10.6: Tidal Currents

Orbits of water particles in tide waves are so flattened that water movements in the tide wave are essentially oscillating currents. The magnitude and periodicity of these currents are as important to mariners as the tidal range is, particularly in coastal and estuarine regions. Although tidal currents are relatively weak in the open oceans, their speed increases as the tide wave moves inshore and its energy is compressed into a shallower depth of water. Tidal-current speed also increases where the tide wave must move large quantities of water through a narrow opening into a large bay or estuary, such as the entrances to San Francisco Bay and New York Harbor.

Open-Ocean Tidal Currents

Tidal currents are generally weak and rotary in character in deep-water areas away from the coast in the large ocean basins (Fig. 10-15). In areas of the Northern Hemisphere with semidiurnal tides, the tidal-current direction rotates progressively, usually clockwise, and completes a 360° rotation in 12 hr 24½ min (Fig. 10-15a). The current speed varies but is never zero.

In areas with mixed tides, the progression of tidal-current speed and direction is more complicated (Fig. 10-15b) because both variables represent a combination of two different tidal components. Note that the times of maximum and minimum tidal-current speed are apparently not related to the times of high and low tide.

Figure 10-15. Changes in the direction and magnitude of open-ocean tidal currents in the Northern Hemisphere. The arrows indicate the direction of the current at approximately hourly intervals, and the length of the arrows is proportional to the current speed. The outer blue line describes how an object would move in the water if there were no currents or water movements other than tidal currents. (a) Where there is a simple semidiurnal tide, the current speed varies little, but its direction rotates clockwise through 360° every 12 hr 24½ min. (b) Where there is a mixed tide, the variations of current speed and direction are more complex. In this case, the current direction rotates clockwise twice within 24 h 49 min, but the rate of rotation and the current speeds follow very different patterns within the two successive 12-h periods.

Tidal currents flow at all depths in the oceans. While current speeds due to tides in the deeper parts of the oceans are slow, these currents interact with seafloor topography in complex ways. Although little is known about these interactions, it is known that increased turbulence as tidal currents flow over seafloor topography is a significant factor in vertical mixing of water masses.

Temporal Variation of Tidal-Current Speeds

Many people mistakenly believe that tidal currents stop when the tide reaches its highest and lowest points. This is rarely true. In fact, if the tide were a pure progressive wave, the tidal current would be at its fastest at high or low water. We can see why by considering the motions of water particles in the tide wave (Fig. 10-16a). However, the maximum tidal currents rarely coincide with high or low tide for several reasons. The most important is that all tides are complex combinations of many components. Some components may be progressive waves, some may be standing waves, and some may have both progressive-wave and standing-wave characteristics. In addition, tide waves are reflected and refracted, so the observed tide may be the result of several different waves moving in different directions.

Figure 10-16. Tidal currents. (a) In a pure progressive wave, the forward motion in the wave is at a maximum as the crest passes, and in the opposite direction when the trough passes. Thus, if the tide wave were a pure progressive wave, the flood currents would reach a maximum at high tide and the ebb currents would reach a maximum at low tide. (b) In a standing wave, the horizontal currents are always zero at the antinodes, and they are reversing currents whose maximum speed increases from zero at the antinode to a maximum at the node. The maximum current speeds are reached at the mid-flood and mid-ebb stages. Because most tides have both progressive-wave and standing-wave components, there is no fixed relationship between the times of high and low tide and the times of maximum tidal current and slack water at different locations. Tidal currents are also complicated by seafloor friction.

In a standing wave, currents are at their maximum midway between high and low tide, when the sea surface is exactly level (Fig. 10-16b). Current speed varies not only with time, but also with location in the wave. It is highest at the node and is always zero at the antinodes, where the vertical water surface displacement, or tidal range, is greatest.

In most places, the tide is a complex mixture of progressive-wave and standing-wave components that vary from location to location. Hence, the relationship between the timing of slack water (minimum current speed) and high and low tide is different for each location. Curiously, at certain times, some locations have tidal currents but no tide (zero tidal range), while others have a large tidal range but no tidal currents. We can envision such locations as the node and antinode, respectively, of a standing wave that is the dominant component of the tides in that location, although the situation is generally more complicated because the tide is the sum of many different components of both solar and lunar tides.

Tide tables that list only times of high and low tide are useless for determining tidal-current speeds because there is no consistent relationship between the times of high and low water and the times of highest current speed and slack water. The only generalization about the tidal currents is that their maximum speed will increase or decrease as tidal range increases or decreases from day to day. Therefore, tidal currents must be measured in each location for which a forecast of the current speed is important, just as tidal ranges must be measured. Tidal-current information from such measurements is subject to harmonic analysis that is similar to the analysis of tidal-height data described later in this chapter. From this analysis, tidal-current tables are produced.

