Science of the Total Environment 816 (2022) 151573

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Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv

A storm-induced ﬂood and associated nearshore dispersal of theriver-derived suspended 137Cs
Yusuke Uchiyama a,⁎, Natsuki Tokunaga a, Kohei Aduma a, Yuki Kamidaira b, Daisuke Tsumune c,
Toshiki Iwasaki d, Masatoshi Yamada e, Yutaka Tadeda f, Takashi Ishimaru g, Yukari Ito g, Yutaka W. Watanabe h,
Ken Ikehara i, Miho Fukuda j, Yuichi Onda kaDepartment of Civil Engineering, Kobe University, Kobe, Japan
Nuclear Science and Engineering Center, Japan Atomic Energy Agency, Tokai, Ibaraki, Japanc
Sustainable System Research Laboratory, Central Research Institute of Electric Power Industry, Abiko, JapandFaculty of Engineering, Hokkaido University, Sapporo, JapaneCentral Laboratory, Marine Ecology Research Institute, Onjuku, Chiba, Japanf
Environmental Science Research Laboratory, Central Research Institute of Electric Power Industry, Abiko, Chiba, Japang
Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Tokyo, Japanh
Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japani
Research Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japanj
National Institute of Radiological Sciences, National Institute for Quantum Science and Technology, Chiba, JapankCenter for Research in Isotopes and Environmental Dynamics, University of Tsukuba, Tsukuba, Ibaraki, Japanb

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T• A storm-induced coastal dispersal ofriver-derived suspended 137Cs wasinvestigated.
• In situ core sampling and highresolution numerical modeling wereconducted.
• The riverine 137Cs is primarily distributed near the mouth and the southerncoasts.
• Breakwater and coastal structures effectively trap the sedimentary 137Cs.

a r t i c l e

i n f oArticle history:
Received 18 August 2021
Received in revised form 5 November 2021
Accepted 5 November 2021
Available online 10 November 2021
Editor: Mae Sexauer Gustin

a b s t r a c t
Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermathof the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011.
Signiﬁcant amount of radionuclides released into the atmosphere were reportedly transported and depositedon land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source ofhighly contaminated suspended radiocesium adhering to sediment particles in the ocean through the rivermouth as a result of hydrological processes. Remaining scientiﬁc questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean ﬂoor derived from the Niida River.
Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution conﬁguration to quantify the transport processes of the suspended⁎ Corresponding author.
E-mail address: uchiyama@harbor.kobe-u.ac.jp (Y. Uchiyama).

https://doi.org/10.1016/j.scitotenv.2021.151573
0048-9697/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Y. Uchiyama, N. Tokunaga, K. Aduma et al.
Keywords:
FNPP1 nuclear accident
Radiocesium-137
Suspended particulate radionuclides
Sediment transport

Science of the Total Environment 816 (2022) 151573

radiocesium. Particularly, we investigated the storm and subsequent ﬂoods associated with Typhoon 201326(Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outﬂuxes to the ocean. Usingin situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identiﬁed three major
accumulative areas, near the river mouth within an area < 1 km, around the breakwaters in the north of theriver mouth, and along the southern coastal area, while offshore and northward transports were minor. The present study clearly exhibits substantial retention of the land-derived radiocesium adsorbed to the sediments in the
coastal areas, leading to possible long-term inﬂuences on the surrounding marine environment.
© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license(http://creativecommons.org/licenses/by/4.0/).

atmosphere and the ocean (Buesseler et al., 2011; Chino et al., 2011;
Kawamura et al., 2011; Yasunari et al., 2011; Kumamoto et al., 2015).
Tokyo Electric Power Company (TEPCO, 2012) reported that the mainradioactive substances released were 131I, 134Cs, and 137Cs, among
which, 137Cs has garnered considerable attention since its half-life isrelatively long at 30.1 years and its activity persists over a long period.

1. Introduction
A nuclear accident occurred at the Fukushima Daiichi Nuclear Power
Plant (FNPP1. Fig. 1) due to catastrophic tsunamis associated with theGreat East Japan Earthquake struck on March 11, 2011. Substantial
amounts of radioactive substances were then released into theFig. 1. Geographical maps of the study area. Bluish colors indicate the depths in the ocean (m) and brownish colors indicate terrestrial altitudes (m). (a) Perimeter of the quadruple nested
ROMS model areas. Black box: ROMS-L1; red box: ROMS-L2; orange box: ROMS-L3; magenta box: ROMS-L4; and yellow line indicates the best track of Typhoon Wipha connecting thelabeled times in UTC. (b) Enlarged view of the entire ROMS-L4 domain. (c) Further enlarged view of the study area near the Niida River mouth. Thick black boxes and lines indicate
boundaries of the sub-regions (NS, HT, RM, OM, SS, and OS) used for the inventory analysis in Section 4.4, thin black contours indicate isobaths (m), and thin black and white semicircles indicate the distances from the Niida River mouth (red triangle marker) and FNPP1 (black triangle marker). Yellow triangle markers correspond to the location of the mouth of
the indicated rivers. Yellow circles indicate the stations selected for the core sampling survey (Sta. 1–8).
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Science of the Total Environment 816 (2022) 151573

processes of terrestrial radionuclides supplied from rivers and to understand their underlying mechanisms in the coastal ocean.
The Niida River is a 62.9 km-long minor river, ofﬁcially categorized
as a second-class river maintained municipally, ﬂowing throughFukushima Prefecture with the river mouth located in Minami-Souma
approximately 24 km north of the FNPP1. Similar to that observed in
the Abukuma River basin, the catchment area is characterized by highterrestrial radioactivity due to the initial atmospheric deposition attributable to the accident (Chartin et al., 2013; Nagao et al., 2015). Hydrological processes have led to the weathering of terrestrial surface
sediments that are subsequently gathered in the river channel for transportation downstream, particularly during ﬂood events. In fact, EyrolleBoyer et al. (2016) and Naulier et al. (2017) reported that a markedly
higher amount of suspended 137Cs than that of dissolved 137Cs ﬂowed
out from the Niida River basin after Typhoon 201326 (Wipha). Hence,
the Niida River has been recognized as a source of highly contaminatedsuspended radionuclides that enter the ocean. The total amounts of thedeposited 137Cs in the catchment of several rivers near the Niida River
were estimated to be 578 TBq for the Abukuma River, 536 TBq for the
Ukedo River, while 189 TBq for the Niida River (Sakuma et al., 2019).
Therefore, the contribution through river ﬂux from the Abukuma andUkedo rivers located at north and south of the Niida River may be larger
than that from the Niida River.
The primary scientiﬁc questions of this study pertained to the extent of suspended radiocesium adhering to sediments released fromthe Niida River into the ocean by storm-induced ﬂooding, and thepossible fate of the river-derived radiocesium in adjacent coastal
seas. Therefore, this study aimed to understand the coastal-scaletransport processes of particulate matter and to quantify the possible
inﬂuences of river-derived suspended radionuclides and their inventory in the marginal seas off Fukushima. In addition to in situ observation of the bed sediments, a high-resolution numerical model oftransport, dispersal, and deposition of suspended radionuclides derived from rivers is useful for examining the governing dynamic
processes in coastal seas. However, such models have not been
applied until now with resolution of river mouths of width of O
(10–100 m) and their surrounding areas to accurately represent theprocesses. In previous studies, numerical models have been used toanalyze the behavior of suspended radionuclides over an extensive
area at coarser horizontal grid resolutions at O (1 km) without considering riverine sediment inputs (Choi et al., 2013; Misumi et al.,2014). Hence, we conducted a two-month numerical hindcast including the storm and ﬂood event associated with Typhoon 201326(Wipha), that passed off the Fukushima coast in October 2013,using a coupled river-ocean-wave-sediment model along with a static
absorption model to assess the amount of radiocesium attached tosediments. The employed 3-D numerical ocean circulation model includes the Regional Oceanic Modeling System (ROMS; Shchepetkin
and McWilliams, 2005, 2009) in a quadruple nested conﬁguration(Fig. 1), that enabled the introduction of a lateral model grid spacingof 50 m in the inner-most, ﬁnest nested domain, coupled with a 3-D
Eulerian multi-size class suspended sediment transport model and a
two-layer stratigraphy bed model based on the study reported by
Blaas et al. (2007). The iRIC-Nays 2DH river model (Jang andShimizu, 2005; Shimizu et al., 2014) was used to model the depthaveraged ﬂow and sediment transport in the Niida River. To precisely
investigate the bed shear stresses, particularly in nearshore areas, athird-generation spectral wave model, SWAN (Booij et al., 1999), subjected to initialization and enforcement by the operational regionalscale spectral wave analysis of GPV-CWM provided by the Japan
Meteorological Agency (JMA), was used. We also performed in situ
sampling of bed sediment cores at several locations around theNiida River mouth, immediately after occurrence of the ﬂood eventdue to Wipha, and compared the results with the model results to assess the reproducibility of the modeled deposition processes of bed
sediments around the mouth.

