2012 seoul umyeonsan(ikeda, japan)

Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012

Preliminary Study on Occurrence of
Debris Flow at Umyeonsan(Mt.)

Sabo & Landslide Technical Center, JAPAN
Akihiko IKEDA

SABO & LANDSLIDE TECHNICAL CENTER


Issues in dispute
1) Whether adequate countermeasures were established for the
Umyeonsan(Mt.) area, after the part of the area experienced
landslides in 2010. Dukwooam valley, for example, suffered
even more severe damages in 2011 again, for which
restoration works have been done after landslides occurred in
2010.
2) Whether the landslides occurred around, and the drained
water from the Air Force base, located at the summit of
Umyeonsan(Mt.), aggravated the damages of the disaster in
2011.
3) Whether the existence and/or misadministration of the Eco-
Eco-
park reservoir in Hyeongchon valley added the damages in
the lower catches.
4) Whether the blasting executed for tunneling underneath the
Umyeonsan(Mt.) caused the landslides in the surrounding
area.
SABO & LANDSLIDE TECHNICAL CENTER 2


Contents
1. Property of Debris Flow (in general)
2. Precondition Data of Umyeonsan Debris Flow
- Based on technical report -
3.Consultation about the process of
Debris flow occurrence
4.Comments



■Classification of Debris Flows

(1) Gravel-filled debris flow
Gravel-
(2) Mud-filled debris flow
Mud-



（1） Gravel-filled debris flow
Gravel-
A. The tip of a debris flow moves
quite straight with a large
concentration of gravel and
boulders that are two to five
meters in diameter, or larger in
some cases.
B. Behind the tip of a debris flow
follows a muddy water flow (a
subsequent flow) that contains
less gravel than the tip of a
debris flow.
June 27,1999 ; Name River, Nagano Pref. , JAPAN
27, Pref. C. Boulders are concentrated at the
tip of a debris flow.
Maximum velocity: 8.48m/sec D. Debris flows rise and flow in an
outer direction around the
corner of a watercourse.



（2） Mud-filled debris flow
Mud-
A. The Mud-filled debris flows are
Mud-
easily generated in areas with
fine-
fine-grained basement rock.
This type of debris flow contains
boulders, but it consists mostly
of fine-grained sand.
fine-
B. Mud-
Mud-filled debris flows move
rampantly entirely from the tip
to subsequent flows, and they
flow in surges.
C. Mud-
Mud-filled debris flows flow
easily compared with gravel-
gravel-
filled debris flows, so there are
cases where mud-filled debris
mud-
flows reached the sea.



■Classification of Debris Flows Source
Based on the location, scale of the landslide and the large
amount of sediment discharge from the river course.
（1） Landslide-type debris flow;
Landslide-
the debris flow generated from the slope failure
（2） Riverbed flow-type debris flow;
flow-
the mobilization of riverbed deposits turns into debris flow
（3） Landslide Dam-type debris flow;
Dam-
the debris flow are assumed to occur under the effects of a landslide dam



（1） Landslide-type debris flow
Landslide-
Minamata City, Kumamoto Prefecture, in 2003



（2） Riverbed flow-type debris flow
flow-
Inabe City, Mie Prefecture, in 1999,2002,2003



（2） Riverbed flow-type debris flow
flow-
Okaya City, Nagano Prefecture, in 2007



（3） Landslide Dam-type debris flow
Dam-
Name River, Nagano Prefecture, 1999

Landslide Dam

Before debris flow(1998.11) After debris flow(1999.6)



cf. Landslide Dam

<Totsukawa Village, Kii Peninsula, in 2011>



■Criteria for sediment movement form
based on riverbed gradient

River
bed

River bed 2° 10°
10° 15°
15° 20°
20°
gradient Approx.
Approx. 1/30 Approx.
Approx. 1/6 Approx.
Approx. 1/4 Approx.
Approx. 1/3

Occurrence area
Flow area
Area
Deposition area
name

Bed load area Debris flow area



2. Precondition Data of
Umyeonsan Debris Flow
These data are based on Korean Society of Civil Engineers,
2012, Interim Report of the “Complementary Studies on the
cause of the Umyeonsan(Mt.) Landslides” (condensed version).
■Location of Umyeonsan Area

<2010> <2011>



■Property of Debris flow
ØMoving distance: avg. 448.8m range 98-1356.1m
98-
ØGradient of initiation slopes: avg. 33° range 11-52°
33° 11-52°
ØVolume of slope failure: avg. 116.7m3 range 1.1-1914.8m3
1.1-
ØBasin area: avg. 151,848m2 (0.15km2)



■Rainfall Analysis



■Runoff Analysis
Peak discharge of the
Seocho and Namhyun
stations are less than
10m3/sec for the
average rainfall.



■Geotechnology

Entire soil layer depth in
Umyeonsan is around few
meters (detail data are
unconfirmed).



