Tides & Currents Flashcards
⟡In Long Beach, where might one experience bank cushion and suction, and how can its affects be reduced?
A pilot in the port of Long Beach can expect to encounter bank cushion and suction in the Back Channel. Reducing speed will reduce the effects of bank cushion and suction. To keep the vessel in the channel, a pilot may also employ the help of the rudder, by steering slightly toward the closest bank to counteract the bank cushion effect on the bow.
⟡Tides and currents - What is Max Low Tide?
What is the Datum?
The mean range of tide in Los Angeles Harbor is 3.8 feet, and in Long Beach inner and outer harbors the mean range is 3.7 feet. The diurnal range of tide is about 5.4 feet for these harbors. A range of about 9 feet may occur at times of maximum tides.
Datum: NAD83
The tidal currents follow the axis of the channels and rarely exceed 1 knot.
⟡Average tidal ranges. Diurnal Tide ranges. Max tide ranges.
The mean range of tide in Los Angeles Harbor is 3.8 feet, and in Long Beach inner and outer harbors the mean range is 3.7 feet. The diurnal range of tide is about 5.4 feet for these harbors. A range of about 9 feet may occur at times of maximum tides (7.3 ft to -2.0 ft from MLLW).
Datum: NAD83
The tidal currents follow the axis of the channels and rarely exceed 1 knot.
Currents
The tidal currents follow the axis of the channels and rarely exceed 1 knot.
Los Angeles and Long Beach Harbors seiche and surge
The most persistent and conspicuous oscillation has a period of approximately 1 hour. In the vicinity of Reservation Point and near the E end of Terminal Island, the hourly surge is very prominent, causing velocity variations which at times may be as great as 1 knot, and which often overcome the lesser tidal current so that the current floods and ebbs at half-hour intervals. Because of the more restricted channel, the surge through Back Channel at the E end of Terminal Island usually reaches a greater velocity than through the channel W of Reservation Point. In Back Channel, the hourly variation may sometimes be 1.5 knots or more. The hourly surge, together with other oscillations of shorter period and of more irregular occurrence, at times causes a very rapid change both in height of the water and the velocity and direction of the current and may endanger vessels tied up at the piers.
What is the general set of the current at the LA Harbor entrance?
The general set of the current is Easterly at the entrance and parallel to the entrance.
Set and current in LA Harbor: The tidal currents follow the axis of the channel and rarely exceed one knot. The set is Easterly.
Surge in the harbor (oscillation) has a period of about 1 hour.
Describe local shoaling and silting in the POLA and POLB.
POLA and POLB do not have a siltation problem like natural river ports. The ports need only dredge occasionally to maintain berth side design water depths.
The harbors usually have very localized shoaling problems. They occur mainly in the immediate vicinity of the pierhead lines, when propeller or bow thruster action cause localized sloughing of the underwharf embankments. Soft bottom conditions mitigate the effect of shoaling.
LALB HSP (V) C
Describe the Tidal Range in LA LB Harbors.
LA Mean 3.8 feet
LB Mean 3.7 feet
LA and LB Diurnal 5.4 feet
Max range 9.0 feet (approx.)
USCP 7, Ch. 4, ¶ 290
Describe the surge encountered in LA LB Harbors.
Both harbors are subject to seiche and surge.
Period is approximately 1 hour.
Hourly surge is very prominent with velocity variations as great as 1 knot and often overcome the lesser tidal current so that the current floods and ebbs at ½ hour intervals:
• In the vicinity of Reservation Point
• Near the E end of Terminal Island
Hourly surges together with shorter oscillations can cause rapid height and current changes endangering vessels at berth, and a 3-minute surge is reported to be responsible for major ship movement and damage. Pilots recommend taut lines at all times.
USCP 7, Ch. 4, ¶ 292
