Coastal dispersal of urban treated wastewater in semi-enclosed estuaries
Yusuke Uchiyama 1, Ryosuke Kanki 1 and Akihiko Nakayama 2
1 Department of Civil Engineering, Kobe University
Kobe, Hyogo, Japan
2 Construction Engineering Research InstituteKobe, Hyogo, JapanABSTRACT
Anthropogenic land-derived materials in urban wastewater in semienclosed, partially stratified estuaries (Osaka Bay and Harima Nada,Japan) are investigated with a triple-nested high-resolution ocean model.
The dispersal and dilution of effluent released from the subsurface
nearshore outfall of Tarumi Sewage Treatment Plant near Akashi Strait
is considered. The effluent dispersal in the northeastern Osaka Bay is
two-layered and highly reversible in the upper and lower layers due tostratification influenced by freshwater input mainly from Yodo River.
The overall effluent load is in the dominant circulation direction of Seto
Inland Sea encompassing the two estuaries, substantially controlled byKuroshio.

KEY WORDS: oceanic downscaling; ROMS: The Regional Oceanic
Modeling System; urban wastewater effluent; buoyancy plume

INTRODUCTIONOsaka Bay and Harima Nada located off the densely populated Kobe
and Osaka areas are encompassed in the northeastern part of Seto
Inland Sea, Japan (Fig. 1). Complex shoreline topography with over3,000 islands in Seto Inland Sea (SIS) and riverine inflows play anessential role in formation of quite complicated tidal and estuarine
circulations. Uchiyama et al. (2012) reported that the overall
circulation in Seto Inland Sea is strongly affected by the seasonallyvarying Kuroshio path. The water quality, coastal marine ecosystem
and aquatic resources in these estuaries are significantly influenced byanthropogenic sewage effluent and river discharges (e.g., Abo et al.,2012). Therefore, for a rigorous hydrodynamic and associated material
dispersal modeling for these shallow estuaries, it is necessary to
precisely account for boundary conditions to convey the SIS
circulations to Osaka Bay and Harima Nada, forcing conditions such astides, river inputs, and surface wind, and the lateral topography andbathymetry as exactly as possible. To this end, in the present study, wedevelop an oceanic downscaling system in a triple nested configuration
to conduct a high-resolution forward modeling of Osaka Bay andHarima Nada with lateral grid spacing of 200 m. A particular attention
is paid to dispersion of sewage effluent released from the subsurface
nearshore outfall of Tarumi Sewage Treatment Plant (TSTP) near

Akashi Strait (Fig. 1 middle) that is considered to be a potential sourceof nutrient loads to the inner Osaka Bay attributed to the clockwise
predominant current in Seto Inland Sea. The modeling system issimilar to the one developed for Santa Monica Bay and San Pedro Bay,California, USA (Uchiyama et al., 2014) while the area of the previous
study is rather at an open coast, differing to the tidally-driven, semienclosed, connected estuaries considered in the present study.

REGIONAL CIRCULATION MODELThe triple nested regional ocean circulation model for Osaka Bay andHarima Nada relies on a state-of-the-art regional ocean model, ROMS
(Shchepetkin and McWilliams, 2005; 2008). The outermost boundary
condition is provided by JCOPE2 (Miyazawa et al., 2009), an eddyresolving, assimilative oceanic reanalysis product with a horizontal grid
resolution, dx, of 1/12º (dx ≈ 10 km) to bring offshore signals inclusiveof influences of Kuroshio into the ROMS-L1 model at dx = 2 km(Table 1). The triply nested configuration (Fig. 1 left) based on ROMS
successively refines the grid spacing from dx = 600 km (L2) to 200 m(L3). The L3 model consists of 800 × 560 horizontal grids with 32
bottom-following, stretched vertical σ-layers. The intermediate L1 andL2 results described in Uchiyama et al. (2012) are exploited for thelateral boundary conditions of the innermost L3 model with a bihourly
update to account for high frequency tidal signals. One-way, offline
nesting approach (Mason et al., 2010) is used to accurately reproduce
internal tides as in the previous studies (e.g., Buijsman et al., 2012).
The L3 grid relies on the high-resolution (at dx = 50 m) dataset of the
topography and the coastal embankments compiled by Japan’s Cabinet
Office. We make use of the hourly GPV-MSM reanalysis product(JMA) for surface wind stresses and the COADS climatology for theother surface forcing. Monthly climatology of freshwater inputs fromthe 10 major rivers in L3 is taken into account. The computational
period is chosen for the four-month period from Aug. 1, 2009 to Nov.
29, 2009.

