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)

Abstract

Debris flows are among the most destructive types of mass movements throughout the world and are not restricted to certain climate zones or geological environments. In opposite corners of the American continent, Canada and Brazil have experienced several historic debris flows. As part of a one-year international exchange program, the lead author compared morphological evidence related to debris flows and the morphometry of watersheds at sites in Canada and Brazil, with the ultimate goal of improving the understanding of debris flows in Brazil. Field surveys carried out in both areas in 2019 and 2020 permitted observation of the debris-flow signatures, as well as the physical aspects of the surrounding areas and morphometric mapping of watersheds. Both areas exhibit similar typical features of debris flows, and the morphometric results indicate differences that may influence the recurrence of events at the sites in Canada as compared to Brazil due to their higher values for the parameters area > 25° (A25), relief ratio (Rr), and Melton ratio (Mr) at the Canadian sites; however, this dataset is limited. Compared to results in the literature from around the world, values of morphometric parameters at Brazilian sites are within the ranges observed in other tropical climates.

KEYWORDS: debris flow; mass movement; field survey; DEM; GIS; morphometry

INTRODUCTION

Among landslide types, debris flows stand out due to their high flow velocities, impact forces, and travel distances. Debris flows are defined by ^{Hungr et al. (2014)} as very rapid to extremely rapid surging flows of saturated debris in a steep channel, which cause damage and casualties when they occur in occupied areas (^{Eisbacher and Clague 1984}, ^{VanDine 1996}, ^{Jakob and Hungr 2005}). Prerequisites for triggering debris flows include an abundant source of sediments, steep slopes, sparse vegetation, and an excess of water, which is typically provided by rainfall events or snowmelt (^{Johnson 1970}, ^{Costa 1984}, ^{Iverson et al. 1997}, ^{Wilford et al. 2004}, ^{Jakob 2005}). Debris flows are not restricted to specific zones; these events occur in different parts of the world, such as Asia, North America, Europe, and South America (^{Jackson et al. 1987}, ^{Marchi et al. 2002}, ^{Carrara et al. 2008}, ^{Gabet and Bookter 2008}, ^{Gomes et al. 2008}, ^{Chen and Yu 2011}, ^{Dias et al. 2016}, ^2021b, ²⁰²²; ^{Picanço et al. 2016}, ^{Sujatha and Sridhar 2017}, ^{Sujatha 2020}, ^{Coe et al. 2021}). Critical volumes of precipitation are the most common factor reported as the main trigger of debris flows. Recent catastrophic occurrences in South America reported precipitation thresholds of at least 130 mm, highlighting the events in Venezuela in 1999, with 911 mm in 48 h (^{García-Martínez and López 2005}), Colombia in 2018, with 130 mm in 3 h (^{García-Delgado et al. 2019}), and in Brazil in 2011, with 250 mm in 48 h (^{Avelar et al. 2013}, ^{Coelho-Netto et al. 2013}, ^{Lima 2017}).

Over the last 60 years, the recorded number of debris flows and shallow landslides has become more frequent in Brazil, including in the Serra do Mar Mountains and on the south and southeast coasts. Remarkable disastrous events occurred in Caraguatatuba (1967), Rio de Janeiro Mountain range and Serra da Prata (2011), and Itaoca (2014), causing economic and social losses and stressing the need for mitigation measures, as well as a susceptibility assessment (^{Ploey and Cruz 1979}, ^{JICA 1991}, ^{Coelho-Netto et al. 2013}, ^{Gomes and Vieira 2016}, ^{Facuri and Picanço 2020}, ^{Lima et al. 2020}, ^{Dias et al. 2021a}). These events caused social damage (more than 3,000 deaths) and economic losses (at least $3 billion (USD)) (^{World Bank 2014}, ^{Kanji et al. 2017}). Nevertheless, debris flow research and practice in the country still lags behind other affected areas worldwide, with few local papers published about this issue (^{Kanji et al. 2008}, ^{Gomes et al. 2008}, ²⁰¹³, ^{Jackson 2011}, ^{Dias et al. 2016}, ^{Campos and Galindo 2016}, ^{Roverato 2016}, ^{Lopes et al. 2016}, ^{Facuri and Picanço 2020}, ^{Cabral et al. 2021}). In comparison, debris flows are one of the most common and well-documented landslide processes in some parts of Canada, particularly in the province of British Columbia on the west coast (^{Thurber Consultants LTD. 1983}, ^{VanDine 1985}, ^{Slaymaker 1990}, ^{Scally et al. 2001}). According to ^{Strouth and McDougall (2021)}, debris flows represented more than 50% of the fatal landslides in British Columbia between 1950 and 2019; however, fatalities caused by the process are considered rare (one death per year in the last decade) despite the population growth, which reflects the development of mitigation measures and the number of detailed studies about the process.

