Jeff Ward, of King's High School, asks :-

Why do some beaches have bigger waves than others?

Bob Kirk, a coastal geographer at the University of Canterbury, responded.

Wave size and wave shape vary with the winds that cause them, the shape of the coast (exposure) and the form of the coastal sea-bed (steep or flat).

The size of waves generated by wind in deep water are controlled by the speed of the wind (the stronger the wind the larger the waves), the duration of the wind (the longer it blows the bigger they get, up to a maximum size for a given wind speed) and the length of water over which the wind blows (the fetch). Waves never become very large on the restricted waters of harbours, estuaries or lakes.

Once formed, waves can travel as swell for thousands of kilometers across the oceans, for example, from near Antarctica to New Zealand. The breakers on the shore are the remnant of bad weather in some remote region days or even weeks earlier.

Winds have well known global patterns and are strongest to the south of New Zealand. Because we lie in the zone of mainly westerly wind flow the west coast is a windward coast, with usually a large surf, and the east coast a lee coast, with often a lighter surf.

Beaches differ in their exposure to the wind and to swell from far distant areas of the oceans. Some face into the winds and swell paths while others have varying degrees of "shelter" that reduces wave size at the shore.

In shallow water the speed of a wave depends on the depth of water. A steep offshore profile will allow waves to make a close approach to shore without much loss of energy to friction. A flatter offshore slope will modify the waves much more extensively before they reach the surf zone. As waves approach a shelving beach their speed slows down so that they normally all end up parallel to the beach and finally they steepen until they break. The rate of steepening depends on the steepness of the beach bottom.