A METHODLOGY FOR HAZARD-AWARE HORIZONTAL SITING AND ALIGNMENT OF AN OPEN PIER - APPPLICATION TO WEST PHILIPPINE SEA CO...
A METHODLOGY FOR HAZARD-AWARE HORIZONTAL SITING AND ALIGNMENT OF AN OPEN PIER - APPPLICATION TO WEST PHILIPPINE SEA COAST Eric C. Cruz1
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Jose Carlo Eric L. Santos2
Water Resources and Coastal Engineering Group, Institute of Civil Engineering, University of the Philippines, Diliman, Quezon City 1101; Email:
[email protected] 2 AMH Philippines, Inc., Bahay ng Alumni Bldg., U.P. Diliman Campus, Quezon City 1101, Philippines
Abstract: This paper aims to present a methodology for the horizontal siting and alignment of an open type of pier along the coastlines of the country. The methodology is adapted to the country’s hazard-tracked location since most of its coastlines are traversed by tropical cyclones annually. The method takes into account site-specific data such as tides, typhoon tracks, cyclone characteristics, and prevailing wave. Optimal siting of an open pier based on prevailing wave climate is reviewed, then its application to an actual project involving a proposed piled pier in a semi-enclosed coastline along West Philippine Sea is discussed to demonstrate the input parameters and the numerical analysis of basis quantities for pier siting. The project application also illustrates the synthesis of engineering parameters that become inputs to preliminary engineering and/or detailed design. It is shown that it is possible to account for storm hazards in the planning and engineering of an open pier.
Key words: pier, siting, waves, storm surge, overtopping
1. INTRODUCTION The Philippines presently has more than 2,467 seaports of various sizes and types. Due to its location and archipelagic nature, the country depends on these ports for intra- and inter-island trade and transportation, as well as for international commerce and tourism. However, a recent study of the causes of damage to the major ports of the country identified the inadequacy of freeboard as one major cause of structural damage and durability problems to these ports. A pier is the most common docking facility of a seaport, and is therefore a crucial element of the port’s master planning. As several new ports are presently in the planning stage and some others are being rehabilitated, it is thus important to have a rational approach to the planning, siting, and preliminary engineering of the port’s pier. Due to the increasing importance of “sun and beaches” in the tourism-related activities, the private sector has long started to develop tourism infrastructures in several beach coastlines of the country. A pier is the primary facility for landing and docking of boats and vessels and is thus crucial in a beach infra master plan. Unlike typical piers, beach piers are of the open type in order not to hamper the littoral processes that sustain the beach and maintain the natural circulation of foreshore waters. Beach piers are also heightlimited to be inconspicuous to guests who normally want to have an unobstructed view of the sea horizon. Archipelagic coastlines are generally bounded by other islands, a
projecting headland, or by some morphological features, such as a spit, that provide some sheltering against waves approaching from certain directions. Such conditions often render the determination of the most suitable location and layout of a beach pier difficult, or at least not straightforward. This paper discusses a methodology of finding the optimal location and layout of a beach pier. It also takes up the accounting of natural hazards from the oceans in the preliminary engineering of the beach pier. 2. NATURAL HAZARDS ON PIER For piers, waves are the primary natural hazards in terms of magnitude and frequency. Wind-generated waves are perpetual loads that all coastal structures must withstand. Astronomic tide is another natural hazard that impact on the siting and engineering of the pier. Tides in archipelagic coasts are generally higher than those in open coasts and can thus considerably influence the engineering of a beach pier structure. Meteorological tide such as typhoon-generated storm surge is also a major hazard that governs the conditions for the pier’s engineering design. While they occur less frequently that wind waves, their action on coastal structures, such as seawall and bulkheads, usually amplify the wave effects such as wave breaking, runup, and wave set-up. As the Philippines lies in a seismically and tectonically active part of the world, tsunamis are also a hazard along most coastlines. While they are the most infrequent of all wave types reaching the coastline, they
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typically have long periods and propagate without breaking and can thus cause catastrophic effects on land, coastal and offshore structures. 3. OPTIMAL SITING OF OPEN PIER The location of a beach pier is optimally determined if it is in the least wave-agitated zone of a local coast. This location is determined on the basis of non-storm or prevailing wind conditions which normally obtain during the pier’s operation. The most important function of the pier is to load or unload cargo or passengers; hence a landing area and mooring zones must be available to safely secure the vessel. The location and layout of the pier are determined by: (1) required draft of the vessel, (2) wave height and directions, and (3) astronomic tide range. For the coastline exposure of a project beach pier, Figure 1 shows the annual wind rose diagram at the nearest wind station of PAGASA in Palawan. This summarizes the directional and frequency distribution of non-storm wind speeds over a 30-year period until 2010. Together with the effective fetches for each direction, a hindcast of the wave condition in deep water is computed as summarized in Table 1. Due to the long fetches, computed non-storm offshore waves are 3-4.4m high, with periods of 6.9-8.1s.