The morphometric evaluation of watersheds affected by debris flows has been used as an important tool for understanding features that may contribute to debris flow occurrence. Many studies evaluate debris flow occurrences in North America and Europe, including studies carried out by ^{Scally et al. (2001}, ²⁰¹⁰), ^{Jakob (1996)}, and ^{Slaymaker (1990)} in different areas of British Columbia, Canada; ^{Cenderelli and Steven Kite (1998)}, ^{Gabet and Bookter (2008)}, ^{Kovanen and Slaymaker (2008)}, ^{Welsh and Davies (2011)}, and ^{Wilford et al. (2004)} in the USA; ^{Portilla et al. (2010)} in the Pyrenees Mountains, Spain; ^{Dotseva and Gerdjikov (2020)} on Stara Platina Mountain and ^{Nikolova et al. (2020)} in the Eastern Rhodopes, Bulgaria; and ^{Ilinca (2021)} in the Carpathians, Romania. Most of these studies evaluate the debris flow process under a specific climatic condition, in this case, a temperate environment, which has very specific characteristics in comparison to a tropical environment. Detailed studies about debris flows in tropical environments are still lacking, especially studies using morphometric parameters and those regarding Brazilian occurrences.

Although far apart, we hypothesize that areas located in different hemispheres may present similarities and specificities related to the occurrence of debris flows, which can help improve the understanding of the process in places like Brazil, where detailed studies are historically lacking. 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}, ^{Picanço et al. 2016}, ^{Cerri et al. 2018}, ^{Lima et al. 2020}).

REGIONAL SETTINGS OF THE STUDY AREAS

Two of the study areas, Lillooet and Mount Currie, are located in North America, in the province of British Columbia, near the west coast of Canada (Figs. 1A and 1B). The other case study areas, Itaoca and Serra da Prata, are located in South America, near the southeast coast of Brazil (Figs. 1C and 1D). Although located in different environments, both sets of areas have geologic, geomorphic, and climatic conditions that contribute to the occurrence of debris flows.

Figure 1
Location of the study areas in (A and B) Canada, (C and D) Brazil, and the respective watersheds.

Geomorphology, geology, soil, and vegetation

Lillooet and Mount Currie are part of the Cordilleran Mountain System. They are situated near the boundary between the Coast Mountains and Interior Plateau. They have a predominance of steep mountains and U-shaped valleys with mass wasting on hillslopes at high elevations, and they range from 100 to > 2,000 meters above sea level (^{Church and Ryder 2010}). The geology in Lillooet includes sedimentary rocks, which are mostly sandstone, conglomerate, argillite, and siltstone from the Cretaceous period (^{Church and Ryder 2010}, ^{Cui et al. 2017}). The Mount Currie geology is mostly igneous rocks, particularly Cretaceous granite (^{Bovis and Evans 1995}, ^{Church and Ryder 2010}). The soil type in both areas is major spodosols and inceptisols (^{Clayton et al. 1977}, ^{Soil Classification Working Group 1998}), covered by montane forest and subalpine forest (^{Natural Resources Canada 2021}).

Itaoca and Serra da Prata are located in Serra do Mar, a mountain system that extends for approximately 1,500 km along the south and southeast coasts of Brazil (^{Almeida and Carneiro 1998}, ^{Vieira and Gramani 2015}). These study areas have predominantly V-shaped valleys, hills, steep mountains, and elevations ranging from 200 to 1,100 meters above sea level (^{Almeida 1964}, ^{MINEROPAR 2006}). The geology in Itaoca and Serra da Prata includes igneous and metamorphic rocks from the Archean-Proterozoic, particularly granite and gneiss (^{Perrotta et al. 2005}, ^{ITCG 2006}, ^{Faleiros et al. 2012}). The soil cover is ultisols and oxisols (^{Ross 2002}, ^{ITCG 2008}, ^{Picanço et al. 2016}, ^{Rossi 2017}) and the vegetation is the Atlantic rainforest (^{IFSP 2009}, ^{ITCG 2009}).

Climate

The climate varies among all the study areas despite their proximities and geological and geomorphological contexts. In Canada, Lillooet is defined as a temperate/Mediterranean continental climate (Dsb) (^{Koppen 1936}, ^{Amani et al. 2019}), with mean temperatures varying between −4 and 20°C in the winter (Dec. to Mar.) and summer (Jun. to Sep.) seasons, respectively. The average monthly rainfall distribution shows little variability in the amount of rain, which does not exceed 50 mm per month, and the average annual total is 316 mm (Fig. 2A). At Mount Currie (less than 100 km south of Lillooet), the climate is defined as a temperate/humid continental climate (Dfb) (^{Koppen 1936}, ^{Amani et al. 2019}), with mean temperatures varying between −6°C in the winter (Dec. to Mar.) and 18°C in summer (Jun. to Sep.) seasons. Despite the relatively similar temperatures to those at Lillooet, the rainfall distribution throughout the year is distinct, with an average annual rainfall of 947 mm; the wet season occurs between October and March (autumn-winter) and the relatively dry season occurs between April and September (spring-summer) (Fig. 2B).

Figure 2
Multiannual rainfall distribution and mean temperature between 1971 and 2000 in (A) Lillooet, (B) Mount Currie, (C) Itaoca, and (D) Serra da Prata. Data were provided by the Pacific Climate Data Portal, (A and B) Station Lillooet Seton BCHPA, CIIAGRO and (C) Department of Water and Electricity of São Paulo (DAEE), and (D) Paraná Water Institute.