EFFLUENT POINT SOURCE SUBMODEL
Sewage effluent is applied in L3 at the location of TSTP (Fig. 1right) as a bottom-released fresh (buoyant) plume at the monitored
hourly volume rate with a Gaussian initial dilution submodel to avoid
undesirable non-hydrostatic effects (Uchiyama et al., 2014). Eulerianpassive tracer is additionally released at TSTP to track the effluent.

Fig. 1. (Left) ROMS-L1, L2 and L3 domains, (middle) ROMS-L3 domain, (right) blow-up of Osaka Bay and Yodo Line (red line).
Table 1. The configlations for the triple nested ROMS model.
ROMS-L1

ROMS-L2

ROMS-L3

Horizontal grids

320× 320 (× 32 vertical)

800× 480 (× 40 vertical)

800 ×560 (× 32 vertical)

Horizontal resolution

2.0 km

600 m

200 m

Surface wind stress

JMA GPV-MSM (hourly)

JMA GPV-MSM (hourly)

JMA GPV-MSM (hourly)

Other surface flux

COADS （monthly）

COADS (monthly)

COADS (monthly)

Major river discharge

29 rivers (monthly)

27 rivers (monthly)

10 rivers (monthly)

Lateral boundarycondition

JCOPE2 (daily)

L1（daily）L2 (two-hourly)

Tides

N/A

Barotropic tides at the open
boundaries using TPXO7.0(Egbert et al., 1994)

Barotropic and baroclinic tides
at the open boundaries withhigh-frequency B.C. updateTopography

SRTM30 + JEGG500

JEGG500 (500 m)

Japan Cabinet Office (50 m)

The near-field initial dilution submodel of Uchiyama et al. (2014) isemployed in the present study. Given an input pollutant concentrationin the outfall pipe Cp [kg/m3] with volume flux Qp (t) [m3/s], we force anormalized tracer concentration equation by an equivalent source P
[1/s]
(1)

∂c+ ∇uc = P∂tHere the unit of c is pollutant concentration normalized by Cp (unlikesalinity which is a mass fraction of seawater), and F = uc (+ Fsgs) is theadvection-mixing flux associated with the flow and subgrid-scale (sgs)parameterizations. The source P is
P ( x, y, z;t ) = Ps (t ) A ( x, y ) H ( z )(2)with A and H are specified spatial functions mimicking the outcome ofunresolved nearfield mixing above the outfall diffusers; their values arenon-dimensional and close to one in the source region and zero outside
it. They have integrals equal to the effective source area and depth,∫∫ A dx dy = A ; ∫ H dz = Hss(3)with As the horizontal area, Hs the vertical size, and Vs = AsHs thevolume. Ps is determined from integrated tracer equivalence between
the model source and the pipe inflow by the relation, Ps = Qp/Vs. For
our simulations we use outfall data for Qp(t), and we could multiply bya mean concentration value Cp for any particular pollutant species (e.g.,CDOM) to translate from our c fields. There is an analogous forcing
source in the model’s salinity S equation, R = RsAH, by the freshwaterflow out of the pipe, where integral balance implies that Rs = − SoQp /Vs
with So = 35 PSU the mean oceanic salinity.
The wastewater effluents are specified as hourly freshwater volume
fluxes of Qp = 1.018 ± 0.0364 m3/s. The nearfield vertical mixing of thebuoyant effluent occurs on a scale smaller than our grid resolution, andas a hydrostatic model ROMS is ill-suited for the large local verticalvelocities associated with the freshwater discharge from the bottommounted pipes. Therefore we assume the outcome of the nearfieldinitial mixing and dilution by imposing the prescribed spatial
distributions of A and H in (2).
A = 1 in the horizontal grid cells that tile the diffuser pipe sections
and zero elsewhere. So As = Nsdx2, where Ns is the number of tilingcells and dx2 = 2.25 × 104 m2 is the cell area. For TSTP, Ns = 9 for thebottom-mounted nearshore outfall pipe. We fit a Gaussian shapefunction to H with parameters guided by nearfield buoyant plumebottom layer (the bottom-most s-layer). On the 16th day, the tracer
begins to emerge in the surface layer in the inner part of Osaka Baynear the month of Yodo River and to subsequently flow westward(Fig.2a). Meanwhile the near-bed tracer has reached the inner bay(Fig.2b). This would be caused by an upwelling associated with anestuarine recirculation in the inner bay near Yodo River that has thelargest freshwater input to Osaka Bay. Once surfaced, the effluent isdispersed over the entire Osaka Bay due to the mean near-surface flowin the WSW direction. This suggests that the estuarine recirculation issubstantial to form a two-layered structure in the inner Osaka Bay thatplays a key role in three-dimensional effluent dispersion.
VERTICAL STRUCTURE
Figure 3 (top panels) shows the monthly-averaged offshore(southwestward) velocity along the Yodo Line (Fig.1). As anticipatedin the preceding subsection, the lateral velocity in the northeastern
Osaka Bay is two-layered and reversible in the upper and the lower
layers due to stratification (not shown) induced by the freshwater input.
The interface between the offshore-directed surface current and theshoreward subsurface current is located approximately at 3-5 m deep,
corresponding to the mixed layer depths computed with the KPP model(Durski et al., 2994). This recirculating reversal clearly leads to theupwelling of the near-bed effluent at the inner Osaka Bay (Fig. 3bottom). In week 1 of Aug., 2009, the tracer distributes in the verticalnear the release site crossing the pycnocline to deeper than 40 m. Inweeks 2 and 3, subsurface intrusion into the lower layer occurs off
Kobe with onshore transport of the concentration maxima, andcontinuously disperses towards the inner bay. In weeks 3 and 4, thetracer surfaces by the upwelling associated with the estuarine
recirculation, and then drifts back to the southwest in the upper layer.
TRACER FLUX BUDGET
Fig. 2. Snapshots of the log10-based normalized passive tracer
concentration on Aug. 16, 2009, in (a) surface and (b) bottomlayers.

