Slack water
Encyclopedia
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 table
s, 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.
, where the amplitudes of the main semi-diurnal tide constituents are almost identical. At neap tides the semi-diurnal tide is virtually absent, resulting in the phenomenon known as a "dodge tide"—a day-long period of slack water—occurring twice a month; this effect is accentuated near the equinoxes when the diurnal component also vanishes, resulting in a period of 2-3 days of slack water.
High Water
High Water is an album by hip hop artist El-P, released on March 9, 2004 through Thirsty Ear Recordings.Made in conjunction with jazz pianist Matthew Shipp and the group for which he is artistic director, The Blue Series Continuum, the album is a striking departure from El-P's usual style, almost...
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
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
stream, and which occurs before the direction of the tidal stream reverses. Slack water can be estimated using a tide table, a tidal atlas
Tidal atlas
A tidal atlas or a tidal stream atlas is used to predict the direction and speed of tidal currents.A tidal atlas usually consists of a set of 12 or 13 diagrams, one for each hour of the tidal cycle, for a coastal region. Each diagram uses arrows to indicate the direction of the flow at that time...
or the tidal diamond
Tidal diamond
Tidal diamonds are symbols on British admiralty charts that indicate the direction and speed of tidal streams.The symbols consist of a letter of the Roman alphabet in a rhombus, printed in purple ink...
information on a nautical chart
Nautical chart
A nautical chart is a graphic representation of a maritime area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land , natural features of the seabed, details of the coastline, navigational hazards, locations of natural and man-made aids...
. Tide table
Tide table
A tide table, sometimes called a tide chart, is used for tidal prediction and shows the daily times and height of high water and low water for a particular location...
s, 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
Scuba diving
Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....
, 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
Visibility
In meteorology, visibility is a measure of the distance at which an object or light can be clearly discerned. It is reported within surface weather observations and METAR code either in meters or statute miles, depending upon the country. Visibility affects all forms of traffic: roads, sailing...
, 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
The Rip
"The Rip", also known as "The Heads", is a dangerous stretch of water in Victoria, Australia, connecting Port Phillip and Bass Strait. It is the only entrance for shipping into Port Phillip and hence into Melbourne. Because of large tidal flows through the relatively narrow channel from the bay to...
between Point Nepean
Point Nepean, Victoria
Point Nepean marks the southern point of The Rip and the most westerly point of the Mornington Peninsula, in Victoria, Australia. It was named after the British politician and colonial administrator, Sir Evan Nepean. It is within the suburb of Portsea...
and Point Lonsdale
Point Lonsdale, Victoria
Point Lonsdale is a coastal township on the Bellarine Peninsula, near Queenscliff, Victoria, Australia, and included in the Borough of Queenscliffe. Point Lonsdale is also one of the headlands which, with Point Nepean, frame The Rip, the entrance to Port Phillip. The headland is dominated by the...
at the entrance to Port Philip Bay, Victoria
Victoria (Australia)
Victoria is the second most populous state in Australia. Geographically the smallest mainland state, Victoria is bordered by New South Wales, South Australia, and Tasmania on Boundary Islet to the north, west and south respectively....
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
; the Menai Strait
Menai Strait
The Menai Strait is a narrow stretch of shallow tidal water about long, which separates the island of Anglesey from the mainland of Wales.The strait is bridged in two places - the main A5 road is carried over the strait by Thomas Telford's elegant iron suspension bridge, the first of its kind,...
between Anglesey and Wales; or the Strait of Gibraltar
Strait of Gibraltar
The Strait of Gibraltar is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Spain in Europe from Morocco in Africa. The name comes from Gibraltar, which in turn originates from the Arabic Jebel Tariq , albeit the Arab name for the Strait is Bab el-Zakat or...
at the entrance to the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
.
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.
Dodge tides
Some localities have unusual tidal characteristics, such as Gulf St Vincent, South AustraliaSouth Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...
, where the amplitudes of the main semi-diurnal tide constituents are almost identical. At neap tides the semi-diurnal tide is virtually absent, resulting in the phenomenon known as a "dodge tide"—a day-long period of slack water—occurring twice a month; this effect is accentuated near the equinoxes when the diurnal component also vanishes, resulting in a period of 2-3 days of slack water.