Justin Flaws of King's High School asks :-
Why are there seven metre waves in Hawaii but none over three metres in New Zealand?
Bob Kirk, a coastal geographer at the University of Canterbury, responded.
It is not strictly correct that waves in Hawaii are larger than they are in New Zealand since waves more than 10m high occur from time to time in both places. However the height of breaking waves is location specific.
The sizes 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).
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 (for example a sandy beach) will modify the waves much more extensively before they reach the surf zone because during the approach they lose energy to the sea bed. 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.
So the height of breaking waves is highly location specific. Much of New Zealand is surrounded by a wide continental shelf hence the waves lose energy before the break zone and consequently are not as high when they break. Hawaii is formed from deep oceanic volcanoes whose steep sided cones rose above sea level and is therefore surrounded by great depth of ocean. Hence the wave heights there are not reduced before they break.