Ad Hoc Expert Meeting on

Climate Change Impacts and Adaptation: A Challenge for

Global Ports

29 – 30 September 2011

Effect of Sea Level Rise and Increase in Typhoon Intensity on Coastal

Structures in Tokyo Bay

By

S. Hoshino, M. Esteban, T. Mikami, T. Takabatake and T. Shibayama

This expert paper is reproduced by the UNCTAD secretariat in the form and language in which it has been received. The views expressed are those of the author and do not necessarily reflect the views of the UNCTAD.

EFFECT OF SEA LEVEL RISE AND INCREASE IN TYPHOON INTENSITY ON COASTAL STRUCTURES IN TOKYO BAY

Sayaka Hoshino1, Miguel Esteban

1, Takahito Mikami

1, Tomoyuki Takabatake

1

and Tomoya Shibayama1

Sea level rise and an increase in typhoon intensity are two of the results expected from future climate change. In the present work a methodology to change the intensity of tropical cyclones in Japan is

developed based on the work of Knutson and Tuleya (2004). An example of how this would affect

one of the worst typhoons to hit the Tokyo Bay area in the 20th century was thus developed, highlighting the considerable dangers associated with this event, and how current sea defences could

be under danger of failing by the end of the 21st century.

INTRODUCTION

Every year, Japan is attacked by a number of tropical cyclones, some of which

can be very strong and cause widespread damage. Apart from wind damage,

these events also generate powerful waves and storm surges, which can inundate

coastal areas and lead to the destruction of property and the loss of many lives.

Global warming as a consequence of increasing concentrations of greenhouse

gases in the atmosphere could lead to an increase in intensity in tropical

cyclones in the future, which would compound the problems already presented

by sea level rise, also expected to accelerate in the course of the 21st century. A

number of authors (such as Knutson and Tuleya, 2004, Elsner et al., 2008,

Landsea et al., 2006, Webster and Holland, 2005) have carried out research

showing how it appears likely that tropical cyclones will increase in intensity in

the future, though there is no broad consensus on the future frequency of these

events.

During the 20th century the global average sea level rose by an average of

around 1.7mm per year, with satellite observations showing that since 1993 sea

level has been rising at a rate of around 3mm per year, according to the 4th

Assessment Report of the Intergovernmental Panel on Climate Change, or 4th

IPCC. Future projections of sea level rise show that sea level could be between

0.18 and 0.59m higher than present by the end of the 21st century. More

extreme scenarios, such as those by Vermeer and Rahmstorf (2009), argue that

sea level rise could be in the range of 0.81 to 1.79m by 2100.

Though the cities around Tokyo Bay are generally well protected by coastal

structures against storm surge, the combination of sea level rise and an increase

in typhoon intensity could eventually require the strengthening of these defences

to mitigate against these effects of climate change. In the present work the

1Dept of Civil and Environmental Engineering, Waseda University, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan

authors present a methodology of how to modify the strength of future typhoons

using the results of Knutson and Tuleya (2004) and based on historical events

use this methodology to determine the significance of climate change to coastal

structures in one case study area (Tokyo Bay).

METHODOLOGY

The present work uses as a case study the typhoon of October 1917 (6th

year of

the Taisho period Typhoon), which was the worst typhoon to affect Tokyo Bay

in the last 100 years. In order to simulate the expected future increase in typhoon

intensity the authors used the probability distribution functions provided by

Knutson and Tuleya (2004), outlining the present and future expected intensity

of typhoons in the Asia-Pacific Region. To determine the storm surge a 2-level

model was used. The governing equations in the model are the mass

conservation equation and the momentum conservation equation. One of the

main problems of this model lies in determining the radius of maximum wind

speeds. To overcome this problem the method of Yasuda et al. (2010) was used,

where the radius is not given a deterministic value but rather follows a

probabilistic curve. Finally, the effect of sea level rise is added to that of the

storm surge to calculate the final level of sea defences that would be required for

a given scenario.

Study Area

The target area of study in the present research are the Tokyo and Sagami Bays.

The simulation uses a nesting approach, and hence the area is divided into 2

levels of calculation, one with a larger grid size than the other. For the large area,

the dimension of grid is 30 seconds (around 3km), and for small grid size, the

dimension is 10 seconds (around 1km), with the small area covering only Tokyo

Bay (Figure 1). Figure 2 and Table 1 show the location of the points which were

studied along the Bay, for which storm surge levels were recorded for each of

the tropical cyclone simulations.

