Some Physical Geography for Ocean Paddlers

 

by Robyn Khorshid  
© 2005, Robyn Khorshid
 




It is mainly constantly changing wind direction and strength that causes waves and swell and the varying impact of these on different types of coastline that affect ocean paddlers.

Ocean Waves or Swell
Waves are caused by the movement of wind over water in the open sea. They represent a forward surge of energy transferred from the atmosphere to the sea by friction. The water does not move forwards (each particle moves only in a circular orbit). Just the energy surge moves through the water.

Ocean waves are usually rounded and symetrical, but in strong winds they can be assymetrical and the water thus moves forward a little in the direction of the strong wind.

Wave Height
This is the vertical distance between the trough and the crest of the wave. It is determined by the strength of the wind, its duration and the distance over which the wind has been forming the waves, known as the 'fetch'.

Wave Length
This is the distance between crests, or troughs.

Ocean waves (or 'swell' ) may form far away from your location ; they travel for long distances - hundreds of kilometres. As they travel they decrease in height but increase in length. White caps form when the wind is strong enough to blow off the top part of the wave crests.

Seas
As well as the above long distance, long wave length ocean swell there will be local wind driven waves which are usually lower in height and of shorter wave length, or greater frequency.

The local wind waves and the swell will be superimposed on each other, resulting in what surfers refer to as 'sets' of waves. For example, there may be swells generated from far away storms which have a long wave length (say, 300m) arriving every 20 seconds and in between these, locally generated wind waves arriving at 5 second intervals. The coastal waters forecast may read something like this: "SSW winds, 13/18knots. Seas to 1.2m. Swell to 2m".

Before attempting launching or landing it helps to wait a while to see if there is a pattern to the waves and then choose your best time and place.

The local winds may come from a near opposite direction to the swell, having a flattening effect on the sea.

Confused seas result when there are storms in different places causing swells in different directions (known as 'interference'). At the same time storms in different places may serve to reinforce other swells (known as 'reinforcement').

See diagram

Refraction around headlands and offshore islands and reefs will also contribute to locally confused seas, as does the backwash from waves striking cliffs in the vicinity. More on 'refraction' below.

Breaking Waves
When waves approach shallow water the circular orbit of the water particles becomes elliptical as the bottom interferes with the circular orbits. This causes the waves to slow down, become shorter in wave length and higher before they break. As a general rule waves start to break at depths of half their wave length. As the water becomes shallower the top of the wave eventually becomes unsupported and crashes down, trapping air and causing turbulence as the water rushes forwards, owing to the pressure of more waves coming from behind. The rush of water towards the beach may reform into a wave before it reaches the beach, so that there may be a surf zone, rather than a single line of breakers. It all depends on the topography of the sea bed before the beach is reached and the size of the waves.

 

The uprush up the beach is called the swash. The backwash is the rush of water back down the beach.

Plunging waves ('dumpers') are mostly associated with steeply sloping bottoms and large waves. Local storm generated waves will also plunge downward more powerfully.

Spilling waves occur when the bottom is more gently sloping.

The location of the surf zone is a guide to the depth of the water. On a steeply sloping bottom, there may be many small waves breaking right at the shore. Sandbars and gently sloping bottoms cause the waves to break further out.

A line of offshore reefs, islands or the coral reefs of tropical waters cause the waves to break there, some distance offshore thus resulting in much protected water and coastline behind.

Refraction
The slowing down of waves as they reach shallow water causes them to bend around the shape of an indented shoreline - the shallower water of headlands and offshore islands is reached first. Thus the waves approach the shore parallel or nearly parallel to the shore.

This results in more turbulent breakers on headlands and promontories than in the neighbouring bays - the energy is more concentrated - not at all inviting as a landing place! The force of the waves on these headlands can be very powerful; 2086lbs/square foot has been measured in winter off the Scottish coast.

See diagram 

Refraction also occurs on long straight coastlines when the prevailing winds and therefore waves approach the beach at an angle. This results in a phenomenon known as 'longshore drift' which affects the steepness of the beach and therefore its suitability for landing or launching. More on this below.

Longshore Drift
Waves approaching the beach tend to be almost parallel, but not quite (depends on wind direction). That small angle of difference is enough to cause the movement of sand along the beach in the direction of the prevailing wind. Here's how it works: the swash up the beach is at an angle and sand is moved in this direction. The backwash, under the influence of gravity, is straight back down the beach, at right angles. The repetition of this action millions of times results in sand moving along the beach and accumulating at the next obstacle, such as a headland or groyne, or at the next bend in the coastline. This makes for a more gently sloping section of beach and just offshore shallow water which may possibly be easier for launching and landing.

Rip Currents
Water piles up along the shore (though it's not noticeable) as a result of longshore drift and the wind. It finds its way back out to sea through shallow depressions in offshore bars as 'rip currents'. See diagram. These may flow at more than 1m/second. After they have broken through the surf zone they fan out and start to dissipate, especially at depth. Rip currents may be suitable launching places to break through surf and head out beyond the turbulent surf zone. They are recognizable by the following:

- may appear as long lanes of foamy water stretching far out
- wave crests are lower and the breakers interrupted and less active
- may be an absence of breaking waves as the current erodes a deeper channel in the sand
- often there will be a bend or indentation in the beach.

The spacing, intensity and location of rip currents are controlled by a range of factors such as the angle, height and period of the waves and the topography of the beach.

If caught swimming in a rip current the recommended mode of exit is to swim parallel to the shore until once again in the line of breakers, perhaps preferable to end up (a bit battered) on the beach rather than out at sea!

Estuaries
Here there is the interplay of river flow and wave action as well as tidal flows. Sediment is brought down by rivers and deposited as the river loses energy as it approaches sea level. In addition, in the SW of WA we have an absence of rainfall in summer months, often with no river flow at all. As a result, mud flats, sand bars and shallow waters are found in most of the estuaries of local rivers. (Dredging is necessary to keep channels open for boating.) Tidal flows become concentrated in the narrow channels between islands and mud flats. As well as deposition of sediment by rivers longshore drift contributes to many rivers having sandbars completely blocking the mouth.

These are broken intermittently when enough water builds up in the inlet behind and there are high tides and big waves from the sea eroding the sand bar. And sometimes they're broken by dynamite! (not joking!)

 

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