A post-disaster assessment on a storminduced flood and associated coastal
dispersal of the river-derived suspended
radiocesium originated from the Fukushima
nuclear accident
Cite as: AIP Conference Proceedings 2157, 020030 (2019); https://doi.org/10.1063/1.5126565
Published Online: 18 September 2019
Yusuke Uchiyama, and Natsuki Tokunaga

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AIP Conference Proceedings 2157, 020030 (2019); https://doi.org/10.1063/1.5126565© 2019 Author(s).

2157, 020030

A Post-disaster Assessment on a Storm-induced Flood andAssociated Coastal Dispersal of the River-derived Suspended
Radiocesium Originated from the Fukushima Nuclear
Accident
Yusuke Uchiyama1, a) and Natsuki Tokunaga1
1

Department of Civil Engineering, Kobe University, Japana)Corresponding author: uchiyama@harbor.kobe-u.ac.jp

Abstract. Niida River, Fukushima, Japan, is well known to be a source of highly contaminated suspended radiocesium tothe ocean, which is originated from the Fukushima Dai-ichi Nuclear Power Plant during the accidental leakage occurred in
March 2011. We examined the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean
floor derived from Niida River, using a quadruple nested JCOPE2-ROMS 3D circulation model in a very high resolution
configuration, coupled with a 3D multi-class sediment transport model, the iRIC-Nays 2DH river sediment model, theSWAN spectral wave model, and a static radiocesium absorption model. A particular attention was paid to the storm andsubsequent flood event associated with Typhoon 201326 (Wipha) passed off the Fukushima Coast in October 2013, which
provoked an enormous amount of precipitation, and subsequent riverine freshwater discharge and sediment fluxes to theocean. With a guide of several in situ observed data, we carried out a quantitative assessment of the accumulation anderosion of the sediments and resultant suspended radiocesium distribution around the river mouth and nearshore areas along
the Fukushima Coast.

INTRODUCTION
A nuclear accident occurred at the Fukushima Dai-ichi Nuclear Power Plant (hereinafter FNPP) in March2011 due to catastrophic tsunamis associated with the Great East Japan Earthquake. Since then, researches have been
carried out with limited measurement during the initial stage of the post-accident aided by numerical models toevaluate that the ocean has received 3-10 PBq as the direct release of radiocesium 137 (137Cs) from the FNPP (e.g.,
Tsumune et al., 2012 [1], 2013 [2]; Kamidaira et al., 2018 [3]). However, the coastal dispersal of the radionuclides isaffected not only by the direct release but also by atmospheric fallout and discharge from the rivers. The last processintroduces a time lag behind the direct release with hydrological process because the radionuclides mostly attach tosuspended particles, i.e., sediments, which are transported quite differently to the dissolved matter in the water (e.g.,
Misumi et al., 2014 [4]). Those suspended radionuclides have been discharged from the river mouths intermittently
with infrequent flood events. They recirculate in the ocean as they readily sink to the ocean bottom due to settling,
counteracted by resuspension back in the water by abrupt increase of bed shear stresses. Therefore, suspended
radionuclides are anticipated to remain largely near the river mouths and coastal areas because of less mobility, leading
to continuous addition of radioactivity to ambient water and marine flora and fauna.
Niida River is a second level (i.e., maintained not by the central but by a local government) river flowing
through Fukushima Prefecture with the mouth located at ~24 km north of the FNPP. It has a watershed characterized
by quite high terrestrial radioactivity due to the initial atmospheric fallout upon the accident. Hydrological processes
have led to weathering of terrestrial surface sediments that are subsequently gathered in the river channel to be

International Symposium on Green and Sustainable Technology (ISGST2019)
AIP Conf. Proc. 2157, 020030-1–020030-7; https://doi.org/10.1063/1.5126565
Published by AIP Publishing. 978-0-7354-1902-5/$30.00

020030-1

FIGURE 1. (a) A hierarchy of the quadruple nested JCOPE2-ROMS downscaling model domain around the Fukushimacoast. The colored rectangle frames account for ROMS-L1 (black, at lateral resolution of 3 km)㸪L2 (red, 1 km), L3 (yellow,
250 m), and L4 (magenta, 50 m). (b) An enlarged area near the mouth of Niida River shown by the red triangle. Mouths ofthe tow neighboring rivers (Mano and Ota rivers) are also presented by blue triangles. Black lines correspond to the perimeterof segments used in the flux budget analysis. Background colors indicate depth in m.
TABLE 1. Numerical configuration of the Niida River model (ROMS-L4)
Computational period
Grid cell numbers
Lateral B.Cs.
Surface wind stress
Other surface fluxes
Sea surface temperature (SST)River discharge
Bathymetry
Wind wave and swell

