THE MECHANISMS OF AFTER-RUNNER STORM

SURGE ALONG THE NORTH COAST OF VIETNAM

Nguyen Ba Thuy Marine forecasting division

Vietnam National Center for Hydro-meteorological Forecasting

Hanoi, 26 Februal 2018

Storm surge

Abnormal water level (Meteorological tsunami)

Spring tide (Hight tide

+ surge induced by moonson)

Coastal risk related to high water level in Vietnam

Hoang Sa

Truong Sa

High Vulnerable storm surge (abundant typhoon +

shallow water)

Less Vulnerable storm surge (rare typhoon + deep water)

Less Vulnerable storm surge

(shallow water +rare typhoon)

Number of

Typhoon

Typhoon and Storm Surge in Vietnam

June

Dec.

Tracks of typhoon in 2017

Typhoon number 2 (July/2017)

6 peoble was die and missing, more than 100 ship was broken by high wave

Coastal inundation due to typhoon No 10 (Sep./2017)

Typhoon number 10 (9/2017)

Typhoon number 12

(Nov./2017)

6 cargo ships was sink, 11 peoble

die and 2 mising. More than 2000

ship broken by high wave

-0.5

0.0

0.5

1.0

1.5

9/29/2006 6:00 9/30/2006 12:00 10/1/2006 18:00

Sur

ge h

eigh

t (m

)

Time (h)

Observation

Without wave

With wave

Numerical Model:

- JMA model -2D ROMS - SuWAT ( Best track or WRF)

Time serial of storm surge

Storm surge research and prediction

Coastal inundation due to tide + storm surge + wave setup

The after runner storm surge on the North coast of Vietnam

Area: The north coast of Vietnam

Typhoon frequency: 1.5 typhoons/year

Maximum storm surge: 3.4m

The after runner storm surge: 6 cases/50 years

Hoang Sa

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-0.2

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1

1.2

9/16/2014 0:00 9/18/2014 0:00

Stoe

m su

rge

(m)

Time (h)

Storm surge at Hondau station during typhoon Wukong (2005) and Kalmaegy (2016)

Typhoon landfall

Typhoon landfall

Normal storm surge

After runner storm surge

The after runner storm surge

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0

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0.8

1

1.2

7/29/2005 0:00 7/31/2005 0:00 8/2/2005 0:00

Stor

m s

urge

(m)

Time (h)

Observation water level, predicted tide and

storm surge at Hondau station

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0

50

100

150

200

250

300

350

400

450

9/16/2014

9:36

9/16/2014

14:24

9/16/2014

19:12

9/17/2014

0:00

9/17/2014

4:48

9/17/2014

9:36

9/17/2014

14:24

9/17/2014

19:12

9/18/2014

0:00

9/18/2014

4:48

9/18/2014

9:36

9/18/2014

14:24

Time (h)

H (

cm)

Total water level

Prediced tide

Surge

Time of typhoon landfall

time interval of

urge exceed 0.5m

The after runner of storm surge cases

Kalmaegi (9/2014)

Mirinae (7/2016)

Typhoon land fall

Inundated

No inundation

- Damage by wind is not much.

- But Storm surge + high tide + high wave

generated inundation at Haiphong coast

after the typhoon landfall 5 hours.

Track of typhoon Kalmaegy

Typhoon Kalmaegi (9/2014) – After runner storm surge case

High wave and inundation at Haiphong coast

- Empirical Typhoon Model:

fDuy

Dgy

Dv

x

Duv

t

Dv

fDvx

Dgy

Duv

x

Du

t

Du

byay

ww

bxax

ww

)(1)(1

)(1)(1

02

02

0

y

Dv

x

Du

t

2

0)/(1

)(rr

PPPrP c

(Fujita model)

r

Pfv

r

v

12

Operational Forecasting Model in 9/2014 (conventional method)

Two dimensional long wave model + Empirical Typhoon Model

Result of Operational forecasting (2D long wave model + Fujita model)

Simulated result showed storm surge not high at Hondau station, and at low tide –> No warning. But infact inundation occurred. We miss warning

Maximum storm surge

Model and observation storm surge

Typhoon track

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0

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1

1.5

2

9/16/2014 12:00 9/17/2014 12:00 9/18/2014 12:00

Stor

m s

urge

(m

)

Time (h)

OBS

FUJITA

Typhoon Landfall

Storm surge observation data

Time profile of total water, tide and storm surge at Hondau (a) and Honngu station (b)

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50

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350

9/16/2014 0:00 9/16/2014 18:00 9/17/2014 12:00

Z (

m)

Time (h)

OBS

Prec. Tide

Storm surge

Typhoon Landfall

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0

50

100

150

200

250

300

350

400

450

9/16/2014 12:00 9/17/2014 12:00 9/18/2014 12:00

Z(m

)

Time (h)

OBS

Predic. Tide

Storm surge

Typhoon land fall

(a)

(b)

Two Dimensional Long wave Model + SWAN

F: The wave force- which correspond to the gradients of wave-induced radiation stress

s The wind stress (including wave dependent drag)

2. Numerical Model (coupled of surge wave and tide)

- The case uncoupled with wave: The conventional CD (Honda and Mitsuyasu, 1980). Sure

induced by wind stress and air pressure only:

- The case coupled with wave: The wave dependent drag CD (Janssen, 1991). The stress on the surface consider wave:

Stress on the surface

10 10W Ws a DC

Computational domains for WRF and Storm surge model

WRF model

Geophysical domains of the study area with three levels. (a) shows the outermost domain of the Vietnam coast. (b) shows the intermediate domain. (c) focuses on the innermost domain with Hondau station.