Radionuclides released into the atmosphere were transported mainly tothe northwest region of FNPP1 by the predominant southeasterly windat the time of the accident, and most radionuclides were deposited onterrestrial areas such as Fukushima and the Kanto Plain (Hirose, 2012;
Chartin et al., 2013). In contrast, atmospheric deposition and direct inﬂow from FNPP1 into the ocean are presumed to be the main pathways
for radionuclide entry into the marine environment. Research has been
conducted with limited measurement in the early stages of the postaccident aided by application of numerical models to evaluate whether
the ocean received 3–6 PBq as the direct release of 137Cs from FNPP1 beginning from March 26, 2011 (Tsumune et al., 2012, 2013; Aoyama
et al., 2016b; Buesseler et al., 2017; Kumamoto et al., 2019), while theatmospheric deposition contributed 12–15 PBq to the western north Paciﬁc Ocean out of the total accidental atmospheric release of 15–20 PBq(e.g., Aoyama et al., 2016a, 2016b).
Dissolved 137Cs in the upper ocean initially tends to be transported
mainly in the southwestward direction along the coastline locatedsouth of FNPP1 (Tsumune et al., 2012). Likewise, concentrations of radionuclides adhering to suspended sediments in water column and marine bed sediments (hereinafter we call them suspended radionuclides
or suspended 137Cs) are also observed to be higher on the southernside of FNPP1 than those observed on the northern side (Ambe et al.,2014; Tsumune et al., 2020). However, zooplankton, ﬁsh and benthos
contaminated with relatively high concentrations of radionuclides
have also been observed near the mouth of the Abukuma River in Sendai
Bay, located on the northern side of FNPP1 (e.g., Kaeriyama et al., 2015;
Narimatsu et al., 2015). The existence of such highly contaminatedfauna suggests that river-derived radionuclides exert a substantial inﬂuence. Nagao et al. (2013) and Kinouchi et al. (2015) reported that dissolved and suspended radionuclides adsorbed by mineral particles(e.g., clay, silt, and sand) were transported through the rivers at every
ﬂood event associated with storms and typhoons, and such radionuclides were eventually discharged into the ocean. The radionuclides derived from rivers strongly depend on the ﬂow rate of rivers and theamount of radioactive materials deposited in catchment areas(Iwagami et al., 2017; Onda et al., 2020; Ueda et al., 2013). Yamashiki
et al. (2014) reported that in the Abukuma River basin, approximately
61.4% of the terrestrial radionuclides discharged into the ocean was attributable to the ﬂood events caused by typhoons. Though the concentration of radionuclides in the bed sediments in the Fukushima coast
gradually decreased, benthic organisms, ﬁsh, and shellﬁsh decreased,
and were found to be contaminated with highly radioactive substances
immediately after the FNPP1 accident (Tsuboi et al., 2015). This may betrue as marine fauna captured in this area show radioactivity that seldom exceeds the regulated value, even as reported in 2021 (Fisheries
Agency, 2021).
Although the dissolved radionuclides deposited into the ocean are
expected to undergo rapid dilution due to mixing with ambient seawater, the suspended radionuclides may accumulate around the rivermouth, and because of settling, they may continue to affect the coastal
environment for a long period. Additionally, this process introduces atime lag following the direct release through hydrological processes, because such terrestrial radionuclides mostly undergo attachment withthe suspended particles and sediments, which are transported via
different mechanisms to the dissolved radionuclides in the water(e.g., Misumi et al., 2014). These suspended radionuclides have been intermittently discharged from the river mouth with the occurrence of infrequent ﬂood events. They recirculate in the ocean as they readily sinkto the bottom of the ocean due to settling, and this process iscounteracted by resuspension back into the water following an abrupt
increase in bed shear stresses. Therefore, suspended radionuclides are
anticipated to persist in considerable amounts near the river mouthand coastal areas because of their low mobility, leading to the continuous introduction of radioactivity into ambient water, thereby affecting
marine ﬂora and fauna. Therefore, it is extremely important for marine
environmental scientists to investigate transport-dispersal-deposition
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Science of the Total Environment 816 (2022) 151573

