Water and disaster in terms of dynamics

Shinji Egashira

International Centre for Water Hazard and Risk Management, PWRI

Casualties resulted from landslides, debris flow , flood, etc.

Governing equations to describe behaviors of flood flow, landslides and debris flow

Some numerical results

Priority plan forsocial capital development

1993

Project to measure steep slope collapse

Ten-year plan for flood control project

1960

The ninth 7-year plan

2003

1983

Project to measure debris flow

Casualties resulted from natural disasters in Japan

3

Elevation

Water surface

< -1m

-1 to < 0 m

Omoto R.Aug/2016,

21

HiroshimaAug/2014,

74

N. KyushuJul/2012,

32Jul/2017,

41

Flood and sediment disasters in the last 5 years, Japan

✓ Each number shows No. of dead and missing

Sendai

Kii PeninsulaSep/2011,

70 (Nara &

Wakayama)

Yamaguchi

& ShimaneJun/2013,4

Izu-OshimaIs.Oct/2013, 39

Mt. Fuji(3776m)

Kinu R.Sep/2015,

2(Joso city)

Flood and sediment disaster resulted from severe rainfall at northern Kyushu in July, 2017

Akadani basin(about 20km2)Chikugo River

Rain

fall in

ten

sit

y(m

m/h

)

Accu

mu

late

d r

ain

fall (

mm

)

Debris flow deposition in the upstream of Akadani basin

Damaged houses due to debris flow

Photos provided by Geographical Survey Institute

Channel change in the middle reach of Akadani

Immediately after the flood Before the flood

River channel

A huge amount of sediment deposited in the downstream reach of Akadani

30

10

8~4

Landslides Debris flows

Sediment erosion

Deposition of driftwoods

Flooding with sediment and driftwoods

Dominant bed load and suspended loadDominant suspended sedimentFlooding of suspended sediment

Flood flow with sediment and drift woods

Scattered driftwood accumulationDriftwood accumulation at bridges

Channel closing dueto sediment deposition

Scattered cobbles and gravels

Chikugo River

Houses, FeldsRoads

Houses, Fields , Paddy fields, Roads

Houses, Paddy Fields

Characteristics of hazards resulted from the rain fall event at northern Kyusyu in July 2017

Channel closing due to sediment deposition

07.0~03.0bi03.0~008.0bi

bi Bed-slope

Yield of drift woodsSediment deposition

Flooding with channel changesSediment floodingDriftwoods flooding

Longitudinal sediment sorting

Bed-slope

Governing equations for flood flow with sediment transportation anddrift wood

- Mass conservation equation for water

- Momentum conservation for water

- Bed load formula, Erosion/deposition rate for suspended sediment(instead of momentum conservation equation)

- Mass conservation equation for sediment in flow body (very dilute sediment)

- Mass conservation equation for bed sediment

river bed

bed-load

suspended load (very dilute sediment)

- Mass conservation equations for drift wood in flow body as well as in bed sediment

Governing equations for debris flow and soil mass released by landslides

- Mass conservation equation for water-sediment mixture

- Momentum conservation for water-sediment mixture

- Mass conservation equation of bed sediment

for mixture for sediment only

Ey

vhc

x

uhc

t

hc

*c

E

y

vh

x

uh

t

h

m

bx

x

Hgh

y

vuh

x

uuh

t

uh

m

by

y

Hgh

y

vvh

x

uvh

t

vh

cos*c

E

t

zb

bz

*c

E

H

by

vu,

c

h Flow depth

Flow velocity

Sediment concentrationin flow body

Sediment concentrationin bed sediment

Erosion/deposition rate

Free surface elevation

Bed surface elevation

,bx Bed shear stress

for coarse sediment

for fine sediment

Initial stage(200m3/s)

Tailing stage(150m3/s)

Around flood peak(350m3/s)

Stream bifurcationdue to sedimentation

Increase of flood area with increase of flow discharge and sediment deposition

provided by Harada et.al.(Nov. 2017)

Sed

imen

t d

isch

arg

e(m

3/s

)

Computed flow patterns

Change of bed elevationDeposition of drift wood

+3.0

0

-3.0

Depth of erosion /deposition (m)

+0.1

+0.05

0

Deposition in unit area(m3/m2)

1.5m at bridge

upstreamupstream

Provided by Harada et.al.(Nov. 2017)

Temporal changes in the number of unstable cellsJuly 5, 2017B

asin

ave

rage

Hou

rly

rain

fall(m

m)

July 66:00 12:00 18:00 0:00

Akadani Basin

Provided by Yamazaki et.al.(Nov. 2017)

15:00 18:00 End

Spatial distribution for erosion and deposition resulted from debris flow

Provided by Yamazaki et.al.(Nov. 2017)

Debris flow resulted from landslides

Flow depthErosion/deposition depth

Provided by Yamazaki et.al.(Nov. 2017)

Critical rainfall conditions to cause severe sediment disasters in different regions

Izu OshimaFukuoka(Akadani)

Hiroshima

18:00 5 Jul. 2017

Rainfall (C-BAND JMA)

17:00

16:00

14:00

Provided by Yamazaki et.al.(Nov. 2017)

Concluding remarks

Numerical models and results on flood flow (inundation processes)

with sediment transportation anddrift wood as well as onlandslides and debris flow

Structural measures:river channel design, debris control dam,whole town’s structures, etc.

Non-structural measures:hazards maps, early warning, evacuations etc.

Education tools: -----------------

Plat form

Countermeasures

Evaluation &decision

Information necessary for modification &development

Governing equations,

A specific closed world?

(Researchers and practitioners) Administrators, beneficiaries(people living in hazardous areas)

enables to provide tools and information for researchers, engineers, administrators and others who are interested in flood and sediment hazards.