Low Crested Breakwaters Using Geotube Alternative Measures For Beach Erosion Control Case Study of Pasir Putih

Beach, Anyer, Banten Province

D. M. Sulaiman1), M.E. Sudjana2), B.S.Prasetyo3} 1 Researcher in Hydraulic Engineering 2 Coastal Engineer Practitioner 1,2 Exp. Station for Coastal Engineering, Research Center for Water Resources Jl. Sapan No. 37 Ciparay- District of Bandung 3 Consultant and Contractor, PT.KPAM Jakarta Jl. Raya Pluit Selatan, Jakarta Utara 14440 e-mail: dedems@ymail.com, mernawan@yahoo.co.id; budiprasetyoid@yahoo.com

Abstract: Low crested breakwaters are well functioning if these structures shall protect the coast behind it with shoreline response showing progress into seaward direction. Retardation of waves that passes the structure shall enhance and increase the sedimentation process and deposit sediment along the coast. The two criteria of successful application of low crested breakwaters are only identified by continuous monitoring results when structure has been constructed. The sedimentation pattern observed at field prototype indicated at contour-2 an intensified retreat of coastline into beach direction that increased the steepness of beach slope. Erosion encountered at contour-1.5 is considered to cause increase of depth of contour line. Due to the erosion at sea bed, sediment is being transported into various directions, but most dominantly sediment is being carried by waves into the coast direction. The deposit of sediment at the coast has increased shoreline and steepness of coastal slope. Retreat of shoreline and increase of coastal slope can be reduced among others by setting up low crested breakwaters off-shore in a depth between -1m to -2m. This paper discuss test results of the low crested breakwater prototype that had used geo-tubes as its material. The field test was carried out at Anyer Beach in Serang, the Banten Province. Keywords : prototype, low crested breakwaters, coastal protection, geotube, Anyer Beach.

1.

INTRODUCTION

The coastal area is the very dynamic transitional zone between land and sea, continuously changing, where most of the water kinetic energy is dissipated by breaking waves, creep, and bed scouring. Dominant results of these processes are erosion and sediment transport in the coastal area. The littoral zone, is very important to the community as a source of socio-economic needs and to other living creatures a place of proliferation and source of foods. Thus, it is very substantial to preserve this strategic area and maintain it by considering the environmental aspects and sustainability. Coastal protection structure is of different type and function and had been developed since long. One of these methods is the breakwater structure which can be installed on-shore or off-shore. The offshore breakwater is a coastal protection structure constructed parallel with the coastline and functioning to dissipate wave energy before it reaches the shore. The use of offshore breakwaters in Indonesia is less common than other coastal protection structures, such as groins, revetments or sea walls. Some factors influencing this condition include among others high construction cost and aesthetic impact to the coast especially at tourist resorts. However, with some technological innovations, for instance on its dimension and material, laboratory and field tests have resulted a low crested breakwater or pemecah gelombang ambang rendah (PEGAR), then called as LCB, of reliable protection effectiveness and less construction cost than that of conventional breakwaters. At present,

more economical geotextile is being produced in large sizes like geotubes or pillow shaped sand containers, and is the material most appropriate for low crested breakwaters. The research and development of the LCB started in the year 2009 with some laboratory tests. Results were then tested on site by construction of a prototype on Anyer Beach , District of Serang, Banten province. The objective of the study include among others (1) determination of a LCB prototype in conformity with the characteristics of the coastal studied; (2) identification of wave reduction characteristics near the LCB based on various geometric shapes, configuration of placement, water depth, wave height and period; and (3) identification of the response of coastline changes due to construction of the LCB structure. Thus, successful results of this study and development of the geotube LCB shall be indicated by the wave reduction after passing the geo-tube structure and beach profile behind the structure. This paper presents analysis results of a LCB prototype, the application of which was made possible through the cooperation between the Research Center for Water Resources, Directorate of River and Coastal Areas, RBO of Cidanau-Ciujung-Cidurian/BBWS C3, and PT. Karya Prima Anugrah Mandiri Jakarta, as a contractor for geotube construction. The coastline response behind the structure and wave dynamic pattern after passing the LCB shall identify the success and effectiveness of structure application, whereas length and breadth of the coast formed due to the existence of the offshore structure is depending very much on its dimension. The discussion in this paper on an environment friendly low crested breakwater is expected to be used as reference for an alternative structure in coastal erosion control.