2. Methods

2.2. Experimental design

2.1. Core sampling survey

In the present study, we used the two-month hindcast result of theinner-most ROMS-L4 model, which contains 32 vertical s-layers and ahorizontal resolution of 50 m, encompassing an area of approximately
25 km × 50 km, including the Niida River mouth, FNPP1, and the coresampling stations (Fig. 1b). This model is based on a quadruple nested
ROMS driven by the JCOPE2 reanalysis (Miyazawa et al., 2009) using
an ofﬂine one-way nesting technique (Marchesiello et al., 2003;
Mason et al., 2010; Uchiyama et al., 2018a). The JCOPE2-ROMS system
has been extensively used in various studies and exhibits high reproducibility on coastal seas located around Japan (Kamidaira et al., 2017,2018, 2019; Kurosawa et al., 2020; Masunaga et al., 2018, 2019; Tada
et al., 2018; Takeda et al., 2021; Uchiyama et al., 2017b, 2018a–c;
Zhang et al., 2019). The model is further coupled with a multi-sizeclass non-cohesive suspended sediment transport model with a twolayered bed stratigraphy model (Blaas et al., 2007; Kamidaira et al.,2021) that additionally accounts for wave effects evaluated by a thirdgeneration spectral wave model SWAN (Booij et al., 1999; Cao et al.,2018). A static absorption of 137Cs to sediment particles was evaluatedwith the empirical formula (He and Walling, 1996) modiﬁed for the
study area (Golosov et al., 2017; Murota et al., 2016; Qin et al., 2012;
Takata et al., 2015, 2020). Riverine sediment supply was quantiﬁed for
the Niida River with the iRIC-Nays2DH river model (Jang and Shimizu,
2005; Nelson et al., 2016; Shimizu et al., 2014, 2019), which computes
depth-integrated river ﬂow, associated multi-grain-size sedimenttransport, bed morphology evolution, and bank erosion in rivers(Iwasaki et al., 2015). Details of the exploited numerical models are described in Appendix in Supplemental Data. The analysis period included
the ﬂood event associated with Typhoon 201326 (Wipha), which was
the closest to the study area on October 16, 2013 (Fig. 1a). Realistic surface wind, waves, swells, barotropic and baroclinic tides, freshwater inputs from rivers, large-scale currents through the open boundaries,
suspended sediments and associated 137Cs from the Niida River, andﬁne-resolution coastal topography and bathymetry were all consideredin the ROMS-L4 model (Section A.1). The model provided data on atemporal evolution of surface elevation, 3-D currents, water density(temperature and salinity), concentration of suspended sediments(sand, silt, and clay), bed stratigraphy, and sedimentary suspended
137
Cs in the coastal ocean around the Niida River mouth.
We investigated two experimental cases using the ROMS-L4 model.
Case 1 is the baseline case used for most of the subsequent analyses,
which represents realistic sediment transport processes in the ocean
without the presence of Niida River-derived sediments. Three size classes of the sediments, namely sand, silt, and clay (Table S2), were considered in Case 1. Case 2 involved the introduction of two more sediments(silt and clay) discharged from the Niida River as additional prognostic
variables to eventually compute a total of ﬁve sediments. The riverderived sand information was ignored in Case 2 because theCore sampling of bed sediments for radioactive 137Cs measurement
was performed at a total of eight points (Sta.1–8), denoted by using yellow circles in Fig. 1c, in the vicinity of the Niida River mouth after thepassage of Typhoon Wipha. Sampling was performed only once ateach sampling station, deﬁned as Sta.1 to Sta.5 in the shallow coastal
areas and Sta.6 to Sta.8 offshore. Owing to the restriction of met-ocean
conditions and logistics, sampling was performed twice in the following
manner: at the three offshore stations, Sta. 6–8 on October 18, 2013, twodays after the typhoon passed, and at the ﬁve coastal stations, Sta.1–5 on
October 31, 2013, 15 days after the typhoon passed. The nearshore samples at Sta. 1–5 were collected manually by scuba divers, while those atSta. 6–8 were collected using a multiple corer obtained from the RV
Shinyo-maru of Tokyo University of Marine Science and Technology(Kubo et al., 2020). Sta.1 and Sta.6 are located near the breakwater ofthe Haramachi thermal power station (HTPS) in the northern part of
the Niida River mouth. Sta.1–6 are located within a 5-km area of theriver mouth, while Sta.2 and Sta.3 are in the immediate vicinity of theriver mouth. The collected sediment cores (Fig. S1) were sliced into 1cm segments and stored in a freezer at −25 °C. Each slice was used
for particle size analysis. The samples were vacuum-dried and placedin U8 containers, and the sedimentary radioactive Cs concentrations
were measured using a gamma-ray spectrometer with a coaxial-type
Ge detector (GR2018, Canberra Industries Inc., USA, CT; 1.7 keV/
1.33 MeV resolution and 26% relative efﬁciency). Data on the concentrations of sedimentary radioactive Cs were then decay-corrected based on
the sampling date (Kubo et al., 2020). The results are shown in Table 1.
The sampled cores of the bed sediments are expected to represent
the history of past ﬂood events and resultant sedimentological processes, while the bed surface primarily reﬂects the most recent event.
Thus, we computed the inventory Inv of 137Cs attached to thesediments in the cores by integrating 137Cs concentration verticallyfrom the bed surface at z = − h down to zv (m) mentioned below, as
represented by
Inv ¼

Z −h
−h−zv

ρm C r dz

ð1Þ

where ρm is dry bulk density of mixed-size sediments in the bed (kg/m3), Cr is particulate 137Cs concentration (Bq/kg), h is local water
depth (m), and zv is the depth (m) at which Cr reaches a value of zero.
We should consider zv to represent samples collected from sufﬁciently
deep regions, whereas zv=0.1 (m) is assumed to compensate for thelimitation of the shallowest core sample. Consequently, the inventory
Inv (Bq/kg) represents the total amount of particulate 137Cs in the bed
sediment per unit area of the seaﬂoor at each core sampling station.

Table 1Results of the core sampling survey. Listed below are the latitude and longitude coordinates of each sampling station, dates of the samplings conducted, local water depth of the stations h,
the Euclidean distance from the Niida River mouth (DNR) and FNPP1 (DFI). Surface137Cs: near-bed surface 137Cs attached to the sediments averaged over the depths of 0 to 1 cm from the
bed surface (Bq/kg), Inventory: the total amount of 137Cs per unit area integrated vertically from the bed surface down to 10 cm (Bq/m2), and d50: median grain size of the near-bed surface
sediments.
Sta

1
2
3
4
5
6
7
8

LatitudeLongitude

(°N)

(°E)

37.65837.63837.63337.62537.61737.65137.63037.630141.025141.028141.027141.025141.025141.061
141.134
141.150Date

10/31/2013
10/31/2013
10/31/2013
10/31/2013
10/31/2013
10/18/2013
10/18/2013
10/18/2013

Depth

DNR

DFI

Surface 137Cs

Inventory

d50

h (m)

(km)

(km)

(Bq/kg)(Bq/m2)

(μm)

5
5
7
5
5
25
35
30

2.39
1.76
1.42
1.41
2.39
5.0
10.0
12.0

26.7
24.5
23.4
23.4
22.3
27.0
25.0
31.0

67.04
27.17
17.73
4.21
3.39
104.0
5.9
5.2

11,616
9,055
3,502
2,581
955
66,155
865
1,108

182
231
231
372
260
86
371
87

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contribution of sand to suspended 137Cs was found to be only 2% of thetotal in Case 1, while introducing nontrivial numerical delicacy. Hence,
the Case 2 model is based on the omission of a mixture of the Niida
River-derived sediments with the initially distributed bed sediments
to distinguish dispersal of the Niida River-supplied sediments fromthat of the bed sediments.

dominated the surface current velocity due to tides and diurnal windﬂuctuations, while low-frequency signals were also visible. The lowfrequency currents and wind were approximately SSW parallel to thecoastline. On the other hand, during the mild climate following the typhoon, the surface current and wind were relatively weak and developed in the opposite direction, NNE, parallel to the coastline.
Fig. 3 shows examples of the horizontal distribution of the Case 1 results. On October 16 (Fig. 3a), when the typhoon was closest to thestudy area, the surface currents (horizontal velocity of the topmost
grid cells) were prevailed by the overall southward drift and weremarkedly enhanced near the river mouth regions, including those of
the Niida River, as well as around the coastline bends, and near the harbor structure (breakwaters) of HTPS located north of the Niida Rivermouth. Near HTPS, the prevailing southward coastal currents bypassed
the north breakwater and intensiﬁed within a few kms off the tip of thestructure. On the northern side (Mano River) and the southern side
(Ohta River) of the Niida River, the coastal currents veered eastward
to form offshore ﬂow due to the inﬂuence of the ﬂoods. On the contrary,on November 30 (Fig. 3b), during a calm period, the surface currents
were noticeably weaker than those observed during the storm. The
prevailing direction turned to NNE. The two month-averaged surface
currents (Fig. 3c) were like those recorded on October 16, suggestingthat the impact of the ﬂood event of the typhoon was signiﬁcantduring the study period. The results clearly showed that the coastal
structures, the river discharge, and near-mouth topography modiﬁed
the nearshore current ﬁeld substantially. In contrast, the near-bed
currents (horizontal velocity of the bottom-most grid cells) formed ina different manner from the surface currents, particularly, in shallow
waters during mild climate conditions. The two month-averaged nearbed currents were headed toward WNW that compensated for thesurface currents. This implied that vertical recirculation occurred near

