Prediction of sediment transport due to detached breakwater

This research intends to predict the pattern and accumulation of sediment transport before and after being built the detached breakwater. This research was conducted on Matras, Penyak, and Pasir Padi shorelines which are located in Bangka Island-Indonesia. Based on the previous research, few empirical methods are used for analyzing the shore response due to the breakwater for fulfilling the erosion and sedimentation in the shoreline. However, there has not been more research that analyzed the effect of breakwater itself on the pattern and accumulation of sediment transport (the formation of tombolo and salient) that happens in the shorelines based on the parameters of structure, physical condition, and characteristics of sediment. The research methodology consists of collecting data, measurement on the field, and Citra satellite observation without and with breakwater. The result is hoped can support the modelling of the distance among breakwaters and the effective formulation of the distance among breakwaters.


Introduction
Shore protection is an effort to protect and secure the shore area and river estuary from damage due to erosion, abrasion, and accretion (PUPR Ministry Rule No 7, 2015).Beach protection can consist of artificial protection, natural protection, and adaptation.However, artificial protection is what is meant is concluding soft structure, hard structure, and a combination of soft and hard structure.A shore building is a structure that is used to protect the shore from damage due to wave and current attacks [1], [2].
One of the shores-protected structures that are the best from wave attack is breakwater [3], [4].Breakwater is a shore-protected construction in which the position is aligned or about aligned to the shoreline [5], [6] with the aim is to dampening the incoming waves (Notification letter of General Work Ministry No 07, 2010).Breakwater is designed to give protection to the shore which is located in the back by breaking the wave energy before reaching the shore [7].In designing construction in a shore area, it is necessary to attend to the volume of material that is transported and precipitated [8].Therefore, abrasion and sedimentation are necessary becoming as attention and will potentially become a serious problem if the impact begins to the impact for the lives of living beings.According to Rijn and Rachman [9], sedimentation is an event of sediment entering in certain watering area through water media and precipitating in the environment.Hydro-dynamic in the shore area [10] is influencing the happening of sedimentation distribution process on the shore.Based on previous research, there have been to carry out analyses to the effectiveness of using breakwater in fulfilling erosion and sedimentation [11] in the shore area.However, there are not many types of research that analyze the effect of the breakwater itself on the pattern and number of sediment transport (the forming of tombolo and salient) that happens in the shore area [12].
This research intends to predict the pattern and number of sediment transport after there is a breakwater building.

Materials and Methods
This research was carried out in Matras Shore, Penyak, and Pasir Padi Shore in Bangka Island-Indonesia.However, analysis of the pattern and number of sediments before and after breakwater, the effect of distance between breakwater, is carried out by using Google Citra satellite and the measurement in the field.
This study monitors changes in the coastline before the breakwater is built and after the breakwater is built.To determine the changes that occur, first determine the initial reference point for measurement.The initial reference point used is the coastline before the breakwater was built.This coastline will be considered a reference coastline and will be digitized using Google Earth so that it can be overlaid.After determining the reference coastline, the coastline is overlaid with the results of Google satellite imagery and comparisons can be made of historical coastline differences.Meanwhile, the latest data was measured in the field.Theoretically, sedimentation behind the breakwater structure will follow the pattern below to form a tombolo or salient.

Sediment Transport
Sediment transport total consists of 2-part components that are cross-shore sediment transport and alongshore sediment transport or littoral drift.The cross-shore sediment transport consists of off-shore transport, for instance, it is happened during a big wave or storm, and in the opposite direction that is on-shore transport that is going on during the normal wave condition.The transport in both directions happens in a significantly different mechanism and in a different time scale.Therefore, there are difficulties in the ability to carry out the calculation that gives the difference substantially.Transport to the sea direction is simpler from both of them and it tends to happen with the bigger speed and as a more regular process with the transport is approximately by the whole active profile.It is profitable because there is bigger technical relevance and attention in the transport into sea direction.After all, there is potential for damage to the structure and area losses.
The cross-shore sediment transport is sediment transport that happens near the shore zone, because it influences the coastal morphology process and as the determination to the place of the shoreline (defeat: to be erosion, advanced: to be accretion or stable relatively).The cross-shore sediment transport becomes one of the main considerations for shore structures located on the alluvial shore, to be caused by the sediment transport process can be hindered by shore structure construction like jetty, groin, or breakwater (and the kind of structure that is built and indented from shore to sea), or even it can be held in inlet and harbor pool.About the jetty or kind of structure, the impact of being hindered by sediment transport is shore expansion along transport upstream side (up-drift) and shore erosion along transport downstream (down-drift) of the structure.The impact that is happening can cause negative problems on the surrounding shore like structure damage, loss of area, and the following impact such as sedimentation of the harbor pool or shallowing of the cruise line.

