Landslides and debris flow, and their countermeasures for enhancing resilient society

Shinji Egashira s-egashira@pwri.go.jp

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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