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1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 107 EVALUATION OF DETACHED BREAKWATER SYSTEM AND GROYNES FOR SUSTAINING THE COASTLINES Lalu Mangal1 , Anitha Joseph2 , Tilba Thomas3 1,2 Professor, Department of Civil Engineering, Thangal Kunju Musaliar College of Engineering, Kollam, Kerala, India- 691 005 3 Assistant Professor, Department of Civil Engineering, St. Joseph's College of Engineering and Technology, Palai, Kerala, India- 686 579 ABSTRACT The effectiveness of submerged, detached breakwaters and transition groynes for the protection of Alappad coast in Kerala is numerically investigated using the software MIKE 21. The study area comprises of a coastal stretch of 3.5 km along shore and 6.5 km offshore. Results reveal predominant longshore transport which favours the selection of groynes as a coastal protection measure whereas the introduction of submerged, detached breakwaters reduced the wave height in its lee to a significant amount. Parametric studies are conducted to compare the effectiveness of detached breakwaters and groynes in reducing the coastline erosion and also to reach a suitable configuration of transition groynes that drastically reduces the sediment transport rate especially during monsoon. Keywords: Coastal Protection, Detached Breakwater, Groyne, Sediment Transport. 1. INTRODUCTION Coastline of a country plays an important role in its development in the various vital sectors such as industrial and residential sectors. But these coastlines are under the threat of severe coastal erosion in almost all parts of the world. Coastal properties and roads are ruined by erosion, particularly during the monsoon. Long lasting coastal protection measures are required to solve the beach erosion problems. The coastal state of Kerala, lying in the western coast of India has a coastline of 588 km and experiences intense wave activity. The coastal properties and roads remain threatened by erosion, particularly during the south-west monsoon. A case study is undertaken at INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 107-116 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2014): 7.9290 (Calculated by GISI) www.jifactor.com IJCIET ©IAEME
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 108 Alappad coast (9°2' N, 76°30' E to 9°4' N, 76°29' E) for a stretch of 3.5 km along shore and 6.5 km in the offshore direction which is one of the most seriously affected regions of the Kerala coast with maximum economic loss due to coastal erosion. The soil of this region has a great mineral value as it is rich in Ilmenite content used for Titanium dioxide production. Detached breakwaters offer an attractive solution in reducing the intense wave activity in the coastal area especially during monsoon. Detached breakwaters are usually preferred as they do not create a total barrier to littoral transport and have a lesser impact on the neighboring shorelines. When these structures are provided as submerged breakwaters, they have the added advantage of improving the aesthetics, as they do not offer a barrier to the vision of outer sea. Groynes are yet another type of shore protection structure which offers significant protection from coastal erosion especially during monsoon as they form a cross-shore barrier that traps sand that moves along-shore, thereby increasing the width of the beach on the upstream side. The groyne also shelters a short reach of shoreline along its down-drift side from wave action. But when groynes are placed, it often results in erosion on the down-drift side of the structure. Down-drift erosion can be reduced by filling the groyne embayment and by the use of transitional groynes both up-drift and down-drift to allow changes in the beach. Thus transition groynes were considered in the study to reduce the negative effects of down-stream erosion in the study area. In the present work, an extensive case study has been conducted to determine the effectiveness of submerged, detached breakwaters and two configurations of transitional groynes in reducing the coastline erosion of Alappad in Kerala. Studies point out to the fact that the effectiveness of various hard coastal protection measures is purely site-specific depending on the various parameters existing in the site such as predominant current direction, sediment characteristics etc. Birben et al. suggested that the fundamental components, namely the breakwater length and the distance to shoreline, have to be given much importance while designing offshore breakwaters . Studies conducted by Pilarczyk found that a reef that allows a large