THEME A: Lagoons and Lakes

77

FLOW AND WATER QUALITY CHANGE IN BRACKISH LAKE
DEPENDING ON THE CHANGE IN METEOROLOGICALCONDITIONS
Kurokawa T.1 and Fukuoka S.2
Department of Civil and Environmental Engineering, Kure National College of Technology
2-2-11, Agaminami, Kure, Hiroshima 737-8506, Japan
Telphone & Fax: +81-823-73-8481, E-mail: kurokawa@kure-nct.ac.jp
2
Department of Social and Environmental Engineering, Global Environment Engineering Division,
Graduate School of Engineering, Hiroshima University
1-4-1, Kagamiyama, Higashi-hiroshima, Hiroshima 739-8527, Japan
Telphone & Fax: +81-824-24-7821, E-mail: sfuku@hiroshima-u.ac.jp
1

Abstract: Lake Nakaumi that is a brackish lake has strong pycnocline, so that dissolved oxygen in thebottom layer is often depleted. Brackish lake is subject to composite meteorological, oceanographic
and hydrological influences that give them complex flows. The meteorological characteristics exerting
influences on Lake Nakaumi, together with the flows in the lake caused by meteorological change andits water quality distribution, were examined in order to clarify the relationships between
meteorological change, flows and water quality environment in the lake. The following results wereobtained; (1) The meteorological changes giving rise to the external forces (wind, and water level
difference between the waters) generating the flows involve combinations of atmospheric pressures
and wind direction at times when strong winds occur. (2) Seawater inflow and wind-driven current byapproach of the low atmospheric pressure carry dissolved oxygen into bottom layer of Yonago Bay. (3)During the period of mild weather, internal wave in the back of Yonago Bay is different from thatgenerated in the other area of the lake. The anoxic water mass moves from the back of the bay towards
the center.
Keywords: brackish lake Lake Nakaumi atmospheric pressure, pycnocline wind-driven current
dissolved oxygen

1. INTRODUCTION
The environs of lakes are the sites for varied human activities. These activities result in various kinds ofwastewater concentrating in lakes. Additionally the fact that water generally dwells in lakes for longtime periods means that there now exist many lakes that have problems of water quality degradation.
Changes in the water quality of lakes are closely connected with the flow fields. As shown in Figure 1,
brackish lakes in particular are subject to composite meteorological, oceanographic and hydrologicalinfluences that give them complex flows. Accordingly the present study sets out to grasp
comprehensively the changes in the flow and water quality of brackish lakes through the example ofJapan's Lake Nakaumi.
The sequence used in the discussion will be the following: External forces - Flow in the Lake - Water
quality changes. To begin with we will clarify the conditions, resulting from frequently occurring
combinations of atmospheric pressures and winds at Lake Nakaumi, that constitute the characteristic
external forces causing flow within this lake. Following that we will examine the flows generated by
the external forces, such as wind-driven currents and inflow/outflow among bodies of water. Finally

XXX IAHR Congress

78

AUTh, Thessaloniki, Greece

oceanographic
sea water level variation
astronomical
spring tide
neap tide

meteorologicalatmosphericpressurehydrologicaltemperature
rainfallgenerating factor
of external force

river water inflow

Windwater level difference between waters

lake surface area, water depth, lake bed shapedensity distribution(water temperature, salinity)

inflow, outflow between waters
interface oscillation

wind-driven current

External force
for the flow in lake

limiting factorof the flow in lake

the flow in lakedensity current

Figure 1 Relationship between meteorological phenomena, hydrological phenomena, astronomical
phenomena, oceanographic phenomena and flow in a brackish lake
we will turn our attention to dissolved oxygen (DO) as indicative of the relationship between the water
quality environment and flow, and examine how flows act on the temporal and spatial distribution ofanoxic water masses.
2. OVERVIEW OF LAKE NAKAUMI
Lake water from Lake Shinji (with salinity around 1/10 that of seawater) flows into Lake Nakaumi via
the Ohashi River, and seawater from the Sea of Japan flows into Lake Nakaumi through the Nakaura
Gate (see Figure 2). With a water surface area of 86.2 km2, Lake Nakaumi is Japan's 5th largest lake.
The southeastern part of the lake is a long, thin projection from the main body known as Yonago Bay,
which receives domestic wastewater from the surrounding urbanized areas, making it the part of Lake
Nakaumi with the highest pollution levels.
Figure 3 gives sample vertical distributions of water temperature, salinity, density and DO in Lake
Nakaumi. In almost the whole of this lake a pycnocline of the kind seen in this figure is present at ornear a depth of 2 to 6m all the year round, so that the lake has a stable stratificational structure1). The
pycnocline inhibits the conveyance of matter between the upper and lower layers, making it difficult
for the lower layer to be supplied with DO. As a result the lower layer tends to become anoxic.
3. CHARACTERISTICS OF EXTERNAL FORCES CAUSING FLOWS
Each lake has its own peculiar topographical features and geographical configuration, and furthermore
is subject to composite meteorological, oceanographic and hydrological influences. This means thatthe characteristics of the dominant external forces in each individual.
The main external forces causing flows in Lake Nakaumi can be cited as (1) differences between thewater levels of the Sea of Japan, Lake Nakaumi and Lake Shinji resulting from water level fluctuations
in each of these waters due to meteorological and astronomical effects, and (2) winds arising fromchanges in the distribution of atmospheric pressure.