October 1, 2013–November 30, 2013
1,024 × 512 × 32 vertical s-layers, lateral grid resolution: 50 m
ROMS-L3 (2-hourly average)
Japan Meteorology Agency GPV-MSM (hourly)
NOAA COADS (monthly climatology)
AVHRR Pathfinder (monthly averaged): for heat flux correction
Niida River: iRIC-Nays 2DH model hindcast (hourly average)
The other rivers: HYDREEMS hindcast (daily average)
Central Disaster Prevention Council, the Cabinet Office of Japan (10 m)
Downscaling CWM-SWAN model hindcast (3-hourly average)transported downstream particularly during flood events. Hence, Niida River has been recognized as a source of highly
contaminated suspended radionuclides to the ocean up until the present. To understand such processes, and to quantify
possible influences of the river-derived suspended radionuclides and their inventory in the ocean off Fukushima, weconducted a two-month numerical hindcast including the storm and flood event associated with Typhoon 201326(Wipha) passed off the Fukushima Coast in October 2013 using a coupled river-ocean-wave-sediment model alongwith a static absorption model to account for radiocesium attached to sediments.

THE COUPLED DOWNSCALING MODEL
A comprehensive, inter-disciplinary approach is essential to deal with such complicated process of theradionuclide in the nature that requires considering atmospheric, hydrological, limnological and oceanic processes.

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TABLE 2. Model parameters for the multi-class sediment transport model

Us

Wcr

kgm -3ws
mms -1Eu
Kgm -2s -1

Nm -2

sand

d
Pm
125

2650

9.4

2.5 u 10 -3

0.15

silt

24

2650

0.4

1.0 u 10 -4

0.07

0.1

1.0 u 10 -4

0.02

class

clay

4

2650

where d: median grain size, s: sediment dry density, ws: settling velocity, Eu: resuspension rate, Wcr: critical bed shear stressfor resuspension, O = 0.4: porosity (constant), a = 3 mm: thickness of the active layer (initial thickness of the substratum: 10m).

FIGURE 2. Time series of (a) hourly discharge from the Niida river mouth evaluated by the iRIC-Nays 2DH model, (b)free-surface elevation at 1 km off the Niida river mouth computed by the ROMS-L4 model, and (c) significant wave heightcomputed by the CWM-SWAN model at the same location as that of (b). The displayed are for the entire two-month analysis
period from October 1–November 30, 2013. Notice Typhoon Wipha approached on October 16.

To this end, we developed a coupled numerical model that investigates the behavior of land-derived sediments andsuspended radionuclides in the marine environment near the river mouths. First, to account for large-scale oceanic
variability responsible for the 3D flow field near the Niida river mount, we configured a downscaling ocean model
based on the Regional Oceanic Modeling System, ROMS (Shchepetkin and McWilliams, 2005 [5]), in a quadruple
nested configuration by 1-way offline nesting approach (Figure 1). Daily-mean velocity and density fields of the
JCOPE2 oceanic reanalysis (Miyazawa et al., 2009 [6]) were utilized as the lateral boundary condition and the initialcondition of the outermost ROMS-L1 model at a grid resolution of 3 km. We successively downscaled it to ROMSL4 model with a horizontal resolution of 50 m by following the well-established methodology described in Mason etal. (2011) [7], Uchiyama et al. (2018 [8]; 2017 [9]), and Tada et al. (2018) [10]. The GPV-MSM operational weather
product of the Japan Meteorological Agency (JMA) primarily provided the atmospheric forcing to the ROMS models.
We introduced the Eulerian passive tracer module (e.g., Uchiyama et al., 2014 [11]; Uchiyama et al., 2018 [12]) for
dispersal of dissolved radioactive 137Cs released from the point source where the FNPP is located by using a leakage
submodel proposed in Tsumune et al. (2012) [1]. The numerical configurations for the ROMS-L4 model are listed inTable 1.
Second, we developed a multi-class non-cohesive suspended sediment transport model with a two-layered

020030-3

(c)FIGURE 3. Time-averaged (a) surface velocity vectors (subsampled) and their magnitude in color, and (b) near-bed velocity
vectors and bes shear stresses in color. The averages were taken for the whole model period (Oct. 1–Nov. 30). Star marks areriver mouths; yellow star is the Niida river mouth; and black contours are isobaths in m. (c) Cumulative deposition (accretion,
redish) and resuspension (erosion, bluish) integrated since Oct. 1 until Nov. 30, 2013 (unit: kgm -2) as a sum of the three sizeclasses.

