Designing a Harbor Entrance to Defeat Swell Wave Agitation. FAX (506) ; 2

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1 Designing a Harbor Entrance to Defeat Swell Wave Agitation Mauricio Wesson, MSc. PE 1 and Jack C. Cox, PE, DPE, DCE, DNE 2 1 Watermark, La Sabana, San Jose, Costa Rica; PH (506) ; FAX (506) ; mwesson@watermark-consultants.com 2 SmithGroupJJR, 625 Williamson Street, Madison, WI 53703; PH (608) ; FAX (608) ; jack.cox@smithgroupjjr.com ABSTRACT A pair of islands are currently being constructed just offshore of Panama City, Panama to create developable land close to the city core. The man-made islands are situated south of Punta Pacifica in front of Panama City A multidisciplinary design team led by coastal engineers was tasked with designing the recreational harbor, to be situated between the islands. The harbor design had to achieve satisfactory tranquility in the berths, provide safe navigation though the entrance for vessels up to 90 meters (300 feet) in length, maintain water quality circulation, minimize siltation, and offer aesthetic and recreational amenities. INTRODUCTION The bay of Panama is exposed to the south Pacific Ocean, water depths inside the bay deepend at a very low slope until they reach a water depth of 200 m, in the 200 km opening of the Bay(figure 1). Wave activity in the Bay of Panama is not the typical, locally generated wind waves. Instead, the Bay is dominated by swell waves generated at far distances from the Southern Hemisphere. Typical wave periods range from 14 to 21 seconds, with periods of swells over 20 seconds appearing annually. Figure 1. Project Location Unlike shorter period wind waves, swells refract and diffract much more severely, propagate much deeper into embayments due to their longer wavelengths, reflect more strongly, and are more

2 difficult to absorb. Furthermore, in shallow waters these swells become highly grouped and exhibit complex non-linear characteristics. Designing a harbor entrance to minimize the propagation of this swell action to the berths is therefore much more challenging and requires innovative engineering to overcome the problem. This paper provides an overview of the state-of-the-art modeling techniques and unique engineering approaches applied to create a successful harbor entrance design in a complex swell-wave climate. The design evolved by modeling various combinations of breakwater lengths and configurations, incorporating beach placements. The analysis was performed using a Non Linear Wave Transformation Model based on the Boussinesq equations to realistically simulate the swell wave behavior and diffraction patterns. Animations of the wave action were created to explore wave behavior patterns not apparent in the simple reporting of s. The results demonstrate a clear and successful indication of the performance to be expected from the constructed harbor design. DESIGN PARAMETERS AND CHALLENGES The twin islands of Santa Maria and La Pinta are currently under construction off the coast of Panama City. The original design of the islands did not have provisions for a marina; however, a basin concept was developed to locate a yacht harbor in the waters between the islands. To provide shelter from the long swell waves of the Bay of Panama, breakwater appendages were added to the south end of both islands. Unfortunately, the diffraction and reflection of these swell waves still caused unacceptable basin agitation. Special island armor edge treatments were recommended to absorb this extra wave energy; however, this armoring could not be incorporated into the final island design for cost reasons. Therefore, alternative means of eliminating or reducing the swell wave agitation had to be developed. A key design requirement stipulated that the solution could only rely on geometry changes to the harbor entrance, since the shape and design of the islands were already permitted, fixed, and under construction. The construction of the breakwater was to be similar to the armoring of the islands; no special wave absorbing features would be considered. The original slip plan for the Ocean Reef marina anticipated a boating market composed of seaworthy vessels in the size range of 12 to 25 m (40 to 80 ft). However, given the value and exclusivity of the overall Ocean Reef development, an expanded boating market was targeted to meet a demand for mega-yacht to gigayacht boats of at least 30 m (100 ft) to as great as 90 m (300 ft). The design needed to provide for this demand while also retaining berthing for the traditional smaller boating market. Of critical importance in providing for the mega-yacht market is assuring adequate water depth, both in the marina and the approach. Vessels of 60 m (200 ft) length require 3 to 3.7 m (10 12 ft) of draft with beams of roughly 11 m (35 ft). Natural low water depth between the islands is roughly 3.5 m (11.5 ft). Ideally, at least 1 meter of water should be maintained under the keel to prevent the water

