THEME C: Density Currents and Stratified Flows389

BEHAVIORS OF LARGE TURBID CURRENT AND
COUNTERMEASURES FOR LONG TERM TURBIDITY
PROBLEM IN THE STRATIFIED RESERVOIRHotta T.1, Fukuoka S.2, Chen F.Y.1, Yamashita Y.1, Shoji H.1 and Tsuruta Y.1
1Engineer, Water Resources Dept., CTI Engineering CO., LTD.
Nihonbashi Honnchou 4-9-11, Chuou-ku, Tokyo 103-8430, Japan
Tel: (081)03-3668-0451, Fax: (081)03-5695-1884, Email: t-hotta@ctie.co.jp
2
Professor, Department of Civil and Environmental Engineering, Hiroshima University,1-4-1, Kagamiyama, Higashi-Hiroshima, 739-8527, Japan
Tel: (081)0824-24-7821, Fax: (081)0824-24-7821, Email: sfuku@hiroshima-u.ac.jp

Abstract: A series of numerical calculation results are shown on dynamic behaviors of turbid
current in the reservoir. With a large volume of the current, it flows down along the bottom of
the reservoir, and the return current after clashing the dam contributes much on mixing of theturbid water with surroundings, causing prolonged turbidity in the reservoir.
The mechanism implied possible treatments to reduce the volume of the turbid water.
Numerical experiments indicate that successful purging the turbid water by a low layer outlet,
such as a selective withdrawal device, reduces the period of turbid water by 88%. Then, some
calculations are conducted to improve the condition of sediment flow in reservoirs. The results
suggest that the behavior of sediment flow can be controlled if the stratification condition or theoutlet elevation in reservoirs can be changed.
Kewords: Large turbid current, long term turbidity problem, reservoir, countermeasures,
stratification, selective withdrawal

1. INTRODUCTION
The request to improve the long-term turbidity problem, for improving the scene and increasing
the fisheries resource, are often from the water’s user and the residents of the downstream
reaches of the river. Especially since the new law of environment assessment has been in force
in Japan, the attentions to the long-term turbidity problem in dam reservoir have also beenraised further more. Therefore, the technology for evaluating this phenomenon is requested
widely.
Generally, if the temperature stratification cannot be formed in the reservoir and the exchange
speed of the water is comparatively fast, the phenomenon of long-term turbidity problem
cannot be generated. On the other hand, the long-term turbidity problem is often appeared in thestratified reservoir, due to its long retention period for the flood currents and the intrusion of
turbid current to the upper layer, which depends on the density flow.
The flowing velocity in the reservoir is usually much faster than the setting velocity of fineparticles, which are induced by the flood. Beside of it, the mixture between the clean water andthe turbidity water is produced due to the turbulent flow, which is generated by the flow and thewind. Therefore, the sedimentation of suspended sediment in the reservoir is obstructed, so thata long-term turbidity problem occurs.

XXX IAHR Congress

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Aki & Shirasuna (1975) found that the turbidity behavior depends on the scale of the flood aswell as the situation of stratification and induced an index to present the turbidity behavior in
the reservoir, named as the scale of the flood:EThe total volume of flood discharge/reservoir capacity

