|Words & Images: J Bateman |
As you know tides have a huge effect on the sport of surfing here in Wales. They can determine what break will be working, how big the waves will be and perhaps how long your surf will last. Below is a basic look at the forces at work and a glance at the Bristol Channel where tidal extremes are at their greatest in Wales.
Tidal range and phases
Tidal range is the vertical difference between the highest high tide and the lowest low tide. In other words, it is the difference in height between high and low tides. The most extreme tidal range will occur around the time of the full or new moons when gravity of both the Sun and Moon are pulling the same way (new moon), or exact opposite way (full). This type of tide is known as a spring tide. Incidentally, veering off topic slightly ..... you've probably heard the term lunacy right? Mad behaviour coinciding with full moons hasn't gone unnoticed down the ages....the same gravitational pull exerted on the oceans exerts itself on us driving some of us over the edge it seems .. we are after all mostly made from water. Back on topic, during neap tides, when the Moon and Sun's vectors make a right angle at the Earth, the difference between high and low tides is smaller. Neap tides occur during the firsts and lasts quarter moons. There is about a seven day interval between springs and neaps.
Tides are predictable but have you ever noticed that big spring tides always occur in the early morning or in the late evening ? Think about a surf spot that needs a very big tide to work .... ever surfed it in the middle of the day ? The opposite obviously applies to the smallest neap tides ... they always occur mid afternoon. It's very unlikely you've surfed two neap high tides in the same day where as, given enough daylight hours it's possible to catch an early morning spring high tide and then cruise down for a evening spring high tide sesh on the same day. That's the lunar cycle in full effect.
Wave / Current interaction
Currents play havoc with a wave's basic mechanics. The maths behind this goes pretty deep so I won't bore you with them here but in short currents change a wave's speed. The wave speed increases when it interacts with a current flowing in the same direction as it's travelling and similarly a wave's speed decreases when it encounters a current flowing in the opposite direction. So why am I talking about wave / current interaction in the context of tides? Well, with big tidal ranges comes big tidal streams and currents so waves heading to our beaches must navigate these extreme currents before finally arriving ashore. Think back to one of the most fundamental contributors of wave heights at your beach 'swell direction'....if a swell direction is perpendicular to the shore wave heights will be greater than that of an equally sized swell arriving at a more oblique angle. Now, the increase or decrease in wave speed caused by the tidal currents is rarely uniform and as a result waves bend and distort as they move through varying current speeds. In short tides have the ability to change wave direction in that they literally bend waves away or towards the shoreline thus changing the swell direction. I'm sure we all know a surf spot or even a stretch of coast that breaks at a particular state of tide, wave / current interaction maybe solely responsible for this.
The Bristol Channel
East of the line from St.Govan’s Head to Hartland Point is the Bristol Channel
A large number of Welsh surfers call the Bristol Channel their home, South East Pembrokeshire, Gower and Porthcawl all fall within it’s boundaries and all know you must be in tune with this constantly changing stretch of water to maximise your surf time. The first subject that usual crops up when you talk with anyone about the Bristol Channel is the massive tides that you have to 'endure'. But why and how does this tiny coastal sea produce such monumental tides and currents that so uniquely shape our waves?
The answer is simple enough, the Bristol Channel is shaped like a funnel – wide and deep at its mouth, and shallow and narrow at its head. As the semi-diurnal [twice daily] tidal wave enters the channel the enormous volume of water moves up the ever-narrowing topography, a “funnel effect” occurs pushing the water up in height resulting in the monster tides you see everyday. The further from the mouth of the Channel the greater the tidal range.
The techy name for the tidal range is determined by >
Micromareal, when the tidal range is lower than 2 meters.
Mesomareal, when the tidal range is between 2 meters and 4 meters.
Macromareal, when the tidal range is higher than 4 meters.
Surfers living closer to the Severn Estuary [ie West of Lavernock Point] experience the full effect of that funnel effect. Spring tides can reach heights of 50ft, a tide on steroids, magnifying the wave / current interaction to extremes. Tidal streams are at their maximum at mid-tide .... and subsequently at this time wave / current interacion is at it's greatest. This can have a positive or negative effect from a surfers point of
view. If a current is bending waves away from the shoreline then at mid tide, on a huge spring tide, wave heights can be massively reduced. Even a solid 6ft swell has trouble propagating into a tidal stream of that magnitude. On the positive side, if a current is bending waves towards the shoreline a small swell can be magnified and perhaps be bigger than forecast. Other contributing factors affecting wave height in shallow seas will be discussed at a later date … preferable when you’re having trouble sleeping.
Check this clip of the Severn Bore, a tidal phenomenon caused by the leading edge of the incoming tide. This creates a wave of water that travels up the River Severn, wacky song to go with it too!