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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
797
DOI: 10.4408/IJEGE.2011-03.B-087
GEOMORPHIC CHANGES OF A LANDSLIDE DAM
BY OVERTOPPING EROSION
K. YOSHINO
(*)
, T. UCHIDA
(*)
, T. SHIMIZU
(*)
& K. TAMURA
(*)
(*)
Incorporated Administrative Agency, Public Works Research Institute, Japan
INTRODUCTION
Outburst flooding due to overtopping failure of
landslide dams can cause catastrophic events (e.g.,
C
osta
et alii, 1988; k
oRuP
, 2004). Understand-
ing the processes of overtopping erosion and sedi-
ment transport is needed for future mitigation of the
downstream effects of these phenomena. Since the
1980s, various numerical models have been used to
analyze the processes of overtopping erosion (e.g.,
t
akaHasHi
et alii, 1987; 1993). To date, erosional
processes at landslide dams have been examined
mainly by hydraulic modelling experiments (e.g.,
t
akaHasHi
et alii, 1983; m
izuyama
et alii, 1989;
o
da
et alii, 2006, 2007), with the governing equa-
tions of these numerical simulations being theoreti-
cal analyses derived from hydraulic models.
Since 2004, detailed investigations of overtop-
ping of large-scale landslide dams formed by the
2005 Typhoon No. 14, the 14 June 2008 Iwate and
Miyagi inland earthquake, and the 2004 Chuetsu
earthquake have been published (k
ato
et alii, 2005;
o
sanai
et alii, 2007; s
atofuka
et alii, 2010; u
CHida
et alii, 2009). Moreover, numerical simulation mod-
els have been verified by observed data (e.g., m
oRi
et alii, 2010; s
atofuka
et alii, 2010). However, in
modelling the processes of overtopping erosion in
these studies, the pre-failure shape of the landslide
dams was inferred from the volume and distribu-
tion of collapsed sediment. Therefore, although dis-
charge rates were reproduced well by these numeri-
ABSTRACT
Clarifying geomorphic changes to landslide
dams following overtopping is important for miti-
gation of future disasters. Past modelling experi-
ments have not provided adequate information about
geomorphic and grain size distribution changes at
landslide dams. We used field survey data, aerial
photographs, and LiDAR imagery to analyze the
geomorphic changes of a landslide dam formed dur-
ing the 2008 Iwate and Miyagi inland earthquake and
the effect of subsequent overtopping erosion on the
dam morphology and grain size distribution of the
sediments. Our field survey data showed clear evi-
dence of armour coating in the stream bed. We used
LiDAR data sets recorded before overtopping, short-
ly after overtopping, and around 120 and 140 days
after overtopping to clarify geomorphic changes.
Our LiDAR data analysis revealed that around 30%
of sediment eroded by overtopping was deposited
within about 200 m of the lower end of the landslide
dam, and that after overtopping, erosion and deposi-
tion gradually returned the riverbed gradient to close
to that which existed before the landslide. Moreover,
we showed that near the flat head of the landslide
dam, the erosional channel was narrow and deep.
Farther downstream from the flat head, the erosional
channel was shallower and wider.
K
ey
words
: landslide dam, overtopping erosion, LiDAR data
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k. YOSHINO, T. UCHIDA, T. SHIMIZU & k. TAMURA
798
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
cal simulations (e.g., m
oRi
et alii, 2010; s
atofuka
et
alii, 2010), the geomorphic change of landslide dam
and the deposition of downstream of the dam were
not adequately modelled.
Recent advances in LiDAR (Light Detection and
Ranging) technology have allowed the acquisition
of highly accurate topographic information that has
been used to reveal the detailed geometry of landslide
dams (e.g., u
CHida
et alii, 2009). In particular, detailed
LiDAR-based measurements were taken shortly after
formation of landslide dams triggered by the 2008 Iwate
and Miyagi inland earthquake. Several of these land-
slide dams have since been breached by overtopping
erosion and then re-measured using LiDAR data. These
data allow us to see in detail the geomorphic change to
landslide dams as a result of overtopping erosion.
