E3S Web of Conferences 415, 03019 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341503019

A multi-year record of topographic changes on debris-flow fansin south-western British Columbia, Canada
Andrew Mitchell1*, Sophia Zubrycky1, Eimile McSorley2, Scott McDougall2, and Mitch D’Arcy2
1BGC Engineering Inc., Suite 500, 980 Howe Street, Vancouver, BC, Canada
2University of British Columbia2, Vancouver, BC, Canada

Abstract. Repeat observations of four debris-flow fans in south-western British Columbia, Canada, weremade using a UAV-lidar system. Detailed measurements of deposit thicknesses and volumes have beengenerated from the data. We present channel measurements and characteristics for one of the sites todemonstrate the utility of the repeat lidar scanning technique to provide insights into where avulsions occur
during debris flows. Through continued monitoring, we plan to obtain greater detail on a wider variety ofevents and the characteristics of avulsion locations.

1 Introduction
Debris flows are periodic, surging flows of water anddebris, and are a common hazard in mountainous terrain[1]. Debris-flow mobility is affected by the channel andfan topography, the characteristics of the flowing
material, and the number and type of surges within anevent. The periodic nature of debris flows, with multiple
events happening over time in the same channel, meansthat past events can be used as an indication of potential
future events. The events are separated by days to years
of inactivity or channel modification by fluvial
processes, which may also affect the behaviour of future
debris flows.
Modern techniques make it possible to collectrepeated, high-resolution topographic surveys of debris
flow channels. Airborne lidar surveys before and after
debris flows have allowed the quantification of erosionand deposition with a level of precision not previously
possible (e.g., [2], [3]). Photogrammetric techniques
have been used to collect repeat surveys of the Gadriachannel in Italy [4] and the Illgraben channel in
Switzerland [5]. In both of these cases, the channel has
been modified with control structures such as checkdams. Physical modelling experiments have been
conducted to examine fan evolution over time without
the effects of control structures in the channel [6], [7].
This work focuses on the evolution of debris flowfans with little anthropogenic modification. We present
the results of four annual, high-resolution topographic
survey and imagery collection. We used a UAV-based
lidar system to allow for accurate surface mapping, on
heavily vegetated slopes where photogrammetricmethods would have limited utility. Our goal is tocapture detailed data on the event volume, erosion anddeposition patterns of individual debris-flow events, andto examine the effects of small events and ongoing

fluvial processes between the large events. Long-term,
we want to use these data to better understand how
previous debris flows and channel modifications
between events control the future evolution of fans,particularly with respect to identifying potential
avulsion locations.

2 Methodology
We used an uncrewed aerial vehicle (UAV) lidar system
to collect high-resolution topographic data for four
debris-flow fans. The system utilized a lidar scanner,inertial measurement unit (IMU), global navigational
satellite system (GNSS) receiver, and a downward
facing camera mounted to a UAV. A static GNSS basestation was used to provide stable reference data forpost-processing. Commercially available survey
software was used to combine the data sources to createa lidar point cloud, refine the point cloud, and classify
the points.
We compared lidar datasets at each site to quantify
topographic changes between surveys. We performed
the analysis using software that automatically completes
the iterative closest point (ICP) alignment andmultiscale model to model cloud comparison (M3C2)
change detection [8].
Finally, we examined the results of the analyses in
GIS software to identify debris-flow events and quantify
their areas and volumes. Locations of channel avulsions,when present, were also recorded.

3 Study sites
Our study sites are within the Coast Mountain range ofsouth-western British Columbia, Canada. We completed
annual surveys on one fan at the base of the north face

