LABORATORY INVESTIGATIONS ON BERM BREAKWATER USING CONCRETE CUBES AS ARTIFICIAL ARMOUR UNIT

Transcription

1 LABORATORY INVESTIGATIONS ON BERM BREAKWATER USING CONCRETE CUBES AS ARTIFICIAL ARMOUR UNIT Subba Rao Kiran G.Shirlal Madhu.M* Professor Assistant professor M.Tech Student Department of Applied Mechanics and Hydraulics,National Institute of Technology Karnataka,Surathkal,Srinivasanagar,D.K.District,Karnataka,India,Pin * ABSTRACT This paper presents the results of experimental studies conducted on the stability of berm breakwater using concrete cube as artificial armour unit. The weight of concrete cube used in the model is about 79.5 gm, for a design wave height of 0.1m. Berm breakwater models were tested for the stability for three different water depths. The dimensionless recession varied from 3 to 6.6 for the design wave height of 0.10 m and 6.6 to 8.85 for higher wave height of 0.12m for the different water depths. Keywords: Run-up, Rundown, Wave steepness, Stability number, Recession, Damage level Notations: d Water depth H 0 /gt 2 Deep Water wave steepness N s Hudson s stability number R u /H 0 Relative run-up INTRODUCTION The berm breakwater is normally constructed with a berm that is allowed to reshape instead of constructing the reshaped profile directly. This is so because it has been considered cheaper to construct the breakwater with a reshaping berm, as it requires smaller size armor stones. After reshaping by severe storms, several breakwaters have been seen to achieve a stable profile and Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 1

2 they withstand later storms without significant further reshaping and damage (Torum et al.1999). This breakwater concept has been used in the construction of breakwaters in several countries (PIANC MarCom WG 40, 2003, Poonawala I.Z. et al., 2004, O.J.Sayao, 1999). The design procedure for the preliminary definition of structure cross-sections are available (Hall & Kao 1991; Van der Meer & Koster 1988; Torum et al. 1999, 2003; PIANC MarCom WG 40, 2003). The uncertainty in wave climate favors a breakwater design that is not too sensitive to the wave height with respect to stability. Hall and Kao (1991) performed basic tests on berm breakwaters, studied the influence of wave height, period, spectral shape, number of waves, grading and rock shape on the profile reshaping. Van der Meer (1992) developed a computational model for the profile development of rock slopes and gravel beaches and this can be used in designing of berm breakwaters. Torum et al. (1999, 2003) developed an equation to calculate the recession of the berm of berm breakwaters based on the wave height, period, and nominal diameter of stones, gradation factor and depth factor. These design procedures have their own limitations because of wide range of armor stone size, gradation of armor stones, water depth, and seaward slope, crest height and wave characteristics. In this paper, result of berm breakwater hydraulic model studies designed to suit the wave parameters of Mangalore coast, are analyzed. DETAILS OF EXPERIMENTAL WORK The present work involves an experimental study on the influence of change in water level in front of breakwater on the stability of the berm breakwater. The weight of concrete cube used in the present model is, W 50 = 79.5 gm and berm width, (B =0.45m). The earlier experiments conducted in the same wave flume on statically stable berm breakwater model with armour weight, W 50 =52gm, and berm width = 0.45m, has shown a stable profile for wave height up to 0.14m (Subba Rao et.al. 2006). The stability of the breakwater is studied by measuring the recession of the berm provided. The recession of the berm beyond the berm width is considered as the failure of the breakwater. Weight of armour unit used, W 50 = 79.5 gm is calculated by Hudson equation (CEM 2002) for a design wave height of 0.1m. In the present model, the Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 2

