Landslides and debris flow, and their countermeasures for enhancing resilient society
Shinji Egashira [email protected]
International Centre for Water Hazard and Risk Management (ICHARM), PWRI
Sediment runoff processes associated with occurrences of landslides Recent disasters due to landslides and debris flow resulted from rainfall events
Method to predict/evaluate their occurrences and runout processes Countermeasures to enhance regional resilience
Contents
Elevation
< -1m
-1 to < 0 m
Omoto R.Aug/2016,
21HiroshimaAug/2014,
74
N. KyushuJul/2012,
32Jul/2017,
42
Flood and sediment disasters since 2011 in Japan
Each underlining number shows No. of dead and missing.
Sep/2011, 85
(Nara & Wakayama)
Yamaguchi
& ShimaneJun/2013,4
Izu-OshimaIs.Oct/2013, 39
Mt. Fuji(3776m)
Kinu R.Sep/2015,
2(Joso city)
Recent disasters resulted from heavy rainfall events in Japan
Okayama
Jul/2018,114
Jul/2018,64
Kii PeninsulaEhime
Jul/2018,29
19 th of Aug. 2014 4th to 6th July, 2018
Rainfall events in the sediment disasters at Hiroshima Data provided by Japan Meteorology Agency (JMA)
Tokyo
Hiroshima
2014 Hiroshima Landslides & debris flow
Photos taken by GSI (Geospatial Information Authority of Japan)
Asa-North Area
74 victims
Tokyo
Hiroshima
2018 Western Japan Torrential Rainfall Disaster
300m
(Saka Town, Hiroshima Pref.)
Sozu River
Prepared by GSI
Saka Town
114 victims
Northern Kyusyu
July 2012 July 2017
Data provided by Japan Meteorology Agency (JMA)
2012 Northern Kyushu Torrential Rainfall Disaster
(Aso volcanic area, Kumamoto Pref.)
Mt. Aso Cardera
Prepared by GSI
KurokawaR.
Tokyo
Aso, Kumamot
o
32 victims
2017 Northern Kyushu Torrential Rainfall DisasterMap prepared by GSI
Chikugo River
Photos taken by ICHARM
Tokyo
Asakuracity,
FukuokaFlood inundationLandslide, Debris flowDamaged roadDamaged railwayUnreadable area
Legend
42 victims
SummaryOccurrence density of landslides and debris flow
Temporal, spatial characteristicsof rainfall
reflected
Disaster may take place more frequently, and in addition,
it may take place even in areas where sedimentdisaster have not ever been experienced.
Due to climate change,
There have been some devastating caseswhere sediment disasters took place in two adjacent regions only several years apart.
Many residences in hazardous areas
Difficulties of evacuation behaviors
Severely damaged due to sediment runoff by landslides and debris flow in Akatani basin (Northern Kyushu - severe rainfall event in July, 2017)
Akatani basin(about 20km2)Chikugo River
Rain
fall
inte
nsity
(mm
/h)
Accu
mul
ated
rain
fall
(mm
)Sediment runoff processes associated with occurrences of landslides
Photo taken by GSI (Geospatial Information Authority of Japan)
Debris flow deposition in the upstream of Akatani basin
Damaged houses due to debris flow
Channel change in the middle reach of AkataniImmediately after the flood Before the flood
River channel
Photos taken by GSI (Geospatial Information Authority of Japan)
A huge amount of sediment deposited in the downstream reach of Akatani
Sediment particle sizes along the Akatani reach
0.9, 1.5, 1.7km
1.5~2.4kmUpstream region
0.1 1 10 100Diameter(mm)
Sediment sampling sitesfor sieve analyses
Clear, longitudinal sediment sorting
(2017 event in Nothern Kyusyu)
Perc
ent f
iner
o30>θ
o10>θ
oo8~4=θ
Landslides Debris flows
Sediment erosion
Deposition of driftwoods
Flooding with sediment and driftwoods
Dominant bed load and suspended load Dominant suspended sedimentFlooding of suspended sediment
Flood flow with sediment and drift woods
Scattered driftwood accumulation Driftwood accumulation at bridges
Channel closing dueto sediment deposition
Scattered cobbles and gravels
Chikugo River
Houses, FeldsRoads
