Tides in the Oosterschelde

How does the tide work?

A complete tidal cycle from high tide to low tide and back to high tide takes on average about 12 hours and 25 minutes.

Within one tidal cycle there are two moments of high tide and two moments of low tide. There are also generally two slacks, where the local current changes direction and temporarily decreases sharply.

High tide and low tide describe the water level. Slack relates to the current. These moments therefore don't automatically coincide.

The exact time also differs per location in the Oosterschelde.

How do high and low tide arise?

The tide arises mainly from the gravitational pull of the moon and, to a lesser extent, the sun. The moon's gravitational pull causes the water on Earth to not be distributed at the same height everywhere.

In a simplified model, two areas of relatively high water arise: one on the side of the Earth facing the moon and one on the opposite side. Between them are areas where the water level is relatively low.

As the Earth rotates and the tide moves across the oceans and coastal areas, different locations successively end up in an area of higher and lower water.

In reality, the pattern is much more complex. Oceans, continents, the shape of the coast and the depth of the seabed influence how the tide moves. In the Oosterschelde, the shape of the channels and branches also plays an important role.

That's why you can't simply deduce the moment of high or low tide from the position of the moon. The local tide forecast remains decisive.

High tide and low tide

At high tide, the water level is at its highest point within a tidal cycle. After that, the water level starts to fall again towards low tide.

At low tide, the water level is at its lowest point. After that, the water level starts to rise again towards the next high tide.

High tide and low tide therefore tell you something about the water level, but not directly about the strength of the current.

The moment when the current is at its minimum therefore doesn't need to exactly coincide with high or low tide.

Read more about this difference at Slack in the Oosterschelde.

The moon and the tide

The moon has the greatest influence on the tide, but the sun also contributes. The position of the moon relative to the Earth and the sun partly determines how strongly the two tidal effects reinforce or partially counteract each other.

There are four important moon phases:

• New moon 🌑 — the moon is positioned roughly between the sun and the Earth.
• First quarter 🌓 — seen from Earth, the moon and the sun are roughly at a right angle to each other.
• Full moon 🌕 — the Earth is positioned roughly between the sun and the moon.
• Last quarter 🌗 — the moon and the sun are again roughly at a right angle to each other.

These moon phases partly determine the rhythm of spring and neap tide.

Spring tide and neap tide

Around new moon and full moon, the sun, Earth and moon are roughly aligned. The tide-generating effects of the sun and moon reinforce each other. The difference between high and low tide is then greater.

We call this spring tide.

Around first and last quarter, the sun and moon, seen from Earth, are roughly at a right angle to each other. Their tide-generating effects then reinforce each other less strongly. The difference between high and low tide is smaller.

We call this neap tide.

Spring tide and neap tide alternate roughly every two weeks.

The larger or smaller tidal difference also affects the strength of the current. During spring tide, the current is generally stronger than during neap tide.

Why do the times shift every day?

The times of high and low tide aren't the same every day. The moon moves relative to the Earth and returns to the same position in the sky on average about 50 minutes later.

Because the tide is linked to this, the moments of high and low tide also shift every day.

The same applies to the slack. The exact moment of slack differs per location, because the current doesn't behave the same way and at the same time everywhere.

That's why you can't assume, for example, that "high tide is every day at 12:00". The times keep shifting continuously.

How far ahead can the tide be predicted?

The astronomical tide can be calculated reliably far in advance. The movements of the Earth, moon and sun are known, which means high and low tide can be calculated months and even years ahead.

The actual conditions can, however, deviate from this astronomical calculation.

Wind and air pressure in particular can affect the actual water level. As a result, the actual current can also deviate from the calculated forecast.

That's why you need to distinguish between an astronomical tide prediction and a forecast of the actual conditions at a dive site.

The Tide Planner shows the expected conditions per location and offers a forecast horizon of up to thirteen months.

What does the tide mean for divers?

The tide determines the rhythm within which conditions in the Oosterschelde change.

For a diver, three things in particular are important:

• Tide determines how the water level rises and falls.
• Current determines how quickly the water moves.
• Slack is the moment at which the local current changes direction and temporarily decreases sharply.

These three are connected, but don't describe the same thing.

Want to know when it's best to dive at a specific dive site? Don't just look at high and low tide, but also check the expected current for that location.

The Tide Planner translates the expected current per dive site into practical dive windows.