2nd Kobe U. Brussels EC Symposium
September 19, 2011
Vrije Universiteit Brussel

The Behavior of 3.11 Tsunamis
in the Sendai Plainand the Damageof the Disaster Prevention SystemAkira Mano
Disaster Control Research Center
Tohoku University

Background and objectives
 Background Damage by 3.11 tsunamis 20,000 people killed or missing
 100,000 houses totally destroyed Coastal disaster mitigation system destroyed Governmental policy of the disaster mitigation
 For L1 class hazards (RP of 100 to 150 y), to rebuilt coastal levees to secure
lives and assets
 For L2 class hazards ( >150 y), to secure lives and mitigate the asset
damage by multiple mitigation countermeasures
 Issues for L2 class hazards
 How to secure lives?
 How to rebuilt the levees not seriously destructed?

 Objectives
 To show the causes of the damage occurrence To find the possible solutions for the issues

Contents

 Background and objectives
 Geology and tsunami deformation
 Human and house damages for the tsunami height Tsunami behavior near the coasts
 Obstacles for the evacuation
 Mechanism of coastal levee destruction Damage of coastal forest Summary: Lessons from the disasterGeology and tsunami deformationin the three coasts•Sariku Coast•Crustal uplifting in the north•Cliff coast•Tsunami collision•Crustal subsiding in the south•Submerged valleys•Energy concentration•Bay water resonance•Frequent tsunami attacks•Sendai Bay CoastSanriku Coast

Sendai Bay Coast•Alluvial plain• Sandy beach and shallow water• Propagation delay and energy
dissipation• Low frequency in large tsunamis•Joban Coast
Joban Coast
100 km• Crustal uplifting• Cliff coast and pocket beach• Tsunami collision• Low frequency in large tsunamis

Tsunami height in 4 major events
after Shigihara (2011)

Recent study finds Jogan Tsunami in 869 is similar to the 2011 tsunamis in the Sendai Plain through thesedimentological analysis, Sugawara (2011). This estimates RP of the tsunamis as a1000y.

Human damage*** Fudai V. was secured by the 15.5m-high levee**** Kamaishi C. was mitigated by the tsunami breakwater***** Ofunato C. was mitigated by the tsunami breakwater****** A part of Toni-Hongo in Kamaishi C. was secured by mass
movement to highland after 1933 Showa Sanriku Tsunamis.

Tsunami characteristics of the coasts
Sanriku CoastMorphology

Sendai Bay Coast

Joban Coast

Rocky coasts andsubmerged valleys

A long sandy beachRocky coasts and pocket
beaches

Major hazards inrecent 100 ybefore 2011

Tsunamis

Storm surges

Wind waves

Height of coastal
levee

5 to 16 m

6 to 7 m

4 to 6 m

Major land use in
the coasts

Fishery

Agriculture, fishery andindustry

Fishery and NPP

Max tsunamis inrecent 150y

10 to 25 m in 1896

3 m in 1960 Chili

4 m in 1960 Chili2011 tsunamis

15 to 35 m

8 to 14 m

6 to 10 m

a9600

a8500

More than 1700

Dead andmissing

High rareness in large tsunamis gave inattention in Sendai Bay and Joban areas.
No tsunamis larger than levee height in one century gave relief in Sanriku area.

West

House damage in Ishinomaki C. West
Koshimura(2011)

Inundation area
house washed away
house survived

Tsunami inundation depth varies from 3m to 10m. More than 80%
of houses were washed away for the tsunami inundation height > 5m.

WestHouse damage in Onagawa T.
Koshimura(2011)

Sendai Bay Coast

Oshika Peninsula

Sanriku Coast

Inundation area
house washed away
house survived

Tsunami inundation depth reaches 18 m. More than 70% of the housesin this town were washed away by the tsunamis.

Tsunami shape near the source

water elevation (m)http://www.pari.go.jp/files/items/3527/File/results.pdf

JLT on March 11
Measured by GPS wave gauge installedat the place with the depth 200m off Kamaishi

The first wave is the maximum with 6.5 m amplitude at 26 minute after the quake6 waves follow with a1 m amplitude and 50m periods

WestTsunami shape along Sendai Bay Coast

The first wave is bore like and has
the maximum at 15:50 which
estimates the peak time 15:49 at
the coast, 63 minutes after thequake.