Houses, Fields , Paddy fields, Roads
Houses, Paddy Fields
Sediment runoff processes and hazards resulted from the rain fall event at northern Kyusyu in July 2017
Channel closing due to sediment deposition
07.0~03.0=b
i
03.0~008.0=b
i
bi Bed-slope
Yield of drift woodsSediment deposition
Flooding with channel changesSediment floodingDriftwoods flooding
Longitudinal sediment sorting
θ Bed-slope
summary
Methods for evaluating occurrences of landslides and debris flow, debris flow behaviors and flood flow with channel changes and drift wood
(1) Occurrences of landslides and debris flow
Surface topography Surface soil layer model
Thickness of surface soilHydraulic, hydrological parametersPhysical parameters such as
angle of repose and cohesion
Surface and subsurfaceFlows as well as runoff
Stability analysis of surface layer
at each grid elementand at each stream
Occurrences of landslides anddebris flow
Rainfall runoff model
Drainage model
Rain
fall
Temporal and spatial distributionof landslides
Runout process of sediment and wood debris released byeach landslide
Behaviors of debris flow and driftwood alongthe torrents
Behaviors of debris flow and driftwood in basin
scale/level
Depth averaged 2-D model
Mass point system
(2) Run-out processes of sediment and wood debris released from landslides
(3) Flood flow with active sediment transportation and drift wood
Depth averaged 2-D governing equations are employed;
# Mass and momentum conservation equations for flood flow
# Mass conservation equations of sediment and drift wood in flow body as well as in bed
Bed-load formulaLayer model to predict sediment particle size distribution of bed sedimentErosion / deposition rates of suspended sediment and wash load
Numerical models need to evaluate non-equilibrium behaviors of suspended sediment and wash-load,longitudinal and lateral sediment sorting,channel changes such as stream bifurcation
Disc
harg
e(m
3 /s)
Tim
e
Tim
e
Debris flow
Landslide
Disc
harg
e(m
2 /s)
Rain
(mm
/h)
Grid
num
ber
hour
hour
hour
Grid
num
ber Debris flow
Landslide
(h)
(h)
(h)
(h)
Rainfall runoffat downstream end
Izu-oshima event2013(Yamazaki et.al 2017)
Debris flow simulation by means of 2-D model
Target torrent
Erosion/deposition (m)
Erosion/deposition resulted fromrun-out process of debris flow
(谷スケール)
(Yamazaki et.al 2018)
Deposition of driftwood carried by debris flow (Yamazaki et.al 2018)
Target torrent
Deposition of driftwood(m)
Flow depth (m)Erosion/deposition depth oftorrent bed (m)
2-D behaviors of debris flow (Yamazakli et.al 2018)
Driftwood volume involved in the flow body in unit area (m)(Concentration ×Flow depth)
Driftwood volume deposited in unit areaof river bed (m)
(Yamazakli et.al 2018)
Depth averaged flow computed in case 4 (Movable bed with fine sediment- and driftwood-supply) (Harada, et.al 2018)
Computed results of driftwood distribution along the reach at the end of the flood (Harada, et.al 2018)
+0.0
1
+0.0
05
0
Countermeasures to enhance regional resilience
# Warning and evacuation system suitable for each unit region
# Hazard maps based on new ideas; sediment and driftwood runoffs, flood flow with channel changes
# Land uses according to hazard levels
# Structure of the city by means of road system, drainage system,park, green belt, etc.
# Development of sediment control structures based on recent debris flow study
Priority plan forsocial capital development
1993Project to measure steep slope collapse
Ten-year plan for flood control project
1960
The ninth 7-year plan
2003
1983Project to measure debris flow
Casualties resulted from natural disasters in Japan
Sediment Disaster Prevention Act
1999
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