Exploring new methods to analyse spatial impact distributions on debris‐flow fans using data from south‐western British Columbia

Abstract

Predicting the spatial impact of debris flows on fans is challenging due to complex runout behaviour. Debris flow mobility is highly variable and flows can sporadically avulse the channel. For hazard and risk assessments, practitioners typically base the probability of spatial impact or avulsion on their experience and expert judgement.To support decision-making with empirical observations, we studied spatial impact distributions on 30 active debris-flow fans in south-western British Columbia, Canada. We mapped 146 debris-flow impact areas over an average observation period of 74 years using orthorectified airphotos, satellite imagery, topographic base maps, LiDAR data, orthophotos, and field observations. We devised a graphical method to convert our geospatial mapping into spatial impact heat maps normalized by fan boundaries, enabling comparison of runout distributions across different fans. About 90% of the mapped debris flows reached beyond the mid-points of fans, while less than 10% avulsed more than half-way across the fan relative to the previous flow path. Most avulsions initiated at distances of 20% to 40% of the maximum fan length from the fan apex and upstream of the fan intersection point. Large volume events tend to be more mobile in the down-fan direction, but the relation between volume and cross-fan runout (e.g., avulsions) is more complex. Differences in spatial impact distributions can be explained, in part, by the degree of fan incision and whether a fan is truncated at its toe by a river or lake. There were no significant differences in spatial impact distributions based on the geology of the source area, sediment supply condition, or hydrogeomorphic process classification.

Differences in the occurrence of debris flows in tropical and temperate environments: field observations and geomorphologic characteristics in Serra do Mar (Brazil) and British Columbia (Canada)

“…The distinct Rr values may contribute to differences in the frequency of debris flows in the watersheds. The MD watershed had the highest value (0.75) and number of occurrences ( 19), as mapped by Zubrycky et al (2021), which suggests it is the site that is most prone to debris flows. Values obtained by Jakob (1996) for weathering-limited basins and transport-limited basins also show this variance.…”

“…3A) and Fountain South (FS). At least 13 events have been identified at these sites since 1948, the most recent of which occurred in 2018 (Zubrycky 2020, Zubrycky et al 2021). The study site at Mount Currie is south of the town of Pemberton; this study focuses on a watershed designated Mount Currie D (MD) (Fig.…”

“…To this end, this study aimed to compare debris flow field observations and morphometric characteristics of example watersheds in Canada with recently affected areas in Brazil, highlight their main differences and similarities, and compare them to results from different parts of the world reported in the literature. The parameters area (A), drainage density (Dd), area > 25° (A25), relief ratio (Rr), length (L), basin relief (Br), and Melton ratio (Mr) were selected due to their use in the literature and practice for the evaluation of debris flow areas in the northern hemisphere, in a temperate environment (VanDine 1985, Jakob 1996, Scally et al 2001, Wilford et al 2004, Kovanen and Slaymaker 2008, Portilla et al 2010, Welsh and Davies 2011, Zubrycky et al 2021, Ilinca 2021) and the southern hemisphere, in a tropical environment (Augusto Filho 1993, Vieira et al 1997, Kanji and Gramani 2001, Chen and Yu 2011, Dias et al 2016, Gomes 2016, Cerri et al 2018, Lima et al 2020.…”

Towards an automated acquisition and parametrization of debris‐flow prone torrent channel properties based on photogrammetric‐derived uncrewed aerial vehicle data

“…One of the fundamental difficulties in debris‐flow mitigation is to determine debris‐flow volumes and mixtures which control in large part the possible inundation area and the likelihood of channel outbreaks (Zubrycky et al., 2021). Hazard assessment methods are used by natural hazard experts to estimate both release and erosion volumes.…”

Episodic sediment supply to alluvial fans: implications for fan incision and morphometry

“…For many fans, short, frequent disturbances (such as debris flows or sediment-laden floods) aggrade the fan or its channels locally but not every event triggers a wholesale avulsion (e.g. de Haas et al, 2018a;Field, 2001;Zubrycky et al, 2021); sequences of multiple events may be required to affect whole-fan morphology, implying that these individual events are shorter than the adjustment timescale. Conversely, on fans experiencing longer (decadal) periods of heightened sediment supply, prolonged aggradation and progradation across the whole fan have been observed (De Rose et al, 1998;Leenman and Tunnicliffe, 2020;Marden et al, 2018).…”