B210

The seasonal variation of Hasaki Beach, Japan and the interaction between beach and wave
〇Xinyu CHEN, Nobuhito MORI, Tomoya SHIMURA, Banno MASAYUKI, Takuya MIYASHITA

Introduction

the year (Figure 2). Meanwhile, the shoreline is not

Climate change has been proven to influence the wave

only influenced by the current significant wave height

climate which consequently influences the beach's

but also by the past wave climate, which decides how

morphodynamic change as the primary driver. Themuch available sediment could be eroded from the bed.

impact of wave on beach morphology is still to bediscovered due to the lack of bathymetry data. To
achieve a more confident prediction of the shoreline in
the future, a better understanding between wave andbeach is needed.

Figure 2 - bi-weekly averaged wave height and shoreline
location. (Dark bar: positive median correlation; white bar:negative median correlation)

Monthly averaged nearshore wave experiences adecrease from March to July meanwhile the correlation
between wave and shoreline increases, which can also
be explained by the sediment availability. The dailysampled bathymetry data at Hasaki Beach makes it
possible to compare the time scale difference of theerosion and accretion process. The more severe storm
Figure 1 - The location of Hasaki beach and the observatory
HORS.

DatasetThe research area of this study, Hasaki Beach, is an
open coast located on the east coast of Japan, which has
been collecting bathymetry data for nearly 30 years.
500-meter bathymetry in cross-shore direction ismeasured daily before 2012 to weekly after 2012. Thewave condition considers both the hindcast wave datafrom the state-of-the-art atmospheric model JRA-55
and in-situ measurement at Kashima port.
Results
the shoreline position and biweekly significant wave
height show a negative correlation generally throughcould erode the deeper bed, but the beach also takes
longer to restore from it. The beach morphodynamic
behaviors with respect to different reference levels
have quite various erosion/accretion pattern. When astorm is coming (significant wave height increases 1 m),
the shoreline with respect to mean high water level(MHW) is more likely got eroded but the shoreline withrespect to mean sea level (MSL) doesn’t show a clear
erosion pattern. That’s because the higher berm’ssediment is eroded and settles down to the lower
shoreface meanwhile the local sediment is stirred up by
breaking waves. The two processes counteract eachother leading to the complicate shoreline changepattern on MSL.

By comparing the observation data, the wave cannot
erode all the surface sediment from the bed instantly
and when the wave remains on the same level, theerosion process may last more than one day. However,when storm passes, the post-storm accretion due to thesediment settlement from the water column only last
for approx. 1 day. And the sediment is constantly
transported by the wave from offshore after that. In theend, based on this, the current morphodynamic
reduced-complexity model, ShoreFor, is modified withtwo different time scales for erosion and accretion
when simulating shorter time-scale variance.

Figure 3 - Correlation between shoreline change and significant
wave height change within one day with respect to MHW.