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Hydrodynamic Simulation In Bali Strait With MIKE21:

Pollutant DistributionTest Case

Muhammad Arief Rahman

Abstrac

A two dimensional numerical model were performed to know the ocean current pattern and

pollutant distribution due to the hydrodynamic processes in Bali Strait. A scenario were made

to investigate how is the behavior and how far it will be distribute if pollutant were included

in the simulation. Using the tidal elevation variation as the forcing at the boundary, the

simulation simulated the ocean current in spring and nipe condition for the whole area of Bali

Strait. Based on the model result, the ocean current flows to the south in flood tide conditon

and flows to the north in the ebb tide condition. In spring condition the ocean current reach

1.4 m/s and in the neap condition the current was 0.4 m/s. As the current pattern follow the

direction of the tidal propagation direction, the pollutant distribution behavior follow this

current pattern.

Background

Ocean currents are a mass movement of ocean water is horizontally due to the driving force

working on the sea water. The driving force is comprised of stes wind, pressure gradient,

ocean waves and tides. Movement of tidal current, there are three types, namely rotational

motion, the motion has changed direction, and hydraulic type. Off at sea, tidal current motion

is rotational elliptical motion which is in line with the direction of rotation clockwise and

counterclockwise. Tidal currents move continuously by continuously changing direction to

follow the direction opposite to the clockwise or clockwise in one tidal period.

To simplify the processing of the data, a numeric model called MIKE21 was used, which is a

program that can model a 2-dimensional surface flow system by being able to simulate the

hydraulic phenomena such as in lakes, estuaries, bays, coastal, and marine. In the

workmanship used MIKE21 flow modules FM models with ability can model with flexible

mesh approach based system that was developed to be applied in the field of environmental,

marine and estuary.

The size of the finite elements used can be adjusted to the user. Usually set to the size of the

finite elements in high seas areas is greater than in the area that is to be examined. With such

arrangement thereby causing calculations can be performed more effectively than the size of

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the finite elements are the same in all areas so that the simulation time can be

shortened.Location

Purpose

The purpose of this modeling was to determine the surface pollutant distribution that occurs

inside the Bali Strait so that the behavior of the distribution can be seen and to find out how

far the pollutant will spread from its source.

Location

The location of the domain area for the model is at Bali Strait, Between the Java Island and

Bali Island, 8.131 – 8.765 South Longitude and 144.34 – 115.169 East Latitude. The area was

shown in the following maps in Figure 1

Figure 1. The location of the domain area for the model is at Bali Strait

Methodology

Mike21 developed by DHI (Danish Hydrodynamic Institute), is a modelling system for 2

dimentional, free-surface flows, and is used for many purposes. The module used is

Hydrodynamic Module that have Full non-linear equations of continuity and conservation of

momentum, ADI finite difference solution of second-order accuracy, Smagorinsky eddy

formulation and Advection-dispersion module that have Conservation of mass equation,

Transport of conservative, heat dissipating and linear decaying matter.

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All primitive equations in Hydrodynamics scheme from the three-dimensional Navier-Stokes

equations used are:

Continuity

Momentum

Free surface condition

Model Domain

Models are made with the study area is a strait Bali (8.131-8.765 South Longitude and

144.34-115.169 East Latitude) reviewed as a channel with a depth of program data obtained

from ETOPO global releif data. Data was simulated for 2 weeks taken in 15 May 2015 until

30 May 2015. Pollutant was placed in a position in the Jembrana (8.43 South Longitude and

114.82 East Latitude) with continuous pollutant concentration of 100 mg / l. CFL value used

as a condition for the stability, dx dt worth 100 meter with 3600 seconds and assumed the

discharge of pollutants worth 5 m3/s and the dispersion coefficient is 5 m

2/s.

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Figure 2. Domain bathymetry and pollutan source

Input Data

The input data for the model is the tidal elevation prediction data from the global data Tide

Model Drive (TMD) that process in Matlab. pollutant concentration of 100 mg / l. CFL value

used as a condition for the stability, dx dt worth 100 meter with 3600 seconds and assumed

the discharge of pollutants worth 5 m3/s and the dispersion coefficient is 5 m

2/s.