3. Met-ocean conditions and resultant sediment transport
Typhoon 201326 (Wipha) commenced near the Mariana Islands at
12:00 UTC on October 10, 2013, and intensiﬁed into a Category 4 typhoon with a minimum pressure of 930 hPa. It advanced toward thenorthwest region, then veered to the east, while maintaining its intensity, and traveled further north with acceleration (Fig. 1a). The typhoon
was sufﬁciently intense to be classiﬁed as a once-in-a-decade storm
when it approached and landed in the Kanto region of Japan. Typhoon
Wipha weakened as it skirted Japan's eastern coastline on October 15,
while leading to considerable precipitation. It also spawned a massive
landslide on Izu-Oshima Island. On October 16, Wipha passed throughthe north region of the island before dawn, and then subsided into an
extratropical cyclone off Sanriku at 15:00 on the east coast of the Boso
Peninsula.
The met-ocean data shown in Fig. 2 clearly demonstrate the prominent inﬂuence of Typhoon Wipha during the analysis period. When
Wipha was closest to the study area on October 16, wind speed increased to approximately 20 m/s, wave height reached approximately
4.6 m, and the associated heavy precipitation induced the most substantial discharge of approximately 180 m3/s at the Niida River mouth,
followed by the occurrence of two minor ﬂoods for several days thereafter. Sediments were discharged into the ocean from the Niida River,with ﬂoods peaking during the typhoon. High frequency variability

Fig. 2. Meteorological and oceanic conditions near the Niida River mouth from October 1 to November 30, 2013. Typhoon Wipha was observed to be the closest to the study area at aroundnoon on October 16, JST. (a) Niida River-derived suspended sediment concentrations (mg/L) of three size classes (sand, silt, and clay) at the river mouth, (b) hourly freshwater discharge at
the Niida River mouth in m3/s, (c) daily freshwater discharge in m3/s at the mouths of four selected adjacent rivers, namely Mano, Ohta, Ukedo, and Tomioka Rivers (Fig. 1) computed byusing the HYDREEM run-off model, (d) wind velocity (m/s) at 10 m above the surface at 141.04°E and 34.6405°N, 1 km off the Niida River mouth (same for e–i) based on the GVM-MSM
atmospheric analysis, (e) sea surface height (SSH, m), (f) ocean surface current velocity (m/s), (g) signiﬁcant wave height (m), (h) signiﬁcant wave period (s), (i) mean wave direction(degree) measured counter-clockwise from the east, where (a) and (b) are obtained by using the iRIC-Nays 2DH river model, (e)-(f) are derived from the ROMS-L4 model (Case 1), and(g)–(i) are derived from the SWAN wave model.
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Science of the Total Environment 816 (2022) 151573

Fig. 3. Results of the numerical model. Displayed variables include (a) daily average for October 16 when the typhoon was observed to be the closest (the stormy condition), (b) daily
average for November 30, six weeks after occurrence of the typhoon (a mild condition), and (c) time average for the entire two-month analysis period (the mean condition). In eachof the conditions from (a)-(c), upper left panel indicates surface current velocity vectors with their magnitude (m/s) in color, lower left panel indicates surface velocity vectors with surface
silt concentration (kg/m3) in color near the Niida River mouth, upper right panel indicates near-bed current velocity vectors with the bed shear stresses (N/m2) in color, and lower rightpanel indicates near-bed current velocity vectors with near-bed silt concentration (kg/m3) in color near the Niida River mouth. Notice vectors are subsampled. The upper panels indicate
results derived from Case 1, while the lower panels indicate results derived from Case 2.

bed. This suggests that the silt settles rapidly upon discharge from theNiida River mouth. Nevertheless, the spatial distributions of the silt differ on a daily basis, which eventually results in the meridionally biased
silt distributions extending to the south in the period-averaged silt distributions. This ﬁnding implied that the river-derived silt was mainlytransported southward and deposited on the southern shores, leading
to increased silt concentration in the water by enhanced τb andresultant resuspension due to waves.

the shore to provoke a coastal upwelling mode. However, depthdependent horizontal currents were less evident on October 16 owing
to intensive vertical mixing associated with the storm.
When the typhoon passed, the bed shear stress considerably increased over the study area as compared to that observed on November
30. The bed shear stress τb was extremely high, especially in coastal
waters shallower than approximately 35 m isobaths attributable tothe strong inﬂuence of the waves. Over extensive areas, τb reaches avalue of 2.0 N/m2, which is remarkably greater than the critical shear
stress τcr of sand (i.e., 0.15 N/m2; Table S2) that is considered the mostimmobile sediment in the model. Hence, resuspension and erosion occurred prominently in all three sediment classes in the entire shallow
waters. In contrast, on November 30, τb was reduced to a value of
0.1 N/m2 overall, whereas it exceeded a value of 0.2 N/m2 in thenearshore areas shallower than 10-m isobaths. The two monthaveraged τb was qualitatively similar in its spatial patterns to those
documented on November 30, but to a greater degree. A high τb isformed periodically at intervals of approximately 10 to 16 km in thealongshore direction, where the coastline is shaped concave to theoffshore (e.g., at 37°24′N, 37°34′N and 37°41′N near the shore).
Silt was the main contributor to the Niida River-derived sediments(Fig. 2a). A signiﬁcant amount of riverine silt was distributed mostly
near the river mouth on October 16 (Fig. 3a). On November 30, the silt
underwent considerable dilution near the river mouth, settled near
the shore, and dispersed broadly offshore by approximately 25-m
isobaths. A certain amount of the river-derived silt exists in the breakwaters of the HTPS and along the coastline on the southern side of theNiida River mouth. On the other hand, silt concentrations are rather
less along the coastline on the northern side of the HTPS and in deep
ocean. The silt is distributed similarly in the surface and bottom layers,
although the near-bed concentrations are generally higher than thosenear the surface because of vertical settling and resuspension from the

