Study of Water Quality Changes due to Offshore Dike Development Plan at Semarang Bay

Now, coast of Semarang Gulf is experiencing rapid growth because Semarang as a center economic growth in Central Java. On the other hand, coast of Gulf Semarang also experience a variety of very complex problems, such as tidal flood, land subsidence, as well as coastal damage due to erosion and sedimentation process. To overcome these problems BPPT and other institutions proposed construction of offshore dike. Construction of the offshore dike is a technology intervention to the marine environment that will certainly affect the hydrodynamic balance in coastal water including water quality in the Gulf of Semarang. Therefore, to determine changes in water quality that will happen is necessary to study the water quality modeling. The study was conducted by using a computational modeling software MIKE-21 Eco Lab Module from DHI. Based on this study result knowed that development offshore dike will change water quality in the west and east dam that formed. In west dam the average value of the DO decline 81.56% - 93.32 % and the average value of BOD rise from 22.01 to 31.19% and in the east dam, there is an increase average value DO of 83.19% - 75.80%, while the average value of BOD decrease by 95,04% - 96.01%. To prevent the downward trend in water quality due to the construction of the offshore dike, its necessary precautions at the upstream area before entering the Gulf of Semarang.


Background
Coast of Semarang Gulf is area that consist land and water areas along the coast that stretches from the mouth of K. Bodri-Kab. Kendal in the west, the coast of Semarang City up to the estuary of K. Wulan-Kab. Demak in the eastern part. This area has approximately 104 km long coast. Coast of Semarang Gulf is currently experiencing rapid growth. As a consequence of these developments and natural conditions this area, now coast of Semarang Gulf experiencing complex problems. The first problem that occurred in the Gulf of Semarang city is tidal flood and river flood. Now, if occurrence of high tides, most areas of the Semarang City especially coastal areas (+ 3,100 ha) will be floods [1].
The second problem is the occurrence of land subsidence. Land subsidence occurs in the range of 2008-2009 reached -12.4 cm, then increased in 2009-2010 became -20.4 cm and fell back to 10.5 cm in 2010-2011 [2]. According to the Indonesia Geological Agency, land subsidence speed range of 0-1 cm/yr (in the district. Tugu, West Semarang, Semarang, Central and East of Semarang) and 8-9 cm/yr (North Semarang and Genuk) [1].
The third problem is the occurrence of abrasion along the coastline due to the dynamics of the coast. Between the years 1972 -1992 in some places along the coast of Semarang back about 500 m, between the years 1992-2001 partially some beach experience abrasion large enough [3].
To overcome these problems has been several concept that proposed to regional governments both to Central Java Government and Semarang City Government. Concepts that are currently the most prominent is the concept of offshore dam and belt/ coastal dike along the shoreline and offshore dike concept proposed by BPDP-BPPT. These concepts beside to overcome tidal flood, land subsidence and coastal erosion, also offer solutions of transportation infrastructure development.
Construction of coastal infrastructure such as offshore dikes that proposed by BPPT is a technology intervention to the marine environment that will certainly have an impact on coastal dynamic balance includes the impact on water quality in the Semarang Gulf. Until now the concepts proposed for addressing concerns in the Semarang Gulf has been no systematic study of the impact on existing water in particular the water quality. Therefore this study will be conducted studies on changes in water quality that would occur if the construction of the offshore dike actually built.

Location and period
The area of this study is along the Semarang Gulf that stretches from the mouth of K. Bodri in Kab. Kendal, the coast of Semarang City until the estuary of K. Wulan in Kab. Demak (figure 1).

Figure 1. Area of study
The period of this study is the rainy season (January) with consideration the data required for modeling is complete compared to other months.

Method
The main step in this study is field surveys and computational modeling.

Steps of Study
The stages of the activities to be undertaken are as follows: • Literature and secondary data collection; field surveys, including bathymetric surveys, sampling and basic sediment drift, currents measurements and tidal measurements • Analysis of secondary, primary data and preparation of scenario computational modeling • Computational modeling and analysis the modeling results

Water Quality Modeling
Numerical advection-dispersion modeling was conducted to determine the distribution of water quality parameters that wants to be reviewed, based on hydrodynamic conditions of local waters. The   [4]. Builder equation for water quality modeling based on the advection dispersion process. Dynamism of a substance to be modeled in the advection process can be expressed by the equation of its transport, which for non-conservative form can be written as follows [4]: (1) where

Result and Discussion
Discussion of the results of water quality modeling is done per-scenario to try to compare the existing condition and ultimate. Existing condition is a condition that exists today (in the absence of giant dike and islands reclaimed) and the condition after the construction of the offshore dike proposed by BPPT.