79

THEME A: Lagoons and Lakes

N

The Sea of Japan
Sakai Channel

Lake Nakaumi
Lake Shinji

0 5
5 6
6 7
7 8
8
Depth(m): Current,Temp.
,Salinty: Water level: ADCP,STD

Yonago Bay

Ohashi River

0

5

10 km

N

0

4km

Figure 2 Location of Lake Nakaumi and observation points and the water depth distribution.

Depth(m)Salinity(psu)
DO(mg/l)(1) WATER LEVEL FLUCTUATION IN THE
18 22 26 30
0 4 8 12WATER BODIES AND WATER LEVEL00RESPONSIVITY BETWEEN THEM
2
2
Since Lake Nakaumi is a body of water that forms a
connected system with the Sea of Japan (Miho Bay)
Salinity
44and Lake Shinji, fluctuations in the sea level andDOinflow of floodwater give rise to water levelσt
6
6 Water Temp.
differences between these three bodies of water.
The influences exerted by astronomical tide and
14 16 18 20 22
101214161820
Water Temp.(oC)σtmeteorological conditions on fluctuation in thewater levels of the Sea of Japan and Lake Shinji Figure 3 Vertical distributions of DO, density,
salinity, and water temperature.
may be characterized as follows1), 3). (1) The phasedifference between Miho Bay and Lake Nakaumi isaround 2 hours, so that changes in the sea level are transmitted to Lake Nakaumi with hardly any
attenuation. (2) In Lake Shinji the phase lag is around 8 hours, so that there is hardly any transmissionof astronomical tides from the sea. Thus, changes in atmospheric pressure are the only influencecausing changes in that lake's water level. (3) When atmospheric pressure changes occur continuously
for several days, water level differences of several tens of centimeters occur between Lakes Nakaumi
and Shinji. (4) Precipitation also results in a relatively higher water level in Lake Shinji.

(2) WIND CHARACTERISTICS IN LAKE NAKAUMI AND ITS ENVIRONS
Figure 4 shows the proportions of wind of various speeds and directions occurring with various
atmospheric pressures at Lake Nakaumi. Atmospheric pressures are categorized into high, "middle"(medium) and low atmospheric pressure, with the latter further divided into falling and rising low
atmospheric pressure.
There is a tendency for strong winds to blow along an east-west axis throughout the year. This is due tothe effects of the surrounding geography; the lakes are sandwiched between mountain land on thenorth and south. When atmospheric pressure is in the medium and high ranges (Figure 4 (c) and (d)),
meteorological conditions are calm and the winds are mostly weak, with speed no more than 6m/s,
their direction being mostly northeasterly to east-northeasterly and southwesterly to westerly. Thesecan be considered to be land and sea breezes. When air pressure is low, by contrast, strong winds occur

80

AUTh, Thessaloniki, Greece

XXX IAHR Congresswith high frequency. At times of low and falling
atmospheric pressure (Figure 4 (a)) there arehighly frequent strong west winds as well asnortheast winds, while at times of low but
recovering (rising) atmospheric pressure ((b))
there is a tendency for even stronger west winds
to blow.
From the above, the combinations ofatmospheric pressure, wind direction and windspeed that are prone to occur may be stated as
the following three: (1) Calm meteorologicalconditions during medium to high atmosphericpressure. (2) Strong northeasterly winds at times
during medium to high atmospheric pressure.
(3) A transition from northeast winds (during
falling low atmospheric pressure) to westerly
winds (during pressure recovery) when low
atmospheric pressure passes.