Mode of Sediment Transport
Mode of shore sediment transport is classified into 3 types regarding the mechanism of sediment particle movement as follows: 1. Bed load 2. Suspended load 3. Wash load Bed sediment transport is moving by sliding, jumping, or rolling along the shore bed.In other words, during transport, there is contact between sediment particles and the riverbed continuously.Mode of bed transport happens during the current speed is not too big and it only happen for rough sediment particle (diameter between 0.2-2.0mm or less bigger).If the particle is soft enough (diameter < 0.2 mm) and the current speed is big enough, there is a turbulent flow so the soft sediment particle will be stirred and lifted to form the suspension and it is transported as the flown load sediment transport.In this process, the sediment particle can be transported in the suspension form at a far distance and precipitated in the new place after the hydro-dynamic condition in this place supports the happening of precipitation, e.g., in the harbor pool or bay.Wash sediment transport consists of very soft particles that are transported by water and usually do not come from the shore bed.Therefore, to estimate the sediment transport total, there is only bed load sediment transport including flown load sediment to be calculated, and wash load transport is ignored.Although conceptually, the classification between bed load sediment and flown load sediment can be carried out in the field, however, it is difficult to differentiate separately the two sediment transport modes.

Process of Happening the Transport
Longshore sediment transport is part of the Littoral Process which is the interaction result between wind, wave, current, tidal ebb, sediment (sand), and the other events on the long shore.The process of long-shore sediment transport can cause erosion that impacts the retreating shoreline (shore abrasion), or cause shallowing that impacts the advanced shoreline (shore accretion) which by the end is decreasing the shore function or structure.The happening of erosion or accretion on a shore is dependent on the sediment transport in this area.In a stable shore, it can happen the process of erosion and accretion during this area can build a shore structure.
The current that is happening in the littoral zone is caused by the combination of tide-ebb current, current due to the wind, current due to the temperature difference, mass density, and current due to the wave.Current due to the wave happens because there is the elevation difference of water level (the reason for setting up the difference is because there is a difference between waves depth along the shore.This current is the biggest in the center of the surf-zone area and is known as a littoral current.
Broken wave causes turbulence that lifts the bed load sediment to the top and mixes with water.Because there is a littoral current, this sediment is also getting carried away by the current that is moving along the shore.The sediment moving is mentioned as littoral sediment transport.
The current circulation that is caused by waves can depend on the shore geometry and also the wave direction.The current that moves aligned to shore is mentioned as a long-shore current, and the current concentration that moves from shore and stops in the sea center is mentioned as a rip current.
Wave is the main reason for sediment transport in the littoral zone.The big wave will be broken farther from shore, so: 1) the Surf zone is getting wide; 2) There is increasing sediment transport on the shore.
The littoral transport can happen by the way as follows: 1) bed load transport, and 2) suspended load transport.Generally, the two ways are happened together.The littoral transport can be classified into a) on-shore (S-on) and off-shore (S-off) sand transport; and b) long-shore sand transport (S-o).

Breakwater and the Fitting
Breakwater or crib is classified based on the type as follows: 1. Upright sight: plaster, caisson 2. Hypotenuse sight: rubble mound Based on the shape, is classified as follows: a. Perpendicular shore (for changing the direction of incoming current and wave) (Figure 1)