Modeling Scenarios
Generally, scenarios is divided into three major parts of the existing condition and the condition after the construction of the offshore dike according Concept of BPPT 1 and BPPT 2 (see figure 2). Due to limited data and time available, simulating is only done for the west season (represented by the month of January 2014), because the existing data for the month of January 2014 is more complete compared with the data in September 2014 so expect the resulting condition can approach the actual conditions. Simulation chosen dates 1 to 11 January 2014, with the consideration that this happens timescales low tide and high tide.  To determine the changes in more detail that caused by infrastructure development like BPPT 1 concept and BPPT 2 concept, it needed extraction at 4 locations, namely : outside the west dam; inside the west dam; in the navigation channel Tanjung Mas; outside the east dam (see figure 4). And to know changes in the value of the DO and BOD during the simulation process (time series) then made extraction at several points for observation (see figure 5).   particularly at positions 1 km of the coastline (see figure 6.a). This is probably occur caused of the movement of water due to the lack of infrastructure that can improve the aeration process at locations in the west infrastructure.
The sea waters at the west, that according to the concept of BPPT 1 and 2 will be formed dam, a decline in the average value of DO is significant and bit an increase in the average value of BOD (see figure 6.b). The decreasing DO value occurs because the presence of dam cause the waters became calm because it is not affected by tidal and wave from the offshore waters so the aeration process is much reduced resulting in dissolved oxygen is also reduced. While the BOD value will raise because with the dam organic substances polutant will be concentrated in the dam and can't spread to the open waters). The magnitude of decrease in the average value of the DO in the dam west this amounted to 81.56% (due to the BPPT 1 Concept) and 93.32% (due to BPPT 2 Concept) of the average DO value of the existing condition of 4.42 mg/l. As for the average BOD value increased by 31.19% (due to the BPPT 1 Concept) and amounted to 22.01% (due to the concept of BPPT 2) of the average value of BOD existing condition of 10.45 mg/l.
The waters in the shipping channel Tanjung Mas Port decline in the average value of the DO and the average value of BOD (see figure 6.c). A decrease in the average value of DO is due to the limited contact with the waters of the open sea so will reduce aeration process.. The magnitude of decrease in the average value of the DO in the waters around Tanjung Mas about to 35.79% (due to the BPPT 1 Concept) and 46.51% (due to the concept of BPPT 2) from the existing condition of 5:16 DO mg/l. As for the average value of BOD decrease about to 48.25% (due to the BPPT 1 Concept) and about to 19.79% (due to the concept of BPPT 2) from the existing condition of 23.27 mg/l.
The waters at the east, which according to the concept of BPPT 1 and 2 will be formed dam, an increase in the average value of the DO as well as a decrease in the average value of BOD significantly (see figure 6.d). Increasing DO value in the dam is caused by the high value of the DO of river water into the dam east though the process airation as tidal and wave did not happen because isolated by dikes. While the decline in value caused by the low BOD value of river water into the dam is compared to the value of the initial BOD sea waters before. The magnitude of the increase in the average value of the DO in deep waters east dam is at 83.19% (due to the BPPT 1 Concept) and 75.80% (concept of BPPT 2) of the average DO value of the existing condition of 3.17 mg/l. As for the average BOD value dropped by 95.04% (due to the BPPT 1 Concept) and about to 96.01% (due to the concept of BPPT 2) of the average value of BOD existing condition of 6.25 mg/l. An example of extraction results at some observation point such as outside the dam (point 1) and inside the dam (point 2) as a function of time will be discuss at below.
At the observation point outside the west dam is seen that the pattern of changes in the value of the DO was not significant (see figure 7.a). At the end of simulation, DO value changes less than 1%. It is caused at this location is still directly connect with the open sea as the existing condition. Slight changes seen in the BOD value (about 1.37%) due to the concept of BPPT 1 because of the shifting of the flow K. Siangker.
At the observation point in the west dam is seen that the pattern of changes are very significant (see figure 7.b). This is probably due to the existing condition is very dynamic beside the influence of the incoming river flow is also due to the tides, waves and wind. So with the concept of BPPT changes in the value of the DO and BOD become more stable does not fluctuate like when it is open to the ocean. At the end of the simulation, DO value decreasing about 97%. While the BOD values fell between 49.4% (due to BPPT 1 concept) and 65.67% (BPPT 2 concept). At the observation point no. 3 in the shipping channel of Tanjung Mas Port, shows that the pattern of changes in the value of the DO and BOD as a result of infrastructure development is significant (see figure 7.c). This is probably due to that the existing condition upheaval of sea waters at this location is large enough, the opposite occurs when it is already built infrastructure, at this location although it still can be in direct contact with the waters of the open sea, but the condition is relatively quiet because shielded from waves. At the end of the simulation decreasing DO value approximately 53.56% (due to  7.d). This is probably due to that the existing condition and has built the infrastructure can still be in direct contact with the open sea waters despite all effect is not large. Thus the fluctuations of DO and BOD values change as a result of this infrastructure is relatively uniform.
At the end of the simulation impaired DO approximately 1.92%. While the BOD values fell between 19.78% (BPPT 1 Concept) and 33.66% (BPPT 2 Concept).

Conclusion
Based on the result of this study, can be concluded as follows : • In general, the concept of BPPT I and BPPT II will form the dam that isolated from open waters, the input of water from river discharge will affect the quality of water in the dam formed. In the dam to be formed to the east of an increase in the average DO value of 83.19% -75.80%, while the average BOD value dropped by 95.04% -96.01%.