Water and disaster in terms of dynamics
Shinji EgashiraInternational 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Ten-year plan forflood control project

The ninth 7-year plan

1960

1993

Priority plan forsocial capital development

2003

Project to measure steep slope collapse
Project to measure debris flow
1983

Casualties resulted from natural disasters in JapanKinu R.
Yamaguchi&Jun/2013,
Shimane4Omoto R.

Sep/2015,
2(Joso city)

Aug/2016,
21

Hiroshima
Aug/2014,
74

N.
Jul/2012,Kyushu

SendaiMt. Fuji(3776m)

32
Jul/2017,
41

Kii Peninsula
Sep/2011,
70(Nara &Wakayama)

Elevation

Izu-Oshima
Oct/2013,
Is.
39-1 to < 0 m✓ Each number shows No. of dead and missing

<1m
3
Water
surface

Flood and sediment disasters in the last 5 years, Japan

Accumulated rainfall (mm)Rainfall intensity(mm/h)

Chikugo River

Akadani basin(about 20km2)

Flood and sediment disaster resulted from severe rainfall at northern Kyushu in July, 2017Debris flow deposition in the upstream of Akadani basin

Damaged houses due to debris flow

River channel

Photos provided by Geographical Survey
InstituteBefore the flood
Immediately after the flood

Channel change in the middle reach of AkadaniA huge amount of sediment deposited in the downstream reach of Akadani

Landslides

Flood flow with sediment and drift woods

Debris flows

Sediment erosion Sediment
Yield of drift woods deposition  30

Flooding with channel changes
Sediment flooding
Driftwoods flooding

Channel closing due to sediment deposition
Dominant bed load and suspended load
Deposition ofScattered cobbles and gravels
driftwoods
Scattered driftwood accumulation  10

Flooding with sedimentand driftwoods
Channel closing due
to sediment deposition
Dominant suspended sediment
Flooding of suspended sediment
Driftwood accumulation at bridges

Longitudinal sediment sorting  4  ~ 8

ib  0.03 ~ 0.07

Houses, Felds
Roads

ib  0.008 ~ 0.03

Houses, Fields , Paddy fields, Roads

Chikugo River

Houses, Paddy Fields

 Bed-slope ib Bed-slope
Characteristics of hazards resulted from the rain fall event at northern Kyusyu in July 2017

Governing equations for flood flow with sediment transportation anddrift wood
- Mass conservation equation for water- Momentum conservation for water- Mass conservation equation for sediment in flow body (very dilute sediment)- Mass conservation equation for bed sediment- Bed load formula, Erosion/deposition rate for suspended sediment(instead of momentum conservation equation)- Mass conservation equations for drift wood in flow body as well as in bed sedimentsuspended load (very dilute sediment)
bed-loadriver bed

Governing equations for debris flow and soil mass released by landslides- Mass conservation equation for water-sediment mixture h  uh  vh E t  x  y c*
for mixture

 c h   c uh   c vh


E
t
x
yfor fine sediment

for sediment only

- Momentum conservation for water-sediment mixture uh  uuh  vuh
H  bx  ghtxyx  mvh uvh  vvh
H  by  ghtx
yy  m- Mass conservation equation of bed sediment

 zbEt
c* cos

for coarse sedimenthFlow depthu, vcFlow velocity
Sediment concentrationin flow body
Sediment concentration
in bed sedimentc*

EErosion/deposition rate

HFree surface elevationzb

Bed surface elevation

 bx ,  by Bed shear stressSediment discharge(m3/s)

Initial stage(200m3/s)

Increase of flood area with increase offlow discharge and sediment deposition
Around flood peak
(350m3/s)

Tailing stage(150m3/s)

Stream bifurcation
due to sedimentation

Computed flow patterns
provided by Harada et.al.
(Nov. 2017)

upstream

upstream

1.5m at bridge

Deposition inunit area(m3/m2)+0.1+0.05

Depth of erosion/deposition (m)

+3.
00
-3.00

Deposition of drift wood

Change of bed elevation

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

15:00Basin average
Hourly rainfall(mm)

18:00

EndAkadani Basin

6:00

12:00

18:00

July 5, 2017

0:00

Temporal changes in the number of unstable cells

July 6
Provided by Yamazaki et.al.
(Nov. 2017)

Spatial distribution for erosion and deposition resulted
from debris flow

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

Erosion/deposition depth

Debris flow resulted from landslides

Flow depth

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

Hiroshima

Fukuoka(Akadani)

Izu Oshima

18:00 5 Jul. 2017
17:00
14:00
16:00

Critical rainfall conditions to cause severe
sediment disasters in different regions

Rainfall(C-BAND JMA)

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

Concluding remarks

A specific closed world?

Countermeasures
Governing equations,
Numerical models and resultson flood flow (inundation processes)with sediment transportation anddrift wood as well as onlandslides and debris flow

(Researchers and practitioners)

Evaluation &decision
Informationnecessary formodification &development

Structural measures:river channel design,debris control dam,
whole town’s structures, etc.
Non-structural measures:hazards maps, early warning,
evacuations etc.
Education tools:----------------Administrators, beneficiaries(people living in hazardous areas)

Plat form enables to provide tools and information for researchers, engineers,

administrators and others who are interested in flood and sediment hazards.