Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Evaluation of Probability of External Force Acting on Shore Protection Facilities
Kanta Okamoto, Kazuhiko Honda
Coastal, Marine and Disaster Prevention Department, National Institute for Land and Infrastructure Management
Yokosuka, Kanagawa, Japanwater level, whereas the HWL is used constantly as the astronomical tidelevel in the conventional method.

ABSTRACTThis study proposed a methodology to evaluate the occurrence
probability of the external force acting on the shore protection facilities,
considering the time variations of the waves, the storm surge, and theastronomical tide level. In a case study, we estimated the occurrence
probability distributions of external forces acting on an existing shore
protection facility with the proposed method. In addition, we calculated
the design external force with the conventional design method, and thenevaluated its return period by referring the occurrence probability
distribution.

Another question is what is the occurrence probability of the design
external force which is calculated with the combination of the design
wave and the design water level; the design wave has 50-year return
period, whereas the design water level has unknown return period. This
question is the same as the external force which exceeds the design
external force.
In order to resolve this question, we need to estimate the occurrence
probability distribution of external force with datasets of the waves andstorm surge induced by storm events. In particular, the reliable
estimation of the long-term return period needs many datasets including
datasets induced by the storm events with severe intensity. However, thenumber of the storm event in the historical data is limited and the historyof the observations for wave, storm surge and sea level is still short.

KEY WORDS: Return period; external force; wave; storm surge;
stochastic typhoon model; shore protection facility.
INTRODUCTION
The structural type and scale of shore protection facilities are determinedby the external force acting on them, which mainly depends on waves
and sea level. The conventional design method applies the wave heightwith the return period of 50 years and corresponding wave period to theparameters of the design wave for shore protection facilities. These wave
parameters are related with offshore wave. The offshore wave height andwave direction are converted to the wave height and wave direction in
front of the shore protection facilities through the wave transformation
analysis. On the other hand, the design water level is the storm surge ofa scenario typhoon on the mean monthly-highest water level (HWL) or
the highest high water level. Typhoon Vera is generally adopted as ascenario typhoon. In the conventional method, therefore, the external
force is calculated with the combination of the design water level in a
scenario typhoon and the highest wave height in another storm event. Inaddition, it is assuming that the peaks of the waves and the storm surge
appear simultaneously.

Under this circumstance, the stochastic typhoon model (STM), which
simulates typhoon parameters during a certain long period with theMonte Carlo method, can be an efficient tool for the collection of a large
number of storm events. For instance, the occurrence probability
distribution of the storm surge in Seto Inland Sea in Japan (Kawai, 2006)
and in Osaka Bay (Higaki et.al, 2020) with the STM were reported. Also,
the occurrence probability distribution of the rate of wave overtopping infour bays in Japan with the STM was reported (Kato et.al, 2003).
Although they have not reported the occurrence probability distributionof the wave force, it could be important for the evaluation of the stabilityof the shore protection facilities.
In this study, we proposed the methodology to evaluate the occurrence
probability of not only the rate of wave overtopping, but the wave force
with the STM. In the methodology, we considered the time variations of
the waves, storm surge, and astronomical tide level by using the timeseries data of them. In addition, we carried out a case study for theestimation of the occurrence probability distributions of external forces
acting on an existing shore protection facility with the proposed method.
Besides, we calculated the design external force with the conventional
design method, and then evaluated its return period by referring theoccurrence probability distribution.

In real situation, however, the external force is determined by thecombination of the waves and the storm surge in the same storm event.
Moreover, the peaks of them may not appear simultaneously due to thedifference of the mechanism of the development (Kawai, 2002).
Considering the astronomical tide level is not related with the storm
surge, we also should consider the time variations of them for the design

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