A nearshore wave model is used to determine the local waves as these offshore waves reach the cost of the proposed beach pier. Figure 2 shows a snapshot of the propagation of the prevailing waves from the north, revealing the complicated shoaling, refraction and diffraction transformation in the shallow waters of the project nearshore zone (red rectangle). The resulting wave heights at high tide (MTL+0.54) are shown in Figure 3 for the first 2 cases in Table 1. The synthesis of all offshore wave approach directions (not all shown in Table 2) clearly indicates an optimal siting, i.e. the least agitated foreshore zone where the needed beach pier extending to 4m depth, as laid out in Figure 3.
Figure 3 Optimal location and layout of beach pier
Figure 1 Wind rose diagram Table 1. Offshore hindcast of deepwater waves Annual Fetch Deepwater Waves Wind speed m/s Freq. % (km) Hs (m) Ts (s) N 13- 16.9 0.1 231 4.38 8.07 NNW 9 - 12.9 0.2 272 3.15 7.10 NW 9 - 12.9 0.1 273 3.15 7.11 W 9 - 12.9 0.7 230 3.00 6.88 Dir.
4. PRELIMINARY ENGINEERING OF PIER Preliminary engineering of the pier structure starts with the vertical siting of the pier deck, which is typically designed not to be overtopped or impinged by surface waves. This condition is thus based on the highest displacement of the sea surface, which occurs during typhoons. The vertical siting of the beach pier follows the methodology used for vertical siting of bigger piers in the country (Cruz and Luna, 2014), which uses the combination of the storm tides and wave effects (Figure 4) to determine the minimum vertical siting of shore-detached offshore structures.
Figure 4 Definition sketch for vertical siting
Figure 2 Simulated waves from north at mean tide
Based on the online database of typhoons of Japan Meteorological Agency (Digital Typhoons, 2015), the tracks of all historical typhoons within 150km radius around the project coast are shown in Figure 5.
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Table 4. Synthesis of required Pier Deck Elevation
Figure 5 Tracks of historical typhoons Table 2 summarizes the parameters of 3 of the recent strong typhoons that traversed the project beach. The parametric prediction method (CEM, 2005) is applied to calculate the cyclone-induced wave conditions in offshore deep water summarized in Table 3, and a stormy wave model to simulate the propagation and transformation of these waves in the project nearshore.
Typhoon Astro. Storm Wave Tide Surge crest
Wave Required setup PDE
Ruping 0.54 Pepang 0.54 Bising 0.54 (unit: m)
-0.02 -0.02 -0.01
1.40 0.48 0.59
1.20 1.36 0.81
3.13 2.37 1.93
5. PIER OVERTOPPING RISK In order to address the height limitation of beach piers, it is necessary to carry out an overtopping risk assessment, if the non-overtopping PDE exceeds this height based on the beach development objectives. An overtopping pier design condition (Figure 7) may satisfy the height constraint but will have the following disadvantages: (1) reduced return period; (2) requires bigger structural elements due to the additional loadings such as wave in-deck forces (Tirindelli et al, 2003) and uplift forces (Gaeta et al, 2012); (3) imposes a more intensive maintenance program for the pier; and (4) may significantly increase its trapping action of littoral materials due to more closely spaced and bigger piles.
Table 2. Meteorological characteristics of historical typhoons Typhoon/ Int’l code Yr/mo/day/ hr Ruping/Mike 1990/11/13 1800 Pepang/Zack 1995/10/29/ 0600 Bising/Nelson 1982/3/27 1200
Vmax (mps) 38.6 30.8 25.7
Rmax (km) 185 74.1 0
Pc (hPa) 960 980 990
Figure 6 shows the simulated wave heights and sea surface snapshot during typhoon Mike. At the optimal location of the beach pier, some wave energy concentration of 2m wave heights can be seen. By computing the storm tide, i.e. astronomic tide plus storm surge, and combining the wave effect, i.e. wave crest elevation and wave set-up, from the stormy wave simulation results, the required nonovertopping Pier Deck Elevation (PDE) is determined for each of the 3 historical typhoons, as summarized in Table 4. It is seen that while the wave effect is greatest under Pepang
the required PDE is governed by Ruping at MTL+3.13m. Table 3. Offshore wave conditions in deep water Wave Wave Typhoon Height (m) Period (s) Ruping 5.1 12.34 Pepang 4.0 10.93 Bising 3.15 9.7
Wave direction WNW WNW NW +15o PIER
Figure 6 Wave fields due to typhoon Ruping
Non-overtopping
Overtopping
Figure 7 Non-overtopping and overtopping pier An overtopping PDE is evaluated by computing the exceedance probability of a lower vertical siting. Since the storm tides and wave effects are driven primarily by the wind, a frequency analysis of wind speeds of all historical cyclones that tracked within 150-km radius around the project beach was undertaken. The online database for maximum wind speeds of Japan Meteorological Agency (Digital Typhoon) was used, drawing about 37 annual maximum speeds of 55.6-167.7kph over 42 years. Regression plots and coefficients of determination were used to determine the best probability density functions (PDF); the best 2 plots are shown in Figure 8. It is seen the LogNormal PDF yields the best fit in the low-speed range, although the Weibull PDF yields the best overall fit with a coefficient of determination of 0.97, from which the return periods of wind speeds of the 3 critical typhoons, summarized in Table 5, were computed. Thus a nonovertopping PDE of +3.13 will be exceeded every 12.8 years on the average, while an overtopping PDE of about +1.73m will be overtopped about every 2.2 years.