3 intakes from ingesting sediments and clogging filters. This clearance is also needed because of the wave agitation in the basin. Based on this, the basin needs to be dredged to a depth of at least 4.5 meter mean lower-low water (MLLW). If the basin is to be accessible during all stages of the tide, then an approach channel must also be maintained to at least a 5 m (16.4 ft) depth. If sailboats are used in the basin, then the required depth increases at least 2 m (6.6 ft) more in both the approach and the berth. Because of sedimentation, an advance allowance of 1 m (3 ft) for shoaling should be provided. Therefore the basin depth needs to be initially set at 5.5 to 7.5 m (18 to 25 ft) MLLW. MODELING APPROACH AND METHODOLOGY The initial study of wave propagation into the proposed harbor was performed using the computer model CGWAVE. Because precise knowledge of wave agitation levels was needed based on changes to entrance geometry, and because diffraction and reflection of waves into and around the basin were critical, the second modeling effort employed a more sophisticated Non Linear Wave Transformation Model. This model was based on Boussinesq s equations for the numerical model of wave propagation and structure interaction, accounting for partial absorptions. The same wave conditions were input for both modeling efforts. In addition, an initial base case wave agitation calibration was performed to confirm that the predictions of the original model and the new effort were exhibiting comparable results for the same conditions. Because of the long period swell waves, a basic wave reflection coefficient of 0.7 was assumed for all rock armored sloping flanks of the islands and the breakwater. The modeling wave grid resolution was 6 meters (20 ft) square, and wave heights were graphically reported in meter (0.65 in) increments. Previous hindcasting of waves reaching the site suggested incident swell wave conditions of 1.5 m (5 ft) at 18 seconds (HDR, 2008). Design criteria for conditions desired inside the marina basin followed the industry guidelines as shown in Table 1 (Cox, 2008). Peak swells here are an annual and well-documented phenomena. Therefore, a target swell height at the berth, assuming a head sea condition, is 0.6 meter (2 ft) horizontal excursion. This is associated with a 0.25 m (0.75 ft) swell of 18 to 21 seconds. Therefore the ideal agitation level at the berths is 0.25 meters or less. Table 1. Recommended Harbor Tranquility Standards for Marinas Provisionally Recommended Criteria for a Good Wave Climate in Small Craft Harbors Direction and Peak Period of Design Harbor Wave Wave Event Exceeded Once In 50 Years Wave Event Exceeded Once A Year Wave Event Exceeded Once Each Week

4 Head Seas less than 2 second These conditions not likely to occur during this event Less than 0.3 m/1 ft Less than 0.3 m/1 ft Head Seas between 2 and 6 seconds Less than 0.6 m/2 ft Less than 0.3 m/1 ft Less than 0.15 m/0.5 ft Head Seas greater than 6 seconds Oblique Seas Beam Seas less than 2 seconds Beam Seas between 2 and 6 seconds Beam Seas greater than 6 seconds Less than 0.6 m wave height or 1.2 m/4 ft horizontal wave motion Less than (2-1.25sinθ) ft where θ is the wave angle from head sea The conditions not likely to occur during this event Less than 0.25-m (0.75 ft) Less than 0.25-m wave height or 0.6 m/2-ft horizontal motion Less than 0.3 m height or 0.6 m/2 ft horizontal wave motion Less than (1-0.5sinθ) ft where θ is the wave angle from head sea Less than 0.3 m/1 ft Less than 0.15 m/0.5 ft Less than 0.15 m wave height or 0.3 m/1 ft horizontal motion Less than 0.15 m wave height or 0.5 m/1.5 ft horizontal motion Less than ( sinθ) ft where θ is the wave angle from head sea Less than 0.3 m/1 ft Less than.08 m/0.25-ft Less than 0.08 m wave height or.25 m/0.75 ft horizontal motion 1 For criteria for an excellent wave climate multiply by 0.75; for a moderate wave climate multiply by 1.25

