debris-flow activity near populated areas and an associated increase in risk of debris-flow damage to local communities.…”

“…In recently burned areas, debris flows related to short-term impacts of fires can be triggered on time scales ranging from a few days to 2 years or more after the fire (DeGraff 2014;DeGraff et al 2015) by relatively frequent (less than 2-year recurrence) storms . Debris flows related to longerterm impacts of fires including root decay from severely burned vegetation and associated loss of soil strength may occur as much as ten or more years after a fire (Cannon and Gartner 2005;DeGraff et al 2015).…”

“…As indicated by DeGraff (2014), risk must be assessed quickly to allow for selection, design, and implementation of mitigation measures before a possible triggering storm. In the USA, Burned Area Emergency Response (BAER) crews are directed to identify elements-at-risk and proposed mitigation measures within as little as 10 days after containment of a fire (DeGraff 2014). This rapid emergency response aspect of post-fire debris-flow risk management requires different strategies and methods than those commonly employed for managing risk from other types of debris flows, e.g., landslide-generated debris flows, volcanic debris flows (lahars), or large rock avalanches.…”

Avaliação Da Suscetibilidade À Rutura E Propagação De Fluxos De Detritos Na Bacia Hidrográfica Do Rio Zêzere (Serra Da Estrela, Portugal)

“…CANNON et al, 1998, 2001a, 2001b, São Paulo, v.18, n.1, (Jan-Mar) p. 81-106, 2017, 2011GARTNER et al, 2008;SANTI et al, 2008;PARISE e CANNON, 2012;STALEY et al, 2013STALEY et al, , 2014DE GRAFF, 2014;NYMAN et al, 2015). Por este motivo, e tendo em conta os incêndios fl orestais que defl agraram na área de estudo dois meses antes do desencadeamento dos fl uxos de detritos, considerou-se importante a integração da variável respeitante às áreas ardidas no verão de 2005.…”