TidesandCurrents
APRA HARBOR, GUAM
TIDES AND CURRENTS
Currents in the vicinity of the Mariana Islands are west. They are strongest near to and south of Saipan Island, and gradually become weaker north of that island. In June, the Equatorial Drift Current was reported to be strongest during the season at 13 ° N and to run to the northwest at a maximum rate of 1 kt.
Variable currents are sometimes encountered near the Mariana Islands. These are caused by the physical makeup of the island and by the additional force of the tidal currents.
An almost constant southwest set has been reported along the northwest coast of Guam during the Northeast Trades. The current has been felt up to 10 nmi offshore.
Astronomical tides in Apra Harbor range from a high of 2.4 ft (0.7 m) to an extreme low of minus 3.5 ft (1.1 m). The mean tidal range in Apra Harbor is 0.5 m, while the spring range is 0.7 m.
As Orote Point is approached, the southwest current associated with the Northeast Trades tends to curve to the south and east. The rate of current is greatly affected by the force of the wind. During the typhoon season, the ebb current from the Harbor augments the southwest current and reduces any northeast current that may occur. Strong rips are observed under these conditions.
The prevalent set of the current in the Apra Harbor entrance is usually south or southwest regardless of the tidal currents, but a set to the north or northeast may be experienced, especially during the summer months.
The flood current in the Apra Harbor entrance sets north to northeast at a maximum rate of 1.5 kt. The ebb current sometimes attains a maximum rate of 3 kt setting southwest. Slack water occurs 30 minutes before low tide and 45 minutes before high water.
Currents within the main portion of Outer Apra Harbor are generally clockwise with velocities of 0.1 kt or less, but countercurrent eddies also exist in the harbor (Defense Mapping Agency Hydrographic/Topographic Center, 1993) (Figure II-32). The currents and tidal currents within the Inner Apra Harbor are weak and variable.
Heavy west swells sometimes make the entrance of Apra Harbor Outer Harbor dangerous for several days in a row. This condition occurs when a typhoon builds up in this area, progresses to the northwest, and then curves to the northeast. Beacons and buoys are sometimes destroyed or carried away at such times.
Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/guam/apra/text/sect6.htm