stratigraphy model based on Blaas et al. (2007) [13]. In the present study, we consider three classes of sediments,
namely fine sand, silt and clay fractions. The corresponding settling velocity and other model variables are
summarized in Table 2. Sediments are suspended when the bed skin stress exceeds the prescribed critical stress. The
bed skin stress is evaluated from a combined wave-current bed shear stress model of Soulsby (1995) [14] as a functionof an empirical combination the log-layer current stress and a wave-induced bottom stress. The two-layer stratigraphymodel consists of an active layer of 3 mm thick sitting on the top of a substrate of 10 m. The active layer is defined as
the thin interface between water column and sediment bed. When the bed erodes, the sediments in the substrate is
mixed with the active layer to compensate for the sediment loss. On the other hand, when sediments deposit, thebottom portion of the active layer is merged with the substrate to keep 3 mm thickness of the active layer. This regional
3D sediment transport model was activated in the innermost ROMS-L4 model. Third, to properly account for
spatiotemporally varying wave effects on the bed shear stress at the ocean floor in the sediment model, a third
generation spectral wave model SWAN (Booij et al., 1999 [15]) at a horizontal resolution of 1 km was used in thepresent study, nested in the JMA GPV-CWM wave reanalysis product at 5–6 km resolution. A static absorption
submodel of 137Cs to sediment particles were employed to evaluate behavior of suspended 137Cs in the ocean.
Finally, the river discharges to the ocean were basically obtained as daily-averaged volume fluxes at eachriver mouth using a surface run-off model HYDREEM. A regional HYDREEM hindcast was conducted at Central
Research Institute of Electric Power Industry, CRIEPI (courtesy of Dr. D. Tsumune), driven by measured precipitation
field for the entire Tohoku/Fukushima region. On the contrary, for Niida River, we exploited a more sophisticated
iRIC-Nays2DH model, which accounts for depth-integrated shallow-water river flow and associated multi-classsediment transport (http://i-ric.org/en/software/?c=17). The iRIC-Nays2DH hindcast was performed at the Civil
Engineering Research Institute for Cold Region and at Hokkaido University (courtesy of Dr. T. Iwasaki and Prof. Y.
Shimizu). It provided hourly discharge and three-class sediment concentrations (viz., fine sand, silt, and clay) at theNiida river mouth. These river model hindcasts introduced freshwater inputs from all the river mouths located in theL4 domain and multi-class sediment fluxes at the Niida river mouth. The developed river-ocean-wave-sediment020030-4

FIGURE 4. Temporal evolution of suspended 137Cs in the marine bed sediments derived from Niida River (Bqm-2). Daily
averaged radioactivity in unit bed area is depicted for Oct. 14 (before Wipha), Oct. 16 (Wipha was closet to Niida River), Oct. 18(two days after Wipha showing influences of heavy precipitation), Oct 31 (two weeks after Wipha), and Nov. 30 (six week afterWipha). The radioactivity is computed for all the three sediment size classes at 0:00 UTC on each day.

coupled model was utilized to carry out a two-month hindcast including the storm and flood event associated with
Typhoon 201326 (Wipha) passed off the Fukushima Coast in October 2013.

RESULTS AND DISCUSSIONS
Figure 2 displays time series plots of the Niida river discharge, sea surface height (SSH) and significant
wave height near the mouth. On Oct. 16 when the typhoon came closest to the Niida river mouth area, the riverdischarge increased as large as 180 m3/s due to heavy precipitation associated with the storm (Figure 2a), during
about four spring-neap tidal cycles (Figure 2b). The typhoon provoked a quite stormy condition with maximum windspeed at 20 m/s and significant wave height of 4.6 m (Figure 2c). The river discharge was increased twice again after
the typhoon passage, while the second and third peaks show only ~60 and 40 m3/s.
Temporally averaged surface currents were approximately in the alongshore direction toward SSE (Figure3a). The current speed was markedly enhanced near the river mouths, coastline bends, and the harbor structures(breakwaters) located north of the Niida river mouth. By contrast, time averaged near-bed currents indicate prevailingshoreward currents that compensate for the surface currents, leading to coastal upwelling (Figure 3b). The associatedbed shear stress (color) exhibits quasi-periodicity in the alongshore direction in the nearshore shallow area with thedepths less than ~10 m. Overall, the nearshore bed shear stress exceeded 0.5 N/m 2 due mainly to waves, whichcorresponds to the resuspension dominant condition for all the three sediment size classes.
Resultant cumulative resuspension and deposition are represented by bed accretion and erosion due tocombined influences from all the three sediment size classes (Figure 3c). Accretion (redish colors) is prevailed in anarrow area right in front of the Niida river mouth, while erosion (bluish colors) dominates extensively in the offshoreof the mouth. Near-surface seaward currents developed near the mouth (Figure 3) serves as a node for sedimentationin the shallow area; erosive (accretive) areas are formed in the north (south) of the mouth. Topographies near the rivermouths and harbor structures entrain alongshore sediment transport effectively, resulting in alternative appearance oferosive and accretive patterns in the alongshore direction. Gravitational circulations associated with buoyant riverine
freshwater input into the ocean induce shoreward near-bed currents (Figure 3b) and thus promote the shoreward
sediment transport. As a result, sedimentation predominantly occurred near the river mouths.
Figure 4 shows temporal evolution of suspended 137Cs in the marine bed sediments derived from Niida River.
Instantaneous radioactivity contained in all the three sediment size classes in unit bed area (Bq/m2) is depicted for Oct.
14 (before Wipha), Oct. 16 (Wipha was closet to Niida River), Oct. 18 (two days after Wipha showing influences ofheavy precipitation and the associated flood), Oct 31 (two weeks after Wipha), and Nov. 30 (six week after Wipha) at
0:00 UTC on each day. The radioactivity is computed for all the three sediment size classes. Before the Wipha-driven
flood (Oct. 14 and 16), suspended 137Cs was confined near the shore and distributed largely on the south of the mouth

