Thursday, May 23, 2013

The landslide story (Nature Geoscience Journal)

The landslide story
May 22, 2013
Probe International

http://journal.probeinternational.org/2013/05/22/the-landslide-story/

Chinese experts in landslide and geohazard protection fear debris flows,
triggered by an epic 2008 earthquake in Sichuan Province, may pose a
threat to the region for two decades. A tremendous amount of loose
material from the landslides is suspended on hillslopes, ready to be
washed away by rain. The potential for ongoing landslides and secondary
hazards, such as flooding and blocked rivers, they argue, warrants
further investigation.

The full version of this report is available from the publisher here:
http://www.nature.com/ngeo/journal/v6/n5/full/ngeo1806.html [by
subscription only]

The Landslide Story
By Runqiu Huang and Xuanmei Fan
Nature Geoscience Journal
6, 325-326 (2013), doi:10.1038/ngeo1806
Published online: April 29, 2013

In "The Landslide Story," geohazard experts Runqiu Huang and Xuanmei Fan
look at the substantial rise in geological hazards experienced by
China's southwestern Sichuan Province following a powerful magnitude-7.9
earthquake on May 12, 2008 in the region's Wenchuan County. Ranked as
China's largest seismic event in more than 50 years, the devastating
quake killed at least 80,000 people.

The Wenchuan quake, they say, triggered more than 60,000 landslides over
an area of 35,000 km2 and caused about one-third of the total number of
fatalities recorded. But the slippage isn't finished as a tremendous
amount of loose material remains suspended on the hillslopes, ready to
be eroded and transported by rain.

The dangers are already evident. The authors note the incidence of
debris flow in the four years following the 2008 quake has increased
three-fold.

Moreover, they estimate, the risk of more debris flows that directly
result from sediment movement during the 2008 earthquake may remain
active for another two decades.

Though an attempt was made by the authorities following the 2008 quake
to assess future hazards, "perhaps not enough attention was paid to the
cascade of geohazards following the earthquake. For example, landslides
triggered by the earthquake blocked rivers, which in turn generated
risks of floods."

The long-term effects of a "cascade of potential hazards were not fully
taken into account in the post-seismic hazard assessment and in the
selection of sites for the reconstruction of destroyed buildings," the
authors caution. In one instance, "following intense rainstorms more
than two years after the earthquake, two large-scale debris flows
partially dammed the Minjiang and Mianyuan rivers. When the dams burst,
the newly reconstructed towns of Yingxiu and Qingping were
catastrophically flooded."

In addition, Huang and Fan say, the long-term impact of the 2008
Wenchuan earthquake on sediment flux in the affected watersheds was also
underestimated initially:

"We now realize that increases in sedimentation as a result of the
shaking will pose a significant problem for rivers and their downstream
reaches. Some river beds have already been elevated by more than 10 m.
These changes raise the probability of floods in the future, and could
severely affect the generation of hydropower."

In order to anticipate the potential short- and long-term risks
associated with future seismic events, the authors say "focused research
efforts must be invested into quantifying the impact cascade following a
large earthquake."

Runqiu Huang and Xuanmei Fan are at the State Key Laboratory of
Geohazards Prevention and Geoenvironment Protection, Chengdu University
of Technology, Chengdu, Sichuan, China.

Xuanmei Fan is also at the Faculty of Geo-Information Science and Earth
Observation (ITC), University of Twente, 7500 AE, Enschede, The Netherlands.
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