2.

LITERATURE REVIEW

2.1 Structure Dimension and Hydro-dynamic Process

The dimension adaptation of an offshore breakwater has resulted a low crested breakwater. Some literature show a trend of use of these low crested breakwaters in North America, Japan, and Europe (Durgappa, 2008). Moreover, in Japan, these type of breakwaters are more commonly used than the conventional structure (Pilarczyk, 2003). The use of low crested breakwaters can among others reduce some aesthetic problems, suppress construction cost, improve water circulation and simultaneously increase the water quality and biological productivity, and decrease obstructive effects of sediment transport. (Kularatne et al., 2008). Wave energy dissipating on beach can be achieved by constructing offshore breakwaters. These structures have adopted the natural protection principle of coral reefs. Big waves that attack the beach will be obstructed and are scattered before reaching the coastline so that wave energy is reduced. With the reduction of wave energy in the vicinity of breakwaters, sediment transport is also reduced and sediment deposit may occur (Figure 1).
Wave direction Breakwaters gelom salient

gap

gap tombolo Original coastline

salient

erosion

Figure 1 Typical of offshore breakwaters and coastline formed Submerged breakwaters are low crested breakwaters with initial crest elevation being below the sea water level. Such structure may probably not be effective during high tide and for effective results,

these breakwaters are to be installed at location during low tide. Main function of the low crested breakwater is to dissipate wave energy coming into the coast through a mechanism of breaking waves, dissipation, scouring, and wave reflection. The design of low crested breakwaters has thus to consider the transmission and reflection of waves flowing over the structure crest. Transmission waves can be caused by the overflow and creep flowing over the structure and is influenced by various factors like width of crest, water elevation at structure toe, slope of structure wall, porosity, and nominal diameter of the armor. According to several literature review water elevation and design wave height are decisive factors in designing breakwaters. Generally, performance of breakwaters is related with structure stability to wave energy. The design of breakwaters include also the decision of armor weight that will be strong enough to withstand the design wave. Breakwater stability is influenced by two factors, namely condition of the coastal environment and structure physical characteristics. Coastal environment factors include among others wave height (Hs), wave period (Ts), duration (volume) of wave, direction of incoming waves, and wave group.

2.2 Geometry and Degree of Submergence

Major parameters used in illustrating the LCB geometry can be seen in Figure 2. In this case, h = structure height; d = water depth, and F = h d, the freeboard or difference between structure height and water depth. An important parameter in designing and defining the breakwater effectiveness is the level of submergence explained by the parameters : (1) submergence = d/h; (2) relative structure height = h/d; and (3) ratio between the free board against water depth = F/d. Whereas, submergence of structure involves the ratio between water depth to LCB structure height. For exposed conventional structures where crest peak exceeds water depth, the ratio is less than 1 or d/h> 1.0. Relative structure height which is the ratio between structure height to water depth (h/d) is also used as a non-dimensional parameter representing the submergence level and its exposure. Application of this relatively high ratio will result a structure submergence level less than 1 (h/d<1.0), and for an exposed structure, the ratio will equal 1 (h/d> 1.0). Freeboard is defined as the height difference between structure and water depth: F ...............................................................(1)

Where F is the freeboard, h the structure height, and d the water depth in front of structure. Equation (1) will result a positive freeboard value for an exposed breakwater and a negative freeboard for the LCB structure. The non-dimensional parameter for relative freeboard is the freeboard ratio defined as the comparison between freeboard and water depth, that is:
( )

..............................................(2)

Figure 2 Low crested breakwater (adapted from www.artificialreefs.org)

By defining the freeboard ratio, an exposed breakwater shall indicate positive value for the freeboard ratio (F/d > 1,0), contrary to the LCB structure showing negative freeboard ratio (F/d < 1,0). These three non-dimensional parameters (d/h, h/d, and F/d) show the relative height of breakwaters compared to water depth and are used to decide the wave energy and current to structure and its effectiveness in dissipating wave energy.