proportion of wave energy to pass over the obstacle can be positioned closer to the shoreline than an emergent feature . Ranasinghe et al. suggested that the structure crest width does not affect the mode of shoreline response when the crest is deeper. It was also observed that in the absence of strong tidal currents, tides appear to have a negligible impact on the net mode of shoreline response to submerged breakwaters . Sayah et al. found that structural protection against erosion will not restore the eroded beach whereas beach nourishment project was proposed to be more effective for beach restoration for the selected study area . Yuliastuti and Hashim proposed permeable submerged rubble mound breakwater in order to overcome the negative effects of the emerged rubble mound breakwater such as degradation of water quality, concerns for natural habitats partly enclosed inside the bay, aesthetic considerations, limited water circulation etc.. Thus a submerged breakwater can be considered more effective than an emergent structure. Studies conducted by Schoones et al.  found out that groynes acted as total traps to sediment transport and the accreted sand was only slightly coarser than that before the groynes were constructed. Joseph et al.  suggested that groyne system with two groynes which go up to 5 m water depth with spacing of twice the length was most effective for the coast of Paravoor located on the south-west coast of Kerala. Thus groynes function best on beaches with a predominant along-shore transport direction. 2. ANALYTICAL INVESTIGATION The feasibility of detached breakwaters and transition groyne field in the study area are numerically investigated using the 2D free surface numerical model, MIKE 21- Near-shore Spectral Wave (NSW), Hydrodynamic (HD) and Sediment Transport (ST) modules. The net littoral transport and shoreline evolution due to the introduction of breakwaters and groynes are determined using LITPACK. Bathymetry of the study area is generated in MIKE 21 from the water depth data, and is the input in NSW module. This module is used to calculate the refraction and shoaling of the incident
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 109 waves. The outputs from NSW module are the significant wave heights and radiation stresses in the study area. These results are used as boundary conditions in order to generate flows due to waves in the module MIKE 21 HD. This module calculates the wave generated orbital currents in x and y directions. The results of MIKE 21 HD are then used to generate the bed load sediment flows in the x and y directions in MIKE 21 ST module. The net annual littoral drift occurring in the area is calculated using LITDRIFT module in LITPACK over a selected cross-shore profile and the shoreline evolution occurring in the study area over a given period of time due to the introduction of breakwaters and groynes are obtained in the LITLINE module in LITPACK. In order to obtain the shoreline evolution due to the application of breakwaters and groynes, the wave climate data is to be given as input in the form of a time series whose accuracy is to be ensured by calibrating with the output obtained from the net annual sediment drift from LITDRIFT. The seasons are described as ‘pre-monsoon’ for February-May, ‘monsoon’ for the south-west monsoon period of June-September and ‘post-monsoon’ for September–December. Four cases each for the three seasons have been analyzed i.e., (1) without protective structure, (2) with detached breakwater, (3) with nine transition groynes (L= 200 m, 158 m, 125 m) of spacing 2L and (4) with seven transition groynes (L= 200 m, 142 m) of spacing 2L, where L= length of individual groyne. 2.1 INPUT DATA The data required for the modelling are wind and wave data, tidal data, sounding data and sediment characteristics. Some of these data were collected from published literature and others were obtained from Port Office at Neendakara and Danish Hydraulic Institute (DHI India). The wave data viz. significant wave heights, mean wave direction and mean wave period used for the study taken from Nair and Kurian  is tabulated in Table 1. Offshore Wave Data of Alappad Area Season Mean sig. wave ht. Mean wave direction Mean wave periods Pre monsoon 0.4 m 240-2700 5.6 – 7.2 s Monsoon 2 to 2.2 m 250-2720 8 – 8.8s Post monsoon 1.6 to 1.8m 270-2850 7 – 8 s 2.2 BATHYMETRY The bathymetry of the study area modelled is depicted in Fig.1 for the four cases; viz. Case 1- without protective structure, Case 2- with detached breakwaters, Case 3- with transition groynes of spacing 2L, Case 4- with transition groynes of spacing 2.75L.