solutions obtained by the model of Roberts et al. (1989) with measured
currents near the outfall:" ( z − z )2 %s'
H ( z ) = exp $−ds2 '&$#(4)where zs is the plume center height, and ds is its vertical scale. A halfspace integral of this H(z) is 0.5 ds√π. For the TSTP outfalls, wepresume that the plume penetrates the stratification and rises up to thesurface: zs = 0. We further choose ds = 10 m, hence Hs = 8.9 m
neglecting the bottom boundary limit.
A feasible future generalization of this nearly-steady effluent source
is to use monitored Qp(t) and run the nearfield buoyant plume modelwith modeled local currents. This would contribute additionalvariability to the effluent distributions seen in the present solutions.
EFFLUENT DISPERSAL
Significant differences between the bottom and the surface layers areobserved in the passive tracer concentration (i.e., wastewater effluent)released from TSPT. Initially, the tracer in the surface layer (the topmost s-layer) is mainly transported toward Osaka Bay and spreadsouthward, whereas the prevailing transport occurs eastward in theA tracer flux budget analysis is conducted at the three straits (viz.,Akashi, Kitan and Naruto). Predominant effluent transport is found tooccur westward to HN at Akashi Strait, while in the southward
direction to the Pacific at Kitan and Naruto Straits. The cumulative
normalized tracer fluxes for the four-month period are 1.04 × 106 m3 at
Akashi, 1.63 × 106 m3 at Kitan, and 2.28 × 105 m3 at Naruto Strait,respectively. The estimated inventory reveals that 75% and 8% out of
the total amount of the released sewage water in the four months areaccumulated in Osaka Bay and Harima Nada, albeit the rest (about17%) disperses towards the Pacific Ocean through Kii Channel.
SUMMARY
Anthropogenic land-derived materials in urban wastewater have beenrecognized to influence on the water quality and the marine ecosystem
in Osaka Bay and Harima Nada, Japan. Towards a rigorous
environmental assessment of these two semi-enclosed estuaries, a
triple-nested high-resolution oceanic modeling framework based on
ROMS is developed to investigate the dispersal and dilution of treatedswage water effluent released from the subsurface nearshore outfall ofTarumi Sewage Treatment Plant near Akashi Strait. The effluent
dispersal in the northeastern Osaka Bay is two-layered and highly
reversible in the upper and lower layers due to stratification influenced
by freshwater input mainly from Yodo River. The overall effluent load
is in the dominant circulation direction of Seto Inland Sea
encompassing the two estuaries, substantially controlled by Kuroshio.
The estimated cumulative tracer flux for a four-month period is foundto accumulate by about 75% in Osaka Bay, while merely 17 % exits
towards the Pacific Ocean.

Fig. 3. Cross-sectional plots of the monthly-averaged offshore (to the SW) velocity (top panels) and of the weekly-averaged normalized tracer
concentration in Aug., 2009 (bottom panels). The while curves indicate the corresponding mixed layer depths.

ACKNOWLEDGEMENTS
This work is financially supported by the Grant-in-Aid for Scientific
Research (KAKENHI, 24560622).

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