Here we show new LiDAR data that demonstrate
the geomorphic changes in response to overtopping
erosion of a landslide dam on a tributary of the San-
hazama River and the effects of downstream sediment
erosion and deposition that followed. We investigated
the detailed geometry of the water channel formed
by overtopping and clarified the erosion processes
involved. The terminology we use in this paper to de-
scribe the geometry of the landslide dam is as shown
on the schematic longitudinal profile of Fig. 1.
STUDY SITE
We investigated a dam formed by a large landslide
in the Numakura-Urasawa area during the 2008 Iwate
and Miyagi inland earthquake (Fig. 2). The landslide
Fig. 1 - Schematic longitudinal profile of a landslide dam
Fig. 2 - Regional location map and topographic map
showing locations (grey shading) of two landslide
dams in the Numakura-urasawa area. The larger
of the two dams is the one investigated in this
study. And contour interval are 10 m
Fig. 3 - Landslide dam of Numakura-Urasawa after the overtopping
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GEOMORPHIC CHANGES OF A LANDSLIDE DAM BY OVERTOPPING EROSION
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
799
ANALYSIS
We defined the width of the river (riverbed and
bank) from geomorphic features observed on LiDAR
data, aerial photographs, and orthographic photo im-
ages (Fig. 7). We then determined the centre line of
the river for measurements of longitudinal profiles of
the riverbed. We also calculated a erosion and deposi-
tion amount for the river by taking the difference in
altitude of two times LiDAR data set.
We measured the detailed geometry of the eroded
water channel using the LiDAR data measured in Sep-
tember. First, we determined the extent of the eroded
water channel from the difference between the river-
bed cross sections of 16 June and 8-9 September (see
Fig. 14). Then, we measured the widths at the bottom
(B3) and top (B2) of the eroded channel, its depth (H),
and the width of its banks (L) (see Fig. 7).
RESULTS AND DISCUSSION
GEOMORPHIC CHANGES TO AND NEAR THE
LANDSLIDE DAM
Before the breaching, the longitudinal length of
dam formed on a tributary of the San-hazama River
about 5 km upstream from Kurikoma Dam. There is
also a small SABO dam (sediment control dam) about
450 m downstream of the lower end of the landslide
dam. The landslide occurred on the valley slope above
right side slope of the river channel. The catchment area
upstream of the landslide dam is 18 km
2
. The rainfall
between 14 June (when the earthquake occurred) and
the end of November is shown in Fig. 4. Maximum dai-
ly rainfall during this period was 128.5 mm (24 Oct.),
and maximum hourly rainfall was 35 mm/h (7 Sep.).
There was no rainfall in the Numakura-Urasawa
area from 20 to 21 June 2008, and overtopping ero-
sion occurred on 21 June (Fig. 5.). At 0030 on 21 June
2008, discharge of water into the Kurikoma- Dam
showed an exponential increase in response to over-
topping of the landslide dam. At 0120 on 21 June,
discharge into the dam peaked at about 100 m
3
/s rate
(Fig. 5.; u
CHida
et alii, 2009) We measured the grain
size distributions of sediments from the landslide
dam after overtopping and from the landslide dam be-
fore overtopping (Fig. 6). The ratio of sediment less
than 2mm in the landslide dam after overtopping was
smaller than that of the landslide dam before overtop-
ping. Therefore, it shows armour coating due to over-
topping erosion was clearly evident in the sample.
METHOD
DATA
We used three LiDAR data sets to provide in-
formation about the geometry of the landslide dam
before the breach (data recorded 16 June), after the
breach (8-9 September), and after heavy rainfall after
the breach (12-13 November).
Fig. 4 - Hyetograph for the period 14 June- 29 November
2008 (AMeDAS data by Japan Meteorological
Agency). Arrows indicate LiDAR data sets used
in this study
Fig. 5 - water inflow rate for kurikoma-Dam for 21-22
June 2008
Fig. 6 - Grain size distribution of samples from the dam
before overtopping and from the dam after overtop-
ping Locations of sampling sites are shown in Fig. 3
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k. YOSHINO, T. UCHIDA, T. SHIMIZU & k. TAMURA
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
The longitudinal gradient of the landslide dam
surface between the lower end of the flat head and
the point 200 m upstream from it was changed from
10° to 2° by overtopping erosion (Fig. 9). There was
minimal change of the longitudinal gradient (6°) of
the landslide dam surface between a point 200 m up-
stream from the lower end of the landslide dam and
the lower end of landslide dam (Fig. 9).