The climate in Itaoca is defined as a humid subtropical climate (Cfa) (^{Koppen 1936}), with the mean temperature varying between 17°C in the winter season (Jun. to Sep.) and 25°C in the summer season (Dec. to Mar.). The monthly multiannual rainfall distribution is high in the summer season and reaches 200 mm in January, which is only 115 mm less than the annual value in Lillooet. The average annual total rainfall in Itaoca is 1,345 mm, which is more than 4 times higher than that in Lillooet (Fig. 2C). In comparison, Serra da Prata is defined as having a Cfa climate at altitudes below 700 m and a temperate oceanic climate (Cfb) at altitudes above 700 m, with mean temperatures varying between −3°C in the winter season (Jun. to Sep.) and 22°C in the summer season (Dec. to Mar.) (^{Koppen 1936}, ^{Blum 2006}, ^{Blum et al. 2011}). Monthly rainfall varies between 61 mm (Aug.) and 222 mm (Jan.), with an average annual total of 1,463 mm, according to 30 years of data from 1970 to 2000 (Fig. 2D).

Occurrence of debris flows

The study site near the town of Lillooet is on Fountain Ridge, east of town. Debris flows occur in two different watersheds, designated Fountain North (FN) (Fig. 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. 3B). At least 19 events have been identified at these sites since 1946 (^{Zubrycky et al. 2021}).

Figure 3
Study areas at (A) Lillooet Fountain North and (B) Mount Currie D.

In comparison, only one debris flow has been observed in Itaoca, in the Guarda-mão watershed (GW), in 2014 (Fig. 4A) (^{Brollo et al. 2015}, ^{Gramani and Martins 2016}, ^{Dias et al. 2022}), and only one event has been observed at Serra da Prata in the Tingidor watershed (TW), in 2011 (Fig. 4B) (^{Picanço and Nunes 2013}, ^{Ferreira et al. 2016}, ^{Facuri and Picanço 2020}). Both events mobilized a large volume of sediments, affecting infrastructure and causing human fatalities. According to ^{Brollo et al. (2015)}, since 1997, Itaoca has been affected by at least 5 critical events, most of which were related to floods and flash floods, with the last one in 2014 being the only one involving debris flows and shallow landslides (^{Dias et al. 2022}).

Figure 4
Study areas in (A) Itaoca and (B) Serra da Prata.

MATERIAL AND METHODS

Field surveys

The data used in this paper were obtained by field surveys conducted between 2019 and 2020. A total of five debris flow-affected watersheds were visited, two in Brazil and three in Canada. The focus was on identifying the main characteristics related to debris flow activities in the geomorphology and stratigraphy of fan deposits. Sedimentological evidence of debris flows on the fan includes sorting, imbrication, levees, and buried logs. Similarly, in the transport zone, the presence of well-defined levees and large boulders that could not be moved by other flow-type processes such as floods can be signs of debris flows in an area (^{Johnson 1970}, ^{Costa 1984}, ^{Jakob 2005}).

In Brazil, two watersheds were visited in the fall (July) and winter (August) of 2019: Guarda-mão (GW) and Tingidor (TW). Due to climatic conditions and the relatively low frequency of debris flows, vegetation growth began to cover the signs of debris flow activity. However, in the study areas, it was still possible to identify and map most of the features related to debris flows. In comparison, at the sites in Canada, vegetation regrowth is not as fast as in a tropical environment, and the relatively high frequency of occurrence (^{Jordan 1994}, ^{Zubrycky et al. 2021}) favors the identification and mapping of debris flow features. Three distinct watersheds were visited, namely, Fountain North (FN) and Fountain South (FS) in the fall (October) of 2019, and Mount Currie D (MD) in the summer (August) of 2020.

Morphometric parameters

Morphometric parameters have been used to study debris flows since the 1980s for a variety of applications, including susceptibility analysis and magnitude-frequency estimation (^{VanDine 1985}, ^{Johnson et al. 1991}, ^{Cenderelli and Steven Kite 1998}, ^{Gabet and Bookter 2008}, ^{Kovanen and Slaymaker 2008}, ^{Portilla et al. 2010}, ^{Chen and Yu 2011}). Several authors have shown their usefulness for evaluating processes such as floods, debris floods, or debris flows (^{VanDine 1985}, ^{Slaymaker 1990}, ^{Johnson et al. 1991}, ^{Wilford et al. 2004}, ^{Welsh and Davies 2011}, ^{Ilinca 2021}), and for identifying debris flow-prone watersheds when geomorphologic and stratigraphic evidence is not visible in the landscape (^{Welsh and Davies 2011}, ^{Ilinca 2021}). Based on the literature, the following parameters were selected for analysis: area (A), drainage density (Dd), area > 25° (A25), relief ratio (Rr), length (L), basin Relief (Br), and Melton ratio (Mr) (Tab. 1).

Table 1
Description of morphometric parameters.

Morphometric Parameters	Abbreviation	Unit	Descriptions
Area	A	km2	Planimetric area of the watershed
Drainage density	Dd	km/km2	Total stream length divided by the watershed area
Area > 25°	A25	%	Percentage of the watershed area with a slope angle above 25°
Relief ratio	Rr	m/m	Basin relief divided by the watershed length
Length	L	km	Horizontal distance in a straight line between the upstream and downstream drainage limits.
Basin relief	Br	km	Difference between the higher and lower altitudes in the watershed
Melton ratio	Mr	DN*	Basin relief divided by the square root of the watershed area

• ^*
  Dimensionless.

