In brief
Tides are the slow rise and fall of the sea caused by the pull of the moon and sun. Waves are made by wind. Together they decide where sand is dropped, where rock is worn away and which plants and animals can live in the band between the tides.
What it is
Tides
A tide is a long, slow wave of water that moves around the oceans. It is caused mainly by the moon’s gravity and, to a smaller degree, by the sun’s. The moon pulls hardest on the water on the side of Earth facing it, less on Earth’s middle and least on the far side. The difference in pull stretches the ocean into two bulges, one facing the moon and one on the opposite side, and Earth turns through them.
Most coasts therefore have two high tides and two low tides in a lunar day, which lasts about 24 hours and 50 minutes. That is why high tide arrives roughly 50 minutes later each day. Some coasts have only one high and one low a day, and others have two unequal ones. These are called diurnal, semidiurnal and mixed tides.
Waves
Most waves are made by wind blowing over open water. Their size depends on the wind speed, how long it blows and the fetch, which is the distance of open water the wind crosses. A wave carries energy, not water: a floating cork mostly moves in a small circle and ends up near where it began. Wave energy grows with the square of wave height, so a wave twice as high carries about four times the energy.
Erosion and deposition
Erosion is the wearing away of rock and sediment. Deposition is the dropping of that material where the water slows. Every shore is a balance between the two.
Spring tides and neap tides
The sun’s pull is a little under half as strong as the moon’s. When the sun, Earth and moon are roughly in a line the two pulls add together and the tidal range, the height difference between high and low water, is largest. When the sun and moon pull at right angles, they partly cancel and the range is smallest. The names have nothing to do with the season spring.
| Moon phase | Sun, Earth and moon | Tidal range | Name |
|---|---|---|---|
| New moon | Roughly in a line, moon between Earth and sun | Larger than average | Spring tide |
| First quarter | At right angles | Smaller than average | Neap tide |
| Full moon | Roughly in a line, Earth between moon and sun | Larger than average | Spring tide |
| Last quarter | At right angles | Smaller than average | Neap tide |
Spring tides come about every two weeks. When a spring tide happens while the moon is also at its closest point to Earth, the range is larger still.
Why it matters
The tide decides how much of a shore is wet or dry, so it draws the invisible lines that sort life into bands (see tide pools). It also sets the safety rules for anyone walking on a shore: a flat sand bar that is dry at low tide can be cut off by rising water.
The size of the tidal range changes the whole character of a coast. Common classes, often credited to a 1964 scheme by J. L. Davies, are listed below.
| Class | Tidal range | What it tends to mean for the shore |
|---|---|---|
| Micro-tidal | Less than 2 metres | Waves do most of the shaping; narrow intertidal zone |
| Meso-tidal | 2 to 4 metres | Waves and tides share the work; barrier islands are common |
| Macro-tidal | More than 4 metres | Tides do much of the shaping; wide flats and strong tidal channels |
The range varies enormously from place to place. In the Mediterranean it is often only a few tens of centimetres, while in the Bay of Fundy it can exceed 15 metres.
How the shore is shaped
Longshore drift
Waves often arrive at a slight angle. Each wave pushes sand and pebbles up the beach at that angle (the swash), but gravity pulls the water straight back down the slope (the backwash). Grain by grain the material zigzags along the coast. This slow movement is called longshore drift, and it can carry large amounts of sand along a coast.
How waves wear rock
| Process | What happens | Typical result |
|---|---|---|
| Hydraulic action | Waves force water and air into cracks in rock and compress it | Cracks widen; blocks break off |
| Abrasion | Waves hurl sand and pebbles against a cliff or platform | Smooth, scoured rock surfaces; undercut notches |
| Attrition | Pebbles knock against each other as they move | Stones become smaller and rounder |
| Solution | Seawater slowly dissolves certain rocks, such as limestone | Pitted, honeycombed rock |
Where hard and soft rock lie side by side, the soft rock wears back faster. Hard rock is left behind as headlands, and the softer rock forms bays. Over a long time a cliff can be undercut into a notch, collapse, and be worn into a flat wave-cut platform. A cave can be widened into an arch, and the roof of an arch can fall to leave a stack.