Simulation and Result

Tidal Elevation

The elevation due to tidal variation shows in figure x.x we can see that the tidal in Bali Strait

occurs two times in a day or the tidal type is mix tide. In the spring period the tidal range

reach 2 meter with the highest tidal elevation in flood period is 1.1 meter and the lowest tidal

elevation in ebb period is -1 meter. In the neap period the tidal range reach 0.7 meter with the

highest tidal elevation in flood period is 0.3 meter and the lowest tidal elevation in ebb period

is -0.7 meter.

Pollutant Source

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Figure 3. Tidal elevation for input in the open boundary condition

As we can see, the tidal wave in flood period propaget from the south to the north (Figure 4)

and in ebb period the tidal wave propaget from the north to the south (Figure 5).

Figure 4. tidal wave in flood period propaget from the south to the north

Figure 5. ebb period the tidal wave propaget from the north to the south

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Ocean Current Pattern and Pollutant Distribution

2-dimensional hydrodynamic modeling results presented in Figure 6 and 7. At low tide

conditions, the current is moving southward toward the Indian Ocean and at high tide and

currents moving north toward the Java Sea.

Figure 6. low tide conditions, the current is moving southward toward the Indian Ocean

Changes speed very large currents in both conditions. It is caused by tidal forces that work

led to mass movement of water. Changes occur due to differences in the speed of movement

of the bathymetry and water level.

Figure 7. high tide and currents moving north toward the Java Sea

In the northern area of the Strait of Bali, current speed fluctuation due to differences in the

topography where the northern part of the strait width narrower than the width of the

southernpart of the strait.

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Figure 8. current speed fluctuation in the northernparth of Bali Strait

In the spring tide period, current speed at high tide reaches 1.4 m / s in the northern part of

the strait and 0,04 m / s in the southern part of the strait and the velocity at low tide reaches

1.2 m / s in the northern part of the strait and 0,02 m / s in the southern part of the strait. In

the neap tide period, current speed at high tide reaches 0.3 m / s in the northern part of the

strait and 0,01 m / s in the southern part of the strait and the velocity at low tide reaches 0.2 m

/ s in the northern part of the strait and 0,01 m / s in the southern part of the strait.

Figure 9. current speed fluctuation in the southernpart of Bali Strait

Pollutants are incorporated into the model area is continuous and moves affected by the

process of the movement of ocean currents advection and diffusion processes due to the

difference between the concentration of pollutants in the water. It can be seen from the

comparison between the first day until day 7, the spread of pollutants is more inclined to the

southeast. On the first day of broad distribution of pollutants ranging from 0.013 km2, on the

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second day with a wide spread of pollutants extends 0.062 km2. On the fifth day with a wide

spread of pollutants 0.13 km2 and on the seventh day broad distribution of pollutants reaching

0.18 km2.

Figure 10. Pollutan distribution in day 1, 2, 5 and 7.

If the area of distribution of the four area known earlier plotted in one graph, the increase in

the area of distribution of pollutants over time can be described by a linear equation with the

equation y = 0.056x - 0.045.

Figure 9. increase in the area of distribution of pollutants over time can be described by a

linear equation

D D

D D

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Conclusion

From the simulation result is that for simulation of hydrodynamics in the waters of the Strait

of Bali which is influenced by the tides, obtained during spring tide riding the elevation can

reach 2 meters at a speed of tidal current reaches 1.4 m / s in the north and 0,04 in the south

and at the neap tide the elevation reaches 1 meter with a speed of tidal current reaches 0.3 m /

s in the north and 0,01 m / s in the south. Distribution of pollutants due to advection and

diffusion processes more dominant moving towards southeast with increasing area of

pollutants on time can be expressed by the equation y = 0.056x - 0.045.

References

Klein, R., 1998, Hydrodynamic Simulation With MIKE21 Of Mele Bay And Port Vila

Vanuatu, SOPAC Technical Report 263

Saputra, J., Purwanto, A. Ismanto, 2015, Studi Pola Arus Dan Potensi Energi Arus Laut Di

Perairan Ujong Pancu Aceh Besar, Jurnal Oseanografi Volume 4 No. 2, 492-498

Surbakti, H., M. Purba, I. W. Nurjaya, 2011, Pemodelan Pola Arus Di Perairan Pesisir

Banyuasin Sumatera Selatan, Maspari Journal 03 (2011) 09-14