2.3. Non Woven Geotextile Material as Substitute of Natural Rock

There are some reasons why this Geotube system is so interesting to be discussed, researched, developed and implemented, there are 1) the size of this GSC is available upon design request, 2) as a substitution of natural rock, automatically its save our natural resources, 3) as the fill material is using existing sand material, no classified filled material required (Saathoff and Witte, 1994), then its environmentally friendly, 4) no skilled labor required for installation, 5) flexible wave response, 6) and the interesting part is, because the overall cost of this system very often less expensive than rigid conventional material (Recio and Oumeraci, 2008).

Figure 3 Typical LCB with geo-tube 2.3.1. Technical Performance & Characteristics of Non Woven Geotextile Geotube Geotube system usually can be made from Woven or Non Woven material. But as the experiences gained from scientific investigations and tests over the last five decades, have lead to an acknowledgement as being state of the art for the use of filtration and revetment purposes in hydraulic engineering. The lifetime as long-term stability, definitely corresponds to the usually planned lifetime of the coastal work (Kohlhase, 1997). To make sure that this LCB Soft Structure system is following long term design life, the performance of the (geotextile) material to be used must be seriously proven from engineering point of view and test report. There are some selection criteria, which significantly relevant and necessary to put into account, such as : UV resistant, puncture test, abrasion resistant, elongation behavior & flexibility, and filtration (filter stable).

2.3.3. Testing Material as a System Subject to Hydraulic Structure After considering and getting selection criteria of material through some testing of the Geotextile material, then the next step is, to find a research or testing performance of the Geotextile material as a system, applied in Hydraulic Structure, especially for coastal structure, in relation with our current project application. This test is important, since the same as natural material for construction, this geotextile system should have done a physical modeling to know how this material works, and what

will be the affect to the structure. Without model test such like this, it will be difficult to get a reliable engineering design. This model test could not be valid in general subject to all geotextile material, but must be carried out by each material from each manufacturer. Since the material from different manufacturer has its own technical data (coefficient of interlocking, friction angle, etc.), which will give a different result and performance. This physical model test for Non Woven Geotextile Container, has been intensively carried out by one of German manufacturer, together with Liechtweiss Institute University of Braunshcweigh Germany. After this intensive research and testing for some years, it gives a formulae to calculate GSC material stability for hydraulic structure (see Figure 4). This research is being done by J.A.Recio as his doctoral dissertation and Prof. Dr. Ing. H. Oumeraci as the main counselor, see (Recio and Oumeraci, 2008). Further research activities related in this topic is still carried out until today.

Figure 4 Physical model test for non-woven geotextile coantainaer

2.4 Advantage and Disadvantage of the LCB

The LCB structure, considering its crest elevation and dimension, has some advantages like (1) aesthetically: LCB is not disturbing sea view, because it is set up at low water level, and not seen during spring tide, (2) wave movement is not completely halted so that response behind the LCB is relatively uniform into beach longitudinal direction, (3) wave energy behind the LCB has been reduced so that the water is safe for swimming, and (4) because being environment friendly, impact caused by the LCB is much smaller than the conventional breakwater. With a lower elevation than the conventional breakwater, change of beach line and formation of salient will be much slower.

3.

METHODOLOGY

3.1 Design Criteria

Survey and design results indicate that waves are the most influential hydro-oceanografic parameters to beach line change and currents generated by the breaking of waves at Pasir Putih Anyer Beach. Dominant waves come from west and southwest direction with respective maximum wave height 2.56m and 1.93m for a period of 4-10 seconds. Tidal waves in the coastal waters of Pasir Putih, Anyer, are semi-diurnal, with two times of neap tide and two times of spring tide in 24 hours. The tidal range is about 1.14m. The dominant current at spring and neap tide, moves from east to west direction at a velocity ranging between 0.00 m/s to 0.33 m/s. Coastal water bathymetry at Pasir Putih Beach, Anyer shows flat coral plains with a depth of 0-13 m. The beach slope ranging between 0.07 to 0.09, is considered as slightly sloping where beach slope of 0.1 is catagorized as flat. The geo-tube low crested breakwater consists of five units, constructed as follows: four units placed in 2 m depth at

lowest water level/LWL at a distance of approximately 50 m from the beach; and one unit in a depth of 1.5 m at mean sea level/ MSL at a distance approximately 40-45 m from the Beach. The location of the LCB prototype at Anyer Beach can be illustrated as in Figure 5 and 6.

