沙漠研究 22-1, 91 -94 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－

Sedimentation Trend and Behavior of Turbid Water in the Reservoir
Mitsuteru IRIE*1), Atsushi KAWACHI1), Jamila TARHOUNI2), Ahmad GHRABI3 ) and Hiroko ISODA1)
Abstract: A half of the water demand for agriculture and urban water supply is covered by surface water resources even in arid land,
however the capacity of the reservoirs is gradually degraded due to sedimentation. Therefore, sedimentation control in reservoirs is
an important issue in the sense of sustainable surface water resource management. In the case of North African countries, due to theclear precipitation difference between rainy season in winter and dry season in summer, the flood water in rainy season is stored for
irrigation and potable use in the dry season. This longer retention time than humid area causes more rapid sedimentation.
In this study, the detailed behavior of turbid water which is discharged from upper catchment in rainy winter season is discussedwith a numerical simulation model applied to a reservoir in northern Tunisia. Because of the low solar irradiance, thermal
stratification was not found in the water body in winter and turbid water flowed along the bottom of the reservoir as a density flow.
Understanding the speed and thickness of this density flow is very important for proper management of reservoirs, whether the turbid
flood water can be removed by opening the spill way or other measures. The numerical simulation was carried out with somedifferent cases of density of inflow and intake rate at the dam, which define the thickness and speed of the bottom flow.
Key Words: Density flow, Flood, Reservoir, Sedimentation, Turbidity
and the subsequence spatial distribution of sedimentation wasaffected. The sedimentation on downstream side (P1-P5) wasdominant from 2000 to 2009 (Irie et al., 2011)

1. Introduction
Securing stable water resources is the most important issue
in arid areas. However, the degradation of surface water
resources in arid land is very serious due to sedimentation inreservoirs. For example, in North African countries, theannual reduction rate of water storage capacity reaches 0.5% in
Morocco and Algeria, and 1% in Tunisia (Remini, 2006).
The spatial distribution of sedimentation in a reservoir is
important for the discussion of the countermeasures tosedimentation. The behavior of the turbid water in the water
body determines the spatial distribution of sediment in thereservoir.
In this study, the effect of the change of the discharge
amount from reservoir to downstream on the behavior of theturbid water is discussed by numerical experiment with ahydraulic simulation model.

2.2.Simulation model
The behavior of turbid flood inflow in the reservoir isdiscussed with a numerical simulation model. ELCOM(Estuary and Lake Computer Model) developed by the Centrefor Water Research at The University of Western Australia(Hodges, 2000) is a three-dimensional hydrodynamic model
used for predicting the velocity, temperature and salinity
distribution in natural water bodies subjected to external
environmental forcing such as wind stress, surface heating or

2. Materials and Methods
2.1. Study site
Joumine dam is located on the northern side of Tunisia.
This reservoir is known as where the environmental discharges
were started 10 years ago for the conservation of Ichkeul wetland (Djebbi, 2009). Due to this change of the managementof Joumine dam, it is inferred that behavior of turbid water
which inflows to the reservoir when floods occur is changed

Fig. 1. Comparison of the cross sections of Joumine dam between
1986, 2000 and 2009.

* Corresponding Author: irie.mitsuteru.fu@u.tsukuba.ac.jp
Tennodai 1-1-1, Tsukuba, Ibaraki, Japan
1) Alliance of Research on North Africa, University of Tsukuba, Japan
2) National Institute of Agronomy Tunisia

3) Centre de Recherches et de Technologies de Eaux, Technopole de Borj-Cedria

Fig. 2. Gridding of bathymetry data for simulation.

