The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.
On 2-3 December 2023, a series of devastating channelised debris flows were triggered by heavy rainfall on the slopes of Mount Hanang in Tanzania, causing immense devastation. At least 89 people were killed, mainly in the towns of Katesh and Gendabi. I blogged about this dreadful event at the time, but noted that the available Planet Labs imaghery meant that a full picture of the disaster was not available.
There is a good article on the Daily News website that discusses the disaster, if you want a local perspective.
Planet Labs has now captured a good image of the site using their Super Dove instrument, which provides a good sense of the order of events. In particular, there is almost perfect image from 21 December 2023 that provides startling clarity.
This is the image of Mount Hanang that was captured on 21 December 2023:-
This is a dramatic and terrible picture. Many shallow landslides have been triggered across the slopes of Mount Hanang, some of which are perhaps surprisingly large. Some of the landslides in the cluster in the northwest, for example, are 300 m in length before they have intercepted the drainage channel. I count at least eleven major landslides in this 1 x 1 km block alone, and there are many, many more failures.
To visualise these landslide better, I have draped the image onto the Google Earth DEM:-
In many cases these shallow landslides have runout directly into a channel, and multiple landslides have then coalesced to form channelised debris flows. The image below shows the was in which these flows have travelled down the drainage lines and then have inundated the inhabited farmland and towns:-
This an appalling image – to give an idea of the scale, the distance along the channel from the landslide source areas to the toe of the main landslide is about 7 km.
The Citizen in Tanzania reports that life is returning to normal in Katesh, but the impact of the disaster lives on. Interestingly, it highlights that there is uncertainty in the community there about the causes of the disaster, with rumours of a volcanic eruption. As the above images show, there is nothing novel and surprising about the catastrophe, if the impact of rising rainfall intensity and the presence of people and properties in the flow path are taken into consideration.
Sadly, this event is likely to repeat, perhaps with an even higher magnitude, so management of the channels (with check dams for example) and avoidance of construction in the runout zone will be key. to that end, I want to give a shout out to the INYUAT E MAA NGO, who are working to understand the impact of the 2-3 December 2023 landslides on the livelihoods of indigenous people in the region. This is vital work.
Reference
Planet Team (2024). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/