4.

RESULTS AND DISCUSSION

From previous laboratory test done on the physical model, the results indicated that wave run-up at a low crested breakwater shall retard the current pattern formed between the structure and beach so that settlement of sediment behind the structure shall form a new coastline either salient or tombolo. In some cases, such current may turn into reverse direction because of the rise of water level behind the structure generating a flow from behind structure into gap direction. This condition may cause erosion behind the structure, whereas accretion may occur in front of the gap and outflow from this gap may cause scouring at the base near edge of the structure threatening its stability. The current pattern, erosion and accretion adjacent to the low crested structure is difficult to be interpreted. A more intense study using numeric models may give a better understanding of the hydro-dynamic process in the surf zone. The sedimentation pattern in field shown by the prototype at Anyer Beach was compared with the results of a test done in a wave basin (Pusat Litbang SDA, 2009), showing an almost uniform contour line change pattern. The change started at contour-2, consecutively retreating into beach direction causing a steep coastal slope. The contour line change occurred due to the erosion at contour-2 that had enlarged the depth. Because of the erosion at sea bed, sedimentation in the area dispersed into several directions, in this case into sea and land direction. At the end, sediment settled at the coast advancing the coastline and steepen the coastal slope. The forward move of coastline and coastal slope can be reduced by placing the low crested structure at a depth of -2 m. The effects of LCB structure can dissipate the maximum daily wave energy. Appropriate to being located below the mean sea level, this submerged structure can only function to its optimum during spring tide. Constructing these low crested structures at a depth of -1.5 m is the most appropriate location for Anyer Beach. However, several possibilities such as a decrease of stability due to wave reflection in front of structure can cause change of coastline and sea bed that result in the geotube LCB move seaward into a deeper location.

5.

CONCLUSION AND SUGGESTIONS

5.1 Conclusion
1) Effectiveness of low crested breakwaters in reducing wave energy is highly influenced by the geometric shape and configuration of the LCB construction, as well as water depth, wave height and period. The LCB structure shall show a good sedimentation performance when the tidal range in the area is les than 2 m (micro-tidal). Whereas, length of the breakwater and the distance from beach line shall determine the change of beach line. 2) Offshore coastal protection is affected by its size and location. Therefore, size of salient or tombolo formation varies in compliance with the dimension of the constructed off shore structure. This change of beach morphology shall certainly only occur as long as sediment transport is encountered along the beach or when aggravated by sand nourishment.

3) By designing the dimension and volume, use of low crested breakwaters as beach protection

measures can provide some advantages like (1) aesthetically: LCB is not disturbing sea view, because it is set up at low water level, and not seen during spring tide, (2) wave movement is not completely halted so that response behind the LCB is relatively uniform into beach longitudinal direction, (3) wave energy behind the LCB has been reduced so that the water is safe for swimming, and (4) because being environment friendly, impact caused by the LCB is much smaller than the conventional breakwater. With a lower elevation than the conventional breakwater, change of beach line and formation of salient will be much slower

Figure 6 Location of the geotube LCB structure at Pasir Putih Beach, Anyer

Figure 7 Cross-sectional profile and placement of geo-tube

5.2 Suggestions
1) The use of geo-tube as substitute material of natural rock has aroused the interest of practitioners and became a challenge in the development of coastal protection technology in Indonesia. Thus, study of the LCB structure stability has to receive special attention in the next activity. 2) To motivate successful use of this innovative technology, physical and numerical models are to be put forth so that beach characteristics, and current and wave dynamic patterns forming the shoreline shall be fully understood.