The Naruse River, 0.5 km upstream from the Sendai Bay Coast

The first wave is bore like and has
the maximum at 16:20 which
estimates the peak time 15:55 at
the coast, 69 minutes after thequake.

The Abukuma River, 10km upstream from the Sendai Bay CoastTsunamis intruding into the residential areaat Yuriage, Natori C, photo by Mainichi News240m

Modified tsunami warning transmission

High underestimate in the Initial tsunami warning gave relief to prevent evacuation.
Modified warning did not reach residents in many places by blackout .

Multiple damage chains caused by blackoutThe other obstacles for the evacuation

 Returning back to the home to see the family or to bringsomething.
 Being in the line of duty, such as firemen, community
leaders, truck drivers, etc.
 Nursing homes near the coasts.
 Inappropriately specified asylums and shelters lower than
the tsunami inundation height.
 Fire caused by floating cars, ships, and oils.

Disaster mitigation system along Sendai Bay Coast Coastal disaster mitigation system
 Sandy beach：Natural wave energy dissipation function
 Coastal channels：Construction started in 1601 by Masamune Date and wascontinued by Meiji government. The functions of conveyance and drainage.
 Coastal forests：Black pine trees have been planted to prevent sea water spray
and aeolian sand after Masamune.
 Coastal levees：6 to 7m high levees have been constructed to prevent stormsurges.

 Present vulnerability for huge hazards
 New town developed by cutting coastal forests
 Important buildings such as airport, oil factory, sewage plant, industrial complex
built near coasts
 Disappearance of beaches

Coastal levee destruction
at Ido, Wakabayashi, Sendai C.
Satellite image on 20110314

200m

Mild slope levee

240m

3m erosion

Steep slope levee The first flood tsunami scattered armor blocks on the mild slope levee landward.
 Concentration of the return flow eroded the back of the steep slope levee up to3m, and changed the lands to sea.

Concentration of the return flow to the channels
at Kabasaki, Iwanuma C.

500m

Before, 20090８１４

After, 201103１４

The return flow was collected to the coastal channel
and then to the crossing channel to return back to the sea.
These channels could be used to control the return flow.

Breach of the coastal leveeat the right bank of the Abukuma River mouth

The Abukuma River mouth at JLT 16:33, March 11
Turbid water flowing out from the breach and big
vortices indicate steadiness and strong erodibity of
the return flow.

Aerial photo on March 14 by GSI Japan
Erosion channel at the back of riverine andcoastal levees by the return flow.

Levee destruction
at the right bank of the Abukuma River mouthUndamaged fore-slope and eroded back-slopeof the riverine levee. The vertical concrete blocks
at the top called parapet were flushed away.

The backland of the coastal levee waseroded to form a channel.
The concrete block with wave returning wasbroken at some place.
The boundary of the riverine and coastal
levees was breached.

Catastrophic destruction of coastal levees
in the Yamamoto Coast, Sendai Bay area

2kmBefore the tsunamis

After the tsunamis

T-jettys were constructed to prevent beach erosion.
Thin beaches raise the levee destruction here and there by the flood 2km
tsunamis.
The return flow concentrated to the destructed points to erode the land and levees.

Concentration of return flow to the breach pointat Watari T. , videoed by a helicopter at JLT 16:35

HL11

At the moment, huge amount of sea water filled inundation area.
The return flow along the back of the levee and through the coastal forests
concentrated toward the breach point to erode the land and levees.

Mitigation by the coastal forests
Bird eye view of the fallen woods, Miyagino Bor., Sendai C.

20110406
The tsunamis fell coastal forests to washout the housesbehind.
The houses behind non-fallen forests were secured.

Fallen woods by domino topping?
Miyagino Bor., Sendai C.

There would be feedback effects that fallen trees promote flow concentration which again
promotes tree falling. High pine trees could caused domino topping.

Summary: Lessons from the disaster Decrease of the human damage requires
 disaster mitigation education and evacuation training improvement of the tsunami warning to have mobile radio to avoid the residence in high hazard areas
 Reconstruction of robust levees requires
 restoration of broad beaches
 control of the return flow concentration throughthe development of coastal channels and forests
 anti-erosion armoring at the flow concentration

最後にThank you for the attention