4. Core sampling vs. model
4.1. History of past ﬂood events and grain size effects on 137Cs
Fig. 4a illustrates the comparison of the bed surface 137Cs, which isactivity concentration of 137Cs in the surface 1 cm of the bed sediment,with the inventory Inv (Section 2.1). The inventory is viewed as arecord of the history of past ﬂood events and associated deposition on
the seaﬂoor. By contrast, the surface 137Cs would largely correspond todeposition of the suspended 137Cs derived from the Niida River by themost recent ﬂood event attributable to Typhoon Wipha. A highly positive correlation with a correlation coefﬁcient of 0.89 is found between
the 137Cs inventory and the surface 137Cs. This ﬁnding clearly indicatedthat the suspended 137Cs transport processes and accompanying deposition on the bed during Typhoon Wipha occurred quite similarly tothose having occurred with the previous storm and ﬂood events in thepast two and a half years after the occurrence of the FNPP1 accident.
Both inventory Inv and surface 137Cs values are greater at Station 1
located in HTPS, Sta. 6 off the tip of the breakwater of HTPS, and at Sta.
2 and 3 near the Niida River mouth than those observed at the other stations, though Station 6 demonstrates the maximum values. It should be
noted that Inv of Sta. 6 is larger than the average, and this station has asmaller contribution of the latest event to the cumulative inventory.
6

Y. Uchiyama, N. Tokunaga, K. Aduma et al.

Science of the Total Environment 816 (2022) 151573

Fig. 4. Results of the core sampling survey. Scatter diagrams of the bed surface 137Cs (Bq/kg) against (a) the inventory I (Bq/m2) and (b) median grain size (mm) of the surface sediments ateach sampling station, Sta. 1–8 (see Fig. 1c for the geographical locations), depicted by using different colors in the legend. (c) An example of the vertical distributions of the median grain
size (mm, black square marks) and the suspended 137Cs concentration (Bq/kg, red circles) at each sliced layer in the core sampling data at Sta. 4, where the ordinate indicates thedownward distance (depth, m) from the bed surface.

On the other hand, Inv and the surface 137Cs values are less at Sta. 4 and 5on the southern side of the mouth and Sta. 7 and 8 offshore.
Nevertheless, 137Cs is ﬁnite yet small even at remote sites such as Sta.
5 and 8, which implies that riverine sediments and suspended 137Cs
may be reachable.
It is well known that radionuclides are readily adsorbed on materials
with small particles sizes such as clay or silt (Comans and Hockley,
1992; Nakao et al., 2009). Owing to the chemical inﬂuences of surface
tension and electriﬁcation of suspended particles, the extent of adsorption of 137Cs is proportional to the speciﬁc surface area, which is thesurface area per unit volume, {4π(d/2)2}/{4π(d/2)3/3} = 6/d, for spherical particles. Thus, the smaller the particle size, the more 137Cs isabsorbed. Therefore, it is important to examine the transport anddeposition processes of particles by size to precisely evaluate the transport of suspended 137Cs particles. As shown in Table 1 and Fig. 4b, themedian grain size d50 of the surface sediments at the eight samplingsites ranged broadly between 86 μm (silty) and 372 μm (sandy).
Finer-grained sediments were found offshore at Sta.6 and 8, whilecoarser-grained sand was predominant at offshore Sta. 7. Nearshore
d50 at Sta. 1–5 shows an evident sandy nature ranging between 182
and 372 μm, with an overall increasing trend in d50 from north tosouth directions.
Fig. 4c shows an example of the vertical distribution of d50 and 137Cs
in the core sampled at Station 4, located in the southern region of theNiida River mouth. An apparent negative correlation was found with acorrelation coefﬁcient of −0.93. The subsurface d50 minimum wasobserved at 7.5 cm below the bed surface where 137Cs peaks, whichwere most likely attributed to the past ﬂood events before Typhoon
Wipha. In turn, horizontal correlations were not markedly clear,
whereas d50 and 137Cs concentration of the bed surface sediments
exhibited overall inverse proportionality with the correlation
coefﬁcient of −0.59 (Fig. 4b). Moreover, the correlation coefﬁcient of
the nearshore Sta.1–5 was −0.74, demonstrating that the suspended
137
Cs discharged from the Niida River coarsened and was diluted as
the particles traveled from the river mouth.
The results of the core sampling survey showed that sedimentary137
Cs was largely related to the particle size with complexity in their
spatial distribution, owing to the 3-D transport of the river-derived
and marine bed-originated sediments driven by spatiotemporally varying coastal currents and associated deposition processes. Therefore, inthe subsequent subsections, the model results have been utilized to examine the hydrodynamics and sediment transport processes by focusing on the contributions of two factors, namely the bottom shear

stress that preconditions resuspension on the seaﬂoor, and the sediment budget that determines local erosion and sedimentation to elucidate the mechanism responsible for the 137Cs distribution around theNiida River mouth.
4.2. Bed shear stress for resuspension and deposition
The combined wave-current bed shear stress τb in Eq. (A2) changes
according to the bottom stress due to the phase-averaged near-bed ﬂow
and bottom stress due to the wave orbital velocity on the seaﬂoor.
When τb < τcr, where τcr is the prescribed critical shear stressdepending on the grain size d50 of the mixed bed, sediments in theseawater are deposited on the seaﬂoor at the corresponding verticalsettling velocity ws (Table S2). On the contrary, when τb ≥ τcr, the bed
sediments are re-suspended in the seawater. Thus, it can be inferredthat the smaller the d50 value, the smaller the τcrvalue and thesedimentation rate. Accordingly, resuspension occurs even at smaller
τbvalues, that is, under less energetic bottom currents and waves, forﬁner sediments.
Fig. 5a–d (top row) shows the modeled bed shear stress τb and nearbed velocity vectors for Case 1 overlaid with the observed median grain
size d50 of the surface sediments in Table 1. The monthly mean τb for a
non-typhoon period, spanning from November 1–30 (Fig. 5a), is intensiﬁed in shallow waters (the local depth h < 10 m) along the coast
where the wave action on the bottom is signiﬁcant, but is small offshore.
Moreover, τb is weak in the sheltered area inside the HTPS and in the vicinity of the Niida River mouth, where the observed d50 also tends to besmall. Particularly, τb near river mouths may be affected by thefollowing two competing processes: 1) convergence of shorewardpropagating waves refracted over the shallow terrace topography ofthe mouth, and 2) wave ampliﬁcation by the offshore ﬂow from theriver and by wave shoaling on the terrace (Uchiyama et al., 2010,2017a). The former mechanism is anticipated to promote convergent
sediment accumulation near the river mouths, while the latter may enhance the wave-induced stress τw to result in more resuspension (Blaas
et al., 2007). The model results suggest that the former mechanism issuperior to the latter near the mouths of the Niida and Mano rivers
located north of the HTPS. Thus, τb was reduced and accumulation ofﬁner sediments occurred, while the latter mechanism might be
predominant near the Ohta River mouth at a region approximately
5 km south of the Niida River.
On the day of occurrence of maximum ﬂood (Fig. 5b), an intensesouthward bottom velocity formed near the river mouth. The associated7

Y. Uchiyama, N. Tokunaga, K. Aduma et al.

(a) Average for November

Science of the Total Environment 816 (2022) 151573(b) October 1 6( c) October 1 8( d ) October 31

d50 (µm)
350
200
100

t b [N/m2 ]t b [N/m2 ](e) sand (kg/m2)(f) silt (kg/m2)

t b [N/m2 ]t b [N/m2 ](g) clay (kg/m2)(h) total: sand + silt + clay (kg/m2)

Inv (Bq/m2)
10000
2000
1000

erosion/accretion [kg/m2 ](i) October 1 6

erosion/accretion [kg/m2 ]( j) October 18

erosion/accretion [kg/m2 ]erosion/accretion [kg/m2 ](k) October 31(l) November 30

