Open Access proceedings Journal of Physics: Conference series Civil and Environmental Science Journal Vol. I, No. 02, pp. 088-095, 2018 88 Analysis of Waikelo Port Breakwater Failure through 2D Wave Model Bambang Winarta1, A. A. N Satria Damarnegara2, Nadjadji Anwar2, Pitojo Tri Juwono1 1Water Resources Engineering Department, Universitas Brawijaya, Malang, 65145, Indonesia 2Civil Engineering Department, Institut Teknologi Sepuluh Nopember, Surabaya, 60111, Indonesia bwinarta@gmail.com / bambang.winarta@ub.ac.id Received 26-07-2018; revised 30-08-2018; accepted 29-09-2018 Abstract. Waikelo Port is located in South West Sumba of East Nusa Tenggara. The port facilities are protected by breakwater with a vertical wall construction and it was built in a relatively deep ocean at -15m of Low Water Sea Level (LWS). On 21st of January 2012, an earthquake with magnitude of 6.3 Richter scale occurred around Sumba Island and it caused cracking in the concrete wall of breakwater. Then, 4 days after on 25th January 2012, a heavy wind of 20–23 knots generated a high wave around 4.0–5.0m in Sumba strait. These high waves caused a critical damage on the west part of the breakwater. The damage of port facilities were getting worse when a storm called Lua hit on March 2012. This study was conducted to observe the effect of the extreme event in the failure of breakwater. The result of two-dimensional (2D) wave model shows that the wave heights in the area of breakwater are varied 3.80 to 4.0m. It is quite greater than the wave design of 50 years return period (= 2.00m) which was used in breakwater design and calculation. This observable fact confirms that the failure of breakwater was caused by the continuous extreme events that exceed the design criteria. Keywords: Waikelo port, breakwater failure, extreme event, 2D wave model 1. Introduction The reasons for breakwater failures can be classified in three major categories (a) reasons relate to the structure itself, (b) reasons relate to the hydraulic and loads conditions, and (c) reasons relate to the foundation and seabed change [1]. The possibility causes in term of the hydraulic and load conditions type are exceedance of design wave condition, concentration of wave action at certain zones along breakwater, breaking wave and impact loads and wave overtopping. Waikelo Port is located in North West Sumba, the Province of East Nusa Tenggara and it is used for inter island transportation. This port was established in 2011 and placed by the Waikelo Sea Port. The Waikelo Port is equipped a movable bridge and protected by a vertical wall construction breakwater. The breakwater itself was constructed in -15m of LWS and calculated based on the design wave of 50 years return period which was equal to 2.00m mailto:bwinarta@gmail.com mailto:bambang.winarta@ub.ac.id Civil and Environmental Science Journal Vol. I, No. 02, pp. 088-095, 2018 89 Figure 1. Location of study, Waikelo Port, South West Sumba, East Nusa Tenggara Civil and Environmental Science Journal Vol. I, No. 02, pp. 088-095, 2018 90 The consecutive extreme events hit Waikelo Port was started on January 21st, 2012, when 6.3 Richter scale earthquakes occurred around Sumba Island. This earthquake caused the breakwater to crack in the concrete wall as displayed in Figs. 2, 3. Then, the next 4 days, the second severe events occurred. A heavy wind of 20–23 knot generated a huge wave around 4.0–5.0m and struck Waikelo Port and also its facilities. This high wave caused the west part of the breakwater around 24.95m length fail. The next massive events happened on March 2012; a storm called Lua blew Waikelo Port in 5 days continuously and caused crucial damage in the port facilities as shown in Figs. 4, 5. By considering the chronologic extreme events as menti0ned before, the main objective of this present study is to analysis and review the breakwater failure through 2D wave propagation model. Figure 2. Cracking on breakwater concrete wall after an earthquake on January 21st, 2012. Figure 3. Significant damage of breakwater due to high wave on January 25th, 2012 Figure 4. Major damage of port facilities after Lua storm on March 2012. Figure 5. Major damage on movable bridge after Lua storm on March 2012. 2. Wave Generation and Propagation Heavy winds blow above the sea with enormous fetches acting for lengthy durations, possibly generating high and huge sea waves. Wave storms are usually described as events of significant wave height, Hs, overreaching a predetermined threshold (critical Hs) with a short duration. 