Spring Tides Explained: Why the Biggest Tides Arrive Around the Equinox
TL;DR: Spring tides have nothing to do with the season. They happen twice a month, a day or two after new and full moon, when the pulls of the moon and sun line up and high tides run higher. The largest of the year come close to the equinoxes, when the sun sits over the equator. Add a storm surge and strong winds, and a normal autumn high tide can overtop coastal defences, which is why the Environment Agency’s coastal flood warnings cluster around these dates.
Last updated: 17 September 2026
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Key takeaways
- Spring tides happen twice every lunar month, when the moon and sun line up at new and full moon, and they usually arrive one to two days later.
- The National Tidal and Sea Level Facility says these tides are particularly large close to the equinoxes, when the sun is overhead at the equator.
- The sun’s tide generating force is 0.46 times the moon’s, so the sun adds to the moon’s tide rather than driving it.
- A storm surge can raise sea level by over 3 metres in European seas, and flooding comes from high tide, surge and waves combining.
- On 13 and 14 September 2026 the Environment Agency lifted eight flood warnings, from North Morecambe Bay to the Suffolk coast at Southwold.
What are spring tides?
Spring tides are the larger tides that occur twice a month, when the moon and the sun pull in line with each other. High water rises higher than average and low water falls lower. The name has nothing to do with the season: in the old sense, the tide springs up.
The National Tidal and Sea Level Facility, part of the National Oceanography Centre, explains the mechanism. They happen when the lunar and solar tides reinforce each other, at new moon and full moon. At first and third quarter the two work against each other, producing the smaller neap tides.
There is a lag. Because real oceans have shape and friction, NTSLF notes that they occur between one and two days after new or full moon. It adds a useful local rule: at any given place, spring high tides arrive at roughly the same time of day, so at Liverpool they always fall around midday and midnight.
Why are spring tides bigger around the equinox?
Because the sun is then overhead at the equator. NTSLF states plainly that spring tides are particularly large close to the equinoxes, around 21 March and 21 September. That is why the late September tides, and the March ones, are the ones coastal teams watch most closely.
The sun is the junior partner. NTSLF puts the tide generating force of the sun at 0.46 times that of the moon: the sun is vastly more massive, but the force falls with the cube of distance, and the moon is far closer. So the sun cannot make a big tide on its own. What it can do is add its pull at the moment the moon’s pull is strongest.
Other astronomical factors stack on top. Tides are slightly larger when the moon is at perigee, its closest point to Earth, and the angle of the moon’s orbit to the equator matters too, over a cycle that lasts 18.6 years. When several of these coincide with an equinox, the predicted high waters are among the highest of the year.
When do spring tides cause flooding?
When something else is added to them. A spring tide on a calm day usually stays within defences built for it. The danger comes from weather: NTSLF defines a storm surge as a large scale rise in sea level caused by a storm, as low pressure lets the sea rise and gale force winds push water towards the coast.
The numbers are large. According to NTSLF’s page on storm surges, surges can raise sea level by over 3 metres in European seas, and coastal flooding happens when some combination of high tide, storm surge and waves overtops or breaches defences. A surge arriving at low water may do little. The same surge on a spring high tide is a different event.
This year’s early autumn showed the pattern. On 14 September 2026, the Environment Agency’s flood alerts and warnings service listed eight flood warnings removed in the previous day, most of them on the coast or tidal rivers, including the coast at North Morecambe Bay, the north Norfolk coast, the Essex coast from Clacton and the Colne and Blackwater estuaries, the Suffolk coast at Southwold, and the tidal River Avon at Bristol.
Why does the Bristol Channel see such big tides?
Because of its shape. NTSLF explains that the coastline and the depth of the sea change how the tide behaves around Britain. The Bristol Channel has the third highest tidal range in the world, with a mean spring range of 12.3 metres at Avonmouth, while Lowestoft on the east coast sees a mean spring range of just 1.9 metres.
That contrast matters for anyone managing assets near the coast. The same tides that barely register in one town lift the sea by about the height of a four storey building in another. Local tide tables, not national averages, set the risk.
How do coastal sites monitor tidal water levels?
By measuring the water where it matters to them: in the tidal outfall, the pumping sump or the drainage chamber behind the sea wall. Tide tables predict the astronomical tide, and surge forecasts add the weather, but neither says how high the water is in your own chamber when the outfall is tide-locked.
That local picture is where continuous level monitoring earns its place. The AQUAIOT Radar measures level without contact, to within ±2 mm, and reads reliably through vapour and condensation in enclosed underground chambers, which is exactly the environment of a coastal outfall or pumping station wet well. Its readings arrive continuously, so a rising level during spring tides shows up as a trend rather than a surprise.
Our answer on tide-locked drainage explains why surface water cannot escape while the sea is higher than the outfall, and our guide to flood monitoring systems covers alarm thresholds and escalation for the wider site.
Frequently asked questions
What are spring tides?
Spring tides are the larger tides that occur twice a month, a day or two after new and full moon, when the pulls of the moon and sun line up. High tides are higher and low tides lower than average. They have nothing to do with the season.
Why are tides higher around the equinox?
The National Tidal and Sea Level Facility says these tides are particularly large close to the equinoxes, around 21 March and 21 September, when the sun is overhead at the equator.
How often do spring tides happen?
Twice every lunar month, at new moon and full moon. Neap tides, with a smaller range, happen in between, at first and third quarter.
Do spring tides cause flooding?
Usually only when combined with a storm surge, strong winds or large waves. Coastal flooding occurs when high tide, surge and waves together overtop or breach defences.
What is a storm surge?
A storm surge is a large scale rise in sea level caused by a storm. Low pressure lets the sea rise and strong winds push water towards the coast. NTSLF says surges can raise sea level by over 3 metres in European seas.
Where to start
Check the dates of the next spring tides in your local tide table and put them in the maintenance calendar. Clear outfall flaps and screens in the days before them, and sign up for the Environment Agency’s flood warnings for your stretch of coast.
Then look at the chambers and sumps that back up when the tide is high. If nobody can say how high the water got during the last big tides, that is the first gap to close. Speak to an AQUAIOT engineer about continuous level monitoring for tidal outfalls and coastal drainage.