Audrie Leslie of Dunedin asks :-

Why is it that in the tide tables of the ODT, high tide at the mouth of the Taieri River occurs 17 minutes after the high tide at Taiaroa Head?

Ron Heath, a physical oceanographer at Otago University, responded.

Of equal interest is low tide at Taieri Mouth occurring 16 minutes before that at Taiaroa Head. To understand the reason for these differences we have to consider the tide as a wave.

All objects are subjected to gravitational attraction by other objects. It is this attraction of the moon, and to a lesser extent the sun, which generates ocean tides. This attraction causes bulges in the ocean's surface. which propagate as a wave across the ocean surface.

New Zealand is highly unusual in that the wave generated by the moon's gravitational attraction is trapped to move anticlockwise around the New Zealand coast in a period (the time between high tides) of 12.42 hours. In general terms, high tide then occurs later the further north you go on the east coast and the further south on the west coast. For example, high tide in Lyttelton occurs about 1 hour and 50 minutes after that at the entrance to Otago Harbour which occurs about 30 minutes after that at Bluff.

However, the exact time of high tide depends on the detail of how the wave moves across the continental shelf and into shallow water. In shallow water, the wave experiences friction and interaction with the coast which causes the generation of additional new waves, the most prominent of which has a time between its crests of half that of the original wave. It is the combination of both waves that we see in the observed tides on the coast.

This combination of waves can produce a situation in shallow water where the time of high tide is delayed and the time of l ow tide brought forward. It is likely that the tides at Taieri Mouth experience more friction than those at Taiaroa Head and hence high tide is delayed and low tide brought forward.

Similarly in Otago Harbour, high tide is delayed at Dunedin relative to the harbour entrance by an hour and a half.