According to the analysis of Aki & Shirasuna, the flood scale can be classified into three levels:
big scale flood for E ! 1 , mid scale flood for 1 ! E ! 1 / 2 , and the small scale flood for E  1 / 2 .
The intrusion location of the flood depends on the scale of flood. The turbid current flows into
the mid-layer of the reservoir for a small flood, and to the whole upper layer for a mid-scaleflood. But for a big scale flood, the clean water in the reservoir can be exchanged for the turbidcurrent. For this case, the heavy turbidity flow can overcome the density difference of watertemperature between the two layers, being above and under the thermocline, where the watertemperature slope is steep, and the stratification state can be reformed. As the result, the lower
layer turbid currents are generated.
Fukuoka et.al (1980) studied phenomenon of long term turbidity problem in the stratified
reservoir by experiment. They found that the turbid currents are divided into two currents, one
flows along the thermocline with fine particles, the other flows down along the bottom of thereservoir with heavy particles. The period of turbid discharge outflow depends on the propertyof the two currents.
Yong and Lin (1991) constructed a vertical 2 dimensional model used the finite element method.
Their model reproduced the turbid current in the stratified reservoir, as well as the variation of
the water temperature.
However, this kind of studies is conducted only for middle and small scale of the flood. Theturbid currents induced by a big scale of flood are rarely studied because the countermeasures
are not considered to be effective.
Therefore, we construct a numerical simulation model first, which can express the diversity ofthe turbid inflow along the thermocline. Then, the flow activity of a big scale turbid current andthe behaviors of long term turbidity problem are numerically analyzed. Finally, we provide a
discussion on the method to treat the long-term turbidity problem.

2. MODEL FORMULATIONThis model deals with two-dimension vertical flow and suspended sediment in reservoirs. The
governing equations are based on the uncompressible concept. It consists of the continuity
equation and the momentum equations, which read as:
wu wv
wx wywu
wu
wu
v
uwx
wywt

wv
wv
wv
u v
wywx
wt

g 

1 wp

U wx

1 wp

U wx

0w
wu
w
wu
( Ax )  ( Ay )
wx
wy
wywx
w
wv
w
wv
( Ax )  ( Ay )wx
wywx wy

(1)

(2)
(3)

The water quality model consists of the state equation of temperature (T) and suspended
sediments (C).
wTwTwT
uv
wx
wywt

Hw
wTw
wT( Dx)  (Dy)
wx
wyUC w
wx
wy(4)

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THEME C: Density Currents and Stratified Flows
wC
wC
wC
v
u
wywx
wtw
wCw
wC( Dx)  (Dy)Swx
wywx
wy

(5)

where H and S are the source terms.

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80100Fig. 1 Comparison of the calculation results with the field data

Elevation(m)This model has been successfully applied to more than 50 dam-reservoirs in Japan to simulate
long term turbidity problems (Chen et. al, 2000). Nearly all applications reproduce thetemperature and turbidity very satisfactorily. Fig. 1 gives an example of simulation result fortemperature and turbidity, in which the simulated results are compared with the field data.
Because the sunlight is changed periodically, the water temperature near the surface varies
periodically as well. Rayner (1980) has shown that the water in reservoirs is stratified and thetypical diurnal and seasonal thermoclines are usually formed. Fig.1 suggests that a thermocline
is formed from the beginning of April to the end of September for Dam A in Sikoku region,
Japan. The elevation of thermocline and the temperature difference between the upper and thelower layers of the thermocline are varied during a year. The temperature difference depends
mainly on the variation of the intensity of sunlight. The elevation of the thermoclines is usuallyinfluenced by the inflow, outflow, wind-stress, sunlight, etc. On the other hand, the profiles ofturbidity show that the turbid current flows along thethermocline because of the withdrawal
from surface water.
Then, the division of turbidity current at the thermocline, as an important phenomenon of
inflow turbidity in the stratified reservoir, must
0km
1km
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7km
be reproduced well to present a big scale of206
flood. Because the long term turbidity problem
194
is depended on the inflow location of the182
turbidity currents. Fig. 2 gives an example of170
turbidity division flow Dam B. The figure
158
suggests that this model can reproduce well this146
phenomenon.
Fig. 2 The division flow at the thermocline