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Figure 9 Computed tsunami incursion limits
Figure 8 Plots of probability density functions Table 5. Return periods of pier vertical siting design Vertical Siting Return Period Date (m from MTL) (years) Mike/ Ruping 1979-19 3.13 12.78 Zack/ Pepang 1982-02 2.37 4.35 Nelson/ Bising 1995-21 1.93 2.17 Typhoon
6. ASESSMENT OF OTHER OCEAN HAZARDS Due to the location of the Philippines, most of its coastlines are susceptible to either historical or potential tsunamis, which are infrequent compared to typhoons but are potentially disastrous hazards (Cruz et al., 2010). Beach piers are commonly located along interior coasts, which are generally sheltered from far-source tsunami generator, but can be affected by near-source events. After tsunami susceptibility is established from a study of historical and potential tsunamigenic sources, a tsunami incursion map must be determined for the coast of the proposed beach pier whose site development plan may be impacted. Figure 9 shows tsunami incursion limits of a beach pier at mean tide and high tides based on a historical near-source tsunami event. On account of their low steepness, usually tsunami height will not govern the non-overtopping PDE of a beach pier in the nearshore zone. However, the land end of the pier may unwittingly lie within the incursion zone. This info may not be critical to the site planning, but is useful for disaster mitigation planning.
7. CONCLUSIONS It is necessary to account for the site-specific hazards in the planning and siting of an open pier along a beach coast. Non-storm prevailing winds and astronomic tides generally govern the horizontal siting of the pier so that the structure serves its intended function during regular, i.e. non-storm, operations. Once the location is optimally found, the preliminary engineering shall consider the various natural hazards including typhoons, high waves, and storm tides. It should also account for infrequent but potentially catastrophic hazards such as tsunamis. The preliminary engineering of a open pier should include an assessment of the overtopping risks in order to present a clear and rational basis for selecting design pier elevation to meet the pier and beach site development plans. ACKNOWLEDGMENT The authors acknowledge the assistance of Engrs. Ismael Aragorn Inocencio and Julius Florenz Giron of AMH Philippines in the field inspections, data processing and plotting. REFERENCES Coastal Engineering Manual (CEM, 2005). United States Army Corps of Engineers Cruz, E.C. and R.A.C. Luna (2014) A methodology for rational vertical siting of marine infrastructures application to the preliminary engineering of a power plant along a typhoon-tracked seacoast. Proc., National Midyear Convention and Technical Seminar, Phil. Inst. Civil Engrs., Baguio City, 2014 June 6-7, pp. 1-7. Cruz, E.C., J.C.E.L. Santos and E.P. Kasilag II (2010) Analysis and accounting of coastal hazards in the planning and siting of port infrastructures. Proceedings,
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5th Civil Engineering Conference in the Asian Region and Australasian Structural Engineering Conference 2010, 8–12 August 2010, Sydney, Australia (CECAR5 and ASEC 2010). Session: Disaster Reduction, 1-6 Digital Typhoon: Typhoon Images and Information. (20012014). Kitamoto Asanobu/National Institute of Informatics (NII). http://agora.ex.nii.ac.jp/~kitamoto/. Retrieved 2014 October Gaeta, M.G., L. Martinelli, A. Lamberti (2012) Uplift forces on wave exposed jetties: scale comparison and effect of venting. Proc., Coastal Engineering 2012, ASCE. Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) Wind rose analysis 1970-2010 Pagasa Island, Palawan Tirindelli, M., G. Cuomo, W. Allsop, A. Lamberti (2003) Wave-in-deck forces on jetties and related structures. Proceedings, 13th International Offshore and Polar Engineering Conference Honolulu, Hawaii, USA, May 25–30, 2003
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