Results and Discussion
Pasir padi breakwater design is as follows:  4, 7, and 10.The shoreline change comes from Citra satellite Google, then there is made the graphic from Microsoft Excel that illustrates the initial condition of the shoreline before it is developed, and the shoreline change after is developed the breakwater.
The observation data of location-1: Pasir Padi shore that has the coordinate at 2°7'26.54" S and 106°10'35.70"E is presented in Figure 4.However, Figure 13 presents a graphic of abrasion at the Penyak shoreline before there is developed the breakwater, and Figure 14 and Table 6 present a graphic of shoreline change at Penyak after there is developed the breakwater.Based on the historical observation at Google Citra satellite, it is obtained that there is happened the shoreline retreat at Matras shore by the average retreat is 1.2 m per year; however at Penyak Shore is 4.05 m per year; and at Pasir Padi shore is 11.06 m per-year before there are the breakwater as presented in the Figure 6, 9, and 12. Therefore, in the scheme of shoreline protection at the shores, there are built breakwaters.
The development of breakwater at Matras shore, Penyak shore, and Pasir Padi shore will have a positive impact by improving the shoreline and giving the new mainland the tombolo or salient in the back of breakwater construction.It is proved based on the observation at Google Citra Satellite in the time range of pot breakwater development.Based on the Google Citra Satellite result, the Pasir Padi shore has the biggest sedimentation volume than the Matras shore and Penyak shore.It is caused by the area that is protected being long enough and the distance of breakwater from shore is farther than Matras shore and Penyak shore.In addition, the shoreline change at Pasir Padi due to the development of the breakwater is the fastest among the other 2 shores reaching 38.8 m per year on average.However, the Matras shore experiences progress an average of 5.7 m per year and the Penyak shore is 9.7 m per year on average.However, there is no sedimentation in Tombolo form at Penyak shore like at Pasir Padi and Matras, it is only the salient form.In addition, the length of the shoreline that is protected at Penyak shore is longer than at Matras shore but the volume of sedimentation at Matras shore is more than at Penyak shore.This is caused that the characteristic Matras shore is sandy, and the wave height is higher than the Penyak and Pasir Padi shores.
Although the sedimentation at Matras shore is less than at Pasir Padi shore, the effectiveness of Matras shore can protect and produce the sedimentation better than Pasir Padi shore.It can be seen from the length of the shoreline that is protected at Matras shore is about 1 km than Pasir Padi shore is about 3 km.The sedimentation volume that is produced by the Penyak shore is less than Pasir Padi shore although the length of the shoreline that is protected is relatively the same as Pasir Padi.It is caused by several things that are characteristic of Penyak shore is muddy, however, at Pasir Padi is sandy, the distance between the breakwater gap at Penyak shore is smaller than at Pasir Padi shore and the distance of breakwater at Penyak shore is closer to the shoreline than Pasir Padi shore.

Conclusions
Based on the observation result of the shoreline in Bangka Island before and after there is developed the breakwater at Pasir Padi, Matras, and Penyak shores, there is happened he shoreline change.Before there is developed the breakwater at Pasir Padi shore, the average abrasion was 11.06 m per year; at Matras shore, it was 1.2 m per year; and at Penyak shore it was 4.05 m per year.However, after there is developed the breakwater, it is happened to the Tombolo sediment transport pattern at Matras and Pasir Padi shores, and Salient at Penyak shore.Therefore, it causes the progress of the shoreline an average of 38.8 m per year at Pasir Padi shore; 5.7 m per year at Matras shore; and 9.7 m per year at Penyak shore.The development of a breakwater can add to the shoreline that was abrasive before.The shoreline changes pattern among Penyak Beach, Pasir Padi Beach, and Matras Beach are different.Some characteristics affect those differences such as the sediment load characteristic.For example, Penyak Beach has muddy sediment meanwhile Pasir Padi Beach and Matras Beach are sandy.Therefore, tombolo is easier to form in Pasir Padi Beach and Matras Beach.However, there are some other parameters such as the distance between the breakwater gap at Penyak shore being smaller than Pasir Padi shore and the distance of the breakwater at Penyak shore being closer to the shoreline than Pasir Padi shore.

Figure 6 .
Figure 6.Pasir Padi Shore, the observation location-1 However, Figure 5 presents a graphic of abrasion at Pasir Padi shoreline before is developed the breakwater, and Figure 6 presents a graphic of shoreline change at Pasir Padi after is developed the breakwater.

Figure 7 .Figure 8 .
Figure 7. Graphic of Abrasion at Pasir Padi Shoreline before there is Developed the Breakwater

Figure 9 .
Figure 9. Matras Shore, the observation location-2 However, Figure 10 presents a graphic of abrasion at Matras shoreline before is developed the breakwater, and Figure 11 and Table 4 present graphic and tabular data of shoreline change at Matras after is developed the breakwater.

Figure 10 .Figure 11 .
Figure 10.Graphic of Abrasion at Matras Shoreline before there is Developed the Breakwater

Figure 13 . 10 Figure 14 .
Figure 13.Graphic of Abrasion at Penyak Shoreline before there is Developed the Breakwater

Table 2 .
Pasir Padi Shoreline after there is built the breakwater

Table 3 .
Matras beach breakwater design

Table 4 .
Matras Shoreline after there is built the breakwater

Table 6 .
Penyak Shoreline after there is built the breakwater