Results of SuWAT + WRF model

Comparison of calculated and simulated storm surge at two station using wind and pressure from WRF

SuWAT+WRF is well simulated the storm surge at two stations -0.6

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9/16/2014 12:00 9/17/2014 6:00 9/18/2014 0:00

Stor

m su

rge

(m)

Time (h)

OBS

WRF

Typhoon landfall

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-0.5

0

0.5

1

1.5

9/16/2014 12:00 9/17/2014 6:00 9/18/2014 0:00Stor

m s

urge

(m

)

Time (h)

Observation

WRF

Fujita

Typhoon landfall

Measured and predicted pressure (above) and

wind speed (below) and at Hondau station

The WRF model simulated well wind and pressure than the

empirical typhoon model

Wind and pressure data

Comparison of calculated and simulated storm surge at two station for two case using WRF model. One using front and tail wind field (above), the other using the front wind field only (below).

Effect of front and tail wind field on storm surge

Storm surge at Hondau station is mostly induced by tail wind. But at Honngu station is due to the inertia force

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9/16/2014 6:00 9/17/2014 6:00 9/18/2014 6:00Sto

rm s

urg

e (m

)

Time (h)OBS

WRF

WRF-No wind after land fall

No wind after typhoon land fall

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0

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9/16/2014 0:00 9/17/2014 0:00 9/18/2014 0:00Stor

m s

urge

(m

)

Time (h)

OBS WRF WRF-No wind after landfall

The result at the Honngu station is not so much different between two cases. It means storm surge at Honngu station is due to water moving from North to the south ?.

Maximum storm surge for the case the model using front and tail wind field (left) and front wind field only (right)

Effect of front and tail wind field on storm surge

Honngu

Hondau

At the time typhoon land fall (water level at two station is down) After typhoon land fall 4 hour (water level at Hondau station is going up)

After typhoon land fall 6 hour (water level at Hondau station get maximun)

The tail wind fields of typhoons are more important than front wind fields.

How the surge generated at the Northern part (Hondau station)

Wind and water surface elevation

After typhoon land fall 4 hour (water level at Hondau station is going up)

Water surface elevation and current

Storm surge on the southern part (Honngu station) due to the tail wind

and water moving from the north.

How the surge generate at the Southern part (Honngu station)

Wind and water surface elevation

After typhoon land fall 8 hour

Effect of wind and pressure on storm surge

Comparison of calculated and simulated storm surge at two station for 3 cases of wind + pressure, wind only and pressure only (data from WRF model). The contribution of wind is mainly.

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9/15/2014 12:00 9/16/2014 12:00 9/17/2014 12:00Stor

m s

urge

(m

)

Time (h)

OBS

Wind and pressure

Win only

Pressure only

Effect of tide storm surge

Comparison of simulated and observation storm surge. The simulated results for two case, with and without consider tide.

Effect of wave on storm surge

Comparison of simulated and observation storm surge. The simulated results for two case, with and without consider wave.

Conclusions:

-During the typhoon Kalmaegy, the surge reach up to one metter after the typhoon landfalled and the duration of up to half a day.

-The SuWAT model gave the results closed to the observation data in both the height and the duration when using wind and pressure from the WRF model. Meanwhile, the traditional method of using the empirical typhoon model gave undersimated.

- The strong tail wind is the main cause of the after-runner surge in this case.

Flooding in Ho Chi Minh City in

20/11/2011 Maximum tide amplitude

Spring tide in Hochiminh city

Spring tide in Hochiminh City is related to Surge induced by

monsoon or not?

It related to flooding in Hochiminh

City on 20/10/2013

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10/8/2013 0:00 10/14/2013

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10/20/2013

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10/26/2013

0:00

Time (s)

Su

rge

hei

ght

(m

)

0

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5

Wat

er s

urf

ace

elev

atio

n (

m)

OBS-Total water levelModel-SurgeOBS-Surge

Spring tide in Hochiminh city on 20/10/2013

* Vung Tau

Hochiminh city

Time profile of total water surface elevation, predicted and measure

surge height

Maximum water surface elevation during the monsoon 10/2013

Wind field at 20/10/2013 Spring tide = high tide+ surge (by moonson)

High Oscillation of water level on the middle coast of Vietnam

High tide at Tuyhoa in Dec. 2016

(No typhoon and calm wind. But the

inundunation height up to 1m, and go in

land up to 200m). Meteorological

tsunami?.

Time serial of total water level, tide and surge

To simulate this phenomenal need high resolution of ocean and atmosphere model. Next study

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12/11/2016 0:00 12/16/2016 0:00 12/21/2016 0:00

Z (c

m)

Time (h)

OBS Tide Surge

local/rear/tail winds

front winds