Why a Debris-Flow Workshop?
I’ve been
interested in landslides and related phenomena ever since the mid-1970s when I
worked as geosciences manager on the Environmental Impact Statement for the
completion of U.S. Interstate Highway 93 through Franconia
Notch State
Park in New
Hampshire.
Widening the existing two-lane highway to four lanes would have meant
cutting into the toes of landslide scars in the narrowest part of the notch
next to Profile Lake.
Our geotechnical engineers concluded that the widened highway would be
vulnerable to slope failures without major structural mitigation. The visual impact of retaining walls, etc.
was unacceptable to many local citizens who wanted to maintain the scenic
beauty of the park. As a result, the new
highway was narrowed to a two-lane parkway for several hundred feet past the
old landslides. As far as I know, this
is the only section of two-lane highway in the entire US Interstate Highway system.
For several
years, I have hoped to visit China,
but I’ve wanted to travel there in a professional capacity rather than going as
a tourist. Last year, I began to regularly
check out the listings of conferences and educational programs on the website
of WASWAC (the World Association of Soil and Water Conservation). When I saw an upcoming workshop in August
2012 on debris-flows (a particularly deadly form of landslide) in Chengdu, China,
I enthusiastically contacted the organizers (the Institute for Mountain Hazards
and Environment in Chengdu)
for further details. My application to
attend the workshop was accepted and I secured a business visa for China through a passport and visa expediting
service in Denver.
 |
Flag of the Peoples’ Republic of China
Map of China
with red box around Chengdu
(located in the south central part of the country).
|
But
wait! How could I attend a workshop in China since I
don’t even know how to ask for the location of the men's bathroom in
Chinese? No problem – all the workshop
presentations were in English and most of the participants could speak some
English. Initially, I was surprised that
a Chinese conference would be held in English.
I soon learned that English has become the “lingua franca” (common
language) for professionals in East Asia. The organizers wanted to attract participants
from outside China
so their only choice was a conference in English.
The
approximately 100 engineers and scientists who attended the workshop were
mostly from China but there
were also participants from Japan,
South Korea, Taiwan, and Thailand. Only two Westerners attended: an Italian
engineer and me. I was disappointed that
I was the only workshop participant from the USA. Certainly, there are American academics and
scientists from the US Geological Survey who have research or applied interests
in debris-flows. It is unfortunate that
they are not more actively participating in an exchange of information with
their Asian colleagues.
 |
Top: Dr. Peng Chi, Workshop Chair, from the Institute of Mountain
Hazards and Environment in Chengdu, gave the opening remarks.
Bottom: 2012 International Debris-Flow Workshop participants (photo provided by the Chinese Institute of Mountain Hazards and Environment)
|
Debris-Flows Defined
According to
the website geology.com, a debris-flow is “a loose mass of mud, sand, soil,
rock, water, and air that travels down a slope under the influence of gravity.”
(http://geology.com/articles/debris-flow). One of the workshop speakers, Professor
Ko-Fei Liu (Taiwan
University) distinguished
debris-flows from landslides which are mass movements of solid materials and
from mud flows which are viscous and contain at least 60% silt. Some speakers used the terms “landslide” and “debris-flow”
interchangeably indicating the lack of universally-accepted definitions of the
terms.
 | |
Professor
Ko-Fei Liu of Taiwan University
explained the differences between debris-flows and other mass wasting phenomena
as part of his presentation comparing two models for assessing granular debris
flow hazards.
Relative Seriousness of Debris Flows in East Asia
Debris-flows
are a particularly serious issue in China and other East Asian
countries. They also occur in the
Western USA, Andean South America, Western Europe (particularly Switzerland and Italy),
and India. Their relatively common occurrence and
seriousness in East Asia results from a
combination of factors including:
1. Numerous
areas with mountainous terrain characterized by steep slopes and narrow
valleys.
2. Frequency
of seismic events which act to trigger failures of steep slopes.
3. Bedrock
and sediments which are prone to failure (for example, impervious layers of
volcanic bedrock which create sliding planes when wet with tropical moisture).
4. Severe
rainfall events (sometimes multiple) often associated with typhoons, monsoons,
or other summer storms.
5. High
densities of human populations in valleys which are vulnerable to debris-flows
(hundreds of people sometimes die as a result)
6.
Difficulty in relocating populations from debris-flow-prone valleys in the
crowded parts of East Asia – where would they
go?
 |
| Examples of large Chinese debris-flows (photos from presentation by Dr. Jing Zhang, Sichuan University) |
Managing Risk and Engineering Solutions
Engineers
and scientists in East Asia are modeling
debris-flows using empirical-based inputs to predict the behavior and
seriousness of future events. They are
also conducting risk assessments to help local officials with disaster
planning. For a number of years, they
have been using structural barriers to debris-flows, and they are experimenting
with improvements in their design.
However, major events often overwhelm even the best structural methods.
It seems to
me that mandatory evacuations would be useful when earthquake activity combined
with heavy rainfall conditions point to a high-debris flow risk. However, I would suspect that such preemptive
evacuations would be difficult given the typically poor transportation and
communications infrastructure in mountainous areas of countries like China.
Relationship of Erosion Control to Debris
Flow Management
Are the
techniques used for debris-flow mitigation applicable to erosion control
industry? Yes and no. Debris-flow events are orders of magnitude
larger than the erosion problems we typically deal. Also, the structural methods used to control-debris
flows are more applicable to long-term management of slope stability for
mountain highways, etc. rather than short-term measures employed during
construction.
Thus, I
think reverse is true: Erosion control
BMPs (Best Management Practices) are applicable to prevention or mitigation of
the effects of debris-flows. For
example, improving vegetative cover on debris flow-prone slopes would reduce
runoff and stabilize slopes subject to heavy precipitation. Many of the photos of debris-flows shown in
PowerPoint presentations at the workshop showed poorly vegetated slopes. Hillside
terracing would be applicable in some cases.
Diversion ditches along the tops of slopes and slope drains would divert
runoff from slopes reducing frequency of saturated soil conditions. I’m not suggesting that erosion control BMPs
should replace engineering structural methods to control debris-flows; rather
they could supplement them.
Coming Next: A look at specifics from some of the
presentations at the debris-flow workshop.
