PHYSICAL MODELLING INVESTIGATION FOR DIKKOWITA FISHERY HARBOUR

8 th INTERNATIONALCONFERENCEONCOASTALANDPORT ENGINEERINGINDEVELOPINGCOUNTRIES COPEDEC2012,IITMadras,Chennai,INDIA. 2024Feb.2012 PHYSICAL MODELLING INVESTIGATION FOR DIKKOWITA FISHERY HARBOUR I. G. I. Kumara 1, K. Benders 2, T. D. T. Pemasiri 3, D. P. L. Ranasinghe 4 and K. Raveenthiran 5 Abstract: Dikkowita, which is located approximately 10km North of Colombo. It was a famous site which being used as a fishing anchorage for a long time. This usage lead for a proposal of develop this site as a commercial fishery harbour with all facilities like ice plant, cold storage, fish handling equipment, etc. in early 1990s. The present study discussed in this paper depicts physical modelling carried out for the detailed design of XBLOC armoured breakwater. The 3D (basin) model covers a part of the harbour centred on the harbour entrance with a width of 500m and consisting of the southern reef breakwater, main breakwater and northern reef breakwater. The model is tested for two directions for several representative wave climates. Stability of front, crest and rear armour on breakwater trunks and heads, toe stability, stability of groynes and spending beach, wave overtopping over the breakwater and wave calmness inside of the harbour were also tested in physical modelling. Slight damage level was noticed at rear armour at main breakwater trunk section and toe berm at main breakwater head section. After made a trench cut to place the X-bases, the toe of the southern reef breakwater was stabilized. Keywords:physicalmodelling;breakwaterstability;wavedisturbance;XBLOC. 1. INTRODUCTION 1.1 Background The proposal of developing the Dikkowita as a commercial fisher harbour in early 1990s, lead to carry out a feasibility study, numerical modelling and 2D and 3D physical modelling of the proposed harbour layout. Dikkowita Sitelocation Colombo Figure.1.LocationMapofDikkowita 1 Research Engineer, Lanka Hydraulic Institute Ltd, Sri Lanka, E mail: indika.kumara@lhi.lk 2 Design Engineering Manager, BAM International, Sri Lanka, E mail: k.benders@bamsrilanka.com 3 PhD Student, University of Western Australia, Australia, E mail: thotagam@sese.uwa.edu.au 4 PhD Student, Tohoku University, Japan, E mail: prasanthiranasinghe@gmail.com 5 Senior Engineering Manger, Lanka Hydraulic Institute, Sri Lanka, E mail: ravi@lhi.lk 1868

ProceedingsofCOPEDEC2012,2024February2012 PhysicalModellingInvestigationforDikkowitaFisheryHarbour The Initial feasibility study was carried out by Lanka Hydraulic Institute Ltd (LHI) in association with Niras Port consult, DHI Water and Environment, and Associated Management Services Ltd (AMS) for Ministry of Fisheries and Aquatic Resources during 1995 1997. The harbour site is sheltered by the natural reef system. The coastal stretch of Dikkowita at the vicinity is almost straight and vulnerable to continuous erosion especially during the south-west monsoons (May September). Hence, at the present, it is protected by several anti-erosion measures such as beach nourishment, groynes and detached rubble breakwater system, under the Coastal Resources Management Project (CRMP) Coastal Stabilisation Component (CSC). Location of the site is shown in Figure 1. 1.2 Present Study Lanka Hydraulic Institute Ltd (LHI) has carried out field investigations including topographic and bathymetric survey, CPT investigations and borehole investigations. Wave climate and water circulation were established through numerical modelling whilst the cross sections of the breakwaters were optimized through a 2D (flume) physical model. Figure 2 depicts the layout of the fishery harbour. Figure.2.ProposedHarbourLayout The layout of the fishery harbour (Figure.2) was designed by Delta Marine Consultants (DMC). The proposed harbour basin which will be dredged to 3.0 to 3.5 m below Mean Sea Level (MSL), is about 900 m long and 200 m wide. The basin will be partly dredged up to 5.0 m below MSL in the future. Part of the breakwaters will be located in the shallow reef area, and the deeper seabed sections of the breakwater are in the order of 6.0 to 7.0 m below MSL whilst the crest of the breakwater is about 5.0 m above MSL. The present study discusses physical modelling carried out for the detailed design of XBLOC armoured breakwater. The harbour layout with harbour entrance, breakwater roundheads, part of breakwater trunks, and groynes enclosing the spending beach are tested in the 3D (basin) physical modelling based on the optimized harbour entrance in the mathematical modelling and optimum breakwater sections in the 2D (flume) modelling. 1.3 Objectives The main objectives of these hydraulic physical modelling tests were to verify the design as determined 1869

