4th International Workshop on Modeling the Ocean (IWMO-2012), Yokohama, Japan.

Three-dimensional unsteady wave-driven littoral currents
Yusuke Uchiyama* and James C. McWilliams*Department of Civil Engineering, Graduate School of Engineering, Kobe University,
Kobe 657-8501, Japan
Abstract:
The multi-scale asymptotic theory by McWilliams et al. (2004) with a vortex-force
formalism is extended appropriate for strong current regimes applicable to wave-driven
nearshore currents around surf zones. The wave-averaged, three-dimensional primitive
equations for current and tracers are derived and implemented in ROMS with appending
non-conservative parameterizations that account for momentum transfer and mixing
associated with depth-induced wave breaking and bottom friction (Uchiyama et al.,2010). A set of the narrow-banded WKB wave ray refraction equation and wave action
balance equation is tightly coupled with the slowly evolving currents to provide
wave-induced forcing.

The coupled system is then applied to the Duck94/SandyDuck

field datasets to reproduce three-dimensional unsteady longshore and rip currents.

We

demonstrate that realistic surfzone topography with shallow incident wave angles leads
to unstable rip currents consisting of abrupt ejection of the inner-surfzone water toenhance cross-shore mixing, whereas shear instability of longshore currents, also known
as shear waves, is induced by obliquely incident waves.

4th International Workshop on Modeling the Ocean (IWMO-2012), Yokohama, Japan.

Oceanic dispersion of cesium-137 off the north-eastern Pacificcoast of Japan
Tomomi Ishii*, Yusuke Uchiyama, Daisuke Tsumune and Yasumasa Miyazawa*Department of Civil and Earth Resources Engineering, Graduate School ofEngineering, Kyoto University, Kyoto 651-8540, JapanAbstract:
The 2011 earthquake off the Pacific coast of Tohoku, Japan, and the associated tsunami
caused a severe nuclear accident at the Fukushima Daiichi Nuclear Power Plant (1F),
leading to radioactive materials leaking into the ocean. We conducted a retrospective,
double-nested high-resolution numerical experiment to evaluate oceanic dispersion of
the leaked radioactive 137Cs in the coastal marginal sea to successfully reproduce
fluctuations of the measured 137Cs concentration. Following the method proposed byTsumune et al. (2011), a total amount of the 137Cs from 1F is estimated quantitatively.
Alongshore distribution of the concentration is found to be highly inhomogeneous withlower 137Cs concentration distributed widely at the south of 1F, while medium
concentration in the nearshore area at the north of 1F. The probability density function(PDF) of 137Cs concentration suggests that hotspots exist along the Sanriku coast, a rias
coastline located north of 1F. According to the previous work by Tsumune et al. (2011),
137

Cs leaked from 1F is reported to be transported offshore rather quickly.

two control volumes centered at 1F, a budget of137

Defining

Cs flux is diagnosed.

Time-integrated 137Cs fluxes at the northern and southern (alongshore) boundaries
indicate apparent outgoing tendency, while the net cross-shore flux at the easternboundary (100 km offshore) almost vanishes or even has an incoming flux, suggestingthat 137Cs tend to remain in the coastal area with mostly being transported alongshore
back and forth.

4th International Workshop on Modeling the Ocean (IWMO-2012), Yokohama, Japan.

Effects of wave-current interaction on development of ripcurrents
Hideki Kaida* and Yusuke Uchiyama*Department of Civil Engineering, Graduate School of Engineering, Kobe University,
Kobe 657-8501, Japan
Abstract:	  Haas et al. (1997) found that the offshore extent of rip currents can be significantly
reduced if the current effects on waves (CEW) are considered. Subsequently, Yu andSlinn (2003, hereafter YS03) suggested that it could be led by change of the work doneby radiation stress. Since radiation stress is also affected by CEW, it has been an open
question that how CEW acts to suppress rip currents. Weir et al. (2011) analyzed ripcurrents with a vortex force formalism to answer this question. They showed that rip
currents are modulated due to the wave ray bending associated with offshore-headed rip
currents. However, they limited themselves in the asymptote where alongshore
topography does not change that much, and didn’t consider momentum balance
modified by CEW.
In the present study, a two-dimensional shallow-water model based on Regional
Oceanic Modeling System (ROMS) with wave effects through the vortex-force
formalism, coupled with a set of wave ray equations (Uchiyama et al., 2009), is used toexamine dynamic effects essential to development of rip currents. In prior to theanalyses, the numerical experiment conducted by YS03 is revisited to confirm thevalidity of the present modeling framework.

Then multiple terms attributed to CEW in

the wave action and wavenumber conservation equations affecting the offshore
evolution of rip currents are explored. The result reconfirms that the wave ray bendingby currents is substantial to evolution of rip currents among others.

If CEW is taken

into consideration, wave ray bending occurs with a modified wavenumber field thatchanges the alongshore pressure gradient force and the cross-shore component ofacceleration due to wave breaking through wave shoaling, leading to the reduction ofthe extent of rip current.

4th International Workshop on Modeling the Ocean (IWMO-2012), Yokohama, Japan.

Oceanic responses to surface gravity waves in the Southern
California Bight
Tatsuya Nishii*, Yusuke Uchiyama and James C. McWilliams*Department of Civil Engineering, Graduate School of Engineering, Kobe University,
Kobe 657-8501, Japan

Abstract:	  Influences of surface gravity waves on inner-shelf circulations in the Southern
California Bight is investigated by using a quadruple nested high-resolution modelingframework based on ROMS (Regional Oceanic Modeling System) along with double
nested SWAN (Simulating Waves Nearshore) and WRF (Weather Research andForecasting). The wave effects on the circulation includes an interaction between
waves’ Stokes-drift and Coriolis force (Stokes-Coriolis effect), a correlation between
Stokes-drift and background relative vorticity (vortex force), momentum transport to thecurrents by depth-induced wave breaking, Bernoulli pressure head, etc.	  The primary momentum balance occurs between the pressure gradient and Coriolis
forces, consistent with the geostrophic balance at a small Rossby number. However,Stokes-Coriolis effect comes in at the same order of magnitude if waves are considered.
Advective momentum and vortex force are rather small, while increased to dominate
over Stokes-Coriolis effect in the nearshore area outside of the surfzone.

It is also

found that turbulent kinetic energy associated with mesoscale and submesoscale
momentum fluctuation is strongly affected by waves, in particular in the nearshore area
for the latter. Therefore, waves are found to play a substantial role in changing thedynamic balance in the bight.