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3. BEHAVIOR OF LARGE TURBID CURRENT
When the upper layer (eplimnion) over the thermocline is shallow, the flood can easily
destroyed the thermocline and a lower layer turbid current can be generated.
Up to now, this kind of turbid current is treated as a big scale flood current, and thecountermeasures to prevent the long term turbidity problem is considered to be almost
ineffective. For example, the turbid current of Dam C shown in Fig. 3 generates a long term
turbidity problem from the flood period to the next March (over 200 days). Here the prolonged
turbidity problem is defined as the period of the outflow turbidity is over 10 degree according tothe inflow suspended sediment concentration.
In order to explain the mechanism of this phenomenon and find a method to solve theprolonged turbidity problem, the dynamic behaviors for the space-time variation of turbid
currents are shown in Fig. 3.
As shown in the figure, the turbid current flows down along the bottom of reservoir before theflood peak (Aug. 10th, 12:00), so that some clean water are left near the surface layer. Whenturbid current comes to the dam-site, the flow is lifted due to outflows released from the surface
layer and the mid-layer. As the result, the turbid water appears at the whole dam-site (Aug. 10th,13:00-14:00).
During the decreasing period of the flood discharge, the suspended sediments in the surface
layer begin to return to the lower layer due to the existence of gravity. Then a backward flow tothe upstream appears due to the sedimentation of the turbidity material (Aug. 10th, 17:00).
When the backward flow in the bottom meets the downstream flow induced by the inflow at thelocation near the thermocline, a part of the turbidity water in the backward flow is rolled into
the upper layer and the relative clean water in the upper layer is mixed (Aug. 10th, 21:00). The
left part of turbid current flows back to the dam site again because of the gravity (Aug. 10th,23:00). After many times of flowing forward and backward before the lower layer flow is settle
down, the suspended fine particles are spread to the whole region of the reservoir. Particularly,
the fine particles in the upper layer will stay in this layer for a long period and induce theprolonged turbidity problem.

4. METHOD TO SOLVE THE PROLONGED TURBIDITY PROBLEM
4.1. DISCUSSION OF THE INFLOW LOCATION OF THE TURBID CURRENT AND
THE STRATIFICATION
Now, we try to find a method to reduce the period of turbidity outflow induced by a big scale offlood based on the analysis in the above. First, we take a discussion on Dam C, which shows a
prolonged turbidity problem in the above.
Since the outflow water is usually drained from the surface for Dam C, the thermocline is
generated in the shallow layer for the actual case. In order to make the location of thethermocline to be formed in the mid-layer, we assume that the outflow water is not taken fromthe surface but from the middle stratum and predict the variation of turbid current. The results
shown in Fig. 4 suggest that the thermocline is located near the flood conduit if the outflow isusually taken at this location. Then, the activities of the turbid currents for these two cases arecompared in the followings.
As mentioned above, the stratification is destroyed by the large turbid current when the water isdrained from the surface. On the other hand, the turbid current with fine particles flows
downstream along the thermocline if the water is usually taken from the middle stratum.
However, turbid current with heavy particles flows down along the riverbed still. In that case,turbidity and the temperature in the lower layer are increased after the flood, but there is a peakof suspended sediment in the middle stratum near the thermocline. It suggests that the change ofTHEME C: Density Currents and Stratified Flows393

the thickness of eplimnion will affect the inflow situation of the turbid current.
The variation of the outflow turbidity in this situation is shown in Fig. 6. From the figure, youcan read that on occasion of taking water from the mid-layer in general time, the outflow
turbidity becomes high during the flood, but the period of high turbidity outflow becomes short
after the flood. That is to say, since the location of the high turbidity appears at the mid-layer andmuch more suspended sediment load is released from this location during the flood in the earlystage of flood. It is effective to shorten the period of high turbidity drainage.
4.2. A CASE STUDY TO TREAT THE LARGE TURBID CURRENT
In many cases, the storage capacity of the reservoirs in Japan is around 50 million m3. In order toavoid the cold-water released from the reservoir, discharge is usually taken from the surface. For
this case, a shallow upper layer with high temperature is formed so that the flood flow will
destroy the thermocline easily and form an under layer flow. As the result, the flow and thestratification situation is brought into a dynamic state and the turbid materials are mixed in thewhole reservoir. General, this kind of flood occurs once per two years, the volume of the flood ismore than 50% of the capacity of reservoir. Beside of it, the flood is usually outlet from the crest
near the surface, or the conduit at the middle layer, so that the under layer turbid current will
clash the dam and a strong turbulent mixture is induced. As shown in Fig.6, the volume ofturbidity water is increased to 45 million m3 in the reservoir even if the inflow turbid current of
the flood is just 30 million m3. This can be considered as the main reason of prolonged turbidityproblem
In order to improve the prolonged turbidity problem, two methods can be used. One is to reducethe mixture of the turbidity water and the clean water in reservoir; the other is to reduce theturbidity volume before the turbid current clashes the dam. For the after case, you can set a floodoutlet, such as a withdrawal device or a conduit, at the lower layer of the dam so that a part ofturbid current can be released from it. Fig. 7 shows the results of a case study on this case. We
just suppose that the discharge released from the lower layer by a selective withdrawal device is
100m3/s, 200m3/s, or 300m3/s, the variations of the turbidity of outflow discharge are predicted
and compared for the above 3 cases. The results suggest that the high turbid period of outflow isdecreased from 76 days to 9 days (88% of the actual case can be improved) if a lower layer
outlet is set. Of course, this is just a case study, but we can find the possibility ofcountermeasures to improve the prolonged turbidity problem.

