15 February 2018
The evolution of post-seismic debris flows
One of the most important legacies of large earthquakes in mountain chains is the increased occurrence of landslides, which continue to cause damage and disruption for years after the mainshock. This issue very much came to the fore in the aftermath of the 1999 Chi-Chi earthquake in Taiwan, in particular in relation to the important Central Cross Island Highway. Repeated attempts were made to reopen the highly damaged sections of road after the earthquake, thwarted in each case by landslides that destroyed the infrastructure. This section of the road remains closed almost 20 years later, and there are no plans to reopen it, with traffic being diverted to throther branch to the south.
However, the defining event in terms of post-seismic debris flows is undoubtedly the 2008 Wenchuan earthquake in China, which was the most efficient earthquake in terms of landslide generation that I have known. The area affected by the mainshock has been dogged by post-seismic debris flows, with terrible consequences. This has highlighted the lack of understanding of the fundamental mechanisms that underpin these continued events, and the processes that lead to reductions in their occurrence. In a paper just published in the journal Engineering Geology, Fan et al. (2018) explore post-seismic debris flows in a major gully in the Gaojiagou Ravine, in the heart of the area affected by the Wenchuan earthquake. This area was heavily affected by landslides in the mainshock, and thereafter was struck by four major debris flows, in 2010, 2011, 2013 and 2016. The effects of the earthquake remain as shocking today as they did at the time. This is a Google Earth image of the study site of Fan et al. (2018), collected in 2005 (note there is no imagery for the portion that is blanked out on the left hand side):
Whilst this is the same area after the earthquake:
Fan et al. (2018) studied the four debris flows that have occurred in the ravine since the earthquake. They found that through time there was a change in the initiation mechanisms of the debris flows, transitioning from simple landslides through to channel-bed failure and then to channel-bank erosion. At the same time the location of the initiation of the post-seismic debris flows migrated to lower positions on the landscape. They also observed changes in the runout characteristics of the post-seismic debris flows, with mobility reducing with time. As the mechanisms of the debris flows changed, the triggering threshold rainfall also increased.
With time the volume of loose material stored in the ravine reduced, with successive debris flows flushing sediment out of the system. Fan et al. (2018) include a really nice series of maps of the areas within the ravine that were covered in loose materials:-
This really interesting study provides real insight into the evolution of post-seismic debris flows, and the changes that occur with time in the aftermath of a large earthquake in mountains. This process is now playing out in Nepal. The study indicates that those being affected by post-seismic debris flows in that area have some years of increased levels of risk to come.
R.L. Fan, L.M. Zhang, H.J. Wang, , X.M. Fan 2018. Evolution of debris flow activities in Gaojiagou Ravine during 2008–2016 after the Wenchuan earthquake. Engineering Geology, 235, 1-10. doi: https://doi.org/10.1016/j.enggeo.2018.01.017