Planning and Engineering of Coastal Flooding Mitigation Works of an Airport Runway in a Storm-Tracked Island

Planning and Engineering of Coastal Flooding Mitigation Works of an Airport Runway in a Storm-tracked Island

Eric C. Cruz1,2 Edgardo P. Kasilag II2

1Professor, Institute of Civil Engineering, University of the Philippines Diliman, Quezon City 1101, Philippines; Email: [email protected] 2 Principal, AMH Philippines Inc., Bahay ng Alumni Bldg, U.P. Diliman Campus, Quezon City 1101

ABSTRACT: An existing airport requires a length extension of its runway to meet increased air traffic demand. The extension places the runway termini to be along the coasts. This paper discusses the methodology applied to address the likely coastal flooding at both termini, which are tracked by typhoons. The approach hinges on the quantification of storm tide levels and local wave effects induced by historical typhoons. Aviation clearance requirement and sediment stability around the seawall toe are also addressed. It is found that the required minimum non-overtopping seawall elevation is highly influenced by historical storm and by local seabed features.

Keywords: coastal flooding, runway, seawall, overtopping, typhoons, engineering

INTRODUCTION

With a growing demand for inter-island trade, commerce and tourism, the Philippines is building new or upgrading existing transport infrastructures in major islands. One of the existing airports needs an extension of its runway length to meet the growing domestic and international demand for air travel to Boracay Island, which is famous for its tropical beaches and marine recreation, and as a popular “sun and beaches” destination of Philippine tourism. The airport is in the northern tip of Aklan Province, which hosts the nearest seaport gateway to Boracay. The existing runway has a length of 950 m and runs southwest-northeast (Figure 1) is designed to serve only small-capacity propeller airplanes from major cities such as Manila and Cebu. Due to the runway’s limited length, planes need to meet the short stopping distance, thus compromising air traffic safety, as they risk hitting a mountain hill near the airport terminus. The low-capacity runway and the high demand also necessitate short intervals for plane landing and taxiing which further decreases safety and passenger comfort. Feasibility studies indicated that a minimum runway length of 1,800 m is needed to service the bigger turbo-propeller aircrafts to match the increased air travel demand. While it is recognized that a long-term planning of the expanded airport

Page 1 could have yielded a more optimal airport master plan, the increased length could only be done without reorienting the runway due the presence of hills and the unattractive economic viability of rebuilding the runway infrastructure from scratch. As a result, the runway extension could only be undertaken by extending both termini along the existing orientation. However, this constraint requires the runway termini to be very close to the shores and thus to the waters. This also compelled the re- alignment and rerouting of existing municipal roads to the fringes of the extended runway and thus out of harm’s way in the extended runway. Such proximity to the seas at both ends also called for protection works against hazards from the seas, namely, storm tides, typhoon waves and storm surges, which can lead to coastal flooding of the airport runway.

FIG. 1. Project location DESCRIPTION OF PROJECT COAST

The project runway is located in the northern tip of Panay Island in central Philippines (Figure 2), and is bounded by Sulu Sea to the west, and by Sibuyan Sea to the east. The island hosts 4 major seaports for inter-island trade and commerce. The project airport is one of 3 airports in the island and serves mostly tourism-geared visitors to Boracay Island and Aklan. Sibuyan Sea is frequented by tropical cyclones and typhoons, while Sulu Sea is exposed to strong monsoon winds from May to September, in addition to seismic events in West Philippine Sea.

FIG. 2. Project location (aerial: Google Earth)

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PROJECT DATA

Preliminary engineering for the coastal flooding protection of the airport runway commences with a study of prevailing waves and winds during non-storm conditions. Figure 3 shows the wind rose diagram based on the nearest wind station summarizing the directional and wind speed distributions of surface winds in a typical year for the Sulu and Sibuyan sides/ends of the runway termini. It is seen that the Sulu side is exposed to essentially south-westerly winds, and Sibuyan side to north-easterly winds whose maximum speed is about 24 mps.

