On 23 February, 2020 an enormous, catastrophic debris flow tore down the Salkantay River in Santa Teresa, Peru.
This cataclysmic event has killed at least four people, with a further 13 reported to be missing. Given the magnitude of the flow, this number is probably uncertain.
Over the last 24 hours more information has become available about the Salkantay landslide and mudflow.
The triggering event is being described as an ice / rock avalanche with an initial volume of 400,000 cubic metres.
This has clearly bulked up to form a mudflow with a much higher volume, presumably through entrainment of ice and saturated debris in the channel.
This is similar to the Seti River rock avalanche and debris in Nepal in 2014, which also had devastating effects.
On Twitter, Julio Montenegro G. has posted an interpretation of the event, based upon an image of the scar, which has then been located on pre-event imagery
#Peru:The Santa Teresa flood in #Cusco originated in the #Salkantay glacier (6,271 masl) after an ice-rock avalanche in S slope of W sector with approx. 400,000 m3 (INAIGEM, 2020) that reach a front moraine then Humantay lake, overtopping without beaching. pic.twitter.com/4Ifp4d4NXx— Julio Montenegro G. (@Juliohydraulics) February 27, 2020
I am not sure as to the origin of the image that shows the scar of the initial failure, but a better version was posted to Twitter by Turismo Peru:
Calentamiento global causo el desprendimiento de glaciar del Apu Salkantay este causo el aluvion que hoy día lamentamos tomemos conciencia sobre nuestras acciones pic.twitter.com/jaQrfhjkFH— Turismo Perú (@PeruTravelExpe) February 26, 2020
If this is indeed the scar then my interpretation is that this is a classic wedge failure in the rock mass, with a near vertical fall onto the ice and moraine at the toe of the slope.
The rock slope would have been a mixture of rock and ice, both on the surface and within fractures.
On impact the mass has probably fragmented to form an ice / rock avalanche, which has then entrained debris and ice / snow / water, transitioning to become the mudflow seen in the videos.
This has behaved in a manner that is akin to a lahar, with a large volume, high velocity and long runout. This event is reminiscent of the 2012 Gayari ice and rock avalanche in Pakistan and the 2017 Villa Santa Lucia landslide in Chile.
Reports suggest that Salkantay Cocha lake remains intact, but that waves within the lake, generated by the landslide, have caused some erosion of the moraine dam. This now needs to be monitored.