Before the fire: Assessing post-wildfire flooding and debris-flow hazards for pre-disaster mitigation

Abstract

Abstract. Increasing size and severity of wildfires, and the expanding built environment into the wildland-urban interface makes it imperative that local governments identify, prepare for and reduce risks to people and infrastructure from wildfires and the aftermaths of fires. Here we report on a pre-wildfire assessment of post-fire hazards in Coconino County, Arizona, the mitigation measures identified and implemented as a result of the study, and proposed changes to the assessment methodology for upcoming studies. Results from the Coconino County study indicate that up to 34 % of the buildings, and up to 26 % of the critical facilities countywide are at some level of increased risk of post-fire flooding if no actions are taken to reduce the risk of severe wildfires. As many as 593 homes (2,191 parcels) in Coconino County, as well as 13 dams and other critical facilities, may be impacted by post-fire debris flows. In two smaller areas of detailed study, flood peaks could increase as much as 4–5 times the existing 100-year flood levels, with up to a 350 % increase in the number of buildings in flood-prone areas. Debris flows will likely be limited in aerial extent but could impact a much larger area from following floods and sediment-laden flows. Mitigation measures identified and implemented as a result of this study include County Coconino County coordination with the Kaibab National Forest in regard to forest health projects, development of a post-wildfire emergency action plan for the City of Williams, development of a Post-Fire, Pre-Disaster plan for the City of Williams and educating the City of Williams and County officials and business stakeholders of the post-fire flood risk. Other mitigation measures that are still in the planning stage include installation of additional early flood warning system gages, and increasing building and infrastructure resiliency through channel conveyance improvement and utility protection projects. For two upcoming assessments, we plan to use a new statistical methodology to develop burn severity maps using historical burns, and we plan to employ a new process-based debris-flow model developed for use in Arizona to assess debris-flow inundation limits.

Inundation and flow properties of a runoff‐generated debris flow following successive high‐severity wildfires in northern Arizona, USA

“…The monsoon season following the Schultz Fire was, at the time, the fourth wettest on record in Flagstaff (Youberg et al, 2011), and storms on 20 July, 22 July and 16 August 2010 produced flash floods and debris flows in watersheds burned by the Schultz Fire (Youberg, 2014). In total, 30 watersheds produced floods in the first year following the fire, 19 of which also produced debris flows, including the Copeland watershed (Youberg, 2015) (Figure S1).…”

“…Debris‐flow runout distances varied from 0 to 2.5 km downstream of watershed outlets before transitioning to flood flows. Debris‐flow volumes ranged from 200 to 14 000 m 3 (Youberg, 2015). While Youberg (2015) did not estimate the volume of sediment mobilized by flood flows, multiple floods caused extensive damage to infrastructure as far as 10 km downstream of the burned area (Youberg, 2014; Youberg et al, 2011a).…”

“…Debris‐flow volumes ranged from 200 to 14 000 m 3 (Youberg, 2015). While Youberg (2015) did not estimate the volume of sediment mobilized by flood flows, multiple floods caused extensive damage to infrastructure as far as 10 km downstream of the burned area (Youberg, 2014; Youberg et al, 2011a). Following the Schultz Fire, the Copeland watershed, which is the focus of this study, produced a 4000 m 3 debris flow that ran out approximately 1.2 km from the watershed outlet.…”

Forecasting the Frequency and Magnitude of Postfire Debris Flows Across Southern California

“…Although more complicated physically based models of debris flow initiation have recently been applied to selected small (<1 km 2 ) drainage basins in southern California (McGuire et al., 2017; Rengers et al., 2019; Tang et al., 2019), such models are not yet practical for hazard assessment because they are computationally expensive and require additional input data that are not generally available across large areas (e.g., high‐resolution digital elevation data, hillslope and channel sediment grain size distributions, and soil infiltration properties). Previous studies have used empirical models of postfire debris flow for prefire planning in combination with various methods to define synthetic distributions of burn severity representing wildfire scenarios (Elliot et al., 2012; Hass et al., 2016; Lancaster et al., 2014; Staley et al., 2018; Stevens et al., 2011; Tillery & Haas, 2016; Youberg, 2008). We adopt a similar approach here to assess a larger area, and we emphasize mapping debris flow hazard in a manner similar to well‐established models for quantifying earthquake hazards in California (Third Uniform California Earthquake Rupture Forecast, UCERF3, Field et al., 2014, 2015a).…”

Integrating Remote and In-Situ Data to Assess the Hydrological Response of a Post-Fire Watershed

“…Flood magnitude may also increase after fire, e.g., from a 10-year to a 1-year return interval for the same discharge [20]. Youberg et al [21] estimated that a 2-year return period rainfall in a burned watershed can produce a runoff similar to a 100-year event in pre-fire conditions. Such an exceptional increase in hazard has a catastrophic impact on the economy of a region, although assessing the costs of post-fire flooding is very difficult.…”