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 110 Fig. 1: (a) Case 1 Fig. 1: (b) Case 2 Fig. 1: (c) Case 3 Fig. 1: (d) Case 4 Fig. 1: Bathymetry of the Study Area Derived using MIKE 21
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 111 3. RESULTS AND DISCUSSION 3.1 WAVE PARAMETERS The significant wave height (Hm0) at the study area for the cases investigated, during monsoon season as obtained from MIKE 21 NSW simulation is shown in Fig.2. In Fig. 2 (b), the green colored portion in the lee of the breakwater indicates a lower value of significant wave height. Thus the breakwaters are effective in significantly reducing the wave heights and offer a calm sea in its lee which can be made use of in improving tourism by promoting sea sports such as surfing in the area. It is also evident from Fig. 2 (c) and Fig. 2 (d) that when groynes are used, reduction in wave height is observed only in the immediate vicinity of the structure. This result is expected, as wave height reduction is not a function of groynes, but of breakwaters. The significant wave heights obtained for all the cases and seasons considered are compared in Fig. 3. The wave height reduction obtained by Case 2 where breakwaters are used is maximum when compared to the other three cases especially during monsoon when wave activity is at its maximum. Fig. 2: (a) Case 1 Fig. 2: (b) Case 2 Fig. 2: (c) Case 3 Fig. 2: (d) Case 4 Fig. 2: Significant Wave Heights during Monsoon (July)
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 112 Fig. 3: Wave Heights for the Various Cases during the Three Seasons 3.2 CURRENT SPEED AND DIRECTION The simulation results obtained from HD module for monsoon season for all the four cases investigated are presented in Fig. 4. It could be observed that northerly current is dominant during all the three seasons. The current velocity for the area under study is observed to be between 0.5 m/s to 1 m/s. Fig. 4: (a) Case 1 Fig. 4: (b) Case 2 Fig. 4: (c) Case 3 Fig. 4: (d) Case 4 Fig. 4: Typical Current Circulation during Monsoon (July) Thus from Fig.4, it could be observed that use of groynes would be a better shore protection measure when compared to detached breakwater system from the perspective of reducing the currents, as it could offer a suitable barrier to the northerly currents which is dominant for all the three seasons considered.
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 113 3.3 SEDIMENT TRANSPORT RATES Potential areas of erosion or deposition of the coastal area can be identified with the help of the results obtained from the simulation in MIKE 21 ST module. A positive value in the cross-shore sediment transport indicates accretion in the coastline due to the transport of sediment towards the shore. The cross shore and long shore sediment transport as obtained from the simulation is shown in Fig. 5 and Fig .6 respectively. Fig. 5: (a) Case 1 Fig. 5: (b) Case 2 Fig. 5: (c) Case 3 Fig. 5: (d) Case 4 Fig. 5: Cross-Shore Sediment Transport during Monsoon (July) In the Fig. 5 (a), the blue patch indicates a negative value which implies that the sediment transport is in the offshore direction when no protective structures are present. In Fig. 5 (b), Fig. 5 (c) and Fig. 5 (d), the yellow portion in the vicinity of the breakwaters and groynes indicate a positive value of sediment transport, which means that there is accretion of sediment towards the shoreline. In long-shore sediment transport, negative sign indicates sediment transport towards south and positive sign indicates towards north. Thus from Fig. 6, the intensity of long-shore sediment transport also reduces when breakwaters or groynes are present when compared to the condition where no protective structures are used. A change in the direction of sediment transport could also be observed near the breakwaters and groynes during the three seasons.
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 114 Fig. 6: (a) Case 1 Fig. 6: (b) Case 2 Fig. 6: (c) Case 3 Fig. 6: (d) Case 4 Fig. 6: Long-Shore Sediment Transport during Monsoon (July) 3.4 LITTORAL DRIFT LITLINE in LITPACK module is used for the determination of shoreline evolution occurring over a given period of time. The net annual littoral drift occurring in the study area when no protective structures are present is about 0.8875×106 m3 /year over the selected cross-shore profile.
