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Understanding Waves Pembrokeshire | Outer Reef Surf School

Surfing Lesson On Understanding Different Types of Waves

Here in Pembrokeshire, we are so lucky to have different types of waves right on our doorstep. When the open ocean swell reaches shallower water it becomes steeper and finally breaks into surf. Several factors influence the amount of growth that occurs from a deep water swell to a breaking wave. These include the angle of approach the swell makes with the shore, the configuration of shallow water bottom contours, the slope of the beach, and the deep water steepness of the wave (the ratio of wave height to wavelength).

PLUNGING OR DUMPING WAVES

The greater growth occurs when swells having little steepness and long period approach a strongly sloping beach or reef resulting in Plunging waves.

An example would be Winter waves frequently found on the reef breaks of the North Shore of Oahu in the Hawaiian Islands.

SPILLING WAVES

In areas of gently sloping bottom contours, waves of average steepness break at a water depth of about 1.3 times their height and are known as Spilling waves. These waves are ideal for beginners.

An example would be beach breaks with gently sloping bottom contours found in areas where the continental shelf extends a long way from land.

Surging Waves

This is where a swell hits a very steep obstruction and does not break in a rideable form.

An example would be waves that hit breakwaters, piers, or very steep banks.

Factors Affecting Waves

TYPES OF SHALLOW WATER BOTTOM MATERIALS

As previously noted, the type of breaker produced depends largely upon the slope of bottom contours near shore.  Shallow water coral and rock bottoms are firmly anchored and quite often have steep slopes resulting in hard breading waves.

Sand bottoms consistently move due to currents and surf. From one day to the next the best place to surf may change drastically. The seaward edge of sandbars usually slopes gradually. This often results in less violent spilling breakers that may tend to become "mushy".

TIDES

Most surf sports seem to break best at a preferred water depth that varies according to surf height. In some areas of small tidal range, such as Hawaii, the 2 foot variation in water level is not too important. In some sections of Europe which have 30 feet (9 metres) tides, the time of tide is very important. It can mean the difference between unbroken swells hitting a steep beach, closed out conditions over low tide continuous sandbars, or good surf at a tide stage in between.

 

LOCAL WINDS AND CURRENTS

The effect of local wind direction on waves in Pembrokeshire is well known by any regular surfer. Offshore winds prevent a wave from breaking until it is steep and hollow. Onshore wind causes a breaker to collapse early, producing a spilling, mushy wave. Furthermore, the wave faces will be marred by wind chop produced by the onshore wind.

If you happen to surf at a place exposed to a seaward moving current or a rip, the waves will steepen and break in unusually deep water. Shoreward moving currents reduce wave height much as an onshore wind does. In either case, a strong current will cause choppy, bumpy waves. Currents near shore can be produced by tides, winds, or fluctuations in sea level due to surges of white water.

PRESENCE OF MORE THAN ONE SWELL

At times, especially during the winter storm season, two or more major swell trains may be present simultaneously at a point. This is particularly true for islands in the mid-ocean. If two or more swells arrive simultaneously that produce surf, complex interactions can occur. To determine the resulting wave height, one must account for the size, direction, and period of each of the contributing swells.

SET WAVES

Whenever the wave trains of the swells are in phase with each other (the wave crests of both swells arrive at the same time), the resulting wave height is the same as the sum of contributing wave heights. These are called set waves.

LULLS IN THE WAVES

When they are out of phase (the wave crests of one swell arrive at the same time as the trough of the other swell), the final wave height is the difference between the individual heights. The surfer will recognise this phase as a 'lull' between sets. It is important to recognise the pattern of sets and lulls on any given day. Good surfers will use lulls to paddle out in whereas beginners may find themselves paddling out too far and then being caught by the sets. Or paddling out in the sets.

TYPES OF SURFING WAVES

There are 3 main types of surfing waves of which only the first is generally suitable for teaching beginners.

