The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.
In a recent post, I highlighted the likely role that high atmospheric temperatures, associated with a combination of anthropogenic climate change and the recent (now waning) El Nino conditions, are playing in generating high levels of loss from landslides. However, increased impacts from landslides are not just associated with changes in rainfall patterns, but also with human factors. One of these is the growth of urban areas in dangerous locations.
There is a very interesting open access paper (Bastos Moroz and Thieken 2024) in the punchily-titled journal Natural Hazards and Earth System Sciences that examines this issue in the context of the 2023 rainfall-induced disaster that struck the North Coast of São Paulo (NCSP), Brazil in February 2023. This disaster, which I covered at the time, killed 65 people, the majority of whom died in landslides.
The paper focuses on the municipality of São Sebastião, as 64 of the 65 fatalities occurred in this area. This site is located around [-23.7662, -45.6919] – the Google Earth image below shows the region:-
For example, one area that was particularly badly affected was the community of Barra do Sahy. This Planet Labs image, collected on 26 February 2023 (after the event) shows this location:-
Barra do Sahy is located just to the east and northeast of the marker. Note the multiple landslides in the image, predominantly triggered by the February 2023 rainfall event. Just visible at the northern end of Barra do Sahy is a series of landslides that have encroached into the community. According to Bastos Moroz and Thieken (2024), these landslides damaged 23 houses, 13 of which were totally destroyed. Of these, 21 houses were damaged by a single landslide.
Bastos Moroz and Thieken (2024) mapped the damaged and destroyed buildings in the municipality of São Sebastião. They have also classified them as being precarious (i.e. informal / unplanned) and non-precarious (i.e. formal / planned) urban areas. They show that there has been rapid urban growth in this area over the period since 1985, with a substantial increase in precarious communities.
The results of the analysis of the February 2023 event are very interesting. Although precarious settlements only represent 10.4% of the total urban area, 60 of the 102 buildings damaged (c.58%) were classified as being precarious. Of the 46 houses that were destroyed, 33 were in precarious settlements (c.72%). Most of these houses were located on the margins of the villages, as is the case for Barra do Sahy.
Bastos Moroz and Thieken (2024) also looked at the chronology of the development of the areas impacted by the February 2023 disaster. Again, looking at the 46 houses that were totally destroyed, 24 (c.52%) were constructed since 2001.
Thus, recent development of precarious buildings on the margins of the communities has greatly increased the risk from landslides. The authors show that these precarious settlements have been constructed in areas of higher landslide hazard.
The driver for the increased urbanisation of this area was the construction of the Rio-Santos highway in the late 1970s, which created economic opportunities. People have relocated to take advantage, but the poorer communities have been excluded from safer areas by high land values, forcing them onto areas of higher hazard.
It has long been thought that the development of informal urban communities has been a factor in increasing landslide risk, but there has been little quantitative data to support this notion. Bastos Moroz and Thieken (2024) is a fascinating demonstration that this point is true in Brazil. It is likely to be the case in many other locations as well.
Final word
At the time of writing, Typhoon Krathon has finally made landfall in southern Taiwan. It is expected to sit over the southern part of the island for the next 24 hours. In the first 14 hours of today (3 October), parts of the Taipei City area have received about 450 mm (roughly 18 inches) of rainfall. Whilst the likely impact of this typhoon is unclear (the landscape of Taiwan is adapted to incredible levels of rainfall), this is one to watch over the next two days.
References
Planet Team (2024). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/
Bastos Moroz, C. and Thieken, A. H. 2024. Urban growth and spatial segregation increase disaster risk: lessons learned from the 2023 disaster on the North Coast of São Paulo, Brazil. Natural Hazards and Earth System Sciences, 24, 3299–3314, https://doi.org/10.5194/nhess-24-3299-2024, 2024.
