Proceedings of Coastal Engineering, JSCE, Vol.3, 2012, November

NUMERICAL SIMULATION OF SALINITY
DISTRIBUTIONS IN THE YATSUSHIRO SEA BY
POM COMBINED WITH WATERSHED MODEL
FOR B-CLASS RIVER BASIN
Sha Lou1, Akihide Tada2, Akira Tai3, Shinichiro Yano4,
Seiji Suzuki5 and Shuguang Liu6
1Dept. of System Science, Graduate School of Science and Technology, Nagasaki University, lousha19@gmail.com
2Division of System Science, Graduate School of Engineering, Nagasaki University, atada@nagasaki-u.ac.jp
3Institute for Advanced Study, Kyushu University, tai@civil.kyushu-u.ac.jp
4Dept. of Urban and Environmental Engineering, Faculty of Engineering,
Kyushu University, yano@civil.kyushu-u.ac.jp
5Division of System Science, Graduate School of Engineering, Nagasaki University, ssuzuki@nagasaki-u.ac.jp
6Dept. of Hydraulic Engineering, Tongji University, liusgliu@tongji.edu.cn

Minamata Bay, located in the southeast coastal areas of the Yatsushiro Sea, was heavily polluted dueto the discharge of organic mercury compound from a chemical factory since 1932. Some research has
demonstrated that correlations between seawater characteristics and methyl-mercury concentration are
observed. Based on the analysis of low-salinity water masses in the Yatsushiro Sea, this paper deals withsalinity field in the Yatsushiro Sea under the effect of freshwater from rivers by Princeton Ocean Model(POM) combined with watershed model. Firstly POM is established covering both the Yatsushiro Sea andthe Ariaka Sea. Salinity fields in six cases are calculated under freshwater inflow from a A-Class river:the Kuma River. Then a watershed model is applied in basin of a B-Class river: the Komenotsu River for
Case5 and Case6. Rainfall data and physical characteristics of catchment are utilized to estimate river's
discharge by Kinematic Wave Method (KWM). Through the computation, it's indicated that simulated
salinities in surface layer exhibit the same trends as the measured ones at most cases. The deviations in
Case1-Case4 are less than 20% at most areas, but the deviations become obviously higher in Case5 andCase6 when the measured salinities are lower than 5 and the highest deviations always occur in surface
layer. Adding the Komenotsu River's discharges into POM, salinities at two stations (St.25 and St.26) inCase5 and one station (St.26) in Case6 have decreased owing to the rainfall and closer location to theriver mouth.
Key Words : the Yatsushiro Sea, salinity filed, freshwater inflow, POM, kinematic wave method

1. INTRODUCTION
Minamata Bay, located in the southeast coastal
areas of the Yatsushiro Sea, was heavily polluted
due to the discharge of organic mercury compoundfrom a chemical factory since 1932. Besides controlof the pollution, many observation measures havealso been implemented (Kumagai et al., 1978;
Tomiyasu et al., 2000; Tomiyasu et al., 2008).
Although at present further research still needs
conducting to comprehensively understand the
dynamic behavior of mercury in Minamata Bay,some research has demonstrated that correlations

betweenseawatercharacteristics
andmethyl-mercury concentration are observed(Matsuyama et al., 2011). However, salinity isalways studied for red tide or algae bloom in the
Yatsushiro Sea (Iunoe et al., 2007; Tomoyuki et al.,2009; Murata et al., 2009), only a few research inrespect of flow pattern and runoff have been done.
In 2011, low-salinity water masses are analyzed
in the Yatsushiro Sea based on six years' measured
data between 1979 and 1998 (Tai et al., 2011).
Salinities in the Yatsushiro Sea were measured at 20
stations (Fig.1) and in six vertical layers: L-S0,
L-S05, L-S10, L-S20, L-S30, L-SB (water surface;-6-5m, 10m, 20m and 30m below water surface;bottom). On this condition, this paper deals withsalinity field in the Yatsushiro Sea under the effectof fresh water from rivers by numerical simulation.