FIG. 3. Annual prevailing winds a) Bathymetry Figure 4 shows the bathymetry of the coasts, consolidated from available small- scale offshore topography map and a local bathymetric survey, covering an area of 7.7 km x 4.2 km for Sulu coast and 2.8 km by 2.7 km for the Sibuyan coast. For Sulu, Figure 4 reveals the highly irregular shape of the seabed with an embayment coast to the southwest (zone A), a shallow shoal of 3m depth at around 300 m to the west (zone B), and a wide flat foreshore northeast of the site (zone C). Simulation results of prevailing waves show that the shoal affects the magnitude and direction of waves near the shoreline, depending on the wave approach direction.

600 m FIG. 4. Nearshore bathymetries at Sulu (left) and Sibuyan (right) coasts

In Sibuyan coast, the shore is fronted by a coral reef that edges out to 1-m depth (zone D) at between 300 m to 400 m (zone E) from the shore. Prevailing wave simulations indicate that waves break on the reef and continue to break over the wide

Page 3 reef flat, dissipating most of the wave energy before they hit land. At about 800 m offshore, the seabed has an irregular terrain (zone F), most likely due to past dredging activities. Wave simulations show that this local feature significantly modifies the wave heights reaching shore, causing wave energy concentrations along the reef edge, before being dissipated by wave breaking on the reef. b) Historical Typhoons Figure 5 shows the tracks of potentially critical typhoons in the area of the 2 project coasts. It is seen that all of the critical typhoons originated from the Pacific Ocean, passed through Sibuyan Sea and took one of the following routes after Sibuyan: (a) proceeded west northwest along its original path, (b) proceeded west southwest along its original path, (c) got deflected by Tablas Island to the north, or (d) turned northwest after existing Panay Island. The wind speeds tend to be high in Sibuyan Sea and decrease after passing the Visayan Islands. Relative to the project coast, the tracks are mostly north of the 2 project sites. The tracks and meteorological data are used to determine the critical historical typhoons that generated the maximum storm tide levels and waves that should govern the design of coastal protection works.

Tablas Is.

Site Sibuyan Sea

Sulu Sea

FIG. 5. Tracks of critical historical typhoons

Table 1 summarizes the track and meteorological data of the historical typhoons that traversed the Sulu and Sibuyan Sea ends of the airport runway. Typhoons Utor, Axel/Grading, and Manny/Naning all tracked north of the Sulu project site. Utor induced the highest wind speed and lowest central pressure along the Sulu side, Axel tracked closest to the site, and then went northwest after hitting Panay Island. Manny followed an unusual west-southwest track that potentially could have generated high waves from that direction. Six critical typhoons are found to have critically tracked the Sibuyan end, as summarized. Half of these typhoons tracked north of the site, and the other three south of the site. Typhoon Haiyan (Yolanda), considered the strongest typhoon recorded for the Philippines, induced the highest wind speed on water but passed to the south of the site, which could have reduced its potential strength. Only Faith and Haiyan tracked south, but very close to the site.

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Table 1. Historical cyclones that tracked Sulu (top) and Sibuyan (bottom) coasts Typhoon/ local Vmax Rmax Pc Closest Case name Year/ mo. (mps) (km) (hPa) distance (km) Track Sulu coast 1 Utor/ Seniang 2006 Dec. 42.70 111 955 24 north 2 Axel/ Garding 1994 Dec. 21.10 0 985 12 north 3 Manny/ Naning 1993 Dec. 21.70 0 985 39 north Sibuyan coast 1 Haiyan/Yolanda 2013 Nov. 46.30 129 940 41 south 2 Vanessa/ Edeng 1986 Oct. 23.00 0 994 6 south 3 Fengshen/ Frank 2008 June 45.00 92 965 48 north 4 Utor/ Seniang 2006 Dec. 40.00 111 970 24 north 5 Cecil/ Bebeng 1979 Apr. 35.00 111 980 78 north 6 Faith/ Norming 1998 Dec. 35.00 129 970 9 south

PRELIMINARY ENGINEERING OF RUNWAY SEAWALLS

With site development considerations, preliminary engineering proceeded to determine the cross-section geometry of the seawalls and their dimensions. The vertical siting of the seawall crest is a critical parameter to complete the cross-section and proceed to detailed engineering. The seawall crest level is determined based on (a) typhoon-induced storm tide level, (b) wave-induced runup, and (c) aviation vertical clearance requirements. The storm tide level, or STL, is the uplifted sea surface level due to the meteorological forcing by typhoon winds and pressure gradient. Following the methodology in Cruz and Luna (2014), we computed the STL based on the critical historical typhoons summarized in Table 2 for Sulu coast. Local waves and their interactions with the seawall are considered in the vertical siting. These interactions depend on the seawall’s plan-form, cross-section geometry of the armor layer, seabed profile, and on the STL. Figure 6 shows the plan-forms of the Sulu seawall, and transects along which wave run-ups are computed. The cross- section is a rock mound with a 1:2 sloping armor layer made up of suitable-sized rocks as armor units. Along the Sibuyan coast, considering the existing coral reefs, the seawall has been placed at a more inland location, thus at a higher ground.