Beaches and storms
A beach is a store of sediment that takes the energy of waves. Storm waves often pull sand offshore into bars, and gentler waves can later push it back. A beach that loses more sand than it gains gets narrower, and its cliffs or dunes behind it lose their cushion.
Life on a sandy shore
Moving sand gives nothing to cling to, so life here digs. Burrowing worms, small clams and tiny crustaceans live in the wet sand. Mole crabs, sometimes called sand crabs, shuffle up and down the beach with the swash and filter food from the water. At low tide, look for small holes, coiled casts and V-shaped marks.
Effects and management
Erosion rates range from barely measurable on hard rock to more than a metre a year on soft sediment in exposed places. People change the balance in several ways:
- Less sediment arriving. Dams trap river sediment that would have fed beaches downstream, and protecting a cliff from erosion also removes a source of sand.
- Structures on the shore. Groynes (walls built out across a beach) trap drifting sand on one side and can starve the beach on the other.
- Loss of natural buffers. Removing marshes, mangroves or dunes takes away shoreline that absorbed wave energy.
- Rising sea level. The global mean sea level rose by about 20 centimetres between 1901 and 2018, and the rate of rise has increased. Higher water lets waves reach further up the shore.
| Approach | How it works | Strength | Drawback |
|---|---|---|---|
| Sea wall | A solid barrier reflects waves | Protects what is directly behind it | Waves can scour the base; the beach in front may narrow |
| Groynes | Walls across a beach hold drifting sand | Builds a wider beach updrift | Can starve the beach further along |
| Beach nourishment | Sand is added to a beach | Keeps the natural cushion | Needs repeating; costs recur |
| Dune and marsh restoration | Plants and landforms absorb wave energy | Cheap per metre; supports wildlife | Needs space and time to grow |
| Managed realignment | Defences are moved back and land is allowed to flood | Can create new marsh and room for the sea | Land and property are given up |
No single method is best everywhere. The right choice depends on the rock or sediment, the waves, the sea-level trend and what is being protected.
Field exercise
On a beach, look at the line of seaweed and debris the last high tide left behind. Estimate how far the water will come in the next hour by watching three waves. On a different day, use a printed tide table to predict the time of the next high tide and check yourself.
Check yourself
Read the question, think of your answer, then open it with a tap or the Enter key.
About how much later does high tide arrive each day, and why?
About 50 minutes later, because the moon takes about 24 hours and 50 minutes to come back to the same point in the sky as seen from Earth.
When do spring tides happen?
Around new moon and full moon, when the sun, Earth and moon are roughly in a line. They have no link with the season.
If a wave doubles in height, what happens to its energy?
It grows about four times, because wave energy depends on the square of wave height.
Why does longshore drift move sand in a zigzag?
The swash pushes grains up the beach at an angle, and the backwash pulls them straight down the slope.
Name two ways a groyne can change a beach.
It traps drifting sand on the side the sand arrives from, which builds a wider beach there, and it can starve the beach on the far side.
Which tidal range class is above 4 metres?
Macro-tidal.
Basis of the facts
Where these facts come from. The physics of tides and the spring-neap cycle follow the standard account in Pugh and Woodworth (2014); the 24 hours 50 minutes lunar day and the 50-minute daily delay are standard figures. Tidal range classes follow the common micro, meso and macro scheme credited to Davies (1964). Erosion processes, longshore drift and management approaches follow Pethick (1984). The sea-level figure (about 20 centimetres between 1901 and 2018) is from the Intergovernmental Panel on Climate Change sixth assessment (2021). Bay of Fundy and Mediterranean ranges are given only as rounded orders of magnitude. Details are on the sources list.