FM 55-501 CHAPTER 7

Chapter 7

Tides and Currents

The rise and fall of the tide is the primary cause of currents. Tides originate in the open oceans and seas, but they are noticeable and important only close to the shore. The effects of tides can be seen and felt along coastal beaches, in bays, and up rivers. The resulting current from the tide can have a major effect on the ship as far as steering and speed are concerned. Both the current and the tide are major factors to be considered when involved in any type of beach operation such as a LOTS operation. This chapter covers tides and currents and outlines the procedures for determining their state at any given time.

TIDES

7-1. The tide is the vertical rise and fall of the ocean level as a result of changes in the gravitational attraction between the earth, moon, and sun. It is a vertical motion only.

TIDAL EFFECTS

7-2. Current is the horizontal movement of the water from any cause. Tidal current is the flow of water from one point to another that results from a difference in tidal heights at these points. Besides the basic definitions given, there are certain tidal terms you must understand (see also Figure 7-1):

High Tide or High Water. This is the highest level reached by the rising tide.

Low Tide or Low Water. This is the lowest level reached by the tide going out.

Range of Tide. This is the total difference in feet and inches between high water and low water.

Height of Tide. At any specified time, this is the vertical measurement between the surface of the water and the reference plane (usually mean low water).

Mean Sea Level. This is the average height of the surface of the sea for all stages of tide that is different only a little from half-tide level, which is the plane midway between mean high water and mean low water.

Figure 7-1. Terms Measuring Depths and Heights

TYPES OF TIDES

7-3. The following tides are named according to the characteristics of the tidal pattern occurring at that specific place (see also Figure 7-2):

Semidiurnal Tide. For each tidal day there are two high waters almost equal in height and two low waters almost equal. This type of tide occurs most commonly along the east coast of the United States.

Diurnal Tide. With this type of tide, only one high water and one low water occurs each tidal day. This type of tide is found in the Gulf of Mexico, the Java Sea, and the Gulf of Tonkin along the North Vietnam and China coasts.

Mixed Tide. This type of tide is characterized by a large difference in the high water heights, low water heights, or in both. There are usually two high waters and two low waters each tidal day, but once in a while, the tide may become diurnal. This type of tide is most common on the west coast of the United States.

Figure 7-2. Tidal Patterns

TIDE TABLES

	7-4. Predictions of tidal heights are published annually by the National Ocean Survey. Tide Tables are issued in four volumes:
	Europe and the West Coast of Africa (including the Mediterranean Sea). East Coast of North and South America (including Greenland). West Coast of North and South America (including the Hawaiian Islands). Central and Western Pacific Ocean and Indian Ocean.
	Together, the four volumes contain predictions for 196 reference ports and differences and other constants for about 6,000 stations. Each volume is arranged as follows:
	TABLE 1. Contains a complete list of the predicted times and heights of the tide for each day of the year at a number of places designated as reference stations. TABLE 2. Gives differences and ratios, which can be used to modify the tidal information for the reference stations to make it applicable to a relatively large number of subordinate stations (substations). TABLE 3. Provides information for use in finding the approximate height of the tide at any time between high water and low water. TABLE 4. Is a sunrise-sunset table at five-day intervals for various latitudes from 76° N to 60° S (40° S in one volume). TABLE 5. Provides an adjustment to convert the local mean time of TABLE 4 to zone or standard time. TABLE 6 (two volumes only). Gives the zone time of moonrise and moonset for each day of the year at certain selected places.

REFERENCE STATIONS

	7-5. TABLE 1 (Figure 7-3), TABLE 2 (Figure 7-4), and TABLE 3 (Figure 7-5) are extracts from the Tide Tables, East Coast of North and South America, 1978.
	7-6. TABLE 1 lists the time and height of the tide at each high water and low water in chronological order for each day of the year at a number of important points known as reference stations. There are 48 reference stations ranging from Argentina and Newfoundland to Punta Layola, Argentina.

Figure 7-3. Extract of Table 1 - Times and Heights of High and Low Waters

Figure 7-4. Extract of Table 2 - Tidal Differences and Other Constants

Figure 7-5. Extract of Table 3 - Height of Tide At Any Time

7-7. All times stated in the tide tables are standard times, so adjustments are required for the use of daylight saving time or for any other differences from standard time. TABLE 1 is for the reference station of Hampton Roads for the months of July, August, and September. The time and heights of high and low water at the reference station are self-explanatory. Where no sign is given before the predicted height, the height is positive and is added to the depths given on the chart. If a minus (-) sign is in front of the height, then the number is subtracted from the charted depths.

