Author William Thomson on how currents work and knowing when it's safe to swim in the sea
In my previous article, I explored how tides are made by giant waves flowing along the coast, with high tide at the peaks and low tide at the troughs. Using an analogy of a tidal wave, I explained how at low tide currents are ‘sucked’ towards the peak, hundreds of miles down the coast. Then, when the peak arrives six hours later, it ‘pushes’ the currents along with it; east up the south coast, south down the east coast and north up the west coast. The effect on your local beach is that currents flow along the shore for six hours at a time, changing direction at set hours before and after high tide – a time known as Slack Water, because the currents are literally slack. The exact time of ‘Slack’ is unique to every beach, but on an open coast it is generally around 2 hours before high tide and 4 hours after high tide (+/- 1 hour for local anomalies). On tidal rivers and estuaries, Slack Water is closer to high and low tides with currents flowing inland as the tide rises and back out to sea as it falls.
Far and fast swimming
After Slack Water, currents speed up for three hours then slow down for three hours, changing direction at the next Slack Water. This cycle has a monumental effect on how far and fast we can swim. Where I live in Kent, the northerly currents at high tide allow me to swim three miles up the coast from Kingsdown to Deal without any trouble at all; it would be impossible against the southerly stream at low tide. On a recent session with my swim buddy Danny Burrows, we challenged ourselves to a 500-metre slog against the currents to Deal Pier. On the single bend of the swim, the torrent was so strong that we spent five minutes opposite the same yellow house on the seafront, despite our trick of staying close inshore where the flow is weakest. In the end, I got out and walked up the beach; to Danny’s credit he stuck it out and ‘beat the bend’, a feat partly motivated by his disinclination for walking barefoot on pebbles. Either way, we battled our way to the pier and raced back at an Olympian 6 knots; 3 knots of propulsion on top of a 3 knot current.
Out to sea
On an open coast, tidal currents flow from the surface to the seabed and all the way out to sea, like a great river. However, if you add an obstruction like a jetty, harbour wall or headland, it will create counter-currents that flow the opposite way. These are eddies, created from pressure differences either side of the obstruction. When water hits the wall and is deflected around it, this leaves an area of low pressure immediately downstream of the obstruction. Nature’s way of restoring balance is to move water from the area of high pressure to the area of low pressure; this is the eddy. So if you work out from the tide times that currents on your beach should be flowing to the left, but they take you to the right, look at where you’re being taken and if there’s an obstruction, chances are you’re in an eddy. This is useful to know, but it can also be life saving; when extremely fast currents hit obstructions like bridge legs or small islands, the opposing currents wrap around each other and create whirlpools capable of sucking you to the seabed.
For those who want to avoid any type of challenging current, eddy or whirlpool, this can be achieved by swimming at Slack Water. The drawback is that you only have a short window to swim. A good compromise is to plan your session for when the conditions are calmest, and this is dependant on the wind in relation to the currents. When the wind and currents are travelling the same way, it flattens the sea and keeps it calm – ideal for relaxed downstream cruising [as long as winds are less than 10-15 knots]. But if the wind keeps its course through the day, when the currents change direction the friction of the opposing forces creates a short, choppy sea. In extreme cases, a fast current flowing into a strong wind will create steep breaking waves called Overfalls. These are dangerous to boats, let alone swimmers, and should be avoided at all costs. Luckily, this is easy to avoid by timing your swim for when the currents are flowing the same way as the wind, unless you enjoy a cold, wet face slap every time you come up for breath.
Waxing and waning
The effects of wind against current are more pronounced around Spring Tides, because currents are faster. As we learnt in my previous article on tides, Springs happen at the Full Moon and New Moon when the sun, moon and earth are aligned, increasing the gravitational pull on our oceans and creating steeper tide waves with higher peaks and deeper troughs. This means more water flows between high and low tide, with the result of faster currents – and as a by-product, shorter periods of slack water. In contrast, currents are weaker during Neaps and Slack Water lasts longer. An Imray nautical chart for the waters off Deal tells me that an hour after high tide during Springs the currents reach a maximum speed of 3.2 knots, but at Neaps it only gets up to 1.7knots – a huge difference for a swimmer averaging 3 knots. So in addition to considering the time of tidal day, you need to be aware of the day of tidal month. The solution is simple: let the waxing and waning of the moon become a natural part of your adventure routine.