Maximum deposition as a result of overtopping
(4.2 m) occurred just downstream of the lower end of
the landslide dam (upper panel of Fig. 10). The riv-
erbed gradient in September in the area of deposition
between the large and small landslide dams was almost
the same as that of the initial riverbed (2.3°). There was
less erosion or deposition (less than ~1 m) below the
SABO dam than farther upstream. There were no re-
markable changes in any part of the longitudinal pro-
file during the September to November period (Fig. 8).
We calculated volumetric erosion and deposition at
20 m intervals along the profile for two periods (June-
Sept and Sept- Nov; lower panel of Fig. 10). About
53,000 m
3
of sediments was eroded from the landslide
the landslide dam was 550 m, the maximum height
between the original riverbed level and the landslide
dam surface was 26 m, and the height difference be-
tween the lower end of the landslide dam and its high-
est point was 42 m (Fig. 8). ponding occurred on the
flat surface upstream of the landslide dam. Another,
smaller landslide dam also formed 200 m downstream
of the lower end of the large dam, but is not discussed
in detail here. The longitudinal gradient of the large
landslide dam surface between the lower end of the
flat head and the lower end of the dam downstream
slope of the landslide dam ranged from 2° to 10° and
the mean gradient was 6° (Fig. 9).
Comparison of June and September LiDAR data
showed that about 12 m of landslide debris was eroded
at the flat head during overtopping (Fig. 10). The maxi-
mum erosion in the region 200 m immediately upstream
from the lower end of the landslide dam was 2 m.
Fig. 8 - Geomorphic changes along a longitudinal section
in the region of the landslide dam
Fig. 10 - Longitudinal changes of riverbed deformation and
of volumetric changes of deposition and erosion
Fig. 7 - Schematic illustration of method used to define
river width. The upper panel is a plan view; the
two insets and the lower panel are cross sections
Fig. 9 - Longitudinal changes of riverbed gradient
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GEOMORPHIC CHANGES OF A LANDSLIDE DAM BY OVERTOPPING EROSION
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
801
dam during June to September, and onethird of this
sediment (18,000 m
3
) was deposited between the lower
end of the large landslide dam and the upper end of the
small landslide dam. About 15,000 m
3
of sediment was
eroded from the small landslide dam and about 2,000 m
3
of sediment was deposited between the small landslide
dam and the confluence with the Sanhazama River.
GEOMORPHIC CHANGES OF THE wATER
CHANNEL
The channel bed along the centre line of the water
channel was eroded by about 12 m at the lower end of
the flat head as a result of overtopping erosion (Fig. 11
and upper panel of Fig. 12). The depth of erosion gener-
ally decreased in the downstream direction (upper panel
of Fig. 12). Longitudinal gradients of the channel bed
along the centre line of the water channel were consist-
ently around 2° from the upper end of the flat head to
about 220 m downstream from there (Figg. 11 and 13).
We calculated the amount of erosion and deposi-
tion following overtopping by using the difference of
the June and September LiDAR data, which we meas-
ured at 20 m intervals along the profile (lower panel
of Fig. 12). From the upper end of the landslide dam
to a point 150 m downstream (1,010 to 1,160 m in
Fig. 12), the vertical amount of erosion corresponded
with the volumetric change of sediment, whereas from
150 to 300 m downstream from the upper end of the
dam (860 to 1,010 m in Fig. 12), the depth of erosion
was relatively smaller, indicating that side bank ero-
sion contributed to total erosion in this section of the
profile (see also the 960 m cross section in Fig. 14).
Vertical erosion occurred mainly at the flat head and
on the upper part of the downstream slope of the land-
slide dam (1,010 to 1,160 m), whereas lateral erosion
became dominant farther downstream.
The widths of the bottom and the top of the eroded
water channel (B3 and B2 of Fig. 7) were narrowest
(about 15 and 40 m, respectively) around 40 to 100 m
from the upper end of flat head (upper panel of Fig. 15)
and increased gradually in the downstream direction.