■Ground water
Groundwater level increase several
hours after the rainfall and base flow
run-
run-off occurs when continuous
rainfall occurs above 250mm.
250mm.

<Raemian> <Hyeongchon>



3.Consultation about the process
of Debris flow occurrence
Based on the technical report, each valley that debris flow
occurred has a slope failure at the top of valley.
valley.
Landslide-
Landslide-type debris flow
In general, the process of Landslide-type debris flow
Landslide-
occurrence as below,
Landslide occurrence

Liquefaction of collapsed soil and flow down the valley

Erode riverbed and river bank, surface soil of the slope

Flow down as Debris flow



Ø In general, landslide classified in 2 types; shallow landslide
and deep landslide.
landslide.
Ø Increasing groundwater, pore water pressure inside the
groundwater,
slope, causes the landslides.
Ø Shallow landslide occurrence tends to be influenced by
rainfall intensity.
intensity.
Ø Deep landslide occurrence tends to be influenced by
cumulative rainfall.
rainfall.



Based on the technical report, average volume of slope failure
is 116.7m3, and the range are 1.1-1914.8m3. It is estimated that
1.1-
average depth of slope failure is around few meters.
meters.
Shallow Landslide
Ø In general, shallow landslide occurrence tend to be
influenced by rainfall intensity.
intensity.
Ø Based on maximum rainfall of 1-hour, 6-hour, 12-hour at
6- 12-
Namhyun station, are over 100 year frequency, these rainfall
frequency,
will be the trigger of the landslide and debris flow.
Necessary to confirm the hyetograph




cf. Some cases in JAPAN,
more than 7mm at 10-minute
10-
rainfall,
rainfall, riverbed flow-type
flow-
debris flow tends to be occur
consistent with peak of the
rainfall.



Samples of Hyetograph
Landslide occurrence Occurrence of
Rainfall Peak Deep
Landslide

Occurrence of
Debris Flow

Stream bed flow-type debris Flow
flow-
<Inabe City, Mie Prefecture, in 1999,2002,2003>

Deep Landslides


Changes in rainfall intensity every 6 hrs

Occurrence of Deep
Landslide





Almost landslides
flew down as debris
flow.



Based on technical report,
Ø Maximum rainfall of 1-hour, 6-hour, 12-hour at Namhyun
6- 12-
station are over 100 year frequency.
Ø Peak discharge of the Seocho and Namhyun stations are
less than 10m3/sec for the average rainfall.
Ø Groundwater level increase several hours after the rainfall
and base flow run-off occurs when continuous rainfall
run-
occurs above 250mm.
250mm.
Ø Entire soil layer depth in Umyeonsan is around few
meters (detail data are unconfirmed).



The cause of liquefaction of collapsed soil and flow down the
valley, influenced by huge amount of water which has been
supplied from;
Ø surface water due to torrential heavy rainfall
Ø ground water presumably due to antecedent rainfall and
cumulative rainfall, infiltrate into the slope

Soil layer depth is about few meters and it will presumably
has a retention of water, but in this disaster, the cumulative
water,
rainfall must be over the capacity of it.



Based on the technical report, debris flow flew down
dynamically propagating downslopes, which are captured by
downslopes,
video cameras at the northern side of Umyeonsan.
<Site 1: Raemian> <Site 2: Shindonga>
ØCatchment: 0.13km3 ØCatchment: 0.145km3
ØAverage slope: 15°
15° ØAverage slope: 14°
14°
ØInitial volumes: 4,000m3 ØInitial volumes: 300m3
ØTotal volumes of ØTotal volumes of
debris flow: 32,000m3 debris flow: 32,000m3



Analyzing movie files, the maximum velocity of debris flow
are estimated 28.6m/sec (Site 1) and 18m/sec (Site 2).
These velocity are very fast-moving, which is presumably due
fast-moving,
to the low sediment volume concentration of the flow, related
to huge amount of water supplied from rainfall and ground
water.
Necessary to confirm the flow property, such as occurrence
section, flow(eroded) section and deposition section,
drawing the result on the longitudinal profile and plane
figure.
If possible, using LiDAR data and compare the elevation
(topography) before and after the disaster, and calculate
the volume of sediment budget.


4.Comments
The cause of landslides and debris flow in 2010, presumably
due to a huge amount of water supplying.
The source of this water is supplied from a record-breaking
record-
torrential heavy rainfall intensity, antecedent rainfall and
cumulative rainfall.
The trigger of landslides and debris flow are the heavy
rainfall intensity; such as 1-hour, 6-hour and 12-hour which
1- 6- 12-
is over 100 year frequency.
The antecedent rainfall and cumulative rainfall increased
the ground water level more higher, that influenced debris
flow discharge larger.


2012 seoul umyeonsan(ikeda, japan)

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