cooling. The bathymetry of Joumine dam is formed with aCartesian 3-dimentional grid.
The above contour map is converted to the raster grid map
with 100 m × 100 m square cells for running the numericalsimulation model. The number of cells is V 40 × H 66.
2.3. Period for the numerical experiment
For the numerical simulation, the actual river inflow which
was calculated from the record of water level fluctuation andwater exploitation (outflow) of the dam was used as theboundary condition of inflow. The selected period was thebeginning of 2010. Figure 3 is the water level fluctuation of
the dam and the inflow time series calculated from water level
and water exploitation. This period is the water level risingphase in the rainy season in this area, especially in 2010, thewater level reached to the maximum water level of the dam.
Regarding thermal stratification due to the solar radiation, thisperiod is transition phase from mixed cold single layer todevelopment phase of stratification. Thus, the three floods
shown in the time series of inflow might have different
behavior depending on the stratification condition. In themodel calculations, the three kinds of traces are released frominflow in the discharge increasing phase in order to evaluate
the behavior of the flood water shown in Figure 3. The
intensity of the tracers at the beginning of inflow is1(dimensionless relative number) and keeps constant for 1-2
days. These experimental tracers are transferred with mixing
and dispersion and draw the movement of the turbid water.
2.4. Assumption for simulation model application
The simulation model considers buoyancy effects of thedifference of density as defined by temperature and salinity.
Suspended solids are also one of the causes of density

Fig. 3. Time series of water level, inflow rate and intake during thestudy period

differences especially in the flooding term, but this model
doesn’t include the effect of suspended solid. In this study,
the salinity takes the place of the suspended solid fordescribing the difference of the density in the simulation
model.
The density of turbid water is described as follows:ρ = ρ f 1 +c11 (−)
1000 ρ fρ s 3(1)3

ρ f (Mg/m ): Density of water, ρ s (Mg/m ): Density of solidmatter, c: Suspended solid (‰). Here it is assumed that thedensity of water and solid matter is 1.0 and 2.68 Mg/m3
respectively. The density of saline water at t = 0°C is
described as follows.
ρ =1+ σ0
= 1 − 0.069 + 1.4708Cl − 0.001570 Cl 2 + 0.0000398Cl 3
S = 0.030 + 1.805 Cl

( 2)
(3)

Where S is salinity (‰) and Cl is chloride concentration (‰).
Calculating those equations and eliminating small factors(squared and cubed term in(3)), the equivalent salinity to thesuspended solid density is described as follows.
S = 0.758 c(4)The actual salinity of the inflow and the water body of thereservoir is almost the same, but the salinity concentration
which is equivalent to the density of turbid river water is given
for the flood inflow in this simulation.
The suspended solid and temperature of inflow have notbeen observed in the field. The temporal fluctuation ofTable 1. Simulation cases of inflow density and intake rate.
RelativeConc.

Concentration

Intake rateIntake(m3/sec)

A1aActual intake (ave. 1.72)

B

0.75

b2.5

C0.5

c5.0

D0.25

d10.0

Actual time series of the intake that of the average is 1.72
m3/sec is shown in Figure 3. For the numerical experiment,
the conditions of constant intake, 2.5 m3/sec, 5 m3/sec and 10
m3/sec are also examined. The cases of the condition areshown in Table 1.

3. Results and Discussion

Fig. 4. Behavior of Tracer 3 seen on Lateral cross section along thedeepest line of the reservoir. (a) 28hr, (b) 40hr, (c) 76hr, (d)95hr from the start of flood. Inflow density and intake condition
is A. a. of Table.1

3.1. Outline of behavior of turbid flood water
Figure 4 shows the snap shots of the distribution of Tracer
3 released from the inflow river while the increasing phase of
the inflow of the 3rd flood in the study period, in the case of A.
a. of Table 1. It could be found that panel (a) the flood water
inflows along the bottom. The thickness of the density low
layer is around 6-8 m. In the reservoir, there is contraction
narrow section. Panel (b) shows that the narrow point dams
the density flow and thickness of the density flow is increased.
After reaching the dam body, the water is reflected, found inpanel (c) and raised up to the interface and reversed to themiddle along it, shown as panel (d).

salinity concentration which is given instead of suspended
solid for describing the density difference in the model is basedon the simple assumption that suspended solid is direct
proportional to the inflow rate. Temperature of the inflow isgiven as the same as the air temperature which is measured atBeja city located in the catchment area of the inflow river andthe distance between these two sites is about 40 km. The
meteorological data of Beja are used for the simulation.
In this study, the dependency of the behavior of flood wateron inflow density is discussed with the comparativeexperiment. The different cases of inflow density areprepared. Figure 4 shows the standard time series of inflow
salinity concentration. This series is equivalent to theturbidity level which is around 4000 ppm at the peak of theflood. Based on this standard time series of suspended solid,
the cases of inflow conditions which are relative concentrationof 0.75, 0.50 and 0.25 than the standard time series wereprepared.
The other aim of this study is to determine the influence of
the intake at the dam body on suspended solid transportation.
Therefore, the different intake conditions were examined.