ProceedingsofCOPEDEC2012,2024February2012 PhysicalModellingInvestigationforDikkowitaFisheryHarbour during the 2D tests with respect to: Stability of front, crest and rear armour on breakwater trunks and heads Toe stability Stability of groynes and spending beach Wave overtopping over the breakwater Wave disturbance in the harbour 2. PHYSICAL MODELLING APPROACH 2.1 Scope of the Test The 3D model covers a part of the harbour centred on the harbour entrance with a width of 500 m. The model was tested only for two directions for several representative wave climates. Actual bathymetry was modeled only up to -7m MSL and then the bed is assumed to be having a slope of 1:100 from -7m MSL onwards. 2.2 Model Scale The same scale of 1:41.25 which was used for the 2D model testing is adopted for the 3D modelling as well. It facilitates to use the same Xbloc model sizes used in 2D modelling and hence easy to compare results of 2D versus 3D modelling. In general, there are some key points to be considered in deciding the model scale as follows; Size of the model basin (35m x 25m x 1m) Layout that should be enclosed in the model Wave conditions that should be tested Sufficient distance from the wave generator to the area of interest to ensure fully developed waves at the area of interest Sufficient distance from the model boundaries to the area of interest to avoid undesirable wave reflection Since the model scale has been already pre-defined, all the above points are restricted by the scale. Figure 3 shows the harbour layout within the model basin with bathymetry, wave gauge locations and the paddle positions. Applied model scales are included in Table 1. Parameter 1Table1.AppliedModelScales ModelScale Length 1 : 41.25 Time and Velocity 1 : (41.25) 1/2 = 1 : 6.422616 Forces, Volume and Mass 1: (41.25) 3 = 1 : 7.0189 10 4 1870

Proceedings of COPEDEC 2012, 20Ͳ24 February 2012 Physical Modelling Investigation for Dikkowita Fishery Harbour Figure. 3. Port Layout on LHI Laboratory Wave Basin 2.3 Model Materials Two types of armour units, Xblocs and rock armours, were used for the breakwaters. Several Xbloc and 1871

ProceedingsofCOPEDEC2012,2024February2012 PhysicalModellingInvestigationforDikkowitaFisheryHarbour Xbase types were used and details of them are given in the Table 2. These model units were imported from DMC to LHI for the purpose of model testing and to be resent upon the completion of model testing. 2Table2.DetailsofXblocandXbaseUnits Section PlaceofAmour ArmourType Reef Breakwater Trunk Prototype Volume(m 3 ) Model Weight(g) Armour layer sea side Xbloc 2.0 62 Armour bottom row Xbase 3.6 107 Armour second row Xbloc 4.0 119 Reef Breakwater Head Armour layer front + rear Xbloc 2.0 62 Armour bottom row Xbase 3.6 107 Armour second row Xbloc 4.0 119 Main Breakwater Trunk Main Breakwater Head Armour layer sea side Xbloc 1.5 49 Armour bottom row Xbase 1.35 44 Armour layer front + rear Xbloc 1.5 49 Armour bottom row Xbase 1.35 44 Since fresh water was being used in the basin instead of sea water, the density difference was taken in to account in model unit weight calculation. This was considered in the calculation of model rock armour units. The corrected relative density for the Xbloc units is 2350kg/m 3. The following scale relationship, developed under Hudson's Stability Number Criterion (1979), was adopted in converting the unit weight of prototype to model. ( Wn50 ) ( W ) Where, m n50 p = ( H ) 3 s m ( H ) 3 s p ρ s ρs 1 m ρw ρ s ρs 1 p ρw 3 p 3 m (1) (W n50 ) m = weight of armour in the model (W n50 ) p = weight of armour in the prototype (H s ) m = significant wave height in the model (H s ) m = significant wave height in the prototype ρs m = density of armour in the model ρs p = density of armour in the prototype 1872