In the literature, the A of a watershed has been related to the occurrence of debris flows. According to published studies in different parts of the world, small basins up to 10 km² are susceptible to debris flows (^{Slaymaker 1990}, ^{Scally et al. 2001}, ^{Ilinca 2021}). The Dd has been related to how rapidly a watershed drains. A high value indicates a well-drained watershed, while a low value indicates a poorly drained watershed. Although not as commonly used as other parameters, the Dd contributes to the evaluation of debris flows, as a high value may indicate more intense hydrogeomorphologic processes such as floods, debris floods, and debris flows (^{Augusto Filho 1993}). The A25 provides the percentage of the watershed with a slope angle above 25°. As a gravity-induced process, an angle of 25° is suggested by many authors as the minimum slope to initiate debris flows (^{Costa 1984}, ^{VanDine 1996}, ^{Takahashi 2007}). The Br may influence the intensity and reach of the debris flow (^{Nikolova et al. 2020}). The Rr indicates the ruggedness of the watershed, which has been related to the production and availability of sediments. According to ^{Scally et al. (2001)}, high ruggedness values indicate a watershed that is prone to more intense processes, such as debris flows. The Mr has been used to evaluate the watershed ruggedness and availability of sediments, as well as to evaluate the predominant hydrogeomorphologic processes in watersheds (^{Wilford et al. 2004}, ^{Welsh and Davies 2011}, ^{Ilinca 2021}).

Topographic information was obtained from the ALOS Palsar Digital Elevation Model (DEM), with a 12.5-meter resolution (^{JAXA and EORC 2008}). These data were chosen due to their availability worldwide so that data could be extracted from the same source for all sites. Although more detailed data are available, such as topographic maps and better resolution DEMs, especially for the Canadian sites, free data on the same scale for both areas are not available in Brazil, which could cause inconsistency in the results of the morphometric analysis. ArcGIS 10.2 was used to automatically generate drainage paths for each DEM. Satellite images from Planet and Google Earth were referenced to manually correct the auto-generated drainage paths, for example, to remove duplicates.

RESULTS

Debris flow deposits

The field surveys in the study areas show that all sites have features associated with debris flow activity, specifically local inverse grading, levees, and local lack of sorting. Specific observations of each of the study sites are shown in the following section.

The headwaters in the FN and FS watersheds feature steep slopes and an extensive sediment supply provided by the weathering of the sedimentary rocks in the catchment. Debris flows in the FN and FS watersheds form channels with levees reaching 2 m thick near the fan apex that lack sorting (Figs. 5A and 5B). Conglomerates and argillites are the most common type of clasts found in the deposits; the deposits are mostly composed of fine matrix particles smaller than 4 mm and cobbles and boulders (^{Jordan 1994}, ^{Zubrycky 2020}) (Fig. 5C). On the lower slopes, the deposits indicate flow spreading and avulsions, including the transport of logs and the burial of trees by cemented matrix (Fig. 5D). Despite being located beside one another, the FN and FS watersheds show differences in textures and morphology, with deposits in the FS watershed being more uniform and coarser in comparison to the FN watershed, favoring the formation of prominent levees in the FS watershed (^{Zubrycky 2020}).

Figure 5
(A and B) Debris flow deposit composed of argillites and (C) lobes and (D) levees at Fountain North in Lillooet.

The catchment area of MD is also steep, with an abundance of sediments, similar to the Lillooet site; however, the Mount Currie site is composed of granitic rocks. Debris flows in the MD watershed formed incised channels with levees reaching 15 m thick that lack sorting (Fig. 6A). Deposits are composed of large granite boulders, logs, and fine sediments that show local inverse grading (Fig. 6B). Compared to the FN and FS watersheds, debris flows in the MD watershed tend to remain more channelized, with spreading and avulsions further down the fan where channelization disappears (^{Zubrycky 2020}).

Figure 6
(A and B) Debris flow deposit composed of granite and (C) levees at Mount Currie D.

Compared to the study sites in Canada, debris flows appear to be relatively rare in the long-term in Itaoca and Serra da Prata, and no records of other historical events exist (^{Brollo et al. 2015}). The 2014 debris flow was triggered in GW by rainfall and the occurrence of shallow landslides, which provided initial sediments for transport. Igneous rocks, mostly granite, and fine sediments were identified in the deposits. Large boulders greater than 2 m in diameter were transported in the event (Figs. 7A and 7B). The debris flow in GW was mainly confined to the main V-shaped channel on higher slopes, losing confinement on the lower slopes and in the opening of the valley in the lowland, where spreading and deposition occurred; this spreading and deposition formed levees that are approximately 1.5 m thick with local inverse grading and local lack of sorting (Figs. 7C and 7D) (^{Gramani 2015}, ^{Dias 2021}; ^{Dias et al. 2022}).

Figure 7
(A and B) Debris flow deposit composed of granites and (C and D) levees in the Guarda-mão watershed in Itaoca.

