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Current speed due to the passage of the ‘‘Alaskan Frontier’’. Current fields due to tsunamis were studied using the MIKE 21 HD model, 

Current speed due to the passage of the ‘‘Alaskan Frontier’’. Current fields due to tsunamis were studied using the MIKE 21 HD model, 

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Conference Paper
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Coastal engineering studies were performed to define site and design conditions for the development of new docking facilities and a wave protection structure at Pier F in the Port of Long Beach, California. The new facilities will provide docking to a fireboat, pilot and port security boats with lengths ranging from 30 to 110 feet (9.1 to 33.5 m)....

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Context 1
... of time varying wind waves within the harbor, and corresponding conditions at Pier F, that would be more realistic than those resulting from constant wind speed/direction derived from frequency tables. Pier F is exposed to the west to the POLB West Basin, which has a relatively short 2,000 m fetch; and to the LA-LB harbor to the south which has a longer, approximately 3,000 to 4,000 m fetch. Wave conditions at Pier F generated by the passage of cold fronts in the winter are of particular interest for the assessment of wave protection at Pier F. As cold fronts approach, the prefrontal wind is usually southerly, typically increasing in speed and becoming gusty. As the cold front passes the wind direction changes clockwise to settle usually in a northwesterly direction after the front has passed. The passage of the cold front typically takes 24 hours. Table 2 shows the wind wave conditions at Pier F due to the passage of selected cold fronts in the period from April 2005 to November 2011. Waves generated within the harbor by southerly prefrontal winds exceed the 1-foot (0.3 m) wave height in all cases, and at a rate of more than once per year. After the front has passed, the westerly winds generate 1-foot (0.3 m) wave heights at a rate of approximately once per year. Because boat users at Pier F also reported that swell is also problematic, the magnitude of the swell that reaches Pier F was also estimated by means of numerical modeling. Figure 2 shows the deep water wave windows and sources that affect the LA-LB harbor. Short period local seas and northern hemisphere extratropical storm swells approach the harbor from a westerly direction, while prefrontal seas, tropical storm swells and southern hemisphere extratropical swells approach from the south. The westerly local seas and the southern hemisphere extratropical swells are in general of small magnitude and were not considered for analysis. Table 3 shows a summary of wave conditions at Pier F due to 1-year extratropical and tropical storms and prefrontal winds, derived from a long-term deep water wave hindcast in the vicinity of Angel’s Gate. Resulting wave conditions are dominated by a swell of approximately 0.5 foot (0.15 m) or less, with long waves in the 1 to 4 inches range (0.03 to 0.10 m) and with periods of about 3 minutes. The results of the wave analysis indicated that waves within the harbor generated by passage of cold fronts exceed the recommended wave height criteria (i.e. ASCE, 1994 and PIANC, 1995), in particular when combined with the seas/swell generated offshore by storms and which penetrates into the harbor through the breakwaters. Therefore, wave protection for the planned new docking facilities was considered necessary. Yearly swell conditions, however, are within the wave criteria. Pier F is protected by a rip-rap revetment with a crest elevation of approximately +15 feet (4.6 m) MLLW and a 1v:1.5h slope down to a depth of approximately -20 to -25 feet (6.1 to 7.6 m) MLLW. Beyond the 25-foot (7.6 m) MLLW depth contour the bathymetry features a steep slope ranging from approximately 0.25 between the 20 and 40-foot (6.1 and 12.2 m) MLLW contours, to 0.75 between the 40 and 65-foot (12.2 and 19.8 m) MLLW contours. Beyond the 65- foot (19.8 m) MLLW contour the bathymetry is relatively flat down to the 70-foot (21.3 m) MMLW contour where the bank of the ship channel commences. The waterfront runs approximately in a north-south direction and three docks are currently in operation: a Public dock on the south, the LBFD fireboat dock on the north and the JPS dock in between. Figure 3 shows the bathymetry in feet MLLW and the existing docks. Pier F users reported that large oil tankers and containerships could generate currents of approximately 3 knots (1.6 m/s) , parallel to the shore and in the direction opposite to the ship travel as they enter the port through the gap. These ships, which enter in a laden condition at about 3 to 4 knots (1.6 to 2.1 m/s) , are in the 1,000-foot (304.8 m) length range, with beams in the order of 150 to 250 feet (45.7 to 76.2 m) and drafts of approximately 60 to 65 feet (18.3 to 19.8 m). Currents induced by ships traveling in the outbound direction and in ballast were reported as negligible. The Alaskan Class tanker ship was selected as a representative ship and numerical hydrodynamic simulations were performed using MIKE 21 FM HD, using a time and space varying pressure field to simulate the moving ship. The numerical model was used for two purposes. First, to verify the predicted ship induced currents