ProceedingsofCOPEDEC2012,2024February2012 PhysicalModellingInvestigationforDikkowitaFisheryHarbour ρ w = density of water in the model (Fresh Water) ρs = density of water in the prototype (Salt Water) Table 3 shows the scaled down values for rock materials. As the imported Xbloc units were not sufficient to cover the entire structure, it was decided to use equivalent rock armours for the rest of the model which is not used to get test measurements (The Rock Manual-The use of rock in Hydraulic Engineering, 2nd Edition, 2007). BreakwaterCross Section ReefBW Main BW Trunk at -2m MSL Head at -4m MSL Trunk at -6m MSL Head at -7m MSL Table3.DetailsofModelRockMaterials LayerType Wt.ofUnit Rock Armourin prototype (kg) Equivalent ModelUnit Diameter D n (mm) ModelParameters Armoursizes used(mm) Proportion % Filter layer 300 14 12.5-19.5 60 Rear 300 14 12.5-19.5 60 armour Core(Quarry 0.5 2 5-7 15 run) 250 13 7-12.5 60 500 16 12.5-19.5 25 Filter layer 300 14 12.5-19.5 60 Core(Quarry 0.5 2 5-7 15 run) 250 13 7-12.5 60 500 16 12.5-19.5 25 Filter layer 300 14 12.5-19.5 60 Rear 1000 21 19.5-25 60 armour 3000 31 30 37* 40 Toe berm 300 14 12.5-19.5 60 Core(Quarry run) 0.5 2 5-7 15 250 13 7-12.5 60 500 16 12.5-19.5 25 Filter layer 300 14 12.5-19.5 60 Toe berm 300 14 12.5-19.5 60 Core(Quarry run) * - Only small fraction was used from this grading 2.4 Bathymetry Construction 0.5 2 5-7 15 250 13 7-12.5 60 500 16 12.5-19.5 25 1873

ProceedingsofCOPEDEC2012,2024February2012 PhysicalModellingInvestigationforDikkowitaFisheryHarbour The structure and its immediate vicinity and sufficient seaward distance to allow for generated waves to fully develop were represented within the basin. The bathymetry was constructed by laying metal strips with wooden pegs at the contour heights and area between the strips was filled with sand, compacted and covered with a thin layer of cement mortar. The wave conditions specified were at the -7 m MSL contour, which was located at the main breakwater head. From the -7 m contour to the offshore reef, the seabed was gently sloping to -8 m MSL. (almost horizontal seabed). In order to achieve sufficient depth at the wave paddles, a slope of 1:100 has been assumed from -7 m MSL contour to -12m MSL contour where the paddles were to be placed. 2.5 Model Calibration For this purpose, three paddles combination was used and the output wave heights were measured by three wave gauges placed in front of the paddles. The wave climate in the 3D basin has to be calibrated for the -7 m MSL contour. Calibration runs, 20 minutes each, were performed for different wave conditions so that input wave heights for the actual model testing could be derived by using these calibration results. 2.6 Model Construction 2.6.1 Breakwater construction The breakwater sections provided by DMC and were scaled down to give correct representation of hydraulic characteristics of the structure such as stability, permeability and porosity. Model structure is then constructed according to the scaled down dimensions, in the sequence of bed preparation, placing of core, placing of filter layer, toe and Xbloc layer. Number of available Xbloc and Xbase units specified for the Main Breakwater Trunk section (1.5m 3 Xbolc and 1.35m 3 Xbase units), were not sufficient during the construction. Hence some modified armour units had to be used. 2.0m 3 Xbolc and 1.8m 3 Xbase (modified by breaking one arm of 2.0m 3 Xbloc unit) units were used instead of 1.5m 3 Xbolc and 1.35m 3 Xbase units respectively (Figure. 4). This part of the main breakwater is however not considered as critical. Special attention will be given to the transition between the 2.0 m 3 and 1.5m 3 section during damage assesment after each test. 1.5m 3 Xblocs 2m 3 Xblocs 1.8m 3 Xbases Towards BW head 1.35m 3 Xbases Towards Southern Reef BW Figure.4.ModifiedArmourUnitsatMainBreakwaterTrunkSection 1874