Similarly, the debris flow in TW (2011) was triggered by shallow landslides initiated by intense rainfall, and no historical records of previous occurrences at the site exist (^{Dias et al. 2022}). The deposits are mostly composed of granite, with fine sediments and the presence of logs (Fig. 8A), and large boulders more than 2 m in diameter (Fig. 8B). The debris flow in TW was primarily confined; however, the channel lost confinement at several locations, forming deposits along the channel before reaching lowland areas and spreading near the river at the maximum runout. Levees reaching 2 m thick were formed beside the channel in depositional areas with the presence of local inverse grading and local lack of sorting (Figs. 8C and 8D) (^{Dias 2021}, ^{Dias et al. 2022}).

Figure 8
(A and B) Debris flow deposit composed of granites and (C and D) levees in the Tingidor watershed in Serra da Prata.

Morphometric parameters

The results show that morphometric parameter values may differ depending on the area (Tab. 2). The values of watershed area (A) range from 0.4 to 3.74 km². For the watersheds in Serra do Mar (n = 2), values range from 2.02 to 3.74 km², and for the watersheds in British Columbia, (n = 3), values range from 0.4 to 1.36 km². The Dd values show a different relation, ranging from 3.44 to 5.75 m/km²; they do not show the same distinction as the previous parameter, indicating watersheds with similar values in both environments. A25 ranges from 22.1 to 94.7%, with higher values presented by FN, FS, and MD (with values ranging from 86 to 93.6%) in comparison to GW and TW (with values between 22.1 and 33.5%), indicating one of the most extreme differences among all the parameters.

Table 2
Descriptive statistics of the morphometric parameters.

Parameters	Canada			Brazil
	FN	FS	MD	GW	TW
Area (km²)	0.9	0.4	1.36	3.74	2.02
Drainage Density (km/km²)	4.72	3.75	3.44	5.75	4.45
Area > 25° (%)	86	88.6	93.6	33.5	22.1
Relief ratio (m/m)	0.65	0.69	0.75	0.23	0.25
Basin length (km)	1.76	1.63	2.19	3.18	3.3
Basin relief (km)	1.15	1.139	1.65	0.753	0.83
Melton ratio (DN)	1.22	1.8	1.42	0.39	0.58

The results of the Rr, L, Br, and Mr parameters have the same distinction as observed in the A and A25 values, with Rr values ranging from 0.23 to 0.75. FN, FS, and MD have higher values (ranging from 0.65 to 0.75) than GW and TW (with values of 0.23 and 0.25). An inverse relation is observed for L, with watersheds in Canada showing low values (ranging from 1.63 to 2.19 km) compared to Brazil (with values between 3.18 and 3.33 km). Values for Br range from 0.75 to 1.65 km, and although there was not a large variation among them, FN, FS, and MD have high values (ranging from 1.15 to 1.65 km) in comparison to GW and TW (with values of 0.83 and 0.75). Lastly, the Mr values range from 0.39 to 1.8, and, similar to Br, there was not a large variation among Mr values; however, the watersheds in Canada distinctly have high values (ranging from 1.22 to 1.80) in comparison to the Brazilian watersheds (with values 0.39 and 0.58).

DISCUSSION

Debris flow deposits and comparison with occurrences worldwide

Debris flows occur in different parts of the world; they have some similarities globally but have different characteristics due to local watershed morphometries and climate. Although the rainfall values are relatively low at the Canadian sites, especially in the FN and FS watersheds, the recurrence of events suggests that constant sediment availability contributes to triggering debris flows, which can indicate a transport-limited or supply-unlimited system. In comparison, the Brazilian sites have a high mean annual precipitation, and the lack of other debris flows recorded in the GW and TW may suggest limited sediment availability, indicating a weathering-limited or supply-limited system.

As stated by ^{Jakob (2005)}, in a supply-limited watershed, it may be difficult to identify the occurrence of debris flows due to low frequency. In GW and TW, the events from 2014 and 2011 are the only historical records of recent debris flows in these areas (for the past 100 years), which suggests a low frequency of the debris flow process. The characteristics of the tropical environment also make it difficult to identify paleo-deposits, as the regeneration of dense, tropical rainforest vegetation is extremely fast. Even for the recent occurrence in GW in 2014, the identification of the main deposit features in the field was possible due only to fires caused by human activity that occurred just before the site visit in the area. Additionally, debris flows in Brazil are recorded when they affect occupied areas and cause casualties, which may influence the frequency evaluation. Another problem is the availability of annual aerial photographs for the areas, which makes it difficult to verify occurrences over the years. In comparison, in the study areas in Canada, the combination of historical records of several occurrences and the availability of aerial and satellite photographs makes it possible to identify and map multiple deposits and affected areas.

Although the total number of events may be underreported, because multiple events could have happened between image collections and smaller events may not be visible in the images, the identification of deposit features in the field in the long-term tends to be easier in the FN, FS, and MD watersheds, in contrast to GW and TW; this contrast is mostly due to differences in the time that it takes for the vegetation to regenerate, which tends to be quicker in a tropical environment as a consequence of the climate. The geomorphic evidence of debris flows agrees with the main characteristics pointed out by ^{Costa (1984)}, ^{Jakob (1996)}, ^{Johnson (1970)}, and ^{VanDine (1996)} and a worldwide register of debris flow-affected areas, which includes some occurrences in South America (^{García-Martínez and López 2005}, ^{García-Delgado et al. 2019}), Europe (^{Rickenmann and Zimmermann 1993}, ^{Breien et al. 2008}, ^{Stoffel 2010}, ^{Ozturk et al. 2018}, ^{Ilinca 2021}), Asia (^{King 1996}, ^{Cui et al. 2013}), Oceania (^{Pierson 1986}), and North America (^{Jackson et al. 1987}, ^{Gabet and Bookter 2008}, ^{Riley et al. 2013}, ^{Kean et al. 2019}). The main features present at all sites were levees and local lack of sorting, followed by the presence of local inverse grading and very large boulders. The presence of large boulders in the GW, TW, and MD watersheds may be associated with geological and lithological differences between the FN and FS watersheds. While the Lillooet area has a predominance of highly jointed sedimentary rocks, Itaoca, Serra da Prata, and Mount Currie have a prevalence of more massive igneous and metamorphic rocks (Fig. 9).

