Rainfall-induced debris flows and shallow landslides in Ribeira Valley, Brazil: main characteristics and inventory mapping

. Debris flows are one of Brazil's most frequent mass movement processes, triggered by extreme rainfall events and initial volume provided by shallow landslides. Despite the recurrence of catastrophic occurrences, Brazil still lacks basic data containing the main characteristics of previous events. In this way, this research aimed to make a morphometric characterization of the event and to provide debris-flow and shallow landslides inventories. For the morphometric analysis, a Digital Elevation Model (DEM) was used. For the inventory map of debris flow runout and shallow landslides scars, a post-event free access image from Google Earth Pro and satellite images from RapidEye were used. The results show that debris flows had two main flows that affected different areas of the city of Itaóca. Also, one single shallow landslide contributed as initial volume to the debris flows that reach the city downtown, demonstrating the importance of entrainment. Shallow landslides analysis shows its concentration in slopes between 20.1 – 30°, with orientation South and Southeast, elevation between 600 – 800m, and in concave curvatures. The results helped to better understand debris flows in Brazil, highlighting their relationship with the occurrence of shallow landslides as one of the main triggering factors. Those data are crucial to mitigation action of possible new events.


Introduction
Debris flows, along with shallow landslides, are one of the most destructive types of landslides due to the capability to transport a variety of materials (e.g., large boulders, logs) over long distances [1][2][3]. These characteristics contribute to the heavy damage caused by the process when affecting occupied areas.
In Brazil, most extreme events are concentrated in the Serra do Mar Region, a mountain range with 1,500 km of extension located on the southeast coast of the country. Climate and geology favor the recurrence of debris flows and shallow landslides, highlighting average rainfall of > 2,000 mm/year, and high slopes (> 25°). In this region, the joint occurrence of debris flows, and shallow landslides are responsible for the main previous catastrophes of the whole country, such as 1967 in Caraguatatuba (120 deaths) [4,5], and 1994 in Cubatão (US$ 44 million of economic losses) [6,7], 2008 in Santa Catarina (78,000 homeless and 134 deaths) [8], 2011 in the Rio de Janeiro Mountain Range (> 1,000 deaths and US$ 3.0 billion of economic losses) [9], 2013 in the Imigrantes Highway (1 death) [10], and 2014 in Itaoca (332 homeless, 25 deaths and US$ 2.63 million of economic losses) [11][12].
Despite their high frequency, the Brazilian events lack detailed characterization and inventories, especially just after their occurrences [13,14]. The absence of information such as the size of scars and/or runout, location, and slope characteristics may negatively impact upcoming studies (e.g., modeling analysis) or even mitigation actions for possible new events.
As part of an effort to provide basic data and information about previous occurrences, this research aimed to make a morphometric characterization of the 2014 event and to provide debris-flow and shallow landslides inventories of one of the debris flow and shallow landslides events in Brazil.

Study area: the 2014 event
The Ribeira Valley region is in the southwest part of the Serra do Mar in its continental area, in the boundary of São Paulo and Paraná States. Apiaí and Itaoca are the two main cities affected by the 2014 event ( Fig.1).  On January 12-13 of 2014, shallow landslides and debris flows were triggered in the area by an extreme rainfall event, causing damage to local infrastructure and economic losses, 332 homeless, and 25 deaths [11,12,15]. The debris flows were triggered by the shallow landslides, that acted as the provider of initial material for transport. The flows initiate on high slopes (Fig. 2a), with initial deposition of most coarse material (e.g., large boulders) in the valley of the watersheds (Fig. 2b) and turning into a debris flood and reaching the lowest parts of the relief, the city downtown, and the Lageado neighborhood, with deposit composed mostly by logs and mud (Fig. 2c).

Inventory mapping
Shallow landslides and debris flow mapping were made based on a post-event free access image of Google Earth Pro, dated 2014-OCT-08, and satellite images from RapidEye (5 m).
The visual criteria for shallow landslides mapping were the absence of vegetation, format and size, drainage network distance, slope position, planar rupture surface, and altimetric variation. The scars were divided into two groups, one considering the rupture, transport, and deposition (S1), and another considering only rupture and transport (S2) (Fig. 3). S1 and S2 were identified based on contour lines (10 m) and expert knowledge. The debris-flow runout was made considering visible erosion on the stream, which creates a path through where the flow passed, and the presence of deposits (e.g., large boulders), which have been identified in the image (Fig. 4).

Morphometric analysis
Morphometric parameters were derived from a SRTM Digital Elevation Model (DEM) processed by the Alaska Satellite Facility to remove outlier data and resampled to a 12.5 m spatial resolution. The morphometric parameters derived were: slope (degrees), aspect, and curvature. Also, the scars' total area and basic statistics were calculated (mean, median, minimum, maximum).

Results and discussions
A total of 1.850 shallow landslide scars (SLS) were mapped. They were concentrated in slopes between 20.1 -30°, South and Southeast orientation, 600 -800m of elevation, concave curvatures (Table 1), and in Quartz-Monzonite and Biotite Monzogranite rocks. SLS occurred in several watersheds, but they are concentrated in the Gurutuba watershed (n=289). Debris flows occurred on two main fronts, initiating in the Palmital river, in the north boundary of Itaoca with Apiaí (Fig. 5a), and in the Guarda-mão watershed, in the north of Itaoca (Fig. 5b). The debris flow in the Palmital river reach the Lageado neighborhood, causing damage to buildings and infrastructures (Fig. 6a). At least 5 debris flow in small watersheds reached the main river and contributed to the flow that affected the neighborhood. From its initial point to the final deposition, the debris flows travel about 13.26 km. In the same way, the debris flow initiated in the Guardamão reached the Palmital river downstream and caused damage to the Itaóca downtown, traveling about 10 km (Fig. 6b). The flow had 2 initial points, with the major one being triggered by only one landslide.
The watershed with the highest number of SLS was Gurutuba (289) and no evidence of an expressive debrisflow occurrence able to be identified through the images. On the other hand, the Guarda-mão watershed had an expressive debris-flow occurrence but a smaller number of SLS, with only one landslide being responsible for the initialization of the debris flow in the main channel and with most of the volume generated by the entrainment of material through the channel [12]. The importance of entrainment was already pointed out by several authors as the main factor to be considered in the final volume calculation [16][17][18][19][20][21][22]. However, more studies are needed to better understand its role in this occurrence.

Conclusions and Future work
This study produced results that agree with the findings of many previous works in this field, highlighting here the importance of entrainment in the debris-flow process.
These findings enhance our understanding of debris flows, especially considering the occurrence of the process in a tropical environment. However, more https://doi.org/10.1051/e3sconf/202341505003 , 05003 (2023) E3S Web of Conferences 415 DFHM8 research is needed to better understand the specificities of debris-flow occurrences, especially related to the process of entrainment, the main source of the total volume produced.
More detailed studies are needed in Brazil to compound a database and a panorama of debris flows in the country, especially considering triggering factors, entrainment, and deposit characteristics.