2.1. Wind Wave Generation The height, length and period of wind waves in the open ocean are generated by the fetch, the wind speed, the duration of the wind blows, the distance of the wave travels and the water depth. General speaking, increasing in fetch length, wind speed and or duration will be generating a huge wind waves. Then, in term of the water depth, when it is sufficiently shallow, it will also determine on the size of Civil and Environmental Science Journal Vol. I, No. 02, pp. 088-095, 2018 91 wave propagation. The wind simultaneously creates waves of various heights, lengths and periods as it blows above the sea. Data used in this present study was obtained from Indonesia Meteorological, Climatological and Geophysical Agency (BMKG) which has located in Kupang. Based on BMKG record, it has increased significantly of wind speed and wave height during 10 days since 21st to 31st of January 2012 in Sumba Strait. BMKG recorded data informed that on January 25th, 2012 wind speed blew at 20–23 knots from North West direction with the significant wave height, Hs, was at 2.50–3.00m and the maximum wave height, Hmax, reached 4.0–5.0m. In order to verify recorded significant wave height data, subsequently, the wind wave heights on January 21st–31st, 2012 was calculated by considering fetch length as displayed and tabulated in Fig. 6 and Tabel 1. Table 1. Calculations of Waikelo Port fetch length Direction North West (= 315°)  cos Xi (km) Xi × cos 42 0.743 69.310 51.507 36 0.809 91.338 73.894 30 0.866 77.510 67.126 24 0.914 74.966 68.485 18 0.951 77.255 73.474 12 0.978 81.628 79.844 6 0.995 77.128 76.705 0 1.000 81.476 81.476 -6 0.995 96.492 95.963 -12 0.978 105.134 102.837 -18 0.951 138.176 131.413 -24 0.914 154.359 141.014 -30 0.866 2.891 2.504 -36 0.809 2.351 1.902 -42 0.743 2.050 1.523 cos : 13.511 xi × cos : 1049.667 Figure 6. Mapping scheme for fetch length calculation of Waikelo Port Civil and Environmental Science Journal Vol. I, No. 02, pp. 088-095, 2018 92 Based on the calculation result as written in Table 1, the effective fetch (fetcheff) can be obtained by using a simple formula as written below, fetcheff = ( ) 690.77 511.13 667.1049 cos cos ==    Xi km Then, recorded wind speed (U) was 23 knot or equal to 11.832m/s, thus wind stress factor (UA) = 0.71U1.23 = 14.830m/s and the significant wave height (Hs) = 1.616 x 10 -2 UA fetcheff 0.5 = 2.112m. This calculated significant wave height is near to BMKG recorded data at 2.50–3.00m. Furthermore, maximum wave height can be calculated using simple and practical equation proposed by Goda [2] that Hmax = 1.8Hs = 3.80m. This value is also close to BMKG recorded data at 4.0-5.0m. 2.2. 2D Wave Model Two-dimensional spectral wave model with energy dissipation and diffraction terms was used in this current study. It simulates a steady-state spectral transformation of directional random waves simultaneous with ambient currents in the coastal area. 2D Wave model in here is based on the wave- action balance equation [3]. ( ) ( ) ( ) ( ) SNN CC NCC NC y NC x NC byy g yyg yx −−      −=   +   +        22 cos 2 cos 2 (1) where: ( )   ,E N = (2) is the wave-action density as a function of frequency σ and direction θ. E(σ,θ) is spectral wave density representing the wave energy per unit water surface area per frequency interval. Execution of the numerical scheme of those governing equation are explained in some literatures [3, 4]. Cx, Cy and Cθ are the velocity characteristic with respect to x, y, and θ direction; Ny and Nyy symbolize the first and second derivatives of N with respect to y; C and Cg are wave celerity and wave group velocity; then, κ is an empirical factor which depicted the magnitude of diffraction; εb is the energy dissipation during wave breaking parameter; S means additional sources such as: bottom friction loss, wind strength and interaction of nonlinear wave. 2.2.1. Wave diffraction. The first term on the right side of equation 1 is the wave diffraction term derived from a parabolic approximation wave theory [3]. In application practice, values of κ are in between 0 with no diffraction to 4 for strong diffraction. A value of κ = 2.5 was used by [3, 4, 5] to model wave diffraction for both narrow and wide gaps between breakwaters. κ value = 4 is recommended for wave diffraction at a semi-infinite long breakwater or at a narrow gap case with the opening equal or less than one wavelength. In the case of a fairly wider gap with an opening greater than one wavelength, the value of κ is equal to 3. 