ProceedingsofCOPEDEC2012,2024February2012 PhysicalModellingInvestigationforDikkowitaFisheryHarbour 2.6.2 Xbloc Armour Units and Placement Technique In the physical model construction, Xbloc units were placed on top of filter layer freely with random orientations after placing one line of Xbases. First Xbases were placed as a line with a small space of 1.3 times of overall height / width of a unit and Xblocs were then freely placed in between them with different orientations one step inwards slope allowing them to interlock without pressing by hand, since Xbloc units naturally find a stable position on the slope (www.xbloc.com). The Xbloc units are simple to place and taken a less time duration as no specific orientation of the individual units are required. 3. MODEL TESTING AND RESULTS 3.1 Test conditions Model testing was carried out to meet up the main objectives of structural stability and the wave penetration in to the harbour.the tests were carried out in test series composed of a number of individual test runs with increasing wave heights from one to another. Test series of extreme wave conditions were carried out to check the structural stability of the breakwaters, overtopping and wave penetration in to the harbour. Main purpose of doing the test series for operational condition is to check the wave penetration into the harbour. Wave heights were measured through the wave gauges located as indicated in the Figure 3. for each test of both extreme and operational test series. Each test run was representing the duration of more than 1000 waves in prototype. ttable4.waveconditionsformodeltest Test Condition Extreme Operational Test Series 1 2 3 4 5 Test Case 1B (80%) 1C (100%) 1D (120%) 2A (80%) 2B (100%) 2C (120%) 3B (100%) 4B (100%) 5B (100%) Aim of test case Check Structural Stability, Overtopping and Wave Penetration Check Structural Stability, Overtopping and Wave Penetration Check Overtopping Check Overtopping Check Toe Stability Prototype Parameters at -7 m MSL contour Water Level (m to MSL) DWL + 0.8 DWL + 0.5 Hs (m) Tp (Sec) 2.56 11.2 3.20 11.2 3.85 11.2 2.32 16.0 2.90 16.0 3.48 16.0 Wave Direction 270 0 N Model Duration (min) 50 35 250 0 N 50 MSL 0.0 2.80 11.2 270 0 N 35 MSL 0.0 2.70 16.0 250 0 N DLWL - 0.6 2.50 16.0 250 0 N 6 Check Sea Wave 1.0 6.0 270 0 N 35 7 Penetration 2.0 8.0 270 0 N 35 MHWS + 0.3 8 Check Swell 1.0 12.0 250 0 N 9 Wave Penetration 2.0 12.0 250 0 N 50 50 1875

3.2 Test results ProceedingsofCOPEDEC2012,2024February2012 PhysicalModellingInvestigationforDikkowitaFisheryHarbour Model testing was starting with extreme test cases. At the commencement of the test, instability of the Xbloc armour was observed at the southern reef breakwater section. It was figureout that instability has occurred due to the unsteady Xbases over the steep (around 1:10) bed slope. Hence, the needness of model modifications in order to improve the stability of the structure was identified. As a first solution for that instability, the steep bed slope section was trimmed at the toe level, in order to make the bed horizontal, so that it provided more steady position for Xbases. This modification could only be made at the two steepest parts of the reef breakwater. It was found impossible to scratch a trench at the correct location without changing the structure cross section to a large extent. The toe extended originally to the white line in Figure 5. However, stability was not that improved as expected and not in a satisfactory level. Horizontal bed at toe Rigid Concrete ridge Figure.5.Modificationforthebreakwater Secondly, it was decided to construct a rigid ridge from cement mortar to see the effectiveness of stabilizing the toe units. Figure 5 shows the breakwater section after the modification. The model rigid concrete ridge represent measures such as pilling or trenching in the prototype. The highest extreme wave (120% extreme swell) was performed and the structure was found completely stable. All other test scenarios were then carried out with this toe solution for the southern reef breakwater. No damage was observed in Northern Reef Breakwater for any test run except some displacement of rear armour whereas some damage of the rear armour at the main breakwater head was observed. Toe stability was specially assessed during low water level of -0.6m MSL. The stability of the structure was assessed by counting the displaced number of units and its percentages. Displacement was considered for particular section and the percentage was then calculated with respect to the total number of units at that section. The damage level is then decided according to the proportion of total number of armour units in particular area as described below; 1% of units displaced Slight damage 2% of units displaced Little damage 3% of units displaced Moderate damage 1876

