Hitoshi Tanaka

Senior Director, International Activities Center (IAC), JSCE

Prof. Dr.Eng., Tohoku University, Sendai, Japan

Erosion of River Delta Coastlines due

to Climate Change

Workshop

Japan and the Netherlands

Climate Adaptation and Resilience

Oct 31, 2019. Tokyo,

Cua Dai Beach, Vietnam

Dec 12, 2015

1. Recent flood disaster in Japan induced by Typhoon No.19 (Typhoon Hagibis)

2. Erosion of River Delta Coastlines due to Climate Change

Breaching of river embankment

Breaching

Class A

Sendai

（MLIT)

（MLIT)

(the Asahi Shimbun)

Yoshida R.

Chikuma R.

Breaching

Class A

Sendai

Yoshida R.

（MLIT)

Breaching

Drainage tunnel

Cornelis Johannes van Doorn

(1837-1906)

Stayed in Japan from 1872-1880

https://www.midorinet-fukushima.jp/

JSCE Civil Engineering

Heritage

van Doorn’s technical report

for the drainage system

Erosion

Accretion

Accretion

OCEAN

LAND

Dam

Nile River delta, Egypt

Thu Bon River delta, Vietnam

Delta Formation and Deformation

Two mechanisms of beach erosion

(1) Intercept of longshore sediment movement by coastal structure

Longshoresediment

movement

Wave incidence

erosion

accretion

Shinnecock Inlet, New York

Longshoresediment

movement

Two mechanisms of beach erosion

(2) Reduction of sediment supply from a river basin

River Sediment supply

Cua Dai River mouth, Vietnam

Reduction of River Sediment supply

Erosion

Erosion

Cua Dai River mouth, Vietnam

IPCC AR5 (2014)

Beach Erosion by Sea Level Rise

Mean water level rise

(average from 2081 to 2100)

Scenario

average from

2081 to 2100

Water level rise

From:Best

Scenario

Worst

Scenario

Equilibrium beach profiles (EBP) ( Bruun, 1962,1988)

3/2Ayh =

h : water depth, A : scale parameter, y : horizontal distance from the shoreline

(1)

Dean’s theory (1977) 1. Wave energy dissipation per unit water volume.

2. Wave energy dissipation per unit surface area.

3. Uniform average longshore shear stress across the surf zone.

3/2

2

*

5

24

=

κρ gg

DA (2)

Shoreline Retreat Theory

Shoreline retreat due to sea level rise.

Bruun’s rule (1962) : Response of beach profile to sea level rise

Bh

LSR

+=

∞

*

*

L*

h*

R∞

Shoreline Retreat Theory

Future Shoreline Retreat in Japan

0

20

40

60

80

100

Ero

sio

n

Rati

o

(%)

Ho

kk

aid

o

Ao

mo

ri

Iwate

Miy

ag

i

Ak

ita

Yam

agata

Fu

ku

shim

a

Ibara

ki

Ch

iba

To

ky

o

Kan

ag

awa

Nii

gat

a

To

yam

a

Ish

ikaw

a

Fu

ku

i

Sh

izu

ok

a

Aic

hi

Mie

Ky

oto

0

20

40

60

80

100

Ero

sio

n

Rati

o

(%)

Osa

ka

Hy

og

o

Wak

ay

am

a

To

tto

ri

Sh

iman

e

Ok

ay

am

a

Hir

osh

ima

Yam

agu

chi

To

ku

shim

a

Kag

aw

a

Eh

ime

Ko

ch

i

Fu

ku

ok

a

Sag

a

Nag

asa

ki

Ku

mam

oto

Oit

a

Miy

azak

i

Kag

osh

ima0

20

40

60

80

100

Ero

sio

n

Rati

o

(%)

Ok

inaw

a

Erosion ratio of sandy coast in each prefecture

(prediction for 30.6cm sea level rise)

(Mimura et al., 1994).

0 20 40 60 80 100 120

Sea-level Rise (m)

0

20

40

60

80

100

Are

a

of

Ero

sio

n

(%)

Relationship between sea level rise and area of

eroded sandy coast (Mimura et al., 1994).

Delta Coastline Erosion in Vietnam

Importance of Beach Erosion in Vietnam

(Yomiuri Online, June 6, 2017)

PM Nguyen Xuan Phuc’s visit to Japan in 2017

Beach erosion in the vicinity ofthe river mouth

Tenryu R. (Japan)

0 20km

Thu Bon R.Vietnam

0 20km

Cuspate shape shoreline due to predominant sediment supply from the river basin

Similarity between Japan and Vietnam

Dam

Location of dams

Beach erosion in the adjacent sandy coast

Feb 7, 1976

March 15, 2014

Large number of dams

⇒Reduction of sediment supply to the sea

⇒Beach erosion

Erosion of Tenryu River Delta

River mouth

Structural measure

- detached breakwater

- artificial reef

- groin

Non-structural measure

- increasing sediment discharge

- beach nourishment

detached breakwater(Arahama Coast, Sendai)

groin (Yamamoto Coast, Miyagi)

Countermeasures

2015/12

Cua Dai Beach

Vietnam

Pre-tsunami

Jul. 23, 2010

Post-tsunami

Mar 14, 2011

Beach Nourishment in Rikuzen Takata, Japan

“Miracle pine", a lone pine

tree that

stood as a

symbol of

hope.

+3m AMSL

+12.5m AMSL

Beachnourishment

Pine tree plantation

Restoration of Rikuzen Takata

Groin

Region 1Length: 400m

Beach width: 30m-60mRegion 3Length: 650m

Beach width: 30m-60m

Region 2Length: 700m

Beach width: 60mGroin

Groin

Groin

Beach Nourishment Plan

Iwate Prefectural Government (Aug, 2016)

Beach Nourishment Project

Iwate Nippo (May 28, 2017)

Thank you for your kind attention!