Figure 1. Study area showing Tokyo Bay and adjacent sea areas

Table 1. Name of the places simulated, with respective prefectures

Taisho 6th

year (1917) typhoon

The Taisho (1917) Typhoon was the worst typhoon to affect Tokyo Bay in the

last 100 years. It caused widespread damage in the Tokyo Bay area, and from

past historical records it flooded an area of over 200 km2, with the number of

dead and missing estimated to be over 1300 people. The lowest pressure of the

typhoon was said to be 952.7hPa according to Miyazaki (2003), though the way

in which pressure was measured in 1917 is slightly different to the way it is

Figure 2. Summary of points of interest along Tokyo Bay

done now. The original way to measure the central pressure of a typhoon was to

carry out plane observations. Though this system was used until 1987, from

1977 meteorological satellite have been mainly used to measure it.

Figure 3. Route of Taisho (1917) Typhoon

Storm Surge Simulation Model

To simulate storm surge height, a 2-level model is applied as illustrated in

Figure 4. This 2-level model was introduced into the calculation of storm surge

by Tsuchiya et al (1981). The model has been verified and used by many

researchers in the past (such as by Toki et al. (1990), , who have verified its

validity against a number of historical typhoons. Nevertheless, for the present

simulation the authors also checked that the storm surge results of the historical

typhoon matched the observed storm surges at different locations along Tokyo

Bay.

A 2-level model considers the influence of wind, so it is more effective for

shallow water such as Tokyo Bay to use 2-level model than 1-level model. The

governing equations are the mass conservation equation and the momentum

conservation equation. For the case of storm surge it is important to consider

also the effect of wind induced wave, shear stress due to wind at water surface,

shear stress between upper layer and lower layer, and sea bottom shear stress.

Therefore, governing equations for storm surge can be expressed in Eqs. (1)-(5)

Figure 4. A 2-level model diagram

0)()( 2121

NN

yMM

xt

(1)

for upper layer

w

sx

w x

PHfN

xgH

H

NM

yH

M

xt

M

111

1

11

1

2

11 )()(

0)()(2

1

2

2

1

2

i

w

ix

h uwy

M

x

MA

(2)

w

sy

w y

PHfM

ygH

H

N

yH

NM

xt

N

111

1

2

1

1

111 )()(

0)()(2

1

2

2

1

2

i

w

iy

h vwy

N

x

NA

(3)

for lower layer

w

sx

w x

PHfN

xgH

H

NM

yH

M

xt

M

222

2

22

2

2

22 )()(

0)()(2

2

2

2

2

2

i

w

ix

h uwy

M

x

MA

(4)

w

sy

w y

PHfM

ygH

H

N

yH

NM

xt

N

222

2

2

2

2

222 )()(

0)()(2

2

2

2

2

2

i

w

iy

h vwy

N

x

NA

(5)

Where is water surface profile above still water level, h is the still water level,

H1 equals to + hi, H2 equals to h + hi, g is acceleration of gravity, x, y are

horizontal coordinate, t is time, M and N are x, y component of momentum flux,

s is water surface shear stress due to wind, b is sea bottom shear stress, i is

shear stress between upper layer and lower layer, w is sea water density, and n

is Manning’s friction factor. Subscribe 1 is for upper layer and subscribe 2 is for

lower layer. The pressure term is governed by Myers’s formula (1954) in Eqs. 6.

)exp( max0

r

rPPP (6)

Where P is pressure of point that has distance r from center of typhoon, P0 is

low pressure at center of typhoon, P is different pressure between center of

typhoon and normal air pressure, and rmax is radius of maximum wind speed.

Estimation of Central Pressure

To understand how future increases in tropical cyclone intensity will affect

storm surge it is necessary to estimate what the future central pressure of a given

event will be. In order to do so, the authors used the results of the work of

Knutson and Tuleya (2004). These authors carried out 1300 five-day idealized

simulations using a high-resolution version of the Geophysical Fluid Dynamics

Laboratory (GFDL) R30 hurricane prediction system. These simulations were

carried out for a Surface Sea Temperature change of between +0.8° to +2.4°C,

which assume a linear +1% compounded yearly increase in CO2 over a period of

80 years, (up to the year 2085) in order to calculate the Surface Sea Temperature.

This +1% yearly increase means that CO2 levels would reach 2.2 times the

control value (that of 2004) by the year 2085.