7-8. While there is normally two high and two low tides for each date, they are, on an average, nearly an hour later each succeeding day. So there will be instances when a high or low tide may skip a calendar day, as indicated by a blank space in the tide tables (see 7, 13, 21, and 27 September).

SUBSTATIONS

	7-9. These stations are listed in geographical order in TABLE 2 (Figure 7-4). Each substation is given a number, location, and position in latitude and longitude given to the nearest minute. Under the "differences" column, data are then given which are to be applied to the predictions at a stated reference station (the specific reference station is shown in bold type). If there is more than one reference station shown on a page of TABLE 2, make sure that you use the reference station printed above the substation listed. For Substation Number 2389, Jamestown Island, you will use the reference station on HAMPTON ROADS.
	7-10. To determine the height of tide for a specific time other than those listed in TABLE 1 or computed using TABLE 2, see the extract of TABLE 3 (Figure 7-5). This table is easy to use and the instructions given below the table are explicit.
	Note: The predictions of times of heights of tide are so influenced by local conditions that they are not exact enough to make meaningful any interpolation for a more precise value. For this reason, interpolation is not done when using TABLE 3.

PREDICTING THE HEIGHT OF TIDE

7-11. Use the tide tables to predict the height of tide at a specific spot for a particular time. As you calculate, write down the information as shown in Figure 7-6.

Example: The harbor master wants to know the height of tide off Jamestown Island, VA, at 1000 hours on 10 September 1978.

Solution:

Write down the date and substation.

Refer to Figure 7-7, and go down the alphabetical listing to find Jamestown Island, VA, and its index number (2389).

Refer to Figure 7-8, and locate the substation by number. Locate and record the differences in times for high and low water and the difference in heights for high and low water (high water [+]2h54m, correction [-]0.5 ft; low water [+]3h26m, correction 0.0 ft).

Refer to Figure 7-9, and write down the times for high and low water and their corrections (high water 0210, 2.4 ft and 1454, 2.9 ft; low water 0818, 0.3ft, and 2121, 0.5ft).

Apply the corrections for the substation as given in Figure 7-8. Write down the corrected times and heights for high and low water for the substation opposite those for Hampton Roads (high water 0504, 1.9 ft and 1748, 2.4 ft; low water 1144, 0.3 ft and 0047 [9/11], 0.5 ft).

Compute the duration of the rise or fall of the tide by determining the time difference between the time of the tide prior to the time for which the height is required and the time of the tide after.

Duration of the tide =
11h44m - 5h04m = 6h40m.

Record the time.
Find the difference between the nearest high or low water and the time for which the height is required. The time to the nearest tide (low water):

LW = 11h44m - 10h00m = 1h44m.

Record this information.
Determine the range of tide by subtracting the low water from the high water (1.9 - 0.3 = 1.6 ft) and record.
In Figure 7-10, locate 6h40m under "duration of rise or fall" printed in bold face. Go across on the horizontal line to find the time from the nearest high or low water, which is the same or almost the same as the actual time difference. (In this case 1h41m is the actual time, so you will use 1h47m.)
Come down this column into the "correction to height" column until you are on the line with the range of tide (1.6) or nearest range of tide (in this case 1.5), and the correction is 0.2. List this correction for computing tide.
Apply the correction to determine the height of tide at a specific time.

Note: When the nearest tide is high water, subtract the correction. When the nearest tide is low water, add the correction.

List the height of tide for 1000 hours on 10 September 1978.

Height of nearest tide	0.3 (low water)
Correction from Figure 7-10	+0.2
Height of Tide	0.5 feet

Note: This predicted correction of 0.5 feet would be added to the charted depth of the water around Jamestown Island. This predicted depth would be valid only for 1000 hours on 10 September 1978.

Figure 7-6. Summary of Calculations for Computing Height of Tide

Figure 7-7. Extract of Index to Stations (Jamestown Island, VA)

Figure 7-8. Extract of Table 2 - Tidal Differences and Other Constants

Figure 7-9. Extract of Table 1 - Times and Heights of High and Low Waters

Figure 7-10. Extract of Table 3 - Height of Tide At Any Time

TIDAL CURRENTS

	7-12. A tidal current is the periodic, alternating, horizontal response of the water to the tidal forces which causes the rise and fall of the tide. Tidal currents are so called to distinguish them from ocean or river currents.
	7-13. The horizontal motions of water that reverses direction of flow during a tidal cycle are called flood current and ebb current. The flood current sets toward and the ebb current away from the coast, or the flood and ebb current set parallel to the coast in opposite directions. At each reversal of the current direction, there is a moment of no horizontal motion called slack water.
	7-14. The time of a tidal current’s change of direction does not coincide with the time of changing tide. The change of direction of the current always lags the turning of the tide by an interval that varies according to the physical characteristics of the land around the body of tidewater. For instance, along a relatively straight coast with only shallow indentations, there is usually little difference between the time of high or low tide and the time of slack water. However, where a large bay connects with the ocean through a narrow channel, the tide and the current may be out of phase by as much as three hours. In such a situation, the current in the channel may be running at its greatest velocity at high or low water outside.
	7-15. The navigator of a ship operating in tidewater must know the direction (called set) and velocity (called drift) of the tidal current his ship may encounter. This information is obtained from Tidal Current Tables.