Figure 9
Debris flow deposits in (A) Lillooet, (B) Itaoca, (C) Mount Currie, and (D) Serra da Prata.

Morphometric parameters and comparison with values from sites worldwide

Regarding morphometric parameters, the literature features the use of A, Rr, L, Br, and Mr, with published studies in different parts of the world, particularly in North America (Canada and USA), Europe (Italian Alps, Bulgaria, and Romania), Asia, and Oceania (Taiwan and New Zealand), and some studies in South America (Brazil and Puerto Rico) (Tab. 3). The results obtained in this paper show similarity with published data and their variations, considering, especially, the climate (Fig. 10).

Figure 10
Boxplot of parameters from Tab. 3 categorized by climatic zone.

Table 3
Comparison of parameter values at different sites.

Source	Area (km²)	Relief ratio (m/m)	Length (km)	Basin relief (km)	Melton ratio	Location
Jackson et al. (1987)	----	----	----	----	0.25 to 0.30	Alberta, Canada
Slaymaker (1990)	0.4 to 7	----	----	----	----	British Columbia, Canada
Marchi et al. (1993)	0.20 to 14	----	----	----	0.49 to 1.74	Italian Alps, Italy
Jakob (1996)	0.3 to 14.4	0.27 to 0.76	----	0.73 to 2.06	----	British Columbia, Canada
Cenderelli and Steven Kite (1998)	1.78 to 17.47	0.35 to 0.49	1 to 4.28	----	----	North Fork Mountain, USA
Scally et al. (2001)	0.05 to 10.90	----	----	0.31 to 1.36	0.38 to 1.77	British Columbia, Canada
Marchi et al. (2002)	4.1			1.15		Italian Alps, Italy
Wilford et al. (2004)	0.2 to 4.1	0.3 to 0.49	0.28 to 4.68	0.6 to 1.4	0.66 to 1.21	British Columbia, Canada
Gabet and Bookter (2008)	0.08 to 0.76	----	----	----	----	Montana, USA
Kovanen and Slaymaker (2008)	0.29 to 32.9	0.01 to 0.58		0.58 to 1.2	0.15 to 1.07	North Cascades foothill, USA
De Scally et al. (2010)	0.18 to 9.66	0.25 to 0.88	1.05 to 4.5	0.55 to 1.93	0.45 to 1.59	New Zealand
Portilla et al. (2010)	0.029 to 3.76	0.22 to 0.51	----	0.29 to 1.42	0.45 to 2.91	Pyrenees Mountain, Italy
Welsh and Davies (2011)	----	----	< 2.7	----	> 0.60	New Zealand
Chen and Yu (2011)	0.51 to 8.63	0.20 to 0.44	1.54 to 5.75	----	----	Taiwan
Simoni et al. (2011)	0.28 to 9.40	----	0.9 to 3.9	----	----	South Tyrol, Italy
Dias et al. (2016)	24 and 20	0.07 and 0.13	----	----	----	Serra do Mar, Brazil
Picanço et al. (2016)	1.03 to 3.06	----	1.99 to 3.71	0.66 to 1.11	0.41 to 0.78	Serra do Mar, Brazil
Dotseva and Gerdjikov (2020)	1.52 to 3.76	0.05 to 0.68	----	----	0.54 to 0.73	Stara Platina Mountains, Bulgaria
Nikolova et al. (2020)	0.015 to 39.27	0.05 to 0.68	0.33 to 15.50	0.16 to 0.82	0.13 to 1.59	Eastern Rhodopes, Bulgaria
Ilinca (2021)	0.005 to 1.02	Mean 0.56	< 1.7	----	> 0.55	Southern Carpathians, Romania
Coe et al. (2021)	0.094 to 0.25	----	----	0.13 to 0.25	0.53 to 0.87	Puerto Rico
V.C. Dias et al. (2021)	----	0.07 to 0.11	----	----	----	Serra do Mar, Brazil
This paper	0.9 to 1.36	0.65 to 0.75	1.63 to 2.19	1.15 to 1.65	1.22 to 1.8	British Columbia, Canada
This paper	2.02 and 3.74	0.23 and 0.25	3.18 and 3.3	0.75 and 0.83	0.39 and 0.58	Serra do Mar, Brazil

The A parameter is one of the most commonly used parameters for the evaluation of debris flow occurrence. The results for A show that, although the values are slightly smaller at the sites in Canada than at the sites in Brazil, they are within the typical range for debris flow watersheds evaluated in other areas. The Canadian sites are below the median for Temperate zones, and the Brazilian sites are above the median for Tropical zones (Fig. 10A). Although small areas are considered debris flow prone, the intensity of the process may vary depending on the size of the watershed, and as stated by ^{Ilinca (2021)}, larger watersheds may be more prone to host debris floods. Both occurrences in Brazil initiated as debris flows, but after initial deposition, the processes continued to low-relief areas, turning into debris floods and destroying the city center, which did not occur at the sites in Canada, where the process was restricted to the watershed as a debris flow.