FIG. 6. Plan-forms of seawall at Sulu (left) and Sibuyan (right) runway termini Page 5 a) Design Water Level The following data are used to hindcast the storm surge and the resulting storm tide: 1) typhoon’s meteorological data along its track; 2) direction of typhoon wind relative to the project coast; 3) project coast location relative to the ttrack; 4) wave fetches along typhoon’s main wind direction; and 5) averaged depths of the sea along the wind direction. The wind setup and pressure storm surge components computed from these data are summarized in Table 2. The STL is taken as the sum of the storm surge and the astronomic tide. The astronomic tide is taken as the mean higher high water (MHHW) which is MTL+0.75m. The storm tide is taken as the higher value between this sum and the recorded High Water Level (HWL), which are available in annually published tide tables (NAMRIA 2014). Based on Table 2, the storm surge is highest for typhoon Utor (+1.35 m), leading to a storm tide level of MSL + 2.10m. Axel, in comparison, likely generated a storm surge of +0.51 m and storm tide level of MSL +1.26m. For the Sibuyan coast, Haiyan generated the highest historical STL (up to 2014) at MSL+1.43m consisting of a high pressure surge due to a large pressure difference and its track’s close proximity to the Sibuyan coast.

Table 2. Elements of historical storm tide – Sulu coast

Case Name Vmax Wind Astronomic Pressure Storm Possible Storm (mps) Setup (m) Tide (m) Surge (m) Surge (m) Tide (m) Sulu coast 1 Utor 43.73 0.80 0.75 0.55 1.35 2.10 2 Axel 25.72 0.51 0.75 0.00 0.51 1.26 3 Manny 25.72 0.28 0.75 0.00 0.28 1.06 Sibuyan coast 1 Haiyan 46.30 0.09 0.75 0.59 0.68 1.43 2 Vanessa 23.00 0.01 0.75 0.00 0.01 1.29 3 Fengshen 45.00 0.11 0.75 0.42 0.53 1.29 4 Utor 40.00 0.00 0.75 0.55 0.55 1.30 b) Storm Wave Runup on Seawall The project coasts are located in Sulu and Sibuyan Seas that are known to be frequented by typhoons. Storm waves generated by typhoons propagate into the project coast and cause short-term effects that affect the NOSE. The meteorological conditions of the historical typhoons at their closest approach to the project coast are used to hindcast the offshore wave conditions based on the theory for quasi-stationary hurricane waves (CEM, 2004). These offshore wave conditions are summarized in Table 3 in terms of significant wave height HS, period TS and approach direction for the Sulu coast. As seen, Utor induced the highest offshore waves on account of its high wind speed. These deepwater wave conditions are inputted into a storm wave to determine the local waves near the toe of the seawall. To account for the storm surge simultaneously induced by these typhoons, wave simulations were carried out with the water surface raised to the STL of each storm

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(see Table 2). This water level effectively increased all water depths and translated the shoreline inland.

Table 3. Deepwater wave conditions of typhoon waves – Sulu coast

Deepwater Waves

Typhoon Date Vmax (mps) Rmax (km) Pc (hPa) Hs (m) Ts (s) Dir. Utor Dec 2006 43.75 111 955 6.00 13.38 WNW Axel Dec. 1994 25.72 0 985 3.60 10.37 NW Manny Dec. 1993 25.72 0 985 3.10 9.62 WSW

Figure 7 plots the simulated wave heights for the Sulu coast together with the extended runway layout. Wave energy concentration zones are clearly seen north of the seawall under Axel and to the south under Utor. Due to the low offshore wave height of Typhoon Axel, local waves in front of the Sulu seawall are consistently lower. Under Manny (not shown), wave concentration zones are induce in front of the shallow shoal, break on it, and get dissipated before reaching the seawall’s toe.

