The sea at low tide is a very different animal from the same sea at high tide, particularly on a spring. Firstly the view is entirely different; miles of damp sand and exposed rocks, versus a tiny ridge of weeds and dry sand. The water at low tide often appears sparser, flatter, less dense. High tide seas swell and billow like the sails on a galleon and take on a new depth and richness of colour. What causes this transformation?
The short answer is that the sea, depending on where you are in relation to the earth’s axis, is affected by the gravitational pull of the moon and sun relative to earth.
Spring Tides and Neap Tides
Spring tides occur at full moon and new moon, approximately every 2 weeks. Neap tides occur on the half moons. Spring tides are more extreme, so a low spring will be lower than a low neap while a high spring will be higher than a high neap. The actual heights and intervals of tides vary according to where you are.
Some places have semi-diurnal tides (two high tides at roughly 12 hour intervals, interspersed with two low tides). Others have mixed tides where tides from two directions meets (e.g. in Dorset where the North Sea tides meet Atlantic tides). Know your local tides!
Magic Seaweed is a useful website for checking sea conditions in general, wave and swell heights, tides, and forecasts for your local area and all around the world.
In general, learn to read websites, tide tables and sea charts and to understand them, and ask experienced local swimmers, sailors and other knowledgeable experts in how the water behaves at the place you are considering a swim.
The significance of tides is in understanding that more water moves during the middle two hours of both an incoming (flood) and outgoing (ebb) tide; this results in stronger currents.
The slack tide is in many places one hour either side of high or low tide, and in some places the water barely moves. But this can vary, with different conditions in some places so it’s vital that you always check local conditions. Do also be aware, however, that while it’s generally safer in most places to swim on a slack tide, the actual times of slack water may vary from the official tide times depending on local geography and weather conditions. And there can be other places where swimming is safer at low tide.
On spring tides, where you will remember the low tide is lower, and the high tide is higher, more water is moving further between those two extremes. Thus currents are correspondingly stronger on springs than on neaps.
While uninterrupted tidal streams seldom flow at more than 2 knots (2.3 mph or 3.7 kph), the fastest current in the sea is at the Saltstraumen near Bodø, in Norway. The Skerstadfjord fills and empties through a narrow gap about 150 metres wide. The average speed of the water there is 7-9 knots, increasing to an estimated 20 knots which creates large whirlpools.
Places in the UK with tidal races include the Pentland Firth between the northern coast of Scotland and Orkney at 10 knots, the Menai Straits between mainland Wales and the island of Anglesey at 8 knots.
To put these speeds into perspective, A mature river typically flows at 3 knots and a steep whitewater river at 6.5 knots. A world-class swimmer travels at a sprint speed of around 4 knots, while dolphins cruise at 6-7 knots. Hence stick to sheltered bays unless you understand exactly what you’re doing which involves careful planning using the relevant tidal stream atlas. You should in any case be extremely wary swimming around exposed coasts and straits.