3.2. Dependency of behavior on inflow density
In order to evaluate the dependency of the turbid water
behavior on the densities, the vertical profiles of different
concentration conditions are compared in Figure 5. Theeffect of the difference looks not so strong except the case ofConc. D (0.25). This lowest concentration case has a weakinterface effect of the density difference and diffusion andmixing vertically. In addition, the current of the bottom
density flow is significantly slower than the others. Thedensity flow doesn’t reach at (iii). The difference of thedensity between pure water and the peak density of the Case(Conc. D) is estimated by the equation (1) as 4.66 × 10-4
Mg/m3 at the peak. On the other hand, the difference oftemperature between inflow water and water body in thisseason is around 2°C. It makes around 2.00 × 10-4 Mg/m3 ofdensity difference while there is the thermal difference of 10°C
between the surface layer and the bottom layer of thestratification in summer season, which makes around 2.00 ×
10-3 Mg/m3 of density difference. Thermal stratification insummer makes significant relative buoyancy and river inflow
intrude the interlayer of the thermal stratification. ComparingFig. 5. Vertical profiles of the bottom density flow for the comparisonof the bottom density flow behavior.

increment influences the suspended solid transportation of thereservoir and enhances the sedimentation on the lower side.
In addition, there is a possibility of another effect of intake
on the sediment transportation. After flooding, the turbid
water remains the deepest point beside the dam body andbuilds the density stratified layers. As mentioned in 3.1, flood
water is raised up to the interface which is produced by thepreceding flood and reversed to the middle part of reservoir.
On the other hand, the intake shrinks the size of the stratified
lower layer. It shortens the distance of the interface and thereversed flow which might transport suspended solid. As aresult, suspended solid might precipitate on the near side of thereservoir.

4. Conclusion

Fig. 6. Traveling time of flood water from St. 1 to St. 2.

with that situation, the relative buoyancy due to thermal
difference in winter is not so strong and the density difference
originated from suspended solid is dominant. If thedifference of the density is less than 1.00 × 10-3 Mg/m3, theinterface effect of the density difference is degraded and thediffusion of the turbid flood water on the bottom slope of thisreservoir might be developed gradually.
3.3. Effect of intake condition
Figure 6 shows the change of the reaching time of flood
water from St.1 to St.2. The reaching time is measured by thetraveling time of the tracers. The timings when the relative
concentration of the tracers rises at the bottom of each point are
observed in the simulation and its difference of the time at eachpoint indicates the traveling time of the flood water.
As mentioned in 3.2., the case of Conc. D shows thedifferent level of the interface strength, so that the travelingtime is significantly longer than the others. However, inevery case, the reduction of traveling time of around 12 hours(equivalent to 20-30% of the traveling time under actual
intake) is found in the case of Intake. d (10 m3/sec). With theconsideration of average particle size of the sediment (Irie et al.,2011), the precipitation rate of the sediment particle is around12 hours/m depth at the same level as the reduced travelingtime. Therefore, there is the possibility that the intake

In this study, the overview of the turbid flood water
behavior in the study site reservoir is described by thenumerical simulation. It clarified the spatial and temporalscale of the bottom density flow in the reservoir. However,the detail of the suspended solid transportation cannot bediscussed due to the capability of the simulation model.
Based on the results, in further study, the field observation suchas fixing sediment trap will be carried out and the numericalsimulation including sedimentation process will be applied.
AcknowledgementsThis research was partially supported by the Ministry ofEducation, Science, Sports and Culture, Grant-in-Aid for
Scientific Research (B), 2010, 22404009.and JST/JICA,SATREPS.

References
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