137

Cs (Bq/kg)
100
30
10

137

Cs [Bq/m2 ]1 37

Cs [Bq/m2 ]137

Cs [Bq/m2 ]1 37

Cs [Bq/m2 ]Fig. 5. Top row (panels a–d): Enlarged views of the modeled bed shear stress τb and (subsampled) near-bed velocity vectors around the Niida River mouth for Case 1 overlaid with theobserved median grain size d50 of the surface sediments in Table 1 depicted by using open circles with the variable diameter proportional to d50. (a) Monthly averages for November
2013 as a representative of the no-typhoon conditions, and daily-averaged values for (b) October 16 under the maximum typhoon inﬂuences, (c) October 18, two days after
occurrence of the typhoon when the survey was conducted at Sta. 6–8, and (d) October 31, 15 days after the typhoon passed when the core samples were collected at the Sta. 1–5.
Middle row (panels e–h): Modeled (Case 1) cumulative deposition (accretion, reddish colors) and resuspension (erosion, bluish colors) integrated since October 1 until October 31,
2013, for (a) sand, (b) silt, (c) clay, and (d) a sum of the three-size classes per unit bottom area in kg/m2. The overlaid open circles indicate the observed inventory Inv of the bed-core137
Cs (Table 1). Bottom row (panels i–l): The temporal evolution of modeled suspended 137Cs in the marine bed sediments of the two size classes (silt and clay, Case 2) derived fromthe Niida River (colors, Bq/m2). Daily averaged radioactivity in unit bed area has been depicted for October 16 (Wipha was closest), October 18 (reﬂecting inﬂuences of heavy
precipitation associated with Wipha), October 31 (two weeks after Wipha), and November 30 (six weeks after Wipha). The open circles indicate the concentration of measured bed
surface 137Cs.

and τb were largely diminished (Fig. 5d). Nearshore τb was as highas 0.25 N/m2 in the southern coastal waters, but reduced toapproximately 0.1 N/m2 near the Niida River mouth and around HTPS.
At Sta. 1 within HTPS and Sta. 2–3 located near the Niida River mouth,
d50 was found to be ﬁner with smaller τb than that recorded at Sta. 4
and 5 located to the south. Hence, nearshore τb and d50 weremoderately correlated.

τb was extensively high near the shore including the mouth areas,
exceeding the critical shear stress τcr of the least mobile sand classsediments (0.15 N/m2; Table S2) everywhere including the offshore
areas deeper than 35 m. This suggests that resuspension and erosion
prevailed for all sizes of sediments at all observation sites when thetyphoon passed. Two days later, when the offshore sampling was
conducted at Sta. 6 to 8, the near-bed velocity was observed to be northwestward with a small τb in general (Fig. 5c). All sediments could be
deposited at the offshore sampling sites, Sta. 6 to 8, because τb was
smaller than 0.15 N/m2. The median grain size d50 at Sta. 6 and Sta. 8
was small in accordance with a small τb, whereas Sta. 7 showed
an exceptionally considerable d50 even at a small τb. On October
31, when the nearshore sampling at Sta. 1 to 5 was conducted,
the oceanic conditions were extremely mild, and bottom currents

4.3. Sediment budget analysis
The bed shear stress τb indicates the resuspension potential at the
bed of interest, whereas the bed evolves due to 3-D sediment transport
over time. Hence, we conducted a sediment budget analysis to estimate
erosion and accumulation of bed sediments by considering the8

Y. Uchiyama, N. Tokunaga, K. Aduma et al.

Science of the Total Environment 816 (2022) 151573

the prevailing southward sediment transport (Figs. 3a and 5b). Twodays later, on October 18 (Fig. 5j), a signiﬁcant amount of 137Cs was deposited near the mouth with a radius of approximately 2 km. The northwestward transport (Fig. 5c) promoted near-mouth accretion,
entrainment in the HTPS harbor facility, and offshore transport toward
Sta. 6. In turn, the suspended 137Cs from the Niida River did not reachSta. 7 and 8 by October 18 when core sampling was conducted. Hence,
the in-situ data collected at Sta. 7 and 8 might not fully reﬂect the inﬂuence of Wipha. Subsequently, 137Cs was gradually transported mainly tothe south and offshore (Oct. 31 and Nov. 30, Fig. 5k–l) with peaks located at 6–7 km south of the river mouth (Fig. 6a–e). A substantial
amount of 137Cs was also transported southward beyond −10 km.
Owing to the occasional supply of 137Cs by two small ﬂoods occurring
after the typhoon (Fig. 2a), the overall 137Cs concentration in the marine
bed increased until October 31, whereas it was widely dispersed mainlyin the southern and offshore directions by November 30. Northwardtransport was found mostly near the river mouth, which peaked at approximately 2 km by October 31 (Fig. 6c), albeit to a much lesser degree
than the southward transport.
To quantify the regional transport and resultant redistribution of theinventory of river-derived suspended 137Cs absorbed into the bed sediments, we categorized the ROMS-L4 domain into seven segments, as
shown in Fig. 1b and c, where the 137Cs inventory was calculated
based on the results obtained for Case 2 (Fig. 6f and Table 2). Data on
the suspended 137Cs had been derived from the Niida River (NR) by
32.0 GBq until Oct. 18, 2 days after the typhoon passed, and by approximately 55.7 GBq as of Nov. 30. The suspended 137Cs was signiﬁcantly
deposited within approximately 1 km of the river mouth, n.b., in theRM subregion, accounting for 46.8% (approximately 26.1 GBq) of theriver supply on November 30. On the other hand, 137Cs deposited in
OM, the offshore of RM by approximately 3.5 km from the rivermouth, rapidly increased in a few days since the occurrence of themain ﬂood; however, the inventory was extremely small, i.e., only approximately 0.51 GBq as of Oct. 18. The 137Cs in OM then gradually reduced to approximately 0.49 GBq by Nov. 30, which is merely ~1.8% of
the inventory in RM. Nearshore-suspended 137Cs transport in the regions shallower than 10 m are strongly biased toward the south (SS).
As of Nov. 30, approximately 13.3 GBq exists in the SS, which corresponds to 23.8% of the supplied 137Cs from the Niida River, whereas
NS receives only nearly 0.45 GBq. However, the near-mouth areaaround HTPS labeled HT is highly accretive and receives approximately
9.1 GBq (17.4%) of the river supply due to sediment depositions inducedby the breakwater as of October 31. The outﬂow to the offshore area(OS) is nearly 5.22 GBq (9.36% of the supply), and the outﬂow to the distant area outside the computational domain (OD) is approximately
7.90 GBq (14.2% of the supply).

divergence and convergence of depth-integrated lateral sedimentﬂuxes of the j-th size class, as expressed by the following equation:
∂
∂t

Z η
−h

cj dz þ ∇hZ η−h

cj uh dz ¼ Ej − Dj ,

ð2Þwhere η is free surface elevation (m), ∇h = (∂/∂x, ∂/∂y) is the horizontal
gradient operator, and uh is the horizontal velocity vector (m/s).
Notably, we deduced the following equation for bed evolution:∂hbj∂Sj¼ ρsj ð1 − λÞ
¼ Dj − Ej ,
∂t
∂t