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 115 Fig. 7 shows the expected beach evolution after one year and Fig.8 shows the expected beach evolution after three years due to the introduction of breakwaters and transition groynes. Fig. 7: (a) Case 2 Fig. 7: (b) Case 3 Fig. 7: (c) Case 4 Fig. 7: Plan of Coastline Evolution after One Year Fig. 8: (a) Case 2 Fig. 8: (b) Case 3 Fig. 8: (c) Case 4 Fig. 8: Plan of Coastline Evolution after Three Years In Fig. 7 and Fig. 8, the blue portion indicates water, yellow portion indicates beach material and 0 m in the offshore direction indicates the initial beach position. After three years, Case 3 trapped maximum sediment in the up-drift side when compared to Cases 2 and 4. Fig. 7 (a) shows no improvement in the shoreline due to the installation of breakwaters and also results in down drift erosion. From Case 3 and Case 4, it is clear that increased spacing between the groynes resulted in poor retention and scour within the groyne field. It can also be observed that all the three cases suffered down-drift erosion whose intensity may be reduced by artificial beach nourishment or sea- wall protection in the down-drift side. Results point out to the fact that Case 3 where transition groynes are placed with a spacing of twice the length is a better solution compared to Case 1 where no protective structures are provided, Case 2 where breakwaters are provided and Case 4 where transition groynes are placed at a spacing of 2.75 L.
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 100- 116 © IAEME 116 4. CONCLUSIONS Based on the numerical model studies, it was observed that the introduction of submerged, detached breakwaters reduced the wave height in its lee to a significant amount. The considerable reduction in wave height achieved by the introduction of detached breakwaters can be expected to stabilize the beach and prevent the temporary beach erosion during monsoon season which is observed in this area frequently. But the effectiveness of transition groynes in wave height reduction is limited to its immediate down-drift side. In the Alappad coast, northerly currents are prevalent for all the three seasons in the near-shore region thereby making long-shore sediment transport dominant than cross-shore sediment transport which is suitable for the introduction of groynes as a shore protection measure which also helps in the development of coastlines. It is also found that groynes offer better solution for the coastline erosion of Alappad coast with respect to sediment trapping efficiency as well as shoreline development when compared to breakwaters. It is also observed that spacing between groynes equal to 2 times the length is a better option compared to spacing of 2.75 times the length. Both configurations of the groynes as well as breakwaters exhibited down-drift erosion which might require artificial beach nourishment or seawall protection in the area immediately down-drift of the protective structure. The groyne field can also be extended to the southern breakwater of the Kayamkulam harbor, which will make the entrance channel of the harbor coincide with the down drift erosion area which in turn will prevent the sedimentation of the entrance channel. 5. ACKNOWLEDGEMENT The authors acknowledge the support from All India Council of Technical Education (Research Promotion Scheme), DHI Water and Environment Denmark, and Centre for Earth Science Studies, Thiruvananthapuram. REFERENCES  Birben, A.R., I.H. Ozolcer, S.Karasu and M.I. Komurcu (2007), Investigation of the effects of offshore breakwater parameters on sediment accumulation, Journal of Ocean Engineering, 34, 284–302.  Pilarczyk, K.W. (2003), Alternative Systems for Coastal Protection- An Overview, Proceedings of the International Conference on Estuaries and Coasts, China, November, 409-419.  Ranasinghe, R., M. Larson and J. Savioli (2010), Shoreline response to a single shore-parallel submerged breakwater, Journal of Coastal Engineering, 57, 1006–1017.  Sayah, S.M., J.L. Boillat and A. Schleiss (2004), The use of soft shore protection measures in shallow lakes: Research methodology and case study, Journal of Limnologica, 34, 65-74.  Yuliastuti, D.I. and A.M. Hashim (2011), Wave Transmission on Submerged Rubble Mound Breakwater Using L-Blocks, Proceedings of the 2nd International Conference on Environmental Science and Technology, vol.6.  Schoones, J.S., A.K. Theron, D. Bevis, “Shoreline accretion and sand transport at groynes inside the Port of Richards Bay”, Coastal Engineering, 53, 2006, 1045–1058.  Joseph, A., L. Mangal and P Deepa, “Effectiveness of groyne systems for the preservation of Kerala coast”, Journal of Institute of Engineers, 90, 2010, 12-17.  Nair, L.S. and N.P. Kurian, “Numerical model simulation of coastal processes and shoreline changes along Alappad coast in Kerala”, Proc. of the National Conf. on Coastal processes, Resources & Management, Thiruvananthapuram, 2010, 70-74.  El Saie Yasser Mohamed, “Effect of Using Submerged Rectangular Stepped Breakwater for the Defence of the Shore Line”, International Journal of Civil Engineering & Technology (IJCIET), Volume 5, Issue 2, 2014, pp. 106 - 118, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
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