FIG. 7. Wave heights in Sulu coast due to typhoons Axel (left) and Utor (right)

Reef edge

FIG. 8. Storm wave heights in Sibuyan due to Haiyan (left) and Fengshen (right) Page 7

Figure 8 shows the local storm wave heights in Sibuyan coast due to typhoons Haiyan and Fengshen coinciding with the computed STL in Table 2. It is seen that Fengshen generated the highest waves in near the seawall’s toe. Although Haiyan packed the highest wind speeds and was the biggest historically, its wind speeds was already reduced when it hit Panay Island, and thus induced lower heights along the project coast. Figure 8 reveals that on the reef edge where the depth is rapidly reduced, incident waves are forced to break. The broken waves are further damped by bottom friction on the shallow reef shelf, and thereby become low at the seawall toe. c) Non-overtopping Seawall Height Effects of wave-structure interaction include the wave setup due to the breaking of waves in shallow water, and the runup of waves on the seawalls’ seaward slope.Wave setup is based on the formula for breaking-induced set-up based on the breaking location, breaker height, and seabed slope, which is prescribed at 1:2. Wave runup is based on the semi-empirical formula in CEM (2004) for a sloping revetment of known slope, with rough slopes but no berms, of known toe depth, and assumed narrow-banded frequency spectrum at the toe. The toe depth is taken as the sum of still-water depth and the computed storm tide discussed above. A narrow-banded spectrum is assumed at the toe which is considered shallow water. Based on these, Table 4 summarizes the computed elements of the required Non- Overtopping Seawall Elevation, or NOSE, for the Sulu seawall for the 3 critical typhoons. Wave set-down, which is less critical as it is subtractive on the NOSE, is observed for Utor and Axel, based on the breaker. The results indicate the required NOSE as MSL + 4.80m based on Utor. The table also summarizes the NOSE for the Sibuyan coast. Wave set-up results for all typhoon cases shown, since the seawall toe is shoreward of the breaker. Like the Sulu coast, wave runup varies narrowly, and the NOSE comes out lower than in Sulu, namely MSL + 3.10m, based on Haiyan. In spite of the exposure of Sibuyan coast to stronger typhoons, its NOSE is lower, which is due to the low wave runup caused by the dissipation of wave energy on the coral reef.

Table 4. Elements of vertical siting of seawalls

Storm Local HS Local Wave Setup Wave NOSE (m from Typhoon Tide (m) (m) Hmax (m) (setdown) (m) Runup (m) MSL) Sulu coast Utor 2.10 2.67 3.00 -0.06 2.76 4.80 Axel 1.26 2.09 2.40 -0.05 2.22 3.43 Manny 1.06 1.99 2.80 0.03 2.20 3.28 Sibuyan coast Haiyan 1.43 1.60 2.29 0.05 1.63 3.10 Vanessa 1.29 0.60 0.86 0.02 0.72 2.03 Fengshen 1.29 1.20 1.72 0.03 1.19 2.51 Utor 1.30 1.20 1.72 0.03 1.22 2.56

Page 8 d) Cross-section Design Consolidating the results of design water level and NOSE into the preliminary proportions of the rock-mound seawalls leads to the cross-designs in Figure 9. An operational freeboard considering waves and wakes from passing ships is added to the NOSE to locate the final seawall crest. The drainage system for the inland surface runoff is also integrated into the seawall design by adding a minimum through-flow in the seawall at storm tide level.

(Note: Vertical scale exaggerated) FIG. 9. Cross-section of Sulu (top) and Sibuyan (bottom) rock-mound seawalls

FUNCTIONAL PERFORMANCE OF SEAWALLS a) Aviation Clearance Due to the low elevation at the Sibuyan end of the existing runway and the extension to the Sibuyan coast, the final seawall crest elevation needs to be checked against the local requirement for aviation vertical clearances. The extension to Sibuyan places the runway terminus at a low elevation of +4.40m, and with an imposed take-off angle and runway vertical clearance, and a short available distance to the seawall, the minimum vertical clearance for an obstacle is computed to be about 8.7 m. Table 5 summarizes the aviation clearance requirements. Based on the final seawall crest elevation, the actual clearance at the seawall was computed to be 10 m, which meets the requirement.

