Investigate: predicting, classifying and staying safe

Discover gave you the words. Understand gave you the mechanism. This layer hands you the tools of a real tide-watcher: a tide table to test yourself against, a way to tell one kind of tide from another, and a look at what happens when the ordinary tide turns dangerous.

The rule for this whole layer is simple: predict before you look. Every time you meet a table, a graph or a lab, decide what you think will happen first. Being wrong and finding out why is how tide-watchers — and scientists of every kind — actually learn.

Here is the port from the rule-of-twelfths lesson in Understand: low water 0.6 m, high water 5.0 m, a range of 4.4 m, with high and low water about 12 h 25 min apart. Below is the water height at the start of every hour, beginning at a low tide at midnight.

Before you read the table, predict: at roughly what hour will the water be highest? At roughly what hour will it be back at its lowest? Will the water rise at a steady rate all morning, or unevenly?

The two-bulge picture predicts two equal high tides and two equal low tides every lunar day, everywhere. Go and measure real coasts and you find three different patterns, because continents, currents and the shapes of ocean basins bend the simple picture.

• A semidiurnal tide has two high waters and two low waters each lunar day, of similar height. This is the pattern at most of India's ports, including Mumbai and Kochi, and along most of the world's Atlantic coasts.
• A diurnal tide has only one high water and one low water each lunar day. The Gulf of Mexico (at ports such as Pensacola) and the Gulf of Tonkin, off Vietnam, are classic examples.
• A mixed tide has two highs and two lows, like a semidiurnal tide, but the two highs (or the two lows) are noticeably unequal in height. Much of the west coast of the United States, including San Francisco, works this way, and parts of India's east coast show a milder version of the same unevenness.

Almost everywhere, the tide arrives quietly: the water simply gets a little higher, hour by hour. In a few special places, the incoming tide arrives instead as a single steep wall of water, called a tidal bore, that runs up a river against the current like a wave with nowhere else to go.

India has one, on the Hooghly, the river that flows past Kolkata to the sea. It begins near Hooghly Point, where the river first narrows, and can be felt as far as 35 km upstream, near Naihati. Wikipedia's account of the river describes the bore as often exceeding about 2.1 m in height, with the most extreme bores — in March and September — reaching 2.4 to 6.1 m. It needs two things at once: a bigger-than-average spring tide, and extra river flow pushing back against it. Small boats caught by surprise have been capsized by it.

Why does a bore form in some rivers and not others? Three things have to line up.

A large tidal range. There has to be a lot of water trying to get in.

A funnel-shaped, shallowing mouth. As you saw with the Gulf of Khambhat, a narrowing, shallowing channel squeezes the same water into less room, forcing the level up fast.

Water moves faster where it is deeper. A wave in deep water outruns the same wave in shallow water. As the front of the incoming tide reaches into the shallowing river mouth, the water just behind it is still in slightly deeper water and catches up. Given enough of a head start and enough distance, the gently sloping front of the tide steepens into an abrupt, breaking wall — the bore.

A cyclone crossing the Bay of Bengal does two things to the sea at once, and the combination is what makes coastal flooding from a cyclone so severe.

Low pressure lifts the sea. Right under the eye of a cyclone, air pressure drops sharply, and with less air weighing down on it, the sea surface rises — a small effect on its own, typically tens of centimetres.

Wind piles water onshore. Far more important, ferocious onshore winds physically shove water ahead of the storm and pile it against the coast, sometimes for tens of kilometres in front of the landfall point. Together these two effects are called a storm surge, and it can raise the sea by several metres above its normal level, for hours.

Here is the crucial link to this whole lesson: a storm surge rides on top of whatever the ordinary tide is doing. A surge that arrives at low tide adds to a low starting level; the very same surge arriving at high tide adds to a high one. A cyclone's landfall time relative to the tide can be the difference between a bad flood and a catastrophic one.

This is why India's cyclone warnings, issued by the India Meteorological Department, always give the expected storm surge height alongside the tide forecast for the landfall time, and why coastal evacuation orders are timed around high tide, not just around the storm's arrival. It is also why the Sundarbans and the low-lying deltas of Odisha and Andhra Pradesh have built networks of raised cyclone shelters: buildings tall and strong enough to keep people above a surge that an ordinary house cannot survive.