2.2.2. Wave reflection. The wave energy reflected at a shoreline is computed in assumptions of the incident and reflected wave angles are proportional to the shore normal direction and the reflected wave Nr is assumed to be linearly relative to the incident wave Ni : irr NKN 2 = (3) where Kr is a coefficient of reflection, Kr = 0 for no reflection and Kr = 1 for full reflection. Kr is defined as the ratio of reflected to incident wave height [6]. Civil and Environmental Science Journal Vol. I, No. 02, pp. 088-095, 2018 93 Refraction diffraction analysis were made using two dimensional 2D wave model at steady state condition with model grid 5 x 5m2 as displayed in Fig. 7. 2D wave model analysis in here used BMKG data recorded on January 25th, 2012 when the wave height was varied from 4.0m–5.0m and strong wind blew from North West direction. Two scenarios of wave model have been developed to simulate wave height in surround breakwater. There are 4 (four) examination points (A, B, C, D) to observe wave height generation due to significant wave height, Hs, of 4.00m and 5.00m as displayed in Fig. 8. Figure 7. Numerical model bathymetry grid for 2D wave model of Waikelo Port Figure 8. Examination points (A, B, C, D) of 2D wave model simulation in surround breakwater Civil and Environmental Science Journal Vol. I, No. 02, pp. 088-095, 2018 94 3. Result and Discussion The simulation result of developed 2D wave model from 2 of significant wave height scenarios at 4.0 and 5.0m are displayed in Figs. 9, 10, 11, and 12. Figure 9. Simulated wave propagation due to 4.0m of significant wave height (Hs). Figure 10. Distribution of wave height in surround breakwater due to Hs = 4.0m. Figure 11. Simulated wave propagation due to 5.0m of significant wave height (Hs). Figure 12. Distribution of wave height in surround breakwater due to Hs = 5.0m. Based on the result of 2D wave model as shown in Figs. 9, 10, 11 and 12 above, it can be seen clearly the variation of wave height in 4 examination points (A, B, C, D) in front and back side of breakwater. The details are summarized in Table 2. Table 2. Simulated wave height in surround breakwater Examination points Hs = 4.0 m Hs = 5.0 m Front side (m) Back side (m) Front side (m) Back side (m) A 4.31 2.22 5.87 2.73 B 4.14 1.55 4.79 1.72 C 3.82 1.53 4.65 1.64 D 3.80 1.83 4.75 1.73 Civil and Environmental Science Journal Vol. I, No. 02, pp. 088-095, 2018 95 From the diffraction and refraction analyses inform that the wave height in front side of breakwater is varied from 3.80m to 5.87m. This value is much higher than 50 years return period of wave design (2.00 meters) which is used as a calculation reference of breakwater structure. 4. Conclusions Calculation of wave generation based on wind speed on January 25th, 2012 at 23 knot, produce significant wave height 2.11m and maximum wave height 3.80m. Those calculation results are quite close to recorded data by BMKG. Based on the result of 2D wave model simulation, it can be concluded that the wave height in surround breakwater is higher than wave design of 50 years return period which is used in design and calculation of breakwater structure. The reasons of Waikelo Port breakwater failure can be classified in reasons relate to the hydraulic and loads conditions: exceedance of wave condition and it can be categorized also in force majeure type. References [1] Oumeraci H., 1994. Review and analysis of vertical breakwater failures - lessons learned, Coastal Engineering 22, pp 3-29 [2] Goda Y., 1985. Random Seas and Design of Maritime Structure (University of Tokyo Press) [3] Mase H., 2001. Multidirectional random wave transformation model based on energy balance equation, J. Coastal Engineering 43(4), pp 317-337 [4] Mase H., Amamori H. and T. Takayama T., 2005a. Wave prediction model in wave-current coexisting field, Proc. 12th Canadian Coastal Conference [5] Mase H., Oki K., Hedges T. S., Li H. J. and Morkoc H., 2005b. Extended energy-balance-equation wave model for multidirectional random wave transformation, Ocean Engineering 32 (8-9), pp 961-985 [6] Dean R. G. and Dalrymple R. A., 1984. Water Wave Mechanics for Engineers and Scientists Englewood Cliffs (Prentice-Hall, Inc)