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Runway clearance Obstacle vertical Angle, φ clearance Land clearance Wave runup STL Max. Runway max. elev. +4.40 Heigh Astro.tide Seawall Elev. 0.0 t MSL Hor. Distance

FIG. 10. Definition of variables for aviation clearance

Table 5. Aviation clearance requirements

Parameter Requirement Actual Take-off Angle φ (degrees) 7.0 7.0 (assumed value) Runway clearance (m) 4.50 4.50 Obstacle clearance (m) 8.71 10.01 b) Scour Protection Scour around the seawall toe is an important consideration in seawall design. In order to evaluate the dynamic stability of pre-construction seabed, seabed and coastal sediment samples were taken from the coasts (Figure 11) and analyzed. Table 6 summarizes the sediment properties for the Sibuyan coast, which indicate fine aggregate material with a narrow range of specific gravities, indicating some mixture of native and coralline sand.

FIG. 11. Sediment samples

Table 6. Sediment Data

Project Soil Location Dry Spec. D50 Coast Sample Description Gravity (mm) Characterization S5 Tidal Zone 2.19 1.00 fine aggregates S6 Tidal Zone 2.35 0.69 fine aggregates Sibuyan S7 Coast 2.49 0.66 fine aggregates S8 Coast 2.30 0.51 fine aggregates

Following a methodology for the dynamic stability under wave-induced bottom sediment motion (Cruz, 2008), the simulated storm wave fields were used together with the sampled sediment properties to determine the required residual grain sizes dr under the critical typhoons. Figure 12 plots the residual grains for the 2 coasts. It seen that existing sediment sizes are smaller, i.e. lighter, than the required residual grains Page 10 in certain zones, including those around the shallow shoal and seawall toe in Sulu coast, and the wave breaking zones along almost the entire coral reef for the Sibuyan coast. The results were used to design suitable toe protections for the seawalls, after the contributions of wave-induced currents had been quantified.

dr (mm)

600 m

FIG. 12. Residual grain sizes for Sulu coast under Utor (left), and for Sibuyan coast under Haiyan (right)

CONCLUSIONS

To mitigate coastal flooding at the coastal termini of the project runway, it is necessary to carry out the preliminary engineering of the proposed rock-mound seawalls as a non-overtopping structure. The computed non-overtopping seawall elevation, or NOSE, is the required minimum vertical siting to contain the historically critical storm tide level and storm wave runup on the seawalls’ armor layer. In addition to historical typhoons, local site features determine the NOSE. For the project coasts here, the existence of shallow shoals and sea-extended seawall toe, caused high waves at the Sulu coast, and thereby the wave component of the NOSE. The presence of coral reef and a shallow shelf also reduced the wave effect in the Sibuyan coast due to wave breaking on the reef. Functional requirements, such as aviation clearance and dynamic stability of surrounding sediments to assess the need for toe scour protection, are also evaluated to complete the preliminary engineering of the mound seawalls.

ACKNOWLEDGMENT

The authors acknowledge the assistance of Engrs. Geogy Vizcara and Jayson Pascual of AMH Philippines during the field inspections and surveys, data gathering and civil works drawing production.

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REFERENCES

Coastal Engineering Manual (CEM, 2005). United States Army Corps of Engineers Coastal Engineering Research Center (SPM, 1984). Shore Protection Manual, Second Edition, Volume I. United States Army Corps of Engineers Cruz, E.C. (2008). Computational analysis of hydrodynamic stability of a sandy beach. Proceedings, 2008 Phil. Inst. Civil Engineers National Midyear Convention, Clark, Angeles, Pampanga, 26-28 June 2008, WRE 1-10 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. Proceedings, National Midyear Convention and Technical Seminar, Phil. Inst. of Civil Engrs., Baguio City, Baguio, 2014 June 6-7, 1-7 Google TM Earth software. Version 7.1.1.1888. http://earth.google.com National Mapping and Resource Information Authority (NAMRIA, 2014) Tide and Current Tables – Philippines 2014 Shore Protection Manual (SPM, 1984) United States Army Corps of Engineers United States Army Corps of Engineers (CEM, 2005). Coastal Engineering Manual http://chl.erdc.usace.army.mil/cem

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