* Corresponding author: AMitchell@bgcengineering.ca© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 03019 (2023)
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https://doi.org/10.1051/e3sconf/202341503019

of Mount Currie, Currie D, from 2019 to 2022. Anadjacent fan, Currie C, was surveyed from 2020 to 2022.
The geology of the catchment area is characterized byigneous rocks, primarily foliated quartz diorites.
Weathering of the rock has resulted in extensive talus
slopes, and supply unlimited (also known as transportlimited) conditions for debris flows [9]. The climate istemperate, with warm dry summers and wet cool
winters, including Pacific storms that bring heavy rain
and snow [9].
We completed annual surveys at two fans on thewest face of Fountain Ridge, Fountain North andFountain South, from 2019 to 2022. The bedrock
geology in the catchment area consists of sedimentaryrocks
including
greywackes,argillitesandconglometerates. Extensive talus slopes are present in
the catchment area, leading to a supply unlimited
condition [9]. The Fountain Ridge site is within the rain
shadow of the Coast Mountains, where it is drier
throughout the year, with warmer summers. Debris
flows can be triggered by spring rain on snow events orsummer thunderstorms [9].

4 Results
An approximately 100,000 m3 event occurred at Currie
D between July 3rd and 12th, 2019 [9]. Our first survey
at Currie D was in October 2019, however, there was anairborne lidar survey available from 2017, allowing us
to complete a change-detection analysis, detailed inZubrycky et al. [9].
Debris flows were triggered on both Fountain Northand South in 2021, and on Fountain North in 2022. The
results of the topographic change detection for 2020 and2021 relative to our 2019 baseline for Fountain Southare shown in Figure 1. This event had a deposit volumeof approximately 43,000 m3. The Fountain South 2021
event is the subject of the more detailed characterizationthat follows.
We identified three avulsion locations from the 2021
event (Fig. 2) and extracted topographic data along theprevious flow path (active channel) from the 2019 lidar(Fig. 3). The uppermost avulsion featured flow spillingover both sides of the channel. It is coincident with thepresence of a road, channel curvatures, a reduction inchannel gradient, and a reduction in channel depth (Fig.
3). The middle avulsion occurred at a slight bend in thechannel, and where the channel downslope was slightly
flatter and less-incised (Fig. 3, Fig. 4a). The lowermost
avulsion featured a splitting of the flow, and is locatedupslope of where the previous channel begins to lose
confinement (Fig. 3).

Fig. 1. (a) Topographic change between 2019 and 2020 (noevent), and (b) topographic change between 2019 and 2021.
Background hillshades for both panels are from the 2019
survey.

See Fig. 4

Fig. 2. Deposit mapping and avulsion locations from theFountain South 2021 event and the flow path of the last eventon this fan, showing the 2021 topographic hillshade.

2

E3S Web of Conferences 415, 03019 (2023)
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https://doi.org/10.1051/e3sconf/202341503019

erosion and deposition and identify the topographic
features of avulsion locations. Beyond the geomorphic
analysis of events, the UAV-lidar data has been valuable
for many other applications. The baseline data from bothsites were valuable as part of an empirical dataset ofdebris-flow events in south-western BC [9], and havebeen made available through an online repository [10].
The Currie D event was also used as a test case toevaluate the performance of a numerical model [11].
Through these annual surveys, we hope to gain
insights as to which conditions lead to avulsions, and get
a better sense of the frequency of small magnitude
events not often detected. Collecting full-scale, realworld data is an important part of evaluating predictivemodels, empirical or numerical, and interpreting theresults of physical modelling.
We would like to acknowledge the many field assistants whohelped with data collection: Amin Ahmed, David Bonneau,
Anda Cen, Vivian Cristina Dias, Negar Ghahramani, Ivan Li,
Tess Muller, and Natalie Pedersen. Funding for this research
was provided in part by the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Canada
Foundation for Innovation (CFI), the British Columbia
Knowledge Development Fund (BCKDF), and BGCEngineering Inc.

Fig. 3. Avulsion locations superimposed on data extracted
along the previous flow path using pre-event (2019) lidar
topography.

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Fig. 4. Profiles through the middle avulsion location
highlighted in Fig. 2, (a) going through the channel plugfollowing the previous channel path, and (b) following the newflow path.

We have not identified any definitive small-scalechannel changes leading to avulsions with the dataset wehave to date. More data will likely be required toeffectively evaluate the effects of these more subtle
channel modifications between events.

5 Conclusions
Our first four annual UAV-lidar surveys have provided
high-precision spatial data for four large debris-flow
events. The detailed topographic data before and after
debris-flow events allow us to examine the patterns of3