The FN, FS, and MD sites had high Rr values compared to the GW and TW sites (maximum 0.75 versus 0.25, respectively), which suggests that the watersheds are more prone to intense processes and have greater availability of sediments. 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. Despite presenting similar mean and maximum values, the minimum value for the weathering-limited type was low in comparison to the transport-limited type (0.27 versus 0.38, respectively) (^{Jakob 1996}). Other sites with the occurrence of debris flows also showed this variance in Rr values, particularly the North Cascades in the USA (0.01 to 0.58) (^{Kovanen and Slaymaker 2008}), the Pyrenees Mountains in Italy (0.22 to 0.51) (^{Portilla et al. 2010}), and Taiwan (0.20 to 0.44) (^{Chen and Yu 2011}). In comparison to the distribution of data from the literature, the values for the Canadian sites are outside of the range of those for Temperate zones, while for the Brazilian sites, the values are above the median values for Tropical zones (Fig. 10B).

Whereas the L values at FN, FS, and MD are within the range of values suggested by ^{Welsh and Davies (2011)} for debris flow areas (up to 2.7 km), the watersheds in Brazil have values above this limit (3.18 and 3.3 km). Other affected areas worldwide also show high L values, such as North Fork Mountain in the USA (1 to 4.28 km) (^{Cenderelli and Steven Kite 1998}); British Columbia, Canada (0.28 to 4.68 km) (^{Wilford et al. 2004}); New Zealand (1.05 to 4.5 km) (^{Scally et al. 2010}); and Taiwan (1.54 to 5.75 km) (^{Chen and Yu 2011}). Among the 72 sites evaluated by ^{Ilinca (2021)}, only one watershed associated with debris flows had a length greater than 1.7 km (2.78 km), which, according to the author, may indicate that although these sites can be affected by debris flows, debris flood and flood processes may be more frequent. Considering the results by climate zones, MD is close to the median value and FN and FS are below 25% of the results for Temperate, while the values for GW and TW are close to the maximum considering the distribution for Tropical zones (Fig. 10C).

Unlike L, Br values did not show a high variance among the sites; however, FN, FS, and MD had the highest values, which may contribute to the high frequency of occurrences in comparison to GW and TW sites. Values reported in the literature are similar to the results in this research (Tab. 3), with higher values in British Columbia, Canada (> 2 km) (^{Jakob 1996}) and New Zealand (1.9 km) (^{Scally et al. 2010}), and the lowest values in Puerto Rico (0.13 to 0.25 km) (^{Coe et al. 2021}). Despite having very similar mean values for weathering-limited and transport-limited basins, the latter has a higher value (2.06 km) in watersheds evaluated by ^{Jakob (1996)}, similar to the FN, FS, and MD sites, which may favor the recurrence of the processes at these sites. In Europe, values are more similar to those of the TW and GW sites, with values ranging from 0.16 to 0.82 km in Bulgaria (^{Nikolova et al. 2020}) and from 0.20 to 1.42 km in Italy (^{Portilla et al. 2010}). Considering climate zones, values in FN, FS, and MD are above 75% of the values for Temperate zones, while values in GW and TW are above the median but under 75% of the sample (Fig. 10D).

The Mr is largely used as an indicator of debris flows in small watersheds, particularly since the 2000s. ^{Welsh and Davies (2011)} stated that values above 0.60 indicate watersheds with a predominance of debris flows, while values between 0.30 and 0.60 indicate a predominance of debris floods and floods. The results show that the watersheds in Canada have values characteristic of debris flow-prone watersheds (1.22 to 1.8), including those outside of the distribution for Temperate zones. In contrast, the areas in Brazil are below the limit established by ^{Welsh and Davies (2011)} (0.39 and 0.58); in other words, they are not classified as having debris flows as a major process but are still in the range of values for cases in Tropical zones (Fig. 10E). Nevertheless, other regions worldwide with the occurrence of debris flows have Mr values below 0.60, such as Italy (0.45 to 2.91) (^{Marchi et al. 1993}, ^{Portilla et al. 2010}), New Zealand (0.45 to 2.91) (^{Scally et al. 2010}), Bulgaria (0.13 to 1.59) (^{Nikolova et al. 2020}), Puerto Rico (^{Coe et al. 2021}), and even Canada and the USA (0.25 to 0.30 and 0.15 to 1.07, respectively) (^{Jackson et al. 1987}, ^{Kovanen and Slaymaker 2008}). Taking into account the results from previous morphometric parameters, this difference may indicate that, although debris flows occur in watersheds with values below 0.60, they are not the main process and have reoccurred less frequently. In this way, attributes from FN, FS, and MD indicated an area with debris flow as a dominant process, while results from TW and GW indicated areas with a mixed process, highlighting debris flows and debris flood.

