Last month we 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.