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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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Webster, P. J., G. J. Holland, et al. 2005. “Changes in tropical cyclone number, duration, and
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Yasuda, T., Hayashi, Y., Mori, N. and Mase, H. 2010, A Stochastic Typhoon Model Applicable to
Storm Surge and Wave Simulations for Climate Change, Proceedings of JSCE, 66(1), pp 1241-
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Miyazaki Masamori.2003.”Study of storm surge.” Seizando-shoten Publishing. pp30-33.Shibayama
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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
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