Priya Nair
Why tide tables are your first port of call
A tide table is not just a list of numbers for sailors. For anyone walking the coast, it is the difference between a memorable stroll and a cold, dangerous surprise. Tide tables predict the time and height of high and low water at a reference port. From that, you can work out when a beach will be wide and firm, when a causeway will be dry, and when a rocky cove will be cut off. The key is to understand what those times and heights actually mean, and to allow for the fact that the sea rarely behaves exactly as printed.
Before you set out on any exposed coastal route — a tidal island, a long sandy bay, a harbour wall — check a tide table for the nearest standard port. Then adjust for where you actually plan to walk. A difference of half an hour or half a metre can turn a safe crossing into a scramble.
Reading times and heights without confusion
Tide tables usually show two high waters and two low waters each day. Times are given in local clock time, but heights are measured relative to chart datum — a low-water baseline that is close to the lowest astronomical tide. So a low water height of 0.3 metres means the water is still 30 centimetres above that baseline, not that the beach is bone dry. A high water of 4.8 metres means the sea will rise nearly five metres above chart datum.
For coastal walking, the height matters more than the time. A low water of 0.1 metres at a gently shelving beach may expose hundreds of metres of sand. The same low water at a steep shingle beach might reveal almost nothing. Look for local guides or noticeboards that state something like: “Causeway passable from two hours before to two hours after low water.” That is the practical translation of the tide table.
- High water is when the sea is at its highest — the worst time to cross a low-lying causeway.
- Low water is when it is lowest — the best window for exposed routes, but still check the height.
- Tidal range is the difference between high and low water on a given day. A big range means faster currents and more rapidly changing conditions.
Spring and neap tides: the fortnightly rhythm
Tides do not repeat identically every day. Roughly twice a month, around new and full moon, the Sun and Moon pull together to produce spring tides. These have the highest highs and the lowest lows — the greatest tidal range. This is often ideal for coastal walking, because more beach is exposed. But it also means the tide comes in faster once it turns. A route that is passable for four hours on a neap tide might be passable for only two on a spring tide.
Neap tides, around the first and third quarters of the Moon, have a smaller range. High water is lower and low water is higher. That can be safer for causeways — less water to cover them — but it can also mean that a beach you hoped to walk is never fully exposed. Always check the range, not just the low water time.
Local variations: why your bay is not the next bay
A tide table for a standard port may be accurate for that port, but it can be misleading a few miles along the coast. Estuaries, headlands, islands and shallow bays all distort the tide. In some places, high water arrives 20 minutes earlier or later than the reference port. In others, the shape of the coastline creates a double high water — the sea stays high for an hour or more, which is particularly dangerous for causeways.
Weather adds another layer. A deep low-pressure system can raise sea level by 30 centimetres or more. Strong onshore winds can push water into a bay and delay the ebb. Conversely, high pressure and offshore winds can make the tide lower than predicted. Never treat a tide table as a guarantee. If the wind is blowing hard onshore, add at least half a metre to the predicted height and subtract half an hour from your safe window.
Practical planning: the rule of twelfths and safety margins
Once you have your tide times and heights, you can estimate how quickly the water will move. The rule of twelfths is a simple guide. In the first hour after low water, one-twelfth of the tidal range comes in. In the second hour, two-twelfths. In the third, three-twelfths. Then it reverses: three, two, one over the next three hours. So if the range is 4 metres, the first hour adds about 33 centimetres, the second about 67 centimetres, the third about 1 metre. That tells you when a causeway might become covered.
Always build in a buffer. Aim to be off any tidal route at least an hour before the tide would cut it off. Carry a map that shows the high-water mark, and know your escape routes. If in doubt, turn back. The coast is unforgiving, but it is also wonderfully predictable once you learn its rhythms.
Your pre-walk tide checklist
Before you leave the house, run through this simple routine. It takes two minutes and could save your life.
- Find the nearest standard port tide table for your walk date.
- Note the times and heights of low and high water, and the tidal range.
- Check whether it is a spring or neap tide.
- Adjust for local variations: ask a local, check a harbour noticeboard, or use a trusted coastal guide.
- Factor in weather: wind direction, pressure, and recent rainfall.
- Plan your route to be on exposed ground only between two hours before and two hours after low water — less if the range is large.
- Tell someone your route and expected return time.
Coastal walking is one of Britain’s great pleasures. With a tide table, a little local knowledge and a healthy respect for the sea, you can enjoy it safely for years to come.