The greatest height of the eroded water channel (H of
Fig. 7) was about 13 m near the flat head, and it de-
creased gradually in the downstream direction (middle
panel of Fig. 15). The gradient of the side bank ranged
from 10° to 45° (lower panel of Fig. 15). There was no
clear trend in the relationship between the gradient of
the side bank angle and distance along the profile.
The gradient of the side bank of the water channel
varied considerably (by around 20°), despite the rela-
tively constant height of the water channel (left panel
of Fig. 16). The average gradient of the side bank was
about 35°, regardless of the height of the water channel
and the distance from the upper end of the flat head.
The variations of the gradient of the side bank farther
Fig. 11 - Longitudinal elevation profile of the centre line of
the water channel
Fig. 12 - Longitudinal changes along riverbed centre line of
riverbed deformation and of volumetric changes
of deposition and erosion. Positive values indicate
deposition and negative values indicate erosion
Fig. 13 - Longitudinal gradient profile along the centre line
of the water
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k. YOSHINO, T. UCHIDA, T. SHIMIZU & k. TAMURA
802
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
stream by overtopping erosion ware shown in Fig. 17,
Fig. 18. The depth of the eroded channel around the
lower end of flat head is larger than the downstream.
Moreover, the width of the eroded channel around the
lower end of flat head is smaller than the downstream.
On the other hands, slope gradient is almost constant
between upstream and downstream. And it also almost
constant between initial reverbed gradient and gradi-
ent at the deposited area by overtopping erosion.
CONCLUDING REMARKS
Our study revealed the following:
1. Approximately 30% of the sediment eroded by lan-
dslide dam overtopping was deposited within about
200 m of the lower end of the dam. After a landsli-
de dam breach, erosion and deposition restored the
riverbed’s longitudinal gradient to conditions simi-
lar to those before the landslide dam formed.
2. The eroded water channel was narrow near the flat
head and widened in the downstream direction.
The water channel was shallowest near the flat
head and deepened in the downstream direction.
Erosion at the flat head and on the upper part of
the downstream slope of the landslide dam was
mainly vertical, whereas erosion was predomi-
nantly lateral farther downstream.
We conducted a detailed investigation of chang-
es in sediment transport at, and downstream from, a
landslide dam before and after dam overtopping. This
investigation involved three sets of LiDAR data. We
than 100 m from the upper end of the flat head were
smaller than those within 100 m of it. The width of the
side bank (L in Fig. 6) increased with increasing height
of the water channel (middle panel of Fig. 16). The rela-
tionship between width of the bottom of the water chan-
nel and its height separated into two distinct clusters
reflecting distance from the upper end of the flat head.
It was shown that the geomorphic change of a
landslide dam and sediment deposition of the down-
Fig. 14 - Changes of cross-section of eroded channel at six locations. Locations shown in Fig. 8
Fig. 15 - Longitudinal changes of width and height of water
channel and of bank gradient
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GEOMORPHIC CHANGES OF A LANDSLIDE DAM BY OVERTOPPING EROSION
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
803
• How the physical properties of materials forming
landslide dams (grain size, soil density, etc.) affect
erosional processes.
ACKNOWLEDGEMENTS
The LiDAR data after the earthquake was offered
by Miyagi Prefecture.
also examined how the geometry of the eroded water
channel changed as a result of landslide dam overtop-
ping, and analysed the erosion processes involved.
Future research will be required to verify our re-
sults, and should include hydraulic experiments and
further investigations of the following issues:
• Depositional processes immediately downstream
of landslide dams to assess the effects of various
dam shapes and sizes.
• Armour coating and the influence of the water
channel after a peak discharge is generated by
erosion caused by landslide dam overtopping.
Fig. 16 - Relationships of height of the eroded water channel to the gradient of side bank (left panel), horizontal width (L) of the side
bank (centre panel), and bottom width of the eroded channel (B3). Solid squares are from cross section less than 100 m from
the upper end of the flat head, and open squares are from cross sections more than 100 m from the upper end of the flat head
Fig. 17 - Schematic illustration of geomorphic change of the
landslide and sediment deposition of the downstream
Fig. 18 - Schematic illustration of sediment change of the
landslide and change of longitudinal riverbed slope
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osta
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k. YOSHINO, T. UCHIDA, T. SHIMIZU & k. TAMURA
804
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
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oRi
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