2. NUMERICAL SIMULATION BY POM
Many numerical models have been used tosimulate salinity field in the ocean (Vinayachandran
et al., 2009; Kitazawa et al., 2011; Iakovlev, 2012).
In this paper, Princeton Ocean Model (POM),
developed in Princeton University (Mellor, 2004), isapplied to reproduce the salinity filed in the
Yatsushiro Sea. It has been widely utilized in theworld (Roed et al., 2007, Aschariyaphotha et al.,2008). The basic equations are shown in Eq. (1)~(4).
DU DV  



0
x
y
 t

(1)

DU DU 2 DUV U  fVD  gD
t
x
y

x


0  '
 K M U
 ' D  '[]d ' []  Fx
 D 
 0  x D x  '

gD 2

(2)

DV DUV DV 2 V  fUD  gD
t
x
y

y


0  '
 ' D  '
 K M V[]d ' []  Fy0  y D y  ' D 

gD 2

DS DUS DVS  S
 K H S[
]  FS




t
x
y

 D 

(3)(4)
where U, V are the horizontal velocities, ω is sigma
coordinate vertical velocity, S is salinity, η is thesurface elevation, H is bottom topography and
D=H+η.
The model is established covering all the
Yatsushiro Sea and the Ariake Sea (Fig.1). Numberof horizontal grids is 438 × 468 with 10' interval inboth longitude and latitude, and there are ten layers
in vertical direction. Open boundary is given at thesouthern end of the simulation area using lunar
semi-diurnal tide constituent of tidal motion (M2with amplitude of 1.0m). Time interval of external
mode is 1s and that of internal mode is 5s. Ten
layers in the model are divided into three partsroughly:
surface-water,
middle-waterandbottom-water for setting the vertical initialconditions of salinity and temperature. Initialconditions of surface-water and bottom-water areinterpolated directly from measured data at layers:
L-S0 and L-SB, respectively, while for
middle-water it is interpolated from averaged
measured data of Layer S05, S10, S20, S30.
Horizontal initial conditions of three parts areinterpolated from measured salinity at 20 stations asshown in eq.(5). Real-time discharge of the Kuma
River (A-Class) is specified as the freshwater
inflow. Other calculation conditions are shown in

Table 1.
nmax

nmax

Si , j   ( Sn / Lin, j ) /  (1/ Lin, j )

n 1
n 1
(5)
where nmax is the total number of measured stations,
Sn stands for measured salinity or temperature at
Station n, Lni,j means distance between Station n andgrid (i,j) in numerical model, Si,j is the salinity or
temperature at grid (i,j) in numerical model.
Verification of the flow field by POM has been
carried out at three stations in the Ariake Sea andtwo stations in the Yatsushiro Sea including one
station at Minamata Bay as mentioned in (LOU etal., 2011).
Fig.2 is the comparisons of measured andsimulated surface layer's salinity at 20 stations in
Case1-Case6. The simulated salinity is the averaged
data at the last day of the calculation. The figure
shows simulated salinities in surface layer exhibit
the same trends as the measured ones at most cases
and the magnitudes have slight difference exceptthat at St.26 in Case2. The extreme low measured
salinity at St.26 must be caused by measurement
error. In Case3 and Case4, simulated salinities at
St.40 are much higher than the measured data. St.40
is located around the mouth of the Kuma River. The
influence of freshwater might be more significant
than the calculation. Severe deviations appear in thenorthern part of Case5 and in the south-central partof Case6. And most of these stations (St.23, 24, 25,
26, 28) are just directed toward mouth of the
Komenotsu River.
Fig.3 shows the depth profiles of measured
salinity, simulated salinity and the deviation. It's

-7-

33.2
N

N

33.0

Ariake
Sea

32.8

32.6

Yatsushiro
Sea 21 C

32.4

B
22

E
31D
32
30

Minamata River
23
Minamata Bay
24
26
25
Komenotsu River

32.2

32.0

28

40
M
39 Kuma
River
37 36
35

Open Boundary
129.6

129.8

130

130.2

130.4

130.6 E

Fig.1 Calculation Region and Locations of 20 Stations

Fig.2 Comparisons of Measured and Simulated Salinity at 20 stations in Six Cases
0
(m)