If a tide can float a ship or empty a harbour, it can also turn a turbine. A tidal barrage is a dam built across a bay or estuary with gates and turbines built into it. Water is let through as the tide rises, and again as it falls, and each time some of that moving water spins turbines connected to generators — the same idea you met for hydroelectric dams in Electricity, but powered by the Moon instead of a river.

Nowhere in India runs more completely on the tide than the Sundarbans, the tangle of tidal rivers, mudflats and mangrove forest where the Ganga and Brahmaputra meet the Bay of Bengal, shared between India and Bangladesh. Every one of its hundreds of channels rises and falls twice a day, and everything about life there is arranged around that fact.

Villages are protected from the daily tide by earthen embankments; a breach at high tide, especially a spring tide pushed higher still by a storm surge, can flood fields with salt water for a growing season or more. Ferries and country boats time every crossing to the tide, because a boat that leaves too early or too late can be stranded on a mudbank for six hours. Honey collectors who go into the forest to gather wild honey from giant honeybee combs must also watch the tide as carefully as they watch for tigers, since the same channels they cross by boat can become impassable, or dangerously fast, as the tide turns.

You met salt pans briefly in Discover. Here is the sum behind them. A salt maker lets a spring high tide flood a shallow pan through a sluice gate, then closes the gate and lets nine or ten weeks of sun and wind evaporate the water away, leaving a crust of salt to be raked up and stacked. Missing a good spring tide can mean waiting a whole fortnight for the next chance to refill the pans — one more reason coastal calendars are built around the Moon, not just the Sun.

Gujarat alone produces most of India's salt this way, much of it around the Gulf of Kutch and the Rann, where some of the country's largest tidal ranges make it easy to flood pans high above the working floor.

You already know tide prediction is possible because the Moon and Sun move predictably. Here is roughly how it is actually done.

A tide gauge at a port records the sea level, often every few minutes, for months or years. That record is not a single clean wave — it is a messy squiggle, because the Moon's tide, the Sun's tide, the shape of the local coast and the weather are all mixed into it together. Harmonic analysis is the mathematics that untangles the mix: it treats the messy squiggle as the sum of many simple, steady waves (called constituents), each with its own period, height and timing, and works out what those waves must be. INCOIS, the Indian National Centre for Ocean Information Services in Hyderabad, describes the real curve at a port as built from as many as about 115 such constituents, though a handful of the largest ones — the Moon's main semidiurnal wave, the Sun's, and two daily ones — usually do almost all of the work.

Once you know each constituent, predicting the future is just arithmetic: run each simple wave forward by however many years you like, using the known, predictable motion of the Moon and Sun, and add them all back together. That is why a tide table can be printed with confidence years in advance, even though the ocean itself is turbulent and hard to model in every other way.

Understand offered three explanations for why tidal range varies so much by place: funnelling, shallowing and resonance. A good scientist does not just accept an explanation — they look for a test that could show it wrong.

Test for funnelling and shallowing: if these matter, tidal range should generally grow as you travel further into a narrowing, shallowing gulf. Along the Gulf of Khambhat, ranges do exactly that: about 4–5 m near the open mouth, growing to about 10 m up near Bhavnagar. That is consistent with the explanation — though consistent is not the same as proven; a sceptic could still ask whether something else also changes along that same stretch of coast.

Test for resonance: resonance predicts that basins whose natural sloshing period happens to be close to the tidal period should have unusually large ranges even without an extreme funnel shape, while similarly shaped basins with a very different natural period should not. The Bay of Fundy fits this well: you calculated in the worked example below that a bay of about its real depth naturally resonates at close to the tidal period, which is exactly where its enormous range comes from.

Put the whole layer to work on one imagined but realistic day at a fishing and ferry town on a tidal creek near the Gulf of Khambhat, where the spring range is about 10 m.

Notice how many separate ideas from this layer show up in that single table. The quarter of a lunar day between low and high water is the same arithmetic from Understand's rule of twelfths. The salt pan's sluice gate depends on catching a spring tide, the fortnightly rhythm from Discover. The ferries and boats are running a semidiurnal routine, twice a day, every day. And the cyclone warning is a reminder that the one thing a tide table cannot predict — the weather — can turn an ordinary high tide into the most dangerous moment of the year.