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Wave windows and sources. 

Wave windows and sources. 

Source publication
Conference Paper
Full-text available
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)....

Context in source publication

Context 1
... from 30 to 110 feet (9.1 to 33.5 m). Pier F is located adjacent and on the east side of the POLB main entrance channel. The 75-80-foot (22.9-24 m) deep channel passes through a narrow 900-foot (274 m) gap formed by the southern end of Pier F to the east and the Navy Mole to the west. Figure 1 shows the location of Fireboat Station No. 15 at Pier F and the Middle Breakwater sided by the San Pedro Breakwater to the west and the Long Beach Breakwater to the east. Wave conditions at Pier F are, in general, relatively benign. Waves generated within the Los Angeles-Long Beach (LA-LB) harbor by the average year-round 10- knot (5.2 m/s) southwest wind are not problematic for the boats and docks currently at Pier F. However, the typical 30+ knot (15.5 m/s) southerly (prefrontal) winds in the winter, associated to the passage of cold fronts, generate 1 to 2-foot (0.3 to 0.6 m), 2- second period waves within the harbor that are problematic for the docks and boats, in particular due to their relative broadside angle of incidence. In addition, the offshore 4 to 10-second waves generated by the prefrontal winds in the winter and the 12 to 16-second swell generated by tropical hurricanes along the Pacific Coast of Mexico during the summer penetrate through the LA-LB harbor breakwaters causing an undesirable “surging” that also affects boats and docks. In order to determine the need and type of wave protection that could potentially be necessary for the new docking facilities at Pier F, coastal engineering studies and investigations were performed. These included numerical modeling of wind waves within the LA-LB harbor, the penetration of offshore swell and long waves (infragravity), and ship and tsunami induced currents. The need for wave protection in small craft harbors is generally assessed using guidelines such those developed by the ASCE (1994) and the Permanent International Association of Navigation Congresses (PIANC, 1995) which define allowable levels of wave conditions in harbors. Table 1 shows the wave criteria per ASCE (1994) where the allowable yearly maximum wave event is 1 foot (0.30 m) for 2-second waves and 0.5 foot (0.15 m) for waves of periods greater than 2 seconds. This criteria is the same as PIANC (1995) and consistent with others found in general design guidelines for small craft harbors. In order to determine if wave protection would be necessary according to the ASCE (1994) criteria, wave conditions and frequency of occurrence at Pier F needed to be determined. Unfortunately, long-term wave measurements or hindcasts at the site were not available to perform this analysis. However, hourly wind measurements at Pier J and Pier F from April 2005 to date from the National Ocean Service’s PORTS program, and a long-term wave hindcast outside the LA-LB harbor in the vicinity of Angel’s Gate were available. In combination with numerical wave modeling these data were used to determine wave conditions at Pier F generated by winds within the harbor and those resulting from waves that penetrate through the LA-LB harbor breakwaters. Two numerical wave models were used: a) MIKE 21 FM SW, a spectral wind-wave model, was used to simulate the generation and transformation of waves due to wind within the LA-LB harbor and b) MIKE 21 BW, a Boussinesq wave model, to simulate the transformation of deep water waves to shallow water, the generation of long waves, and the penetration of these waves to the Pier F area. The PORTS’ wind records at Pier J and F allowed for the simulation 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 ...

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