0
(m)

0
(m)

0
(m)

-5

-5

-5

-5

0
(m)
-5

0
(m)
-5

-10

-10

-10

-10

-10

-10

-15

-15

-15

-15

-15

-15

-20

-20

-20

-20

-20

-20

-25

-25

-25

-25

-25

-25

-30

-30

-30

-30

-30

-30

-35

-35

-35

-35

-35

-35

-40

-40

-40

-40

-40

-40

-45

-45

-45

-45

-45

-45

-50

-50

-50

-50

-50

-55
-60

-55

Case1-Measured
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

Case1-Deviation
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

0
(m)

0
(m)

0
(m)

-55

-55

Case1-Simulated

-50

-55

Case2-Measured
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

-55

Case2-Simulated
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

0
(m)

0
(m)

-5

-5

-5

-5

-5

-5

-10

-10

-10

-10

-10

-10

-15

-15

-15

-15

-15

-15

-20

-20

-20

-20

-20

-20

-25

-25

-25

-25

-25

-25

-30

-30

-30

-30

-30

-30

-35

-35

-35

-35

-35

-35

-40

-40

-40

-40

-40

-40

-45

-45

-45

-45

-45

-45

-50

-50

-50

-50

-50

-55
-60

-55

Case3-Measured
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

-55

-55

Case3-Simulated
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

Case3-Deviation
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

-50
-55

-55

Case4-Measured
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

Case4-Simulated
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

-5

-5

-5

0
(m)
-5

-5

-5

-10

-10

-10

-10

-10

-10

-15

-15

-15

-15

-15

-15

-20

-20

-20

-20

-20

-20

-25

-25

-25

-25

-25

-25

-30

-30

-30

-30

-30

-30

-35

-35

-35

-35

-35

-35

-40

0
(m)

0
(m)

0
(m)

0
(m)

-40

-40

-40

-40

-40

-45

-45

-45

-45

-45

-50

-50

-50

-50

-50

-50

-55

-55

-55

-55

-55

-55

-60

-60
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

Case5-Measured
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

Case5-Simulated
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

-60

Case6-Measured
3220 3225 3230 3235 3240 3245 3250 3255 10^-2(Deg)

Case4-Deviation
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

0
(m)

-45

Case5-Deviation

Case2-Deviation
3220 3225 3230 3235 3240 3245 3250 3255 10^-2
(Deg)

0
(m)

Case6-Deviation

Case6-Simulated

3220 3225 3230 3235 3240 3245 3250 3255 10^-2(Deg)

Fig.3 Vertical Profiles of Salinity in Six Cases(X-Axis: latitude; Y-Axis: water depth; Deviation =100*|Simulated - Measured |/ Measured)realized that the tendency of simulated salinities arein accord with the measured data in not only thehorizontal but also the vertical distribution, and thedeviations in Case1-Case4 are less than 20% at mostareas (except 40% in northern part in Case3). But
the highest deviations reach up to 1600% and 100%in Case5 and Case6 respectively when the measured
salinities are no greater than 12. In addition, thedeviations are always higher on the surface layer.

-8-

Table1 Calculation Conditions for Salinity Fields in Six Cases

No.
Case1
Case2
Case3
Case4
Case5
Case6

Initial Time
25th Aug.
14th Aug.
18th Jun.
28th Jun.
13rd Jun.
24th Jun.

End Time
24th Sep.
10th Sep.
23rd Jul.
25th Jul.
6th Jul.
22nd Jul.

