Navigation NAU 102 Lesson 28

Tidal Current The periodic horizontal flow of water accompanying the rise and fall of the tide.

Tidal Current Moon

Tidal Current Two Types: Rotary Currents Reversing Currents

Rotary Currents Occur offshore where direction of flow is not restricted. Coriolis Effect causes set to rotate through all directions. Clockwise in the Northern Hemisphere Counterclockwise in the Southern Hemisphere.

Rotary Currents Water flows continuously but the speed and direction changes Two speed maximums in opposite directions Two speed minimums about halfway between the maximums

Rotary Currents Table 5 of the Tidal Current Tables has further discussion of rotary currents.

Reversing Currents Occur where direction of water flow is constricted: Coastal areas Rivers Bays Straits

Reversing Currents Definitions Flood – the movement towards shore or upstream. Ebb – the movement away from shore or downstream. Slack – the period when speed of the current is zero.

Tidal Current

Current Predictions We can predict set and drift of tidal currents using: Computer Programs Tidal Current Tables

Tidal Currents Horizontal movement of water caused by the tides.

Tidal Current Tables 2 volumes by location Data on tidal currents Privately published based on NOS data

Tidal Current Tables Table 1 - list of predicted times of maximum currents and slack water, with the velocity of the maximum currents, for a number of reference stations. Arranged by Tables

Table 1

Tidal Current Table 2 - gives differences, ratios, and other information related to a relatively large number of subordinate stations. Index numbers of subordinate stations are found in the Index.

Table 2

Tidal Current Tables Table 3 - enables determination of current’s velocity at any time between entries in tables 1 and 2.

Tidal Current Tables Table 4 - gives duration of slack, or the number of minutes the current does not exceed stated amounts, for various maximum velocities. Table 5 (Atlantic Coast of North America only) gives information on rotary tidal currents.

Tidal Current Tables The volumes also contain: Wind-driven currents Combination currents Gulf Stream currents for the East Coast Coastal currents on the West Coast

Example Problem #1 Current at Reference Stations What will be the time, speed and direction of the maximum currents and slack waters at Key West, Florida on 19 November?

Example Problem #1

TimeSpeedDirection ° T ° T ° T

Example Problem #2 Current at Subordinate Stations What will be the time, speed and direction of the first maximum currents and slack waters at Grant’s Tomb on 04 June (DST applies)?

Example Problem #2

MBFFMBEEFE Narrows23:3101:4504:5208: Grant’s Tomb x 0.9X :3003:3806:3710: Flood –025° T Ebb –

Example Problem #3 Current at Any Time What will be the speed and direction of the tidal current at Grant’s Tomb at 0430 on 04 June?

Example Problem #3

MBFFMBEEFE Narrows23:3101:4504:5208: Grant’s Tomb x 0.9X :3003:3806:3710: Flood – 025° T Ebb – Time of Slack Time of Maximum

Example Problem #3 Interval Between Slack & Maximum Current 06:37 (Slack) -03:38 (Max) 2h 59 min Interval Between Slack & Desired Time 06:37 (Slack) -04:30 (Desired) 2h 07 min

Example Problem #3

MBFFMBEEFE Narrows23:3101:4504:5208: Grant’s Tomb x 0.9X :3003:3806:3710: Flood – 025° T Ebb – Time of Maximum

Example Problem #3 1.2 (speed of max current) x 0.9 (factor from table 3) = 1.1 knot flood What will be the speed and direction of the tidal current at Grant’s Tomb at 0430 on 04 June?

Daylight Savings Time Times in Table 1 are in Standard Time If you are keeping Daylight Savings Time, “Spring Forward” Add one hour to times from Table 1.

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