NNW

NW

N

NNE

wind speed 12 9
(m/s)

NE

6

0

ENE

WNW
W

13.4%

4%

8% E

ESE

WSW
SW
SSW

S

SSE

SE

NNW

NW

N

NNE

NE
ENE

WNW
W

(a) in low atmosphericpressure( into falling)

10.3%

8% E

4%

ESE

WSW
SW
SSW

S

SSE

NNW

N

NNE

SE

(b) in low atmospheric pressure (into
NNW

NW

N

NNE

NW

NE

NE
ENE

ENE WNW

WNW

45.9% 4% 8% E

W

W

30.3%

4%

8% E

ESE

ESE WSW

WSW
SW
SSW

S

SSE

SW

SE

SSW

(c) in "middle" atmosphericpressure

S

SSE

SE

(d) in high atmosphericpressure

Cross Correlati
Coefficient

4. CHARACTERISTICS OF FLOWS
The
external
forces(meteorological,
oceanographic and hydrological) causing flows Figure 4 Generation proportion of the wind speed
and direction (proportion as each atmosphericpossess various time scales. Accordingly it ispressure condition for the whole is shown innecessary to ascertain the predominant flow
central circle )phenomena for each time scale.
0.8
0.8
At Lake Nakaumi the main time scale division is
St.1
St.2
0.4
0.4
into half to one day for land and sea breezes and
St.3
St.2
astronomical tides on the one hand, and several
St
.
1
0.0
0.0
days to several weeks for winds and water level
St.4
-0.4
-0.4
St.3
fluctuations due to meteorological changes on
St.4
-0.8
-0.8
the other.
0

12

24

0

12

24

Time Lag (hr)Time Lag (hr)(1)
LAKE-FLOW GENERATED
BY(a) with water level(b) with wind speed
EXTERNAL FORCES WITH PERIOD OF
Figure 5 Cross correlation coefficient of current velocity
HALF TO ONE DAY
upper layer) with (a)water level, with (b)wind
Figure 5 shows the cross correlations of the (St.1-4
speed (the ‘Diurnal period component’).
upper layer main current axial velocity at each ofstations 1 through 4 with the water level and the wind speed. Specifically, this figure gives the "diurnal
period components" for these items. The correlation with water level is high at station 1, located close
to Nakaura Gate, whereas the correlation with wind speed increases the further away one moves from
Nakaura Gate. Thus as regards diurnal period flow components, the effects of astronomical tides are
dominant only in the vicinity of the Nakaura Gate, while in the greater part of the lake's interior it island and sea breezes that are the predominant factor driving the flow.

(2) LAKE-FLOW RESULTING FROM DAY-TO-DAY METEOROLOGICAL CHANGESThe flows resulting from day-to-day meteorological changes are the wind-driven currents due tochanges in atmospheric pressure distribution and the inflows/outflows due to level differences
between the waters that are generated by atmospheric pressure variations and floodwater inflow. We

THEME A: Lagoons and Lakes

81

Figure 6 Temporal variations of (a) atmospheric pressure and water level, (b) water level
difference, (c) wind speed and direction 1997/9/11-10/6
will now look at each of these two flows individually.
a) Wind-driven currents caused by wind arising from changes in atmospheric pressure distribution
Figure 6 shows the temporal variations during the period 11th September to 6th October 1997 in (a)atmospheric pressure and water level, (b) water level difference between the center of Lake Nakaumi
and its Yonago Bay portion, and (c) wind direction and speed. Four characteristic meteorologicalconditions occurred during this period: (1) the passing of a typhoon, when northeast winds of around
16 m/s blew continuously, (2) a time of high-pressure strong winds, when northeast winds of around
10 m/s enveloped in high atmospheric pressure blew continuously, (3) the coming of low atmosphericpressure, when the transition to low atmospheric pressure caused strong winds to blow that
progressively altered their direction from east to west clockwise, and a rise in the water level occurred,and (4) a time of calm weather when the atmospheric pressure recovered again, no strong winds blew,
and land and sea breezes predominated. (1) is a characteristic Japanese meteorological phenomenon
that is seen frequently during the summer and autumn in this country.
b) Inflows/outflows due to level differences between the waters resulting from atmosphericpressure distribution variations
Normally, inflow to Lake Nakaumi from Lake Shinji is prone to occur when atmospheric pressure ison a rising trend, and inflow to Lake Nakaumi's lower layers from Miho Bay when atmosphericpressure is on a falling trend. Figure 7 shows temporal variations (a) in the atmospheric pressure andthe water levels of Lakes Nakaumi and Shinji, and (b) in the salinity at depths of 0.5 and 1m below
surface in the center of Lake Nakaumi during the period 10th to 25th November 1996, plus (c) in thecurrent velocity in the upper and lower layers of Lake Nakaumi at stations 2, 3, 4 and 12 from 18th to
25th of the month stated.
Let us examine as an example the times when atmospheric pressure is on a rising trend (18th-19th and21st-22nd November in Figure 7). As the atmospheric pressure begins to rise, the water level situation
changes from one in which there is a lag in the water level response between Lakes Nakaumi andShinji to one in which the level of Lake Shinji is relatively higher than that in Lake Nakaumi ((a) in the
figure). In response to this, inflow occurs from Lake Shinji to Lake Nakaumi via the Ohashi River ((c)).
Thereafter the salinity of the upper layer at the center of the lake falls some 12 hours later ((b)), andfurthermore the influence of inflows appears at stations 2 and 3 as indicated by the broken arrow lines((c)). On the other hand, no influence of inflows is observed at station 4. This means that the
inflow/outflow caused by water level difference between the waters consists predominantly of flow inthe direction linking the Nakaura Gate with the Ohashi River, and thus has little effect on Yonago Bay.
Moreover the fall in salinity at the center of the lake is greater at 0.5 m than at 1.0 m below the surface,

