Wastewater dispersal in Osaka Bay and Harima Nada
using a multi-nested high-resolution ocean modeling
Ryosuke Kanki 1, Yusuke Uchiyama 1 and Akihiko Nakayama 2
1

Department of Civil Engineering, Kobe University, Japan
2
Construction Engineering Research Institute, Japan
Abstract

Anthropogenic land-derived materials in urban wastewater have been recognized to influence on the water quality and themarine ecosystem in Osaka Bay and Harima Nada, Japan. Towards a rigorous environmental assessment of these twosemi-enclosed estuaries, a triple-nested high-resolution oceanic modeling framework based on ROMS is developed toinvestigate the dispersal and dilution of treated swage 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 found to accumulate
by about 75% in Osaka Bay, while merely 17 % exits towards the Pacific.
Keywords – ocean downscaling, ROMS, urban wastewater effluent
1. Introduction

2. Numerical Model

Osaka Bay (OB) and Harima Nada (HN) located off thedensely populated Kobe and Osaka areas are encompassed in
the northeastern part of Seto Inland Sea (SIS), Japan (Fig. 1).
Complex shoreline topography with over 3,000 islands in SIS
and riverine inflows play an essential role in formation of quite
complicated tidal and estuarine circulations. Uchiyama et al.
(2012) reported that the overall circulation in SIS is strongly
affected by the seasonally varying Kuroshio path. The waterquality, coastal marine ecosystem and aquatic resources in theseestuaries are significantly influenced by anthropogenic 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 OB and HN, forcing conditions such as tides, river inputs, andsurface wind, and the lateral topography and bathymetry as
exactly as possible. To this end, an oceanic downscaling system
in a triple nested configuration is developed to conduct a highresolution forward modeling of OB and HN with lateral grid
spacing of 200 m. A particular attention is paid to dispersion ofsewage effluent released from the subsurface nearshore outfallof Tarumi Sewage Treatment Plant (TSTP) near Akashi Strait(Fig. 1 middle) that is considered to be a potential source ofnutrient loads to the inner OB attributed to the clockwise
predominant current in SIS.

The outermost boundary condition is provided by JCOPE2, an
eddy-resolving, assimilative oceanic reanalysis product with ahorizontal grid resolution, dx, of 1/12º (dx ≈ 10 km) to bringoffshore signals inclusive of influences of Kuroshio into theROMS-L1 model at dx = 2 km. The triply nested configuration(Fig. 1 left) based on ROMS (Shchepetkin and McWilliams,2005) successively refines dx = 600 km (L2) to 200 m (L3).
The L3 model consists of 800 × 560 horizontal grids with 32bottom-following, stretched vertical σ-layers. The intermediate
L1 and L2 results described in Uchiyama et al. (2012) are
exploited for the lateral boundary conditions of the innermost
L3 model with a bihourly update to account for high frequency
tidal signals. The L3 grid relies on the high-resolution (at dx =50 m) dataset of the topography and the coastal embankmentscompiled by Japan’s Cabinet Office. We use the hourly GPVMSM product (JMA) for surface wind stresses and the COADS
climatology for the other surface forcing. Monthly climatologyof freshwater inputs from the 10 major rivers in L3 is taken intoaccount. Sewage effluent is applied in L3 at the location ofTSTP as a bottom-released fresh (buoyant) plume at themonitored hourly volume rate with a Gaussian initial dilution
submodel to avoid undesirable non-hydrostatic effects. Eulerianpassive tracer is additionally released at TSTP to track theeffluent. The computational period is chosen for the four-month
period from Aug. 1, 2009 to Nov. 29, 2009.

Fig. 1. (Left) ROMS-L1, L2 and L3 domains, (middle) ROMS-L3 domain, (right) blow-up of Osaka Bay and Yodo Line (red line).

3. Effluent Dispersal

4. Vertical Structure

Significant differences between the bottom and the surface
layers are observed in the passive tracer concentration (i.e.,wastewater effluent) released from TSPT. Initially, the tracer inthe surface layer (the top-most σ-layer) is mainly transported
toward OB and spread southward, whereas the prevailingtransport occurs eastward in the bottom layer (the bottom-mostσ-layer). On the 16th day, the tracer begins to emerge in thesurface layer in the inner part of OB near 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 an estuarine
recirculation in the inner bay near Yodo River that has thelargest freshwater input to OB. Once surfaced, the effluent isdispersed over the entire OB due to the mean near-surface flowin the WSW direction. This suggests that the estuarine
recirculation is substantial to form a two-layered structure in theinner OB that plays a key role in three-dimensional effluent
dispersion.

Figure 3 (top panels) shows the monthly-averaged offshore(southwestward) velocity along the Yodo Line (Fig.1). As
anticipated in Sec. 3, the lateral velocity in the northeastern OB
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 andthe shoreward subsurface current is located approximately at 3-5
m deep, corresponding to the mixed layer depths computed with
the KPP model. This recirculating reversal clearly leads to theupwelling of the near-bed effluent at the inner OB (Fig. 3bottom). In week 1 of Aug., 2009, the tracer distributes in thevertical near the release site crossing the pycnocline to deeper
than 40 m. In weeks 2 and 3, subsurface intrusion into the lower
layer occurs off Kobe with onshore transport of theconcentration maxima, and continuously disperses towards theinner bay. In weeks 3 and 4, the tracer surfaces by theupwelling associated with the estuarine recirculation, and thendrifts back to the southwest in the upper layer.
5. Tracer Flux Budget
A tracer flux budget analysis is conducted at the three straits(viz., Akashi, Kitan and Naruto). Predominant effluent transport
is found to occur westward to HN at Akashi Strait, while in thesouthward 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, and2.28 × 105 m3 at Naruto Strait, respectively. The estimatedinventory reveals that 75% and 8% out of the total amount of thereleased sewage water in the four months are accumulated inOB and HN, albeit the rest (about 17%) disperses towards thePacific through Kii Channel.
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Fig. 2. Snapshots of the log10-based normalized passive
tracer concentration on Aug. 16, 2009, in (a) surfaceand (b) bottom layers.

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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.