3 primary layer is divided into three zones crest ward slope, berm and toe ward slope, and the units in these regions are coloured as grey,white and red respectively..fig.1 shows the sectional elevation of the breakwater model studied. Geometrically similar scale of 1:30 was selected for the present investigation. The armour layer thickness has been calculated using layer coefficient as explained in the CEM (2002). The same number of layers are provided in the secondary layers with armor weight W 50 = 7.95gm. The minimum crest width adopted is so as to accommodate three cubes, and a crest width of 0.15m. A horizontal berm is provided at a constant depth of 0.425m above the seabed. The seaward slope above the berm and below the berm is kept same (1.5:1). Fig: 1 Cross-section of berm breakwater model Wave Flume The wave flume is 50m long, 0.71m wide, 1.1m deep and has a 42m long smooth concrete bed. Fig.2 shows the sketch of the wave flume used in the present work. A bottom-hinged flap generates waves at one end of the deep chamber which is 6.3m long, 1.5m wide and 1.4m deep. About 15m length of the flume is provided with glass panels on one side. The flap is controlled by an induction motor of 11kW and 1450 rpm. This motor is regulated by an inverter drive (0-50Hz) rotating with a speed range of rpm. Regular waves of height 0.02m to 0.24m, and periods 0.8sec to 4sec can be generated with this facility. Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 3

4 Fig: 2 Details of experimental setup In this analysis a stable berm breakwater is defined when recession of the berm (eroded berm width) is less than the initial berm width provided ((Rec/B) <1 for a minimum storm condition of 3000 waves, or till the breakwater has failed, whichever occurred earlier is the limit for every test run, where Rec is the recession of the berm and B is the initial berm width provided (Fig: 3) Fig: 3- Berm Recession Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 4

5 Table.1 Range of Experimental Variables SL.No. Variable Expression Range 1 Wave height H 0.10,0.12,0.14,0.16m 2 Wave period T 1.6 & 2.0 sec 3 Berm width B 0.45 m 4 Storm duration N 3000 waves 5 Angle of wave attack θ Water depth d 0.37m, 0.40m,0.43m 7 Design Armor unit weight W gm 8 Nominal diameter of primary Armour unit D n m 9 Primary armour layer thickness : Above the berm At and below the berm t p m m 10 Secondary armour layer thickness : Above the berm At and below the berm t s m m Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 5

6 11 Shape of the armor unit - cube 12 Crest height m 13 Slope - 1: Specific gravity of armor unit S r 2.4 RESULTS AND DISCUSSIONS Effect of Storm Duration on Recession The influence of storm duration on reshaping of berm is shown in Fig.4. It shows the recession of berm for different wave heights against the number of waves for T=1.6sec, berm width, B=0.45m and water depth in front of breakwater, d=0.37m. From the figure it is observed that for the wave height, H=0.1m, most of the changes took place during the first 1000 waves and the recession remains almost same for the number of waves more than 2000 and 3000 indicating the stability of the breakwater. When the wave height was H=0.12m the recession of the berm was almost constant after 2000 waves. For the wave height, H=0.14m the recession was found increasing even up to 2000 number of waves. Though the rate of recession of the berm was reduced after 2000 number of waves, for the wave height, H=0.16m, the recession was found to be increasing up to 2500 waves and recession the berm reduced after 2500 waves, up to 3000 waves From the above discussion it can be concluded that the stability of the breakwater is largely influenced by the storm duration. Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 6

7 Fig: 4 Variation of Recession with Number of waves for T=1.6 sec and d=0.43 m Effect of water depth on Damage level (S) The variation of damage level (S) with N s for different water depths is shown in Fig.5. It is observed that the variation of S with N s is linear for all the water depths and it increases with the increase in N s values. Although the variation of S for lower values of N s is higher up to N s =2.3. But for higher values of N s, the variation of S for 0.40m water depth is the highest and for 0.43 m water depth, it is the least. And the variation of S for d=0.37m is intermediate. Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 7