ProceedingsofCOPEDEC2012,2024February2012 PhysicalModellingInvestigationforDikkowitaFisheryHarbour 5% of units displaced Severe damage t3table5.damagelevels Toe Rear Xbloc Test1B Test1C Test1D Test2A Test2B Test2C Test3B Test4B Test5B No Slight Little Slight Moderate Little Slight Slight Slight damage damage damage damage damage damage damage damage damage No No No Slight Slight Slight No No No damage damage damage damage damage damage damage damage damage No No No No No No No No No damage damage damage damage damage damage damage damage damage Overtopping during the testing was collected at two trays (Figure 3) in the reef breakwater sections. The prototype overtopping values (l/s per m run) shown in Table 6. NBW Northern breakwater; SBW Sothern breakwater Table6.ResultsofOvertopping Test1B Test1C Test1D Test2A Test2B Test2C Test3B Test4B Test5B NBW 0.1 1.3 5.1 0.4 1.2 1.1 0.1 0.7 0.2 SBW 1.5 6.0 9.6 2.1 8.4 7.7 0.7 6.3 1.3 Model was tested for all the scenarios in test series for operational conditions and wave heights were measured at locations as in the Figure 3. Main objective of this test series was to assess the wave penetration in to the harbour and to check whether penetrated wave heights are in the allowable range and results are shown in Table 7. Table6.ResultsofWaveDisturbance Test Outsideofharbour Inchannel Spendingbeach Infrontofquaywall no WHM1 WHM2 WHM3 WHM4 WHM5 WHM9 WHM10 6 1.157 1.104 0.596 0.306 0.186 0.075 0.058 7 2.033 2.227 1.488 0.699 0.502 0.279 0.264 8 0.946 1.034 0.632 0.294 0.187 0.080 0.235 9 1.956 2.104 1.372 0.623 0.437 0.255 0.433 4. DISCUSSION OF THE MODEL RESULTS 4.1 Stability of breakwater The stability of above structure components were assessed during the extreme wave condition. For the all wave conditions displacement of armour units were below than 5% with respect to the total number of units at that section. For most test cases, the displacement of rear armour, percentage unit displacements are less than 1% and hence can be considered as slight damage level. Before the stability measures toe has being moderately damage after performed the test 2B. There were no damage of Xbloc after contrustion of rigid concrete berm. No displacements can be seen of the Xbloc at the crest. 4.2 Overtoppping of the breakwater The maximum overtopping occurred in 1D test case, which is having the highest water level and the 1877

ProceedingsofCOPEDEC2012,2024February2012 PhysicalModellingInvestigationforDikkowitaFisheryHarbour wave climate with 120%. The overtopped amounts for northern and southern breakwaters are 5.12 and 9.57 l/s per m run respectively. 4.3 Wave disturbances in side of the harbor The allowable maximum wave height in front of quay wall for loading-unloading and mooring is 0.4-0.5m (Wave Agitation Criteria For Fishing Harbours In Atlantic Canada, Proceedings of the International Conference on Coastal Engineering, 1992). All the wave hieghts have being obesrved infront of quay wall were less than or within the accepctble limit. 5. CONCLUSIONS The following conclusions can be made based on the test results; The Xbases were not stable where local conditions are as such that the bed is hard and sloping steeper than 1:10. It can concluded that Xbases have actually been stabilized by means of pilling base or trenching. The wave height infornt of the southern quey wall was 0.4 0.5m for highest wave condition in mean high water spring (operational test 9). It is possible to increase the calmness by extending the southern groyne. 6. REFERENCES CIRIA, CUR, CETMEF, The Rock Manual-The use of rock in Hydraulic Engineering, 2nd Edition, 2007, London Hughes, Steven A., Physical Model and Laboratory Techniques in Coastal Engineering, Advanced Series of Ocean Engineering.vol.7, 1993, World Scientific: Singapore LHI, Design and Construction of Dikkowita Fishery Harbour Field Investigation Final Report, 2009 LHI, Old-Elkala Fishing port, Algeria, 3D Basin Stability and Wave Agitation Modelling Final Report, 2008 Charles C.P., Michael M.W., Wave Agitation Criteria For Fishing Harbours In Atlantic Canada, Proceedings of the International Conference on Coastal Engineering, 1992 Pope J., Lockhart J., Coastal Engineering Manual: Part VI, Design of coastal project Elements, 2001, U.S.Army Corps of Engineers: Washington Technical information.<http://www.xbloc.com> 1878