Exceptional natural spectacle: discover the dates of France's highest tides
A spring tide occurs when the tidal coefficient reaches or exceeds 90. These spectacular events happen several times a year and produce exceptional differences in water level between high tide and low tide.
The tidal coefficient is a French invention, created by the Service Hydrographique et Océanographique de la Marine (SHOM) based in Brest. This scale, unique in the world, provides a simple way to measure tidal amplitude on a range from 20 (neap tides) to 120 (tide of the century).
A coefficient of 120 corresponds to the theoretical maximum tide, also known as the "Highest Astronomical Tide" (HAT). These exceptional tides are extremely rare: the last one dates back to March 2015, and the next is expected around 2033.
Tides are the result of the combined gravitational pull of the Moon and the Sun on the Earth's ocean waters. Although the Sun is far more massive, it is the Moon that exerts the dominant influence because of its much closer proximity to the Earth.
When the Moon lies between the Earth and the Sun, their gravitational forces combine and reinforce each other. This is the period of spring tides, with high coefficients.
When the Earth sits between the Moon and the Sun, the gravitational forces align once again. Spring tides also occur during this phase, potentially producing very large tidal ranges.
During the first and last quarter phases, the Moon forms a right angle with the Earth-Sun axis. The gravitational forces work against each other, producing neap tides (coefficients below 70).
The largest tides of the year traditionally occur around the spring equinox (21 March) and the autumn equinox (21 September). During these periods, several astronomical factors combine to maximise tidal amplitude:
Spring tides are the perfect occasion for shore fishing. The exceptional retreat of the sea exposes areas that are normally inaccessible, teeming with shellfish, crustaceans and fish. The best conditions arise when coefficients exceed 100, roughly two hours before low water.
Every species has a minimum legal catch size. Bring a measuring gauge to check your catch. Always consult local regulations, as they may vary from one region to another.
Spring tides offer outstanding photographic opportunities. The landscape transforms dramatically: vast expanses of beach at low tide, powerful waves crashing in at high tide. Rocks that are normally submerged emerge from the water, creating spectacular compositions that are impossible to capture at any other time.
Surfers and bodyboarders are particularly keen on spring tides because of the more powerful waves they generate. However, the currents are significantly stronger during these events, so these activities are best left to experienced practitioners who know their local spots well.
Tide predictions take only astronomical factors into account. Meteorological conditions also play a significant role in the actual water level:
When a storm coincides with a spring tide and low atmospheric pressure, water levels can far exceed predictions. It is under these combined conditions that the most dangerous coastal flooding events occur.
The coefficient (from 20 to 120) indicates the theoretical amplitude of the tide, calculated from the average tidal range at Brest. The tidal range is the actual difference in water height between high tide and low tide, expressed in metres. The same coefficient will produce different tidal ranges depending on location: roughly 6 metres at Brest for a coefficient of 100, but up to 15 metres at Mont Saint-Michel.
A tide is generally considered a spring tide when the coefficient reaches or exceeds 100. Between 95 and 99, it is classified as a moderate spring tide. From 110 upwards, these are exceptional spring tides that draw large crowds to the coastline.
Because tides are an astronomical phenomenon, they can be calculated with great precision many years, even decades, in advance. SHOM publishes tide predictions covering a 10-year horizon. Only meteorological factors (atmospheric pressure, wind) cannot be forecast far ahead.
The Mediterranean is a nearly enclosed sea, too small for tidal forces to generate significant amplitudes. The tidal range there is generally less than 40 cm, compared with several metres along the Atlantic coast. The shape of the coastline and the depth of the ocean play a major role in amplifying tides.
The expression "tide of the century" refers to a tide whose coefficient approaches or reaches 120, the theoretical maximum on the scale. These events are rare and occur when all astronomical factors align perfectly: syzygy (Sun-Earth-Moon alignment), lunar perigee (Moon at its closest) and equinox. The last one took place on 21 March 2015 (coefficient 119).
Swimming is possible but requires extreme caution. Currents are much stronger than during normal conditions, increasing the risk of rip currents. It is essential to swim only in supervised areas, between the flags, and to follow the instructions of lifeguards, who may prohibit swimming altogether if conditions are too dangerous.
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