Glossary
| Current | Generally, a horizontal movement of water. Currents may be classified as tidal and nontidal. Tidal currents are caused by gravitational interactions between the sun, moon, and earth, and are a part of the same general movement of the sea that is manifested in the vertical rise and fall, called tide. Nontidal currents include the permanent currents in the general circulatory systems of the sea as well as temporary currents arising from more pronounced meteorological variability. |
| Ebb Current | The movement of a tidal current away from shore or down a tidal river or estuary. In the mixed type of reversing tidal current, the terms "greater ebb" and "lesser ebb" are applied respectively to the ebb tidal currents of greater and lesser speed of each day. The terms "maximum ebb" and "minimum ebb" are applied to the maximum and minimum speeds of a current running continuously ebb, the speed alternately increasing and decreasing without coming to a slack or reversing. The expression "maximum ebb" is also applicable to any ebb current at the time of greatest speed. For the Hudson and East Rivers - when current is moving from north to south along the Manhattan shore line. For the Harlem River - when the current is moving from the East River to the Hudson River. |
| Flood Current | The movement of a tidal current toward the shore or up a tidal river or estuary. In the mixed type of reversing current, the terms "greater flood" and "lesser flood" are applied respectively to the flood currents of greater and lesser speed of each day. The terms "maximum flood" and "minimum flood" are applied to the maximum and minimum speeds of a flood current, the speed of which alternately increases and decreases without coming to a slack or reversing. The expression "maximum flood" is also applicable to any flood current at the time of greatest speed. For the Hudson and East Rivers - when river is moving from south to north along the Manhattan shore line. For the Harlem River - when the river is moving from the Hudson River to the East River. |
| Hydraulic Current | A current in a channel caused by a difference in the surface level at the two ends. Such a current may be expected in a strait connecting two bodies of water in which the tides differ in time or range. The current in New York City's East River, which connects Long Island Sound and New York Harbor, is an example. |
| Reversing Current | A tidal current that flows alternately in approximately opposite directions with a slack water at each reversal of direction. Currents of this type usually occur in rivers and straits where the direction of flow is more or less restricted to certain channels. When the movement is toward the shore or up a stream, the current is said to be flooding; when in the opposite direction, it is said to be ebbing. The combined flood and ebb movement, including the slack water, covers on an average 12.42 hours for the semidiurnal current. If unaffected by a nontidal flow, the flood and ebb movements each last about six hours, but when combined with such a flow, the durations of flood and ebb may be quite unequal. During the flow in each direction, the speed of the current varies from zero at the time of slack water to a maximum about midway between the slacks. |
| Strength of Current | Phase of tidal current in which the speed is a maximum; also the speed at this time. Beginning with slack before flood in the period of a reversing tidal current (or minimum before flood in a rotary current), the speed gradually increases to flood strength and then diminishes to slack before ebb (or minimum before ebb in a rotary current), after which the current turns in direction, the speed increases to ebb strength, and then diminishes to slack before flood completing the cycle. If it is assumed that the speed throughout the cycle varies as the ordinates of a cosine curve, it can be shown that the average speed for an entire flood or ebb period is equal to 2/p or 0.6366 of the speed of the corresponding strength of current. |
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