Knutson and Tuleya (2004) computed histograms of the maximum surface wind

speed for 4 different types of hurricane simulation, all of which result in an

increase in both storm intensity and near-storm precipitation rates related to the

increase in Surface Sea Temperature. The method used in the present paper

simplifies the 2085 histogram into a probability distribution curve, and uses this

to modify the intensity of historical storms as shown in Fig 5. According to this

Figure, for example, a typhoon that is 950hPa nowadays (lowest 5.6% of

typhoons) would correspond to a 945hPa in 2085 (lowest 5.6% of typhoons

according to the probability distribution functions of Knutson and Tuleya, 2004).

Figure 5. Present and future probability distribution function of tropical cyclones,

according to Knutson and Tuleya (2004)

Estimation of Radius of Maximum Wind Speed

One of the main problems of this model lies in determining the radius of

maximum wind speeds rmax, necessary for the correct resolution of the Myers

formula (1954). To overcome this problem the method of Yasuda et al. (2010)

was used, where the radius is not given a deterministic value but rather follows a

probabilistic curve which depends on the central pressure of the tropical cyclone.

There is a slight limitation in using the model of Yasuda et al. (2010) in that the

model only allows the determination of the radius of tropical cyclones which

have a central pressure of 940 hPa or less, and hence weaker typhoons cannot be

simulated using this method.

Sea level rise

Future patterns in sea level rise are highly uncertain due to a lack of

understanding of the precise working of global climate and its interaction with

the physical environment. A lot of this is down to uncertainty in the response of

the big ice sheets of Greenland and Antarctica (Allison et al., 2009). In fact, it is

currently believed that sea level is likely to rise much more by 2100 than the

range of 0.18-0.59m given in the IPCC 4AR. In this report, the coupled models

used for the 21st century sea level projections did not include representations of

dynamic ice sheets, only including simple mass balance estimates of the

contributions from Greenland and the Arctic ice sheets. In fact the IPCC 4AR

assumed that ice was accumulating over the Antarctic ice sheet, though this is

currently losing mass as a consequence of dynamical processes, as shown in

Allison et al., (2009). Recent research such as that by Vermeer and Rahmstorf

(2009) show how sea level rise for the period 1990-2100 could be in the 0.75 to

1.9m range.

In spite of all this, the authors decided to use a rather conservative philosophy in

the levels of sea level rise used. Hence, the authors decided to use a sea level

rise of 0.28m (which would be similar to that of the scenario B1 of the IPCC,

thought the IPCC computes this sea level rise for the year 2100 and the results of

Knutson and Tuleya are given for the year 2085). Using higher levels of sea

level rise will only increase the effects outlined latter in this paper.

Scenarios

The Taisho (1917) typhoon was reported as having a central pressure of

952.7hPa according to Miyazaki(2003). However, in the present simulation the

historical central pressure of the typhoon is assumed to have a value of at least

940hPa. The reasons for this are twofold. First, it is not clear whether the way of

measuring the central pressure of typhoons in 1917 was as accurate as that

nowadays, and hence it is possible that some errors were involved in that

estimation. Second, because although the historical “non-climate change

affected” worst recorded typhoon in Tokyo bay was that in 1917, it could be

possible that earlier typhoons had a lower central pressure. Table 2 shows the

central pressure I'd other typhoons that went through other locations in japan in

the 20th century. As can be seen, some of them are much lower than 952.7,

justifying a cautionary approach in dealing with typhoons in Tokyo bay. They

were not recorded due to lack of adequate measuring equipment.

Table2. Examples of typhoons which have lower central pressure than that of 1917 in the 20

th century

Year Name Central pressure Storm surge

Toll (Place) (Place)

1934 Muroto typhoon 912hPa (Kouchi) 3.1m (Osaka) 2702

1959 Ise typhoon 929hPa (Wakayama) 3.5m (Mie) 4697

1961 2nd Muroto

typhoon 931hPa (Kouchi) 2.8m (Osaka) 194

Thus, the philosophy followed is that the worst ever non-climate change induced

disaster would have had a central pressure only slightly worse (940 instead of

952,7hPa) that the worst actually recorded. On top of that, another 3 different

scenarios were calculated, showing what would hypothetically happen if other

climate-change augmented typhoons were to hit the area. Though up to now

these typhoons have not taken place, history alone is a poor way to predict

natural disasters, as has been shown by the 2011 Tohoku tsunami in Japan

(where the magnitude of the event was much bigger than anything else

experienced in the area in since records began in Japan).