Beach Break

Surfer rebounding of the white water on a beach break wave.  The “mushy” nature of this wave is partly the result of an on-shore wind and a shallow sloping beach.

BEACH BREAKS - SOME COMMON CHARACTERISTICS

Offshore winds and steeply sloping sand banks can give rise to good hollow waves. Water surging towards the beach often causes strong cross currents leading into rip currents as it returns seaward.

Gently sloping contours and a large continental shelf often gives rise to slow beach break waves. Shifting sand banks often create a series of breaking waves and deeper water channels separating them. Beach break waves generally break over sand bottoms and various factors will determine their suitability for the beginner. Ideally the beach will have a gently sloping bottom producing gently sloping spilling waves with several lines of soup.

Dumping Waves

These are often found on beaches at high tide where a steep bank may cause the waves to break almost onto the sand. The waves rise up quite suddenly, break top to bottom and close out. They are unsuitable for beginners or experts.  Other factors e.g. a strong rip current can give rise to dumpers.

Waimea Bay Shore Break - A spectacular and very photogenic Dumping wave

Reef Breaks

A reef may be of rock or coral and may be covered with sand. Because the slope of the reef is generally fairly steep the waves travel fast and break with great power. Reefs can produce excellent waves for more experienced surfers but are unsuitable for beginners.

The Pipeline on Oahu’s North Shore. One of the best known and most challenging Reef breaks in the World.

Reef Breaks - common characteristics

Offshore winds may contribute to the hollowness of these waves. In many areas, the lack of a continental shelf means that the waves do not lose much of their power prior to breaking. If the reef is at an angle to the approaching swells they will "peel off" along the reef edge. The waves often break hard in shallow water and the hard and often uneven nature of the bottom makes a reef break a more dangerous and challenging surfing location. Definitely not a beginner's wave!

 

Point Breaks

In certain places a point of land, projecting into the sea will allow waves to break along its length giving long, good quality waves. Although these waves may break quite gently there is often a strong current running along the point requiring surfers to be able to paddle hard in order to reach to initial take off point. Point break waves often break over and in front of rocks and are frequently fast and challenging waves.  They are often unsuitable for beginners.

A classic Point Break - Jefferies Bay, South Africa

Point Breaks - common characteristics

Underwater the point stretches out to sea causing the approaching swells to slow down and refract around it (the swells in deeper water will continue to move faster). The waves will then peel along the line of the point as they break. The lines of swell radiate out into deeper water and the current created by the breaking waves moves parallel to the shore into deeper water.

Travel Time of Swell

The time a swell takes to travel from its source to any point depends upon the wave period. The swell speed in knots is approximately 1.5 times its period in seconds. The longest period waves travel more rapidly, and the shorter period ones are left behind. This process is known as dispersion.
As the swell moves away from the generating fetch, the period of the waves as observed along a line towards their destination becomes more distinct. This causes "cleaner" more well defined swell waves, the kind that surfers want. At a point several hundred miles or more downwind from a fetch the following sequence of events is usually observed.

• First to arrive are long period (often 15 to 20 seconds or more) waves containing relatively little energy, which are quite small. But to a sharp eye these are signs of a building swell.
• As a slightly slower, somewhat shorter period waves arrive, the swell builds to maximum height. These medium period (often 8-18 second) waves contain the most energy. Gradually the wave period shortens further and the wave height decreases as swells of lesser energy arrive.
• Often the really short period waves (5 seconds or less) never travel more than a few hundred miles beyond the fetch before being flattened out by other prevailing winds or seas in the decay area.

The duration of a swell at a point depends mostly on the length of time the wind has blown through the fetch, and the distance over which wave dispersion has occurred. Surf produced by swells arriving from great distances usually lasts longer that surf from nearby swell sources. e.g. In the previous table it will be seen that swells travelling longer distances will last much longer (100 hours as opposed to 33 hours for the swells of 20 second duration) This is provided that they do not decay too much before reaching the destination!

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