by Dave Hammond
Due to a couple of recent incidents in New Zealand the following may be of special interest to kayakers interested in surfing. It is taken from a British Canoe Union book so some terms may differ.
Up until the moment a wave breaks, there is no forward momentum but an oscillation movement within the wave itself that becomes increasingly unstable until the energy within the wave is released in a forward rush creating a white foamy soup. This water cannot pile up indefinitely on the beach and has to find an escape route back out to sea. It does this in two ways. On steep beaches, the water washes up the slope after the wave breaks and then washes back down again under the force of gravity. The momentum of this returning mass of water then carries on under the next incoming breaking wave, causing what we know as undertow.
On gently sloping beaches undertow does not occur, instead the water finds its way back out to sea through deep water channels. These are called rips. Rips are often found along the edges of bays where there are rocks. They are sometimes found where a stream running off the land scours out a channel on a beach, or they may create their own escape channels as can be seen on long, sandy beaches. In between rips, the sandy bottom is usually a little higher than the rip channels and this results in an undulating surface across the beach. Once the beach becomes covered by water, these contours in the sand in turn have an effect on the wave shape creating peaks and giving good surf waves.
An area of flatter water between the breaking waves may indicate rips as the outgoing current goes against the wave movement.
The break fine is the area where incoming waves start to break. Rips tend to dissipate once they get beyond the break line, so for good surfers they give a welcome lift out through the break fine. For the less experienced, however, and unwary swimmers, they can be death traps, particularly when the surf is large.
Rips flow like a river and it is not uncommon for them to move at several knots. A speed of 12 knots has been recorded in a rip on a Cornish beach during a storm. If caught in a rip as a swimmer, try to remain calm and work out which way it is taking you. Swim at right angles to this and you will soon be out of its influence.
Although rips dissipate outside of the break, they can sweep the unwary paddlers out into strong tidal streams that may be at work just a little way off shore. Be aware of this or a surfing day turn into an unwanted sea trip.