ð3Þ

where Sj is vertical downward sediment ﬂux of the j-th class depositedon (positive) or eroded from (negative) the unit seaﬂoor area (kg/m2/s),
ρsj is the dry density of the j-th sediment (Table S2), hbj is deviation of
the seaﬂoor height (m) due only to the j-th sediment measured upwardfrom the initial bed at z = − h. The time integral of Sj represents thetotal amount of sediment accumulated on the bed owing to the lateralsediment transport over time. Likewise, the sum of hbj over j is thus
the resultant bed height deviation measurable in situ. Note that hb isomitted as hb ≪ H (H = h + η is total depth in m) in Eq. (2).
Fig. 5e–h (middle row) illustrates the comparison of the modeled
cumulative Sj integrated over the month of October 2013 from theCase 1 result with the observed radiocesium inventory Inv (Table 1 andFig. 4). Overall, Inv of sedimentary 137Cs is substantial where
deposition is dominant (reddish color), while it is small in the erosive
area (bluish color). Particularly, the offshore area of the breakwater of
HTPS centered at Station 6, which is characterized by higher 137Cs,
ﬁner d50, and smaller τb (Fig. 5a–d), is accretive even though it is far
from the Niida River mouth. The reddish accretive area of silt and clay
(Fig. 5f, g) extends from the northern area to the southeast region
toward Sta. 6 along the HTPS breakwater, demonstrating that lateraltransport and the resultant deposition of ﬁner silt and clay play an important role in the accumulation of suspended 137Cs. Indeed, the timeaveraged surface velocity for the two months (Fig. 3c), which considerably affects the overall silt and clay transport, shows offshore currents
along the breakwater of HTPS to reach Sta. 6. Therefore, the accretive
area of ﬁne sediments was formed by the weak bed shear stress under
the inﬂuence of horizontal transport from the north of the HTPS.
It is worthwhile to quantify the extents of erosion and accretioncaused by storm events. As the erosion at Sta. 2–5 ranges from −40 to−50 kg/m2 (Fig. 5h), it is translated into the thickness of the eroded
bed sediments of 2–3 cm, considering the porosity of 0.4 and the sediment density of 2650 kg/m3. Similarly, the accretion around Sta. 6 wasevaluated to be approximately not more than 1 cm in thickness. Therefore, the present storm-ﬂood event caused centimeter-scale bed thickness changes near the river month. This ﬁnding also supports theargument made in Section 4.1, which states that only the sediments
present in the top few centimeters of the sampled cores may reﬂect
the present event (Fig. 4c) and the lower portions represent a recordof past events.

5. Discussion
The model results showed that the river-derived suspended 137Cs(NR) attributable to the ﬂood associated with Typhoon Wipha provided
nearly 32 GBq within two days. The supplied suspended 137Cs wereprominently deposited in the vicinity of the river mouth (RM) by approximately 47%, even though 1.5 months had elapsed after occurrenceof the typhoon-induced ﬂood. The offshore mouth area (OM), spanningfrom 1 km to 3.5 km from the mouth, was not substantially affected by
the riverine 137Cs, accounting for only 0.03%–0.75% of the river supply.
The increased 137Cs concentration that was reported at the rivermouth, and at the interface between the river and the ocean, was consistent with the ﬁeld measurement conducted by Kakehi et al. (2016).
Their work showed that >80% of the river-derived suspended 137Csfrom the Abukuma River, located further north of the present study
site, was deposited at the river mouth. These results demonstrate thatthe river-induced suspended 137Cs ﬂux is less dispersive and is thus
barely transported to regions located far offshore. Such a near-mouthaccumulation may be maintained by the landward near-bottom

4.4. 137Cs inventory in bed sediments
Fig. 5i–l (bottom row) shows the temporal evolution of suspended
Cs adhering to the bed sediments derived from the Niida River(colors, Bq/m2) along with the in situ 137Cs inventory (open circles,
Table 1). The modeled 137Cs distributions were then spatially integrated
in the offshore direction and in the alongshore direction at 500-m intervals to depict histograms (Fig. 6a–e). Note that for the present analysis,
we considered results of Case 2, in which the sediments derived fromthe Niida River were separately computed from the bed sedimentdata, enabling us to extract information on the riverine suspended137
Cs contributions.
When the typhoon passed (October 16, Fig. 5i), the river-derived
137
Cs was distributed only in the southern nearshore shallow areas by137

9Y. Uchiyama, N. Tokunaga, K. Aduma et al.

Science of the Total Environment 816 (2022) 151573(f) Cummulave Cs inventories [GBq]Fig. 6. (a–e) The histogram of sedimentary 137Cs attached to the Niida River-derived silt (cyan) and clay (orange), integrated in the offshore direction and at 500-m alongshore distanceintervals (Case 2). The abscissa indicates the northward distance from the Niida River mount shown by the vertical red line. Notice different scales in the ordinate. (f) Time series plot of the137
Cs inventory in the bed sediments in the six subregions deﬁned in Fig. 1b and c and NR: the amount of 137Cs supplied from the Niida River. The displayed acronyms are as follows: RM,
the river mouth region within approximately 1 km; OM, offshore of RM as far as approximately 3.5 km (note that the 137Cs in OM is magniﬁed by a factor of 20); NS, the northern shore
areas; SS, the southern shore areas; OS, the offshore area deeper than approximately 10 m; and HT, the area around Haramachi Thermal Power Station (HTPS).

137

transport (Fig. 5a–d), which is formed in the near-bed lower layer tocompensate for the near-surface offshore freshwater outﬂow from theriver.
Nevertheless, our results further illustrate the importance of alongshore 137Cs transport in shallow nearshore areas. Figs. 5i–l and 6 showthat highly contaminated suspended 137Cs is deposited along the southern coast (SS) and near the breakwater of HTPS in the north of the rivermouth (HT). The former is promoted by the predominant southward

Cs transport due to the southward alongshore currents by thestorm, while the latter is attributed to the northwestward bottom currents formed under ordinary, mild oceanic and atmospheric conditions(Fig. 5a–d). However, the northward 137Cs transport is signiﬁcantly affected by the coastal structure, that is, the HTPS breakwaters locatedin the north of the river mouth. The breakwaters are generally constructed to shelter the port facility (HTPS) from incoming waves aswell as sediment inﬂuxes. Apparently, the latter functions as intended.
10

Y. Uchiyama, N. Tokunaga, K. Aduma et al.

Science of the Total Environment 816 (2022) 151573

Table 2The suspended 137Cs inventory in the bed sediments in the six segments (RM, OM, NS, HT, SS, and OS. See Fig. 1 for the deﬁnition). NR: the amount of suspended 137Cs adhering to thesediments supplied from the Niida River, and OD: the suspended 137Cs ﬂowed out of the model domain. The listed 137Cs inventory is expressed in GBq and integrated in time sinceOct. 1 until at 0:00 UTM on the labeled day.
Date until
Oct. 14
Oct. 16
Oct. 18
Oct. 31
Nov. 30

