●Research Trends and Results
Development of a new model capable of calculating the spatial
distribution of age of seawater which has entered a river mouth
AMANO Kunihiko (Phd(Eng)), HeadONUMA Katsuhiro, Senior Researcher
ENDO Maremi, Researcher
Environment Department, River Environment Division(Key words) Riverine Estuary, retention time, age, water quality analysis
1. Introduction
Riverineestuaries
havecomplex
waterenvironments where fresh water from the river andseawater are mixed in various proportions both intemporally and spatially. This research is intended tosupport water quality and ecosystem preservationplans for riverine estuaries by clarifying themechanism which produces oxygen deficient water
masses in the bottom layers and various other water
quality phenomena which are problems for riverine
estuaries by developing a new model1) which can beused to quantitatively evaluate the elapsed time (age)after sea water flows into the riverine estuaries.

c i
c i
c i
c i
w
u
v
yz
t
x

  c i    c i   
c i AHAHKH
x x  y z y  z C 1 inC 2 in
Cn in

C 1 C 2  CnC   Ci

C 1 out
C 2 out
Cn outFigure 1. Salinity Mass Balance Equation andSalinity Fractionation
2.0

3.0

4.0

5.0

河口からの距離(m)Distance fromriver mouth (m)(days)
30

25 20

15

105

Water depth (m)2. Age calculation model
Normal numerical analysis of salinity in riverine
estuaries and inner bays calculates the mass balance ofsalinity for each grid. This process is expressed interms of advection and diffusion. Necessary values forthis calculation such as flow velocity and diffusion
coefficients are given from a hydrodynamic model
which analyzes the temporal and spatial distribution offlow. The mass balance equation based on advection
and diffusion is shown in Figure 1, but if the flow
velocities and diffusion coefficients are given by thehydrodynamics model, the only unknown variable isthe salinity C, so the equation in the Figure 1 can betreated as a linear equation. Thus, even if thecalculation is done treating the fractionated salinity by
the time each flowed into the river mouth as separate
variables, when these are totaled, the value is identicalto the concentration calculated in the ordinary way
without fractionating the salinity.
If the salinity is fractionated by the time it flowedfrom the sea through the lateral section of theboundary between the ocean and the river into theriver channel, it is possible to obtain the concentration
and age for each of the fractionated salinities, so theaverage age of the existing salinity can be computedfor each calculation grid set in the riverine estuaries.
Figure 2 shows the salinity at the river mouth andthe average age after flow into the river mouth invertically two-dimensional distribution, revealing thatthe age is longer on the upstream side.

Fractionated model
分画モデル

水深(m)(psu)

Figure 2. Average Age (Days) of the Salinity and theSalinity (psu)
3. Conclusion
The elapsed time (age) of the water mass plays animportant role in the fluctuation of water quality in ariverine estuaries. Our new model can quantitativelyevaluate this, permitting the clarification of themechanisms of water quality fluctuation in such areas.
[Reference]
1) Amano et al.: Numerical analysis of environmental
effects on the retention time of sea water in riverine
estuaries, Journal of the Japan Society of Civil
Engineers G (Environment), v. 6, n 7, 2011.