Tidal Currents in Estuaries and Rivers

Tides extend far into many bays, estuaries, and rivers. Tides in such locations are affected by the extremely shallow water depths, freshwater flow, and friction with the seafloor. In very shallow water, the crest of the tide wave moves in significantly deeper water than the trough. Therefore, the high tide tends to catch up to the low tide in estuaries or rivers where the tide travels long distances through shallow water. As a result, river tides can be modified so that there is a long period between high and low tide, but a very short period between low and high (Fig. 10-17).

Figure 10-17. This tidal-height plot for the Hudson River near Albany, New York (about 220 km from New York Harbor), shows that the tide rises much faster (that is, the plot line is steeper) than it falls.

In some areas where tidal ranges are large and the tide enters a channel or bay that narrows markedly or has a steeply sloping seafloor, tidal bores may occur. A tidal bore is created when the currents in the flooding tide are faster than the speed of a shallow-water wave in that depth. The leading edge of the tide wave must force its way upstream faster than the wave motion can accommodate. The wave therefore moves up the bay or estuary as a wall of water, much like a continuously breaking wave (Fig. 10-18). Well-known tidal bores occur in the Bay of Fundy in Nova Scotia and in Turnagain Arm off Cook Inlet in Alaska. Most tidal bores are less than 1 m high, but they can reach as much as 10 times that height. Some bores, notably one in the Qiantang River in China, are high enough that they attract surfers who are looking for the longest ride of their lives.

Figure 10-18. Tidal bore in a channel of the Kent River near Arnside, Westmoreland, England.

Tidal currents in estuaries and rivers are affected by freshwater flowing toward the ocean. Freshwater flowing from a river into an estuary prolongs the ebb tide in relation to the flood tide because more water must be transported out of the river during the ebb than enters during the flood to accommodate the freshwater discharge. River flow rates can substantially change the nature of tides and tidal currents in estuaries. Accordingly, tide tables and tidal-current tables for bays, estuaries, and rivers must be used only as a general guide, particularly when river flow rates are abnormally high or low.

Tidal currents within bays, rivers, and estuaries can behave as progressive waves, standing waves, or, in some cases, a combination of the two. In long bays with progressive tides, tidal currents can flow in opposite directions at the same time in different sections of the bay. Chesapeake Bay has this type of circulation (Fig. 10-19). Slack water from each tide migrates up the bay about 60 km•h^–1 and takes about 10 h to travel the length of the bay. Successive slack tides enter the bay approximately every 6 h 12 min because each semidiurnal tide has slack water associated with both flood and ebb. This progressive wave is not modified significantly by an east–west wave, because the bay is too narrow for a significant wave to be generated directly by the tide-generating forces.

Figure 10-19. Chesapeake Bay has a primarily progressive-wave tide. The tide wave enters the estuary and moves progressively up the bay, reaching the north end about 10 h after entering the bay. Follow the successive slack-water occurrences numbered 1, 2, and 3 in these diagrams as they move north. (a) Slack water occurs at the mouth of the bay just as the flood begins and also at a location just above mid bay. (b) Two hours after the start of the flood, the two areas of slack water have migrated northward. (c) Four hours after the start of the flood, the northernmost slack has reached the north end of the bay and the second slack is almost in mid bay. (d) Six hours after the start of the flood, the slack that entered the bay 6 h earlier is now where its preceding slack had been at that time, and a new slack-water area occurs at the mouth of the bay as the ebb begins.

Tidal currents in Chesapeake Bay are somewhat more complex than those shown by the red arrows in Figure 10-19 that show the relative current speeds, because the bay does not have uniform depth. The tide wave slows more rapidly at the sides of the bay, where the water is shallower and friction with the channel sides is enhanced. In this and other bays with a deep central channel and shallower sides, the tidal current reverses sooner at the sides than in the deep channel. This effect can be useful to boaters who want to avoid the fastest opposing currents. It is also a mechanism for mixing water from the deep main channel with water from the shallow sides, and therefore is of interest in studies of the dilution of discharges.

Long Island Sound has a predominantly standing-wave tide (Fig. 10-20). Tidal currents first flow westward throughout the length of the sound, then slack water occurs throughout the sound, and subsequently the current reverses and flows eastward throughout the sound. The area of nearly permanent slack water at the west end of the sound is the standing wave antinode and has a large tidal range, but little or no tidal current. The east end of the sound is the node and has a small tidal range. 

Figure 10-20. Long Island Sound has a predominantly standing-wave tide, with a node at the entrance to the sound and an antinode at the western end. There is little or no tidal current at any time during the tidal cycle at the western end where the antinode is located. In the remainder of the sound there are reversing tidal currents, with the maximum current speed increasing from the western end to the eastern opening to the ocean, where a node occurs.