82

Figure 7

XXX IAHR Congress

AUTh, Thessaloniki, Greece

Temporal variations of (a) atmospheric pressure and water level, (b) salinity at Center of
Lake , (c) current velocity at St.2 3 4 12 (1996/11/10 11/25)

which demonstrates that inflowing freshwater penetrates near the surface in the manner of a density
current.
c) Correlation of wind-driven currents and the inflow/outflow between waters
Figure 8 summarizes the lake-flowss as those occurring with particular combinations of atmosphericpressure and wind direction/speed. Specifically these are the flows that occur (a) when there is medium
to high atmospheric pressure and strong winds blow from a north northeasterly to easterly direction,and (b) when low atmospheric pressure comes and a change takes place from northeasterly to strong
westerly winds.
The relatively higher water level that occurs in Lake Shinji in the case of (a) makes freshwater prone toflow into the upper layer of Lake Nakaumi through the Ohashi River, and this gives rise to a current in
the upper layer extending from the Ohashi River inlet to the center of the lake and thence to the
Nakaura Gate. But the direction of the wind-driven current produced by the northeast winds is in the
opposite direction, and as a result a horizontal circulating current is prone to occur.
In the case of (b) (coming of low atmospheric pressure), when the atmospheric pressure is fallingseawater is prone to flow into the lower layer of the lake from the sea, and furthermore the east winds
act to make the water around the center of the lake flow out into the lower layer of Yonago Bay; thismay be considered the combination of conditions most favorable for the supply of DO-rich water tothe bay's lower layer. On the other hand when the atmospheric pressure starts to recover (rising again
from its lowest point) the winds change to strong westerly and additionally there begins an inflow into
the lake's upper layer from Lake Shinji, which has a lag of half to one day; if the west winds are
sustained, the flow mobility within Lake Nakaumi will be increased through synergistic effects.

83

THEME A: Lagoons and Lakes
Flow which originates from water level differenc e
between waters by pressure change

Flow which originatesfrom wind

Wind

Wind

Wind

(Upper layer)(Upper layer)(Upper layer)(Lower layer)(Lower layer)(Lower layer)( into rising)( into fallring)(a) high or middle atmospheric pressure
with strong east wind

Figure 840(b) low atmospheric pressure ( it changes to the strong west wind from east wind ).

Summary of the flow under the characteristic weather condition in Lake Nakaumi.
Upper
Lower

20

DO[mg/l]

8

7

6

5

4

3

2

1

9

(St.8) 12 (St.10)

0

0
-20

St.1

Inflow of seawaterby low atmospheric pressure

0
-20
20

St.2

0
-20
20

St.5

0
1
2
3
4
5
6
7
8
9
10

Density stratum

3

6

15

Distance from Nakaura gate(km)

0
-20
20

Depth (m)Current Velocity (cm/s)

-40
20

St.8

Figure 10 Longitudinal distribution of DO (1997/10/3 10:00)

