Pelorus Sound - Tides and Currents

Tides and Currents

There have been several hydrographic studies in Pelorus Sound. These show the importance of the large, variable freshwater input into Pelorus Sound. The Sound has 380 kilometres (240 mi) of shoreline and an area of 290 square kilometres (112 sq mi). Average freshwater input is 99 cubic metres per second, of which the Pelorus and Kaituna Rivers at the head of the sound deliver 43 and 5 cubic metres per second respectively. Rain falling directly on the surface of the sound provides another 14 cubic metres per second, and the remainder drains from adjacent land via small streams.

This freshwater input corresponds to a depth of 10.8 m of water distributed over the area of the sound per year. The freshwater input is highly variable. In a typical year there are several events where the daily rainfall exceeds 50 mm and the river flow exceeds 400 cubic metres per second. The freshwater inflow generates an estuarine circulation in the main channel, with low-salinity water moving outwards at the surface, and higher-salinity water flowing inwards below. The currents in the main channel associated with this circulation are typically 0.05–0.15 metres per second, but immediately after a flood event they can be much larger. Fresh water layers have been observed to spread through the Sound at 0.9 metres per second. After each flood event, the freshwater is slowly flushed out of the sound, with a flushing time estimated to be 21 days.

The tidal regime at the entrance to Pelorus Sound is typical of New Zealand coastal locations, i.e. there is a semi-diurnal tide modulated on a spring-neap cycle, generated by the superposition of M2 (lunar, semi-diurnal) and S2 (solar, semi-diurnal) tidal constituents. Other tidal constituents, notably N2 (semi-diurnal, generated by the ellipticity of the moon’s orbit), introduce further modulation, meaning that the spring-neap cycle is not exactly repeated. The tidal range varies between 1.0–1.3 m at neap and 2.1–2.4 m at spring. (These values are calculated from a sea level time series, July 1997–February 1998, at Pelorus Sound entrance.) The propagation of this tidal signal into Pelorus Sound has not been well described, but models and measurements both suggest that the tidal range within the sound is somewhat larger (by 10–20%) than at the entrance. Because the sound is long, and in places narrow, the tidal currents are fairly strong.

A further factor affecting the hydrodynamics of Pelorus Sound is wind. Winds in the area are generally strong and are channelled by the steep topography. Wind stress acting on the surface generates currents, which have not been well described. The wind stress also generates turbulence, which causes the surface fresh water to be mixed down and results in a brackish layer typically 5–15 metres thick. The interface, called a pycnocline, between the brackish near-surface water and the denser water below supports internal wave motion.

Waves on the pycnocline, called internal waves, are ubiquitous in Pelorus Sound. They propagate at one metres per second and involve vertical displacements of the pycnocline through five metres. They are often generated by the tide, in which case they are called internal tides. They play an important part in driving flow and horizontal exchange in bays off the main channel. For example current meter records at the entrance to Beatrix Bay show a significant semi-diurnal component with velocities up to 0.2 metres per second. This is not simple, uniform tidal flow in and out of the bay because its amplitude does not follow a spring-neap cycle and there is a phase difference of several hours between the surface and the bottom.