against currents estimated from observations and currents measured during a one month program at two locations: one at a potential location for a breakwater, the other at the potential location of the new fireboat boat bay. Secondly, to explore the effects of fixed breakwater alternatives on the current field, and to assess the hydrodynamic loads on the breakwaters (fixed and floating), docks and boats. Figure 4 shows a comparison of measured and predicted current speeds at the planned fireboat boat bay location (bbcm) due to the passage of the “Alaskan Frontier” oil tanker at approximately 3.1 knots (1.6 m/s) with a draft of 50.1 feet (15.3 m). Measured and predicted current speeds are in very good agreement, in particular when considering that the track of the ship has an effect on the magnitude of the induced currents and that an assumed track, instead of the actual track, was used in the model. The comparison of model predictions against measurements and visual observations of drifters led to conclude that maximum current speeds induced by large tankers passing at 3 knots (1.6 m/s) with 50 to 65 foot (15.3 to 19.8 m) draft would be less than 2 knots (1 m/s) along Pier F. breakwater alternatives on the current field, and to assess the hydrodynamic loads on the breakwaters (fixed and floating), docks and boats. Modeling results showed that the inclusion of an approximately 200-foot (61 m) long, shore-perpendicular, fixed breakwater near the south end of Pier F, as expected, would slow down the current along Pier F to approximately 1.5 m/s (3 knots). However, at of the tip of the breakwater, strong 3+ m/s (6+ knots) currents would result from narrowing the Pier F-Navy Mole gap, requiring special considerations in the design of that section of the breakwater. Feasible alternatives for wave protection were difficult to define. While most would be adequate to provide the necessary wave protection, the need to accommodate a relatively large fleet, no interference with navigation, and site conditions such as deep water depths and steep bottom slopes, seismicity, and potentially strong tsunami induced currents proved to be very challenging conditions that limited the feasibility of most structures. A floating breakwater was analyzed along with fixed alternatives such as rubble mound, caissons, concrete sheet pile, cylindrical pile and steel sheet/king pile breakwaters. Because a breakwater layout that would protect the new docks and boats against southerly and northwesterly waves would be impractical from operations and navigation perspectives, preference was given to protect the new docks and boats from southerly waves with a breakwater, and mitigate their exposure to the broadside northwesterly post cold front wind waves by arranging most berths in a shore-parallel orientation, combined with individual slips and four-point moorings for each boat. The analysis of the wave climate indicated that a floating breakwater would be adequate to protect the planned new facilities from wind waves. With transmission coefficients in the order of 40%, a shore-perpendicular 160-foot (48.8 m) long, 15- foot (4.6 m) wide and 5-foot (1.5 m) draft floating breakwater would mitigate the 2- second southerly waves generated in the LA-LB harbor due to the passage of cold fronts and provide a wave climate for the planned new facilities that would satisfy criteria such as ASCE (1994) and PIANC (1995). However, it would offer no protection against swells, also reported as problematic at Pier F, given the low breakwater beam to wave length ratio. The proposed arrangement of the berths in a shore-parallel direction would only mitigate the effects of southerly incoming swells. Mooring the floating breakwater by means of lines (taut or catenary) and piles was analyzed. A catenary system in the deep water depths at Pier F would allow for large lateral excursions that would take needed space from the limited space available for docks and boats. A taut (pre-tensioned) mooring line system, on the other hand, while featuring reduced lateral excursions, would impose obstructions to the small vessel off-channel traffic (i.e. tugs, barges, etc.), in addition to requiring more maintenance, and higher redundancy in the event of mooring line failure. Compared to a pile system, a mooring line system is significantly more compliant and therefore the loads induced by waves will be smaller than on a pile system. The deep water conditions at the site will require significantly long piles, and the potentially high hydrodynamic loads on the breakwater and piles by tsunami currents would required specialized structural design of the piles and breakwater-pile interface. A rubble mound breakwater was discarded early on because its excessive footprint would obstruct small vessel navigation, in addition to encroaching significantly in the limited area available for the new docks and boats. Relative to a rubble mound breakwater, a breakwater consisting of concrete prismatic 20 x 30 feet (6.1 x 9.1 m) base, 43-foot (13.1 m) high gravel-filled caissons would have a reduced footprint, but the massive weight needed to provide stability would create large forces in the event of an earthquake, and overturning moments would create very high pressures at the toe where a well compacted gravel base could provide the necessary foundation strength, but not the ...

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