0
-20

St.10

(a) current velocity in upper and lower layer (St.1) in lower layer(St. 2 5 8 10

5. INFLUENCE OF FLOWS ON
HORIZONTALRIZONTAL AND VERTICAL
6
St.2
DISTRIBUTION OF WATER QUALITY
Accordingly we will examine the pycnocline
4
St.5
St.8
behavior
and water quality variation caused by
2
St.10
wind-driven
currents and inflows of seawater and0(b) DO in lower layer
river water, so as to clarify the relationships
Figure 9Temporal variation of current velocity and between weather, flows and water quality.
DO (1997/9/22-10/6)
Focusing mainly on DO as water quality
parameter, we will examine in what ways theflows act on the temporal and spatial distribution of anoxic water masses.
Figure 9 shows (a) the velocity of the current along the Nakaura Gate to Yonago Bay axis at stations 1,
2, 5, 8 and 10; and (b) the temporal variation in lower layer DO at stations 2, 5, 8and 10. Large scale
inflow of seawater into the lower layer occurs only for a several days at a time, when low atmosphericpressure passes over the Japan sea causing a rise in the level of the sea beyond the lake (period (3),coming of low atmospheric pressure). The influence of such seawater inflow gradually reaches as far
as Yonago Bay (station 10), but only thanks to the synergistic effects of the wind-driven density
DO(mg/l)8

84

XXX IAHR Congress

AUTh, Thessaloniki, Greece

currents. From 25th to 27th September the shift to low atmospheric pressure caused the wind direction
to change progressively clockwise from an easterly starting point and increased the wind speed. In
terms of lake-flows, this probably resulted in the inflowing seawater that headed for the lower layer at
the center of the lake (stations 2 and 3) being effectively carried into Yonago Bay (stations 7 and 8) by
the wind-driven density currents generated by the east-to-south shift in wind direction. Subsequently
the fall in atmospheric pressure bottomed out during the afternoon of 27th September and the winddirection changed to westerly, whereupon inflow into the lower layer of Yonago Bay ceased.
Such inflow of seawater that occurs with the coming of low atmospheric pressure causes a rise in DO
that extends into Yonago Bay. This serves to demonstrate that the horizontal distribution of water
quality within the lake is subject to the influence of meteorological conditions to a large degree.
Next we will look at the relationship between seawater inflow and the vertical configuration of water
quality. Figure 10 shows the longitudinal and vertical distributions of DO from the vicinity of the
Nakaura Gate up to Yonago Bay at a time of calm weather (around 10:00 on 3rd October 1997). It canbe seen from this figure how the high-DO seawater flows in the manner of a density current into thelower layer. This serves to demonstrate that the flows caused by meteorological change can also havethe effect of rejuvenating the vertical configuration of water quality.

6. CONCLUSIONSThe principal conclusions are as follows.
1) The meteorological changes giving rise to the external forces (wind, and water level difference
between the waters) generating the flows involve combinations of atmospheric pressures and winddirection at times when strong winds occur. They are of 2 kinds: a change from northeast to east windoccurring at times of high atmospheric pressure, and a change from east wind to strong west windoccurring together with the coming or passing of low atmospheric pressure.
2) As regards the characteristics of the lake-flows generated by the external forces: Flow in the
direction linking the Nakaura Gate with Lake Shinji predominates in the inflow/outflow of seawaterand Lake Shinji water due to water level differences between the waters resulting from atmosphericpressure changes. Yonago Bay is close to stagnant but wind-driven currents produced by strong winds
increase its slow water replacement rate.
3) As regards the effects of flows on the water quality distribution: One such effect is produced byseawater inflow when low atmospheric pressure comes to the lake. In calm weather oxygen
consumption is continuously in progress in the lower layer of Yonago Bay, resulting in an anoxic statein that layer. This anoxic state is however temporarily alleviated when such seawater inflow produced
by low atmospheric pressure reaches the extremities of Yonago Bay. Further, inflow of seawater orriver water produced by meteorological changes produces the effect of altering the lake's verticaldensity distribution, which subsequently exerts influences on its flow mobility.

REFERENCES[1] Hibino, T., Fukuoka, S., Ikeuchi, K.: Characteristics of Internal Currents in Enclosed Brackish
Lake with Seasonal and Daily Pressure Distribution Change, Annual Journal of HydraulicEngineering, JSCE, No.579/2 -103, pp.93-103, 1997.
[2] Fukuoka, S., Kurokawa, T., Hibino, T., Suzuki, A., Nakamura, T. and Uehara, H.: Flow andAnoxic Water Mass Behavior Accompanied with the Change in Meteorological Conditions in Lake
Nakaumi, Annual Journal of Hydraulic Engineering, JSCE, No.636/7-1 3, pp.61-79, 1999.
[3] Ishitobi, Y., Kamiya, H. and Itogawa, H.: Tidal, Meteorological and Hydrological Effects on the
Water Level Variation in a Lagoon, Lake Shinji, Jpn. J. Limnol., Vol.54, No.l, pp.69-79, 1993.