Research Trends and Results

Development of tsunami damage reduction
technologies by linking oceanographic radar withnumerical simulations
HINATA Hirofumi, Head (Dr. Eng)
FUJI Ryotaro, Guest Research Engineer
Coastal Zone Systems Division, Coastal Marine and Disaster Prevention Department(Key words) Tsunami, oceanographic radar, tsunami inversion
1. Background and purpose of the research
The Great East Japan Earthquake clarified two
major problems concerning the reduction of tsunami
damage: [1] underestimation of tsunami height, and[2] delayed identification of regions that will beseverely damaged. [1] was a result of technical limitsof present tsunami warning system based only on
analysis of seismic waves and [2] was caused by slow
clarification of the state of damage by prefectures as aresult of catastrophic damage to administrative organsof cities, towns, and villages by the Great East Japan
Earthquake. In the event of a giant earthquake such as
the predicted Nankai Trough Earthquake, theseproblems could recur.
Oceanographic radar is land-based remote sensing
equipment that measures flow on the ocean surface up
to 100km offshore. We developed technology toresolve the above two problems by linking
oceanographic radar with numerical simulations. To
overcome challenge [1], we developed a method ofcalculating the sea surface height (SSH) distribution
based on tsunami flow velocities offshore measured
by the radar, and to deal with challenge [2], wedeveloped a method of estimating tsunami initial SSH
distribution with high precision by taking advantage ofthe characteristics of the radar, specifically its ability
to obtain spatial distribution of a flow field. If it ispossible to predict initial SSH distribution with highprecision, it is also possible to clarify the state ofdamage over a wide area using existing tsunami
models.
2. Analysis procedure
The figure shows the observed data processing flow
chart. The double lines indicate technologies alreadydeveloped, including existing technologies. The
oceanographic radar transmits radio waves to the sea
surface and receives the backscattered signals from theocean surface. Frequency analysis (FFT) of the signal
can obtain the velocity component caused by thetsunami (tsunami component) and the backgroundcurrent component unrelated to the tsunami(wind-induced current, density-driven current and tidal
current components). Based on knowledge of fluid
mechanics, the tsunami component is converted towater level distribution, and compared with tsunami

warnings (resolving challenge [1]). This tsunami
component and the least squares method are used toestimate the initial tsunami water level distribution1).
In other words, it statistically estimated the degree of
SSH that ought to be formed in the hypocenter region
to create the tsunami distribution measured by theradar. Then the SSH is applied to the tsunami model toperform flooding calculation and to clarify the state ofwide-area damage (resolving challenge [2]). The
background current component will be used tocalculate drift of heavy oil etc. discharged into theocean.
Figure. Measured Data Processing Flow Chart
Comparison
FMCW

Frequency
analysis
FFT

Least squares
methodTsunamicomponent

Initial sea
surface height
distribution

Velocity
potential

Flooding
calculation

Sea surfaceheight

FMICW

Radialvelocity

Backgroundcurrent
component

Tidal current
component

Stream function

Velocitypotential

Flow velocity
Particle
tracing

Clarification ofdamage state

Warning
decision

Driftcalculation

3. Future challenges
A weakness of our present radar system (FMICW)
is that it is susceptible to the impacts of various kindsof noise resulting in weakening tsunami signals. To
improve this, our research group is now developing
radar that uses another transmission/reception system(FMCW). comparative tests of the present radar andthe new radar system will be performed in the next
fiscal year.
[Sources]
Fuji et al. doron, Journal of JSCE, Ser. B2 (CoastalEngineering) NO. 69, I_436-I_440, 2013