Slack Water

Slack water, which used to be known as 'The stand of the tide', is a short period in a body of tidal water either side of high water or low water essentially when the water is completely unstressed, and therefore with no rise or fall of the tide and no movement either way in the tidal stream, and which occurs before the direction of the tidal stream reverses. Slack water can be estimated using a tide table, a tidal atlas or the tidal diamond information on a nautical chart. Tide tables, which tabulate the time of high and low water, are generally only published for Standard Ports. To determine the time of slack water at less important locations, the time difference (or Tidal Constant) between the time of high water at the Standard Port and at that location also needs to be known.

For scuba divers, the absence of a flow means that less effort is required to swim, and there is less likelihood of drifting away from a vessel or shore. Slack water can reduce underwater visibility, as there is no flow to remove debris such as sand or mud. In areas with potentially dangerous tides and currents, it is standard practice for divers to plan a dive at slack times.

For any vessel, a favourable flow will improve the vessel's speed over the bottom for a given speed in the water. Difficult channels are also more safely navigated during slack water, as any flow may set a vessel out of a channel and into danger.

In many locations, in addition to the tidal streams there is also a current causing the tidal stream in the one direction to be stronger than, and last for longer than the stream in the opposite direction six hours later. Variations in the strength of that current will also vary the time when the stream reverses, thus altering the time and duration of slack water. Variations in wind stress also reflect directly on the height of the tide, and the inverse relationship between the height of the tide and atmospheric pressure is well understood (1 cm change in sea level for each 1 mb change in pressure) while the duration of slack water at a given location is inversely related to the height of the tide at that location.

Slack water is a much misused term, often used to describe a period of equilibrium between two opposing streams when the water is anything but slack, but highly stressed. Although there may be no flow in either direction there may be many eddies, and since this so-called slack water occurs before high water while the tide is still rising, the tide may continue to rise even after the direction of the stream has reversed. Conversely, since it occurs after low water while the tide is rising, the tide may also continue to rise during this so-called low water slack period. Such conditions typically occur at river mouths, or in straits open at both ends where their entrances have markedly different physical characteristics. Examples include The Rip between Point Nepean and Point Lonsdale at the entrance to Port Philip Bay, Victoria, Australia; the Menai Strait between Anglesey and Wales; or the Strait of Gibraltar at the entrance to the Mediterranean Sea.

Slack water may also be misused to refer to a process in caves. This occurs when a stream cave, or fluviokarst, is completely filled with water during flooding.