TIDAL CURRENT TABLES

7-16. The Tidal Current Tables are also published annually by the National Ocean Survey. These tables are similar to the Tide Tables, but the coverage is not so extensive, being given in two volumes. Each volume is arranged as follows:

TABLE 1. Contains a complete list of predicted times of maximum currents and slack, with the velocity (speed) of the maximum currents for a number of reference stations.

TABLE 2. Gives differences, ratios, and other information related to a relatively large number of subordinate stations (substations).

TABLE 3. Provides information for use in finding the speed of the current at any time between tabulated entries in TABLES 1 and 2.

TABLE 4. Gives the number of minutes the current does not exceed stated amounts for various maximum speeds.

TABLE 5 (Atlantic Coast of North America only). Gives information on rotary tidal currents.

PREDICTING THE SET AND DRIFT OF THE CURRENT

7-17. Let us predict the set and drift of the current at Jamestown Island, VA, for 1000 hours on the same day we predicted the height of tide. As you calculate, write down the information as shown in Figure 7-11.

Figure 7-11. Summary of Calculations for Set and Drift of Current

Notes:
1. Time differences are applied to the times of slack and maximum current at the reference station in the same manner that time differences are applied when figuring tides. Application of the time difference to the tabulated time of flood and ebb current produces the time of the corresponding current at the substation. Find the maximum velocity at the substation by multiplying the maximum velocity at the reference station by the correct flood or ebb ratio.
2. Flood direction is the approximate true direction toward which the flood current flows. Ebb direction is usually close to the reciprocal of the flood direction. Average flood and ebb velocities are averages of all the flood and ebb currents.

First look up Jamestown Island in Figure 7-12. Its number here is 3705.

In Figure 7-13, find Substation 3705 (Jamestown Island). Notice near the top of the page that the Reference Station is "Chesapeake Bay Entrance" instead of "Hampton Roads (Sewell’s Point)." You will also see that the time difference for slack water is (+)2h00m and for the maximum current, (+)1h40m. These time differences mean that when slack water or maximum current exists at the Chesapeake Bay Entrance, the same conditions will exist, 2h00m and 1h40m later respectively, at Jamestown Island.

Figure 7-12. Extract of Index to Stations (Jamestown Island)

Figure 7-13. Extract of Table 2 - Current Differences and Other Constants

7-18. The flood velocity ratio is 1.1 and the ebb velocity ratio is 0.9. Before selecting the correct ratio, you must determine whether the current is ebbing or flooding at 1000.

7-19. Under the maximum currents columns you will find the flood direction to be 325° and the ebb 145° (the reciprocal). The average flood velocity is 1.1 knots and the average ebb velocity is 1.3 knots.

Write down these values for your calculations:

Time differences -- (+)2h00m for slack water,
(+)1h40m for maximum current.
Velocity ratios -- 1.1 for maximum flood, 0.9 for
maximum ebb.
Directin of current -- 325° for flood, 145° for ebb.

In Figure 7-14 you will find the time of slack water and maximum current for 10 September 1978.

Write down all the current conditions for the Reference Station (Chesapeake Bay Entrance) and the calculated values for the Jamestown Island substation.

Note that the maximum current at Jamestown Island occurs at 1002 with a velocity of 1.3 knots (E). Therefore the current is ebbing, and its direction or set is 145° .

Determine the interval between slack water and the desired time (1000):

10h00m desired time
-0.6h12m time of slack water
3h48m interval

Determine the interval between slack water and maximum current:

10h02m time of maximum
current
-0.6h12m time of slack water
3h50m interval

In Figure 7-15, locate value nearest to 3h48m under "interval between slack and desired time," locate value nearest to 3h50m under "interval between slack and maximum current."

Apply the factor of 1.0 to the value for maximum ebb velocity: predicted velocity of current = 1.0 X 1.3 = 1.3 knots.

What is the direction or set? You know that the current is ebbing at this time, and the ebb direction is 145° , which is the set.

Figure 7-14. Extract of Table 1 - Daily Current Predictions

Figure 7-15. Extract of Table 3 - Velocity of Current At Any Time

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