5 RESULTS AND RECOMMENDED SOLUTION A preliminary estimate of the modifications required to the breakwater entrance was first attempted using monochromatic diffraction diagrams (USACE, 1984). To achieve a required attenuation coefficient of 0.25/1.5 = 0.15, the initial estimate of needed modifications suggested that the planned east breakwater might need to be lengthened to approximately 395 m (1300 ft) overall. Figure 1 shows the approximate location of the expected 0.25 m (0.75 ft) wave state for each of the breakwater lengths (395, 365, 335, 305, and 235 meter lengths). The 0.25 meter condition occurs along a line defined by a K D value of To the right of each of these lines, s would be expected to be less, not considering reflection. The turning and maneuvering requirements of a 90 m (300 ft) yacht were used to define limits and possible positioning of the breakwater extension. Also indicated in Figure 1 is the tightest allowable vessel turning radii, as well as the minimum width of an entrance channel. This maneuvering path is shown by the curving pathway passing through the entrance channel. Figure 1. Entrance channel design parameters The wave model was analyzed to develop basic geometry requirements to achieve a 0.25 m (0.75 ft) or less wave agitation level at the outer end on Amador dock (90 m/300 ft yacht berthing) for an 18 second wave period. For the initial analysis, the previous breakwater alignment was effectively preserved but lengthened. The results suggest the breakwater should be extended a minimum of 130 m (427 ft) beyond its original design length of 235 to 365 m (770 to 1200 ft) overall, in order to

6 achieve acceptable performance. This is shown in Figure 2. Further reductions in breakwater length degraded agitation levels at the berths outside the tolerable criteria. Figure 2. Required extension of breakwater for T18s The performance of the 365-m (1200 ft) breakwater was then tested for sensitivity to both wave direction and wave period. The previous wave analysis suggested that the troublesome swell waves approach at a compass heading of approximately 163 azimuth. However, the swell direction is largely unidirectional with little spreading. Because the swell wave approach is constrained, a parametric check for wave approaches +/- 5 from the basic azimuth showed little difference in basin agitation. The results based on a longer period 21 second wave period swell were also tested. Unfortunately, at the longer wave period, the agitation increased to greater than the target tranquility criteria. This comparison is shown in Figure 3. Figure 3. Comparison of extended breakwater for T18s and T21s

7 This required further modifications to the entrance geometry to lower wave transmission and agitation. The most significant consideration was the pattern and path of wave propagation through the entrance channel into the berthing area. Animations from the Boussinesq model of these wave propagation patterns provided insight into the reflection-diffraction occurring at the entrance of the harbor. Assessment revealed that the curvature of the east breakwater was a contributing source of the agitation. To encourage greater diffraction of the 21-second swell wave, the entrance design approach evolved into exploring methods for diverting the swell path away from the berthing area. Making the breakwater longer, both straight as well as serpentine, was effective but undesirable as it degraded the primary viewshed from the island properties. A second attempted solution introduced a roundhead at the end of the east breakwater and widened the channel inside the roundhead to create a stilling basin; this approach did not achieve the desired tranquility, however. The results and performance projection for the third and recommended breakwater design approach are shown in Figure 4. This design uses a unique fishtail configuration at the tip of the east breakwater. This fishtail creates a double diffraction of the swell wave as it penetrates the entrance channel. The figure shows that the wave conditions through the entrance channel are expected to be 0.7 m (2.3 ft) or less during major swell events, and that the conditions at the 90-m (300 ft) berths on Amador dock are expected to be less than 0.3 meters (1 ft). Figure 4: Performance for the recommended breakwater design

8 The geometric requirements for the recommended entrance breakwater are summarized in Figure 5. The overall length of the east breakwater is approximately 340 meters. The west breakwater is approximately 217 m (712 ft). The key design feature that makes the entrance work to defeat swell wave penetration into the marina is the fishtail tip of the east breakwater. The outer spur is a half wavelength in dimension. The minimum channel width at low tide is 64.5 m (212 ft), and a 109 m (358 ft) diameter turning circle is accommodated at the inner end of the entrance channel. Figure 5 also incorporates a sloping beach inside the inner breakwater that runs parallel to the entrance channel, a feature that was added to the final design. Figure 5. Geometry of recommended breakwaters The crest elevation of the east breakwater is assumed to be +7 meters (23 ft), with the west breakwater +6 m (20 ft). The crest height recommendation is based on expected overtopping of the breakwater by some of the larger swell waves. The data suggests that the freeboard of the breakwater should be approximately one wave height above the design water level to achieve 10% transmission. Assuming a 1.5 m (5 ft) design wave, the crest height of 5.5 m (18 ft) tide plus 1.5 m wave equals a 7 m (23 ft), crest height. For the west breakwater, the crest can be lower at +6 m, though a transition elevation from EL +7 m down to EL +6 m is recommended at the root of the breakwater where it merges with Santa Maria Island.