3) Activities still delayed due to limited time involve the monitoring, analysis and evaluation of coastline response behind the LCB structure, and to what extent beach profile behind structure is progressing. Therefore, monitoring of this beach profile is to be carried out periodically; seasonal or annually. Monitoring is also to be done on the LCB location itself and its formation to time and season.

ACKNOWLEDGEMENT
The writing of this paper was made possible with the help and assistance of many persons who had provided data, information and other material. For this, the writers like to thank all persons and institutions involved, particularly the Director of the Research Center for Water Resources, and the Head of the Experimental Station for Coastal Engineering, and Officials from Cidanau-CiujungCidurian River Basin. REFERENCES 1) Dattatri, J., Raman, H. and Shankar, N.J, (1978), Performance Characteristics of Submerged th Breakwater, Proc. of the 16 Coastal Engineering Conf., Hamburg, Germany, pp.2153-2171. 2) Durgappa H.R., (2008), Coastal Protection Works, Proceedings of COPEDEC VII, Dubai, UAE, 24-28 February 2008 3) HANSON, H. and KRAUS, N.C. (1991), Numerical simulation of shoreline change at Lorain, Ohio. J.of Waterways, Port, Coastal and Ocean Engrg., Vol. 117, No.1, January/February. 4) Heibaum, M.(1999), Coastal scour stabilization using granular filter in geosynthetics non woven containers. Federal Waterways Engineering and Research Institute (BAW) Karlsruhe Germany, 20 February 1999. 5) Heerten, G. (1980), Long-term experiences with the use of synthetics filter fabrics in coastal th engineering. Proc. 17 International Conference on Coastal Engineering ICCE, Sydney, Australia, 1980. 6) Kohlhase, S.(1997), Some aspect of the Use of Geotextile in the field of Coastal Engineering, Proc.of the first German-Chinese Joint Seminar Recent Developments in Coastal Engineering, Hasenwinkel, University of Rostock, Germany, 1997. 7) Kularatne S.R., J.W. Kamphuis, and M.A. Dabees, (2008), Morphodynamics Around Low Crested Breakwaters a Numerical Study, Proceedings of COPEDEC VII, Dubai, UAE, 24-28 February 2008 8) Pilarczyk, K.W. (2003), Design of Low Crested(Submerged) Structures- an Overview-, Proceedings of COPEDEC VI, Colombo, Sri Lanka. 9) Pina, G.G. and J.M. Valdes F. Alarcon, (1990), Experiments on Coastal Protection Submerged nd Breakwaters: A Way to Look at the Results, Proc. of the 22 Coastal Engineering Conf., Delft, the Netherlands, pp.1592-1605. 10) Pusat Litbang Sumber Daya Air, (2010), Pembuatan Prototip Pemecah Gelombang Ambang Rendah, Laporan Akhir, Bandung. 11) Pusat Litbang Sumber Daya Air, (2009), Uji Model Fisik Pemecah Gelombang Ambang Rendah Pantai Pangandaran, Kabupaten Ciamis, Laporan Akhir, Bandung. 12) Recio J and H. Oumeraci, (2008), Hydraulic Stability of Geotextile Sand Containers for Coastal Structures, Proceedings of COPEDEC VII, Dubai, UAE, 24-28 February 2008. 13) Saathoff, F. & Witte, J. (1994), Use of Geotextile Containers for Stabilizing the Scour Embankment at the Eidersperrwerk, Geosynthetics World, Part 1 September 1994, Part 2 October 1994. 14) Van der Meer, J.W., (1991), Stability and Transmission at Low Crested Structures, Delft Hydraulics Publication No. 453. 15) Vogt H., (2008), Long-term Weathering Behaviour of Geotextiles, Proceedings of COPEDEC VI, Colombo, Sri Lanka, 16) Weerakoon S., Mocke, G.F., Smit, F.,, and Al Zahed,K., (2003), Cost effective Coastal Protection Works Using Sand filled Geotextile Containers, Proceedings of COPEDEC VI, Colombo, Sri Lanka.