science

Slack tide

Slack tide is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Slack tide rather than just read about it. In short: Slack tide or slack water is the short period in a body of tidal water when the water is completely unstressed, and there is no movement either way in the tidal stream. It occurs before the direction of the tidal stream reverses.

Key takeaways

Slack tide belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Slack tide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Slack tide from memory before moving on to harder problems.

Reference excerpt

Slack tide or slack water is the short period in a body of tidal water when the water is completely unstressed, and there is no movement either way in the tidal stream. It occurs before the direction of the tidal stream reverses. Slack water can be estimated using a tidal atlas or the tidal diamond information on a nautical chart. The time of slack water, particularly in constricted waters, does not occur at high and low water, and in certain areas, such as Primera Angostura, the ebb may run for up to three hours after the water level has started to rise. Similarly, the flood may run for up to three hours after the water has started to fall. In 1884, Thornton Lecky illustrated the phenomenon with an inland basin of infinite size, connected to the sea by a narrow mouth. Since the level of the basin is always at mean sea level, the flood in the mouth starts at half tide, and its velocity is at its greatest at the time of high water, with the strongest ebb occurring conversely at low water.

Implications for seafarers 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 following high tide can improve underwater visibility, as the previously incoming tide brings clear water with it. Following low tide, visibility can be reduced as the ebb draws silt, mud, and other particulates with it. 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 into danger.

Combined tidal stream and current 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 directly affect 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.

Misconceptions Slack water is different from the 'stand of the tide', which is when tide levels 'stand' at a maximum or minimum (i.e., at that moment in time, not rising or falling).

Dodge tides Some localities have unusual tidal characteristics, such as Gulf St Vincent, South Australia, 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.

See also Stability theory, a main mathematical concept considering forces in equilibrium.

References

Worked examples

Example 1 — a first encounter with Slack tide

Start with the simplest possible case. Write down what Slack tide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Slack tide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Slack tide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Slack tide

In research: Slack tide appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Slack tide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Slack tide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Tides, Underwater diving environment, so understanding it makes those chapters shorter.
In everyday life: Look for Slack tide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Slack tide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Slack tide means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Slack tide out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Slack tide in simple terms?

Why does Slack tide matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Slack tide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Slack tide.