9 The outer spur in the fishtail of the west breakwater is also lowered to EL +6 m (20 ft), since the added height for overtopping protection is not needed here. Recommended Design The recommended engineering solution is an offset entrance with inner and outer harbor mouths, connected by a navigation channel several wave lengths in extent. The navigation channel is oriented to force maximum diffraction of any entering swell waves away from the sailing line. To enhance this effect, a fishtail tip is applied to the entrance breakwater. Adding this secondary spur at the entrance breakwater tip forces a repositioning of the primary wave diffraction point, creating the desired calm wave climate for the harbor entrance. The entrance channel itself was given a more sinuous shoreline, with undulations on the scale of the wavelengths to disrupt a mach-stem wave action and more effectively scatter the reflecting waves. As a final enhancement, a unique wavedissipating beach feature was modeled and added along the inner bank of the entrance channel. This beach differs from traditional beaches in that it does not slope perpendicular to the channel as would be expected but rather slopes parallel with the channel; this allows much of the swell action to be dissipated across a long, mildly sloping beach requiring only a minimum amount of real estate. Numerical model results for the entrance design with this beach enhancement are shown in Figure 6. Figure 6. Design performance including beach enhancement

10 Computer modeling of the entrance with the proposed beach suggests that the incorporation of the beach is both feasible and stable. The shift in the navigation channel slightly south to accommodate the beach improves the maneuvering safety of yachts transiting the harbor entrance, while the beach provides additional wave damping so that the entrance channel is calmer. The level of wave action projected to develop on the beach, H 0.2 m (0.66 ft), is also considered to be safe recreationally. CONCLUSION A rendered plan view of the recommended entrance design with the beach enhancement is shown in Figure 7. An innovative double head Fish Tail breakwater was found to be the optimal solution to shelter the marina basin from the long travelled swells that affect the Pacific Coast of Panama. These swells which start their journey off the coast of Australia present a difficult engineering challenge due to the strong reflections and diffractions their penetration causes in harbor entrances. The unique solution modeled and proposed here would disrupt the penetration of the long swell waves without requiring the construction of an excessively long or visually dominating breakwater. This approach to the entrance design also brings added capacity and opportunities for amenities to the development. In particular, the re-alignment of the entrance breakwaters and improved tranquility created more development areas for private slips within the harbor. The design also accommodates the maneuvering needs and capabilities of vessels up to 90 m (300 ft) long. The recommendation to use a 1V:1.5H slope, as well as lower the crest height on segments of the breakwater in less critical areas, minimizes volume requirements. While this needs further confirmation based on geotechnical considerations as well as for stability in wave attack, it represents a valuable cost-saving opportunity for the developer. In addition, some of the armor on the marina side of the west breakwater could potentially be deleted completely pending engineering review and construction phasing. The result is an optimized design approach providing clear performance benefits, both technically and financially. Figure 7. Rendering of modeled harbor entrance design

11 REFERENCES CIRIA. Manual on the Use of Rock in Coastal and Shoreline Engineering, Balkema Cox, Jack, Zen and the Art of Marina Design, presentation to Docks and Marinas International Course and Conference, University of Wisconsin, HDR, Ocean Reef Conceptual Marina Study, Preliminary Marina Design Report, HDR report submitted to Grupo Los Pueblos, June 20, Klancnik, Fred, Jack Cox and others. Chapter 2: Entrance, Breakwater and Basin Design, ASCE Manual 50: Planning and Design Guidelines for Small Craft Harbors, Third Edition, American Society of Civil Engineering, USACE, Shore Protection Manual, Coastal Engineering Research Center, Wesson, Mauricio and Cox, Jack. Breakwater Redesign for Harbor Agitation Control, Ocean Reef Islands Marina, Panama City, Panama. SmithGroupJJR report submitted to Grupo Los Pueblos, July 7, Wesson, Mauricio and Cox, Jack. Entrance Redesign for Beach Inclusion, Ocean Reef Islands Marina, Panama City, Panama. SmithGroupJJR report submitted to Grupo Los Pueblos, October 19, 2011.