NR
0.530
0.578
32.0
52.3
55.7

RM
0.282
0.026
22.7
24.2
26.1

OM
0.004
2.13 × 10−4
0.514
0.409
0.489

NS
−5

2.50 × 10
8.20 × 10−6
8.84 × 10−6
0.491
0.450

HT

SS

OS

OD

0.028
0.035
1.11
9.08
2.31

0.150
0.130
1.71
8.51
13.3

0.011
0.002
0.120
8.23
5.22

0.056
0.417
5.79
1.42
7.90

Thus, the suspended 137Cs is considerably accumulated within, and on
the southern side of the HTPS, while further northward 137Cs transport
is hampered.
Another important ﬁnding is that the suspended 137Cs from theNiida River is observed mostly as silt fraction, proportional to the siltassociated sediment ﬂux from the river (Fig. 2a), rather than sand(not shown) and clay (Fig. 6a–e). As the vertical settling velocity andthe resuspension rate of silt are intermediate compared to those ofsand and clay (Table S2), silty 137Cs is dispersed more extensively
than sandy 137Cs, although markedly less mobile than clayish 137Cs, toaccumulate near the source area (i.e., the Niida River mouth). Additionally, based on the signiﬁcant correlation between the surface 137Cs andthe amount of 137Cs inventory of the sampled cores (Section 4.1,
Fig. 4), the present model results, such as those pertaining to lateral dispersal and deposition patterns, would be applicable to other storminduced ﬂood events that have occurred near the Niida River mouth
since the FNPP accident. Therefore, we hypothesize that the riverderived suspended 137Cs may have prominently accumulated within
1 km of the river mouth (RM), in the southern beaches (SS), and aroundthe HTPS breakwaters at each ﬂood event.
The radiocesium concentrations in marine ﬁsh species off the
Fukushima coast have decreased signiﬁcantly as dissolved 137Cs content
in seawater has decreased rapidly since the occurrence of the accident(Buesseler, 2012). However, time-series trends of radiocesium concentrations have been shown to differ among taxa, habitats, and spatial distributions (Wada et al., 2013). The present model did not account forthe desorption of suspended 137Cs. In reality, suspended 137Cs depositednear the river mouth or along a coast persist for a long period, as thehalf-life is approximately 30.1 years. Such accumulated suspended137
Cs particles may leach into seawater and affect the marine environment as dissolved 137Cs. In fact, the Fisheries Agency reported on
January 31, 2019, that a common skete (Raja kenojei), a species of ray,caught off the Fukushima coast had 137Cs with values exceeding theJapanese standard limit of 100 Bq/kg. This was the ﬁrst time that marine
fauna with an exceeded 137Cs value was captured in four years. As common sketes inhabit the bottom of the sea, they are affected bysuspended 137Cs adhering to the bed sediments. Therefore, the evaluation of the distribution of suspended 137Cs in the seabed is a crucial
step toward the protection of marine ecosystems from radionuclides.

The core sampling data revealed that highly contaminatedsuspended 137Cs were distributed on the seaﬂoor near the Niida Rivermouth and the breakwater of the adjacent thermal power station(HTPS). A negative correlation between 137Cs concentration and mediangrain size of the bed surface sediments was identiﬁed; the ﬁner the bed
sediments, the more 137Cs was absorbed. The inventory of suspended137
Cs particles, viz. the vertically integrated bed sedimentary 137Cs,
was positively correlated with the bed surface 137Cs. The 137Cs inventory
reﬂects the history of past storm or ﬂood events occurring since the
FNPP1 accident, whereas the surface 137Cs predominantly represents
occurrence of the most recent event due to Typhoon Wipha. The significant correlation implies that the transport and deposition processes
due to Wipha are like those of previous storms occurred in the past
two and a half years since the accident.
On the other hand, the model enables examination of the dynamics
and mechanisms underlying sediment transport. Concrete and remarkable consistencies were found between the in-situ grain size and themodeled bed shear stress, and between the observed bed surface 137Cs
and the modeled sediment erosion/accumulation. Overall, ﬁner particles accumulate to increase content of the suspended 137Cs in regions
where the bed shear stress is lower. However, the horizontal current
ﬁeld is largely affected by the vertical recirculation occurring due tothe near-surface freshwater outﬂow from the river mouth and by thecoastal structure of the adjacent HTPS, leading to the establishment ofa hot spot of high 137Cs content (e.g., at Sta. 6). Furthermore, themodel results (Case 2) were utilized to conduct an inventory analysisof the river-derived suspended 137Cs present in the sea ﬂoor. A signiﬁcant amount of 137Cs was deposited within an area approximately
1 km of the river mouth (RM), near the HTPS breakwater north of theriver mouth (HT), and along the coast south of the river mouth (SS).
In contrast, a small amount of suspended 137Cs was transported and deposited offshore of the river mouth (OM) and along the coast north of
the river mouth (NS). In summary, the present study demonstrates substantial retention of the land-derived radiocesium adhering to the sediments in the shallow coastal areas near the river mouth, leading topossible long-term inﬂuences of 137Cs on the surrounding marine
environment.

6. Conclusions

Yusuke Uchiyama: Conceptualization, Methodology, Supervision,
Software, Writing – review & editing, Funding acquisition. Natsuki
Tokunaga: Investigation, Software, Validation, Visualization, Writing –original draft. Kohei Aduma: Investigation, Software, Validation, Visualization. Yuki Kamidaira: Investigation, Software, Validation. Daisuke
Tsumune: Investigation, Methodology, Supervision. Toshiki Iwasaki:
Methodology, Validation, Software. Masatoshi Yamada: Investigation,
Methodology, Supervision, Funding acquisition. Yutaka Tadeda:
Investigation, Formal analysis. Takashi Ishimaru: Investigation, Formal
analysis. Yukari Ito: Formal analysis. Yutaka W. Watanabe: Formal
analysis, Visualization. Ken Ikehara: Formal analysis, Visualization.
Miho Fukuda: Formal analysis. Yuichi Onda: Investigation, Methodology, Supervision, Funding acquisition.

CRediT authorship contribution statement

This study investigated the regional transport and resultant deposition of sediments and associated suspended 137Cs on the seabed, derived from the Niida River, Fukushima, Japan. The catchment of thisriver was found to be contaminated by the initial atmospheric deposition of radionuclides following the FNPP1 accident that occurred in
March 2011. We focused on storm-induced ﬂood events associatedwith Typhoon 201326 (Wipha) in October and November 2013. In
situ core sampling data were collected immediately after the typhoon
passed, and a high-resolution downscaling numerical model based on
the JCOPE2-ROMS system was used to examine the detailed processes
and their underlying mechanisms.
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Declaration of competing interest
The authors declare that they have no known competing ﬁnancial
interests or personal relationships that could have appeared to inﬂuence the work reported in this paper.
Acknowledgments
This study was ﬁnancially supported by the Japan Society for the
Promotion of Science (JSPS) Grants-in-Aid for Scientiﬁc Research
15H00977 and 18H03798 at Kobe University, and the Interdisciplinary
Project on Environmental Transfer of Radionuclides among Hirosaki
University, Tsukuba University and Kobe University. We also appreciate
Takafumi Yamanishi (West Nippon Expressway Co. Ltd.), Tomomi Ishii(Tokyo Electric Power Co. Holdings Inc.), and Yasumasa Miyazawa(Japan Agency for Marine-Earth Science and Technology) for their assistance provided at the early stages in the development of the ROMS
models embedded in the assimilative Japan Coastal Ocean Predictability
Experiments (JCOPE2) oceanic reanalysis.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.scitotenv.2021.151573.
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