8 Fig: 5 Variation of Damage level (S) With Stability Number (N S ) for different Water depths Effect of water depth on runup Fig: 6 shows the variation of wave runup (R u /H o ) with wave steepness (H o /gt 2 ) for different water depths, where H o is the deep water wave height. The runup values in the graphs were corresponding to the wave heights from 0.10m to 0.16 m and wave periods 1.6sec and 2.0sec. It is observed that as the wave steepness increases from to the wave run up (R u /H 0 ) varied from 0.52 to 0.79 for d=0.43m, 0.76 to 0.98 for d=0.40m and 0.94 to 1.08 for d=0.37m For the three cases of water depths studied, the dimensionless runup varied from 0.52 to 1.08.when compared the results of present work with literatures of, AVRP Rajesh, showed 0.86 to 1.17,Pralay Kumar Roy showed 0.75 to 1.28 and K.Balakrishna Rao showed 0.73 to 1.58,hence the dimensionless runup of present work is lesser compared to other works, and trend pattern is similar as shown in CEM. Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 8

9 Fig: 6 Variation of relative runup with wave steepness for different water depths CONCLUSIONS Based on the present investigation, the following conclusions are drawn. The stability of berm breakwater model studied is largely influenced by the storm duration. The model has shown a stable profile after duration of 1000 waves for the wave heights of 0.10 m and 0.12 m. The recession of berm is largely influenced by the change in water level in front of the breakwater. The dimensionless recession varied from 0.93 to 2.04 for the design wave height of 0.10 m and 2.32 to 2.72 for higher wave height of 0.12m, for the different water depths. For the higher wave heights of H=0.14m showed a recession of 6.36 and H= 0.16m showed a dimensionless recession of for d=0.37 m, Hence the recession is less with the use of cube as armour unit, for the three cases of water depths studied. The dimensionless runup varied from 0.52 to The variation of damage level (S) with N s for 0.40m water depth is highest and 0.37 m being least. Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 9

10 ACKNOWLEDGEMENTS The authors are thankful to the Director of National Institute of Technology Karnataka, Surathkal, and Head of the Department of Applied Mechanics and Hydraulics for the facilities provided for the investigation and permission granted to publish the results. REFERENCES CEM. Coastal Engineering Manual (2002). Fundamental of Design. EM (Part-6), U.S. Army Corps. of Engineers Hall, K. R.and Kao, J. S. (1991). The influence of armor stone gradation on dynamically stable breakwaters. Journal of Coastal Engineering, Vol.15, PIANC MarCom W G 40. (2003). State-of-the-art of the design and construction of berm breakwaters. PIANC, Report of Working Group 40 - MARCOM, Brussels. Sayao, O.J. (1999). On the profile reshaping of berm breakwaters, Coastal Structures, Subba Rao, Ch. Pramod and Balakrishna Rao K, (2004), Stability of Berm Breakwater with Reduced Armor Stone Weight, Ocean Engineering, An International Journal of research and development, Pergamon, Elsevier Science Ltd. New York, N.Y., U.S.A., Vol. 31, pp Subba Rao, Balakrishna Rao K., Rajesh AVRP (2006). Stability Aspect of Berm Breakwaters. Proceedings of National Conference on Hydraulics and Water Resources, HYDRO-2006, Dec.8-9, Pune, Subba Rao, Subrahmanya K, Balakrishna Rao, and Chandramohan V. R. (2007), Stability aspects of non reshaped berm breakwaters with reduced armour weight and varying slopes, Journal of Waterways, Port, Coastal and Ocean Engineering, ASCE, USA, Vol. 134, No. 2, Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 10

11 March/April 2008, pp Torum, A., Krogh, S.R., Bjordal, S., Fjeld, S., Archetti, R., Jacobsen, A. (1999). Design criteria and design procedure for berm breakwaters. Proc., Coastal Structures '99', Balkema, Rotterdam, Torum, A., Franziska K., Andreas Menze (2003). On berm breakwaters. Stability, scour, overtopping. Journal of Coastal Engineering, Vol.49, Van der Meer & Koster (1988), CIRIA and CUR (1991), state of the art guide by PIANC (2003b) Laboratory Investigations on Berm Breakwater Using Concrete cubes as Artificial Armour Unit 11