The Dd and A25 parameters are not as widely used as the other parameters discussed above; however, values from the sites in Canada and Brazil also contribute to understanding differences in debris flow occurrences in these areas. Regarding Dd, the results in Tab. 2 indicate well-drained watersheds, particularly the FN and GW sites, which have high values (4.72 and 5.75 km/km²). In British Columbia, Canada, ^{Jakob (1996)} found values between 0.6 and 6.8 km/km² in weathering-limited basins and 2.1 and 7.4 km/km² in transport-limited basins. ^{Cenderelli and Steven Kite (1998)} divided the watersheds according to debris flow zones and found higher values for Dd in failure zones (5.62 to 12.40 km/km²), followed by values in transport zones (2.57 to 4.44 km/km²) and deposition zones (2.67 to 4.36 km/km²) in the USA. The high values in failure zones indicate the need for a well-drained area for the initiation of the process. Despite showing lower values in comparison to previous examples (1.57 to 2.12 km/km²), which indicates poor to moderate drainage, watersheds evaluated by ^{Dotseva and Gerdjikov (2020)} also have debris flows, which may indicate the influence of other morphometric features.

All sites in Canada have values higher than 85% for A25, which indicates extremely steep watersheds, in comparison to values found at the Brazilian sites (22.1 and 33.5%). This difference may influence the recurrence of events in the areas, with more frequent events occurring due to the availability of sediments to transport in the FN, FS, and MD sites compared to the GW and TW sites. This difference in the percentages of watersheds with high-angled areas was also observed in other studies. In affected areas in Bulgaria, ^{Nikolova et al. (2020)} use the percentage of watersheds above 30 and 45 degrees as a parameter, with results between 1.28% and 74.1%. Likewise, ^{Coe et al. (2021)} evaluated watersheds in Puerto Rico using areas above 30° as the limit, obtaining results between 54 and 62%. Similar to Dd, this variability among the results in different areas may indicate the influence of other morphometric characteristics in controlling debris flow occurrences in watersheds.

CONCLUSIONS

This study compared watersheds affected by debris flows in two different environments through field surveys, geomorphic characterization using morphometric parameters, and comparing results from occurrences worldwide. Despite significant differences in geology and climate, the study areas had similar main debris flow features and morphometries. Although the watersheds in Brazil had some morphometric variations from the literature values that are characteristic of debris flow-prone areas, other studies showed results in agreement with the values shown here. This may indicate variability in the morphometric characteristics of a watershed concerning the occurrence of debris flows.

Despite having a high average annual rainfall in comparison to areas in Canada, a single debris flow occurrence in the historical record in each of the watersheds in Brazil suggests a weathering-limited system. In this way, regardless of the higher amounts of precipitation, sediment recharge may take more time than in the transport-limited systems observed in the temperate watersheds. This characteristic, in addition to the lower values for A25 (33.5 and 22.05), Rr (0.23 and 0.25), and Mr (0.39 and 0.58), may contribute to the differences in the recurrence of debris flows.

However, it is important to point out the need for more studies about debris flow occurrences in Brazil, specifically regarding morphometric analyses and physical characterization of affected areas, considering slope, soil type, soil thickness, degree of weathering, and hydrogeology. This characterization should ideally be made immediately after the event takes place, aiming for an accurate analysis of the process, adding more data for comparison with future occurrences and for comparison with other events worldwide. The monitoring of susceptible areas using remote sensing, aerial photos, and rainfall intensity, is needed to provide more data about debris flow processes in Brazil. Also, it is necessary to improve rainfall measurement in susceptible areas, such as Serra do Mar, providing more information about rainfall thresholds that trigger debris flow events. Despite the limitations and lack of historical data, this study contributed to the understanding of debris flows in Brazil by comparing deposit features and morphometric characteristics with those found in affected areas in Canada. The similarities and differences between the areas can help develop future studies about debris flows in Brazil, especially considering data availability limitations.

ACKNOWLEDGMENTS

The authors gratefully acknowledge: support for this project from the São Paulo Research Foundation (FAPESP) (Grants #2018/08402-4; #2019/11223-7), the Graduate Program in Physical Geography from the Universidade de São Paulo (USP) and the University of British Columbia (UBC); support for the field surveys in Brazil provided by Professor Claudinei Taborda da Silveira from the Universidade Federal do Paraná, Professor Maria Carolina Villaça Gomes from the Universidade do Estado de Santa Catarina, Fernando Alves de Abreu, and the Itaoca city Hall; support for the field surveys in Canada by Sophia Zubrycky, David Bonneau and Ivan Li; and the Editor-in-Chief, the Associate Editor, and the anonymous reviewers for their comments and suggestions, which helped to improve this paper.

• Funding
  This work was supported by the São Paulo Research Foundation (FAPESP) (grant numbers #2018/08402-4; #2019/11223-7).
• Role of the funding source
  This funding source had no role in the design of this study and will not have any role during its execution, analyses, data interpretation, or decision to submit results.
• Data availability statement
  The raw data required in this study are available upon request by contacting V. C. Dias ([e-mail]).

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Publication Dates

• Publication in this collection
  29 Aug 2022
• Date of issue
  2022

History

• Received
  02 Sept 2021
• Accepted
  21 Mar 2022