Calculation Time
31 Days in 1998
28 Days in 1980
36 Days in 1996
28 Days in 1979
24 Days in 1995
29 Days in 1982

3. WATERSHED MODEL
Based on the results of POM, it's indicated that
improvement is necessary for Case5 and Case6.
Although there is lack of discharge data of B-Class
river, the Komenotsu River which pours into the sea
at the southern areas should be considered toconsistent with practical situation. Therefore, awatershed model using Kinematic Wave Method(Wooding, 1965; Wong et al., 1989; Suzuki et al.,2011) is adopted in basin of the Komenotsu River
whose mouth is located in Izumi, Kagoshima
Prefecture. The river's discharge is related to rainfall
and physical characteristics of catchment such as
areas, slope and surface roughness. The whole basinwould be ideally divided into several elementscomposed of a river channel and two catchments in
KWM. Basic equations are as following.
For the catchment:
h q

 re
t x
(6)

h  Kq

P

(7)

3
P
5
 n 
K  
 i

(8)
P

(9)

For the river channel:
A Q

 2q
t x

A  KQ P

(11)

3
P
2Z  3 n 
K   2/3 
i
 k1

(10)

(12)
P

(13)
in which A is the channel flow area, Q is the channel
discharge, h is the depth of flow, q is the discharge
per unit width, t is the time, x is the distance along
the catchment or channel in the direction of flow, reis the rainfall excess, n is the roughness ofunderlying surface, i is slop, and k1 and Z are theparameters relating to the flow area and flow depth.
Fig.4 is about 73 river channels with 146
catchments under different land-use in the
Komenotsu Basin. In every segment, the lengthinterval is 10m and the time step is one hour. Fig.5
shows the rainfall data and discharge results of the
Komenotsu River in Case5 and Case6. The rainfall
data is averaged from data at three meteorologicalstations in Kagoshima Prefecture: Izumi, Ookuchi
and Shibisan. The maximum instantaneous rainfall
is around 31mm/h in both Case5 and Case6, but theaccumulated rainfall is 767mm in Case5 and
536mm in Case6. Through the utilization ofwatershed model for the Komenotsu River, adischarge of approximately 700 m3/s is caused bylarge amount of rainfall in Case5, while in Case6 themaximum discharge is only 300 m3/s.
Adding the discharges of the Komenotsu River
into POM as a boundary condition, improved resultsof the salinity distribution for Case5 and Case6 areillustrated in Fig.6. Obvious decreases of salinity at
St.25 and St.26 occur in Case5. From the locationsof 20 stations in Fig.1, it's found out St.25 and St.26
are much closer to the Komenotsu River's mouth
than other stations. Even with the abundantprecipitation as in Case5, freshwater from the
Komenotsu River only has some influence in certain
areas around river mouth. And in Case6 under the
relatively lower rainfall, only salinity at St.26 has
been slightly impacted by the Komenotsu River.

4. CONCLUSIONS
Through the analysis of measured and simulated
salinity fields in the Yatsushiro Sea, several
conclusions have been summarized as following:(1) In six cases which are all in summer season,
salinity stratification is observed in evidence and themeasured salinities are lower in the surface layer.
But at some stations, the measured surface layer's
salinities are relatively low like 19.4 at St.26 in
Case2, 11.63 at St.D and 11.16 at St.E in Case6,
moreover, salinities at some stations in Case5 arelower than 5 which cause 1600% deviation in Fig.3.
Reliability of measured data should be confirmed.
(2) POM could describe salinity field in the
Yatsushiro Sea with a certain accuracy. Mostly,
simulated salinities agree with the measured dataquite well. But around the Kuma River's mouth andin the southern part, the simulated salinities arehigher than the measured ones. Deviations reducewith the increasing of water depth.
(3) KWM is a valid method for the watershed
model to estimate river's discharge. Considering the-9-

Fig.4 Land-use in the Komenotsu BasinFig.5 Rainfall Data and Discharge of the Komenotsu River in Case5 and Case6

Fig.6 Improved Results of Salinity Distribution by Watershed Model using KWM in Case5 and Case6

freshwater from the Komenotsu River, little impact
has appeared around river mouth areas in Case5 andCase6 under the precipitation.
Therefore, to heighten the accuracy, more
researches still need to be done, such as consideringother B-Class rivers or scaling down the model.
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(Received June 15, 2012)- 10 -