Table3. Parameters for each scenarios

Scenario

Historical

Central

Pressure

(hPa)

Climate-change

modified central

pressure (hPa)

Radius of

maximum wind

speed

Sea Level

rise

I 940 930 Probability

distribution

function

according to

Yasuda et al.

(2010), 10

computations

for each

Scenario

28cm

II 930 920

III 920 910

IV 910 895

RESULTS

To calculate the storm surge under for the modified tropical cyclones it is

necessary to consider the central pressure, radius of maximum wind speed and

sea level rise, as mentioned earlier. As the methodology of Yasuda et al. (2010)

is probabilistic, this also results in a probabilistic answer, where the storm surge

for a given central pressure takes a range of possible values.

The results in Fig 6 shows the maximum storm surge that could be expected

from the case study climate change altered typhoon at 3 sample points inside

Tokyo Bay for each location and scenario. The graph also shows the level of the

sea defences in each of these points, and how at several points the current level

of sea defences could be breached for some of the more onerous scenarios.

It is also worth considering how close the defences are to failure, as this can give

a feeling of the risks involved. Figure 7 shows the two different cases that will

be considered, with case A being the probability of the storm surge overflowing

the protection structures, and case B the probability it will reach a level of at

least 50cm below the top of the defences. Table 4 computes the probability of

each case being reached for each location and scenario. This table shows how

even for scenario A the structures can be considered to be at risk, as the storm

surge approaches the top of the defences, leaving little in terms of a safety

margin.

Figure 6. Probability distribution function of storm surge for Chiba

Figure 7.Definition of storm defence cases A and B

Table 4. Probability (%) that storm surge height becomes higher than case A or B of defences.

CONCLUSION

The combined effect of an increase in typhoon intensity and sea level rise could

pose significant challenges to coastal defences in the Tokyo area. Current

Japanese construction policy specifies that coastal defences in the Tokyo Bay

area should be constructed to a level of +4m above Tokyo mean water level

(+4m T.P., representing 3m against storm surge and 1m against high tide.),

though many old structures have been designed to a lower level.

In the present work the authors developed a methodology to determine the

change in tropical cyclone intensity around Tokyo Bay, based on the results of

Knutson and Tuleya (2085). The methodology also takes into account sea level

rise, though for the present work the authors only considered a rather limited

case of 0.28cm (equivalent to the average of scenario B1 of the 4th

IPCC)

To showcase the methodology, the equivalent of Taisho (1917) typhoon was

simulated as it moved through the Kanto area. By changing the typhoon

parameters (central pressure, maximum radius) 60 simulation cases were carried

out, giving the probability distribution of storm surge occurrence. Though the

calculated storm surges at Shibaura and Funabashi are high, the risk of overflow

is much higher at Yokosuka, Yokohama, Kawasaki, and Futtsu. It would thus

appear necessary to increase flood defence heights by 0.5m or more at these

locations

The present results thus showcase how the level of defences could be inadequate

by the end of the 21st century, and that consideration should be given to

alternative defensive measures or an increase in the required protection level.

If more onerous sea level rise scenarios are considered, such as those in the

IPCC or those by Vermeer and Rahmstorf (2009), this would then require even

higher sea defences to be built. In this case the likely increase in storm surge

would be small compared to the effect of sea level rise, and would warrant even

more dramatic adaptation measures to be taken, which could even include a

(very costly and probably environmentally controversial) storm surge barrier

across the entrance of Tokyo Bay. This would behave in a similar way to

something like the Thames Barrier, but on a far more massive scale.

ACKNOWLEDGMENT

The present work was supported by the grant in aid for scientific research, Japan

Society for Promotion of Science (JSPS), No.B-22404011.

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KEYWORDS – CSt2011

Abstract acceptance number: p0142

EFFECT OF SEA LEVEL RISE AND INCREASE IN TYPHOON

INTENSITY ON COASTAL STRUCTURES IN TOKYO BAY

1st Author: Sayaka Hoshino

2nd

Author: Miguel Esteban

3rd

Author: Takahito Mikami

4th

Author; Tomoyuki Takabatake

5th

Author: Tomoya Shibayama

Tropical Cyclone

Climate Change

Sea Level Rise

Storm Surge

Tokyo Bay