5. CONCLUSION
On the basis of the numerical stimulation, this paper treats the phenomenon of turbid current in
the dam reservoir. According to the analysis, the following conclusions can be drawn.
1) In the dam which outflow discharge is often taken from the surface, the outflow location
should be changed to the mid layer near the flood outlet (ex: conduit) if the outflow watertemperature can be ensured. For that case, the turbid current will be induced to the mid layer
so that the prolonged turbidity problem will be improved.
2) It is confirmed that the prolonged turbidity problem becomes a serious problem if a big scale
flood current occurs. In this case, the stratification in the reservoir is reformed and a lower
layer turbid current is generated. The turbid materials induced by the turbid current will bemixed with the water around it after the turbid current clashes the dam. The lower layer
withdrawal equipment (such as outlet or a selective withdrawal device), as a countermeasure,is effective to release the turbid current and to improve the long-term turbidity problem394

XXX IAHR Congress

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REFERENCESAki, S. and K. Shirasuna (1975) A Numerical Study on the Flow regime in Dam Reservoir,Journal of Hydropower and Hydraulics, Vol.134.
Chen, F.Y., N. Yoshida, H. Sekine, T. Hotta & K. Murata (2000) A Prediction Method andConservation Measures of Water Quality in Dam Reservoirs, Proceedings of the second
International Summer Symposium, JSCE, Tokyo, Japan.
Fukuoka, S., Y. Fukushima, K. Murata and K. Arai (1980) Experimental Study on Density
Currents Advancing into a two-dimensional stratified reservoir, Journal of JSCE,
Vol.293/II-1.
Rayner, K.N. (1980) Diurnal Energetics of a Reservoir Surface Layer, Joint Report:
Environmental Dynamics, Department of Civil Engineering, University of WesternAustralia, Report ED-80-005, and Department of Conservation and Environment, Western
Australia, Bulletin No. 75.
Young, D.L., and Q.H. Lin (1991) Density currents during a storm in Te-Chi reservoir of Taiwan,
Proc., 24th IAHR Conference of International Association for Hydraulic Research, Delft,
The Netherlands.

395

THEME C: Density Currents and Stratified Flowsinflow

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Elevation (m)Elevation (m)Distance (m)Distance (m)Distance (m)Fig.3 The prolonged turbidity problem induced by a lower layer turbid current

AUTh, Thessaloniki, Greece

XXX IAHR Congress396
surface outlet
m idlayer outlet

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Fig.4 Variation of water temperature(a) Surface drainage(b) Mid-layer drainage

Fig.5 The relation between turbid current and the outlet elevation
Turbidity for outflow

Turbidity at Damsite
m idlayer outlet

surface outlet

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Turbidity

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Fig.6 The improvement of the prolonged turbidity problem by a mid-layer outlet
Q =100m 3/s

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Surface outlet

50Turbidi

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Fig.7 The improvement of the prolonged turbidity problem by a lower layer outlet

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