020030-5

TABLE 3. Inventory of suspended 137Cs originated from Niida River to the ocean. Cumulative radioactivity of suspended
137Cs (unit: GBq) from Niida River (labeled by NR), and those accumulated in the six spatial segments (RM: river mouth at
depths < 5 m, OM: offshore of the mouth at depths ranging between 5 and 10 m, NS: north shore area at depth < 10 m, SS:
southern shore area at depth < 10 m, OS: offshore area, and OD: out of domain) are depicted. The area segmentation with theiracronyms are presented in Figure 1b. Time integral was performed from Oct .1 to Oct. 14, Oct. 16, Oct. 18, and Nov. 30.
Date in 2013

NR

RM

OM

NS

SS

OS

OD

10/14
10/16
10/18
11/30

0.530
0.578
32.00
55.73

0.249
0.023
21.05
25.22

0.002
0.001
1.333
0.365

0.021
0.003
0.855
1.694

0.182
0.132
2.573
14.01

0.021
0.003
0.403
6.541

0.056
0.417
5.790
7.902

due to southward sediment transport. After Wipha passed off the Fukushima on Oct. 18, the subsequent heavy rainfall
and associated flood induced a pronounced 137Cs accumulation in the vicinity of the mouth within a radius of ~2 km,
followed by gradual southward transport and then dispersed and diluted extensively (Oct. 31 and Nov. 30).
Furthermore, the harbor structure (of a power plant facility) at the north of Niida river mouth significantly collected
sediments to induce accretion with the harbor

CONCLUSIONS
To summarize the present study, the suspended 137Cs inventory was analyzed as listed in Table 3. Cumulative
total radioactivity of suspended 137Cs discharged out of Niida River (NR) was about 30 GBq until Oct. 18 (just after
the typhoon passage), and about 56 GBq until Nov. 30. About 25 GBq (45.3 %) of this 56 GBq was deposited near
the river mouth only within a radius of 1 km (RM). The offshore area of the mouth (OM) caught a quite limited amountof the riverine suspended 137Cs; it increased to 1.3 GBq until Oct. 18, but then decreased to 0.37 GBq until Nov. 30,
which is only ~1.5 % of that accumulated in the near-mouth region (RM). The decrease of the 137Cs inventory in OM
stands for erosive characteristic over there, with estimated spatial half-life of suspended 137Cs of about three days.
Alongshore transport of sediments and suspended 137Cs occurred asymmetrically with an evident southward bias. The
southern area of the Niida river mouth (SS) caught 137Cs accumulation of ~14 GBq (25.1 % of the river-derived 137Cs),
whereas only 1.7 GBq was transported northward (NS). By contrast, the offshore and far-field transport were rather
small; the offshore 137Cs transport beyond the 10-m isobaths (OS) occurred ~6.5 GBq (11.7 %) and the far-fieldtransport existing the model domain (OD) was 7.9 GBq (14.2 %), respectively. Therefore, the Niida river-derived
137
Cs mostly stayed in the southern nearshore area of the mouth.
It is worth noting that these model estimates are generally consistent with field measurements conducted near
the Niida river mouth in the fall 2013. The quantitative model assessment of the river-originated radioactivity revealed
that infrequent but non-trivial supplies of terrestrial 137Cs have taken place occasionally to influence subsequentoceanic dispersal and resultant redistribution of deposited sediments and suspended 137Cs in the ocean floor.

ACKNOWLEDGMENTS
The present research was financially supported by Japan Society for the Promotion of Science (JSPS) Grants-in-Aid
for Scientific Research 15KK0207, 15H04049, and 18H03798 (PI: Y. Uchiyama) at Kobe University. We appreciate
K. Aduma, a former student at Kobe University for his support during the initial stage of the model development.
Thanks are also due to members of the ISET-R project funded by the JSPS for their comments on the present study.

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