Tidal Streams - General Information

A tidal stream is the periodic movement of water in a horizontal direction that is due ultimately to the same astronomical causes as the tide (whereas the tide is a movement in the vertical direction). The tidal stream associated with a rising tide is called the flood stream; that associated with a falling tide is called the ebb stream. A tidal stream is often described by the direction to which it runs. There is usually a period when the flow ceases, known as slack water, before the stream changes direction. Interestingly, slack water does not necessarily occur at times of high or low water.

Tidal streams can be analysed and predicted in much the same way as tides. It is generally simpler however, to compare the stream with the tide at a nearby Standard Port, and determine the time difference between high or low water at the port and the beginning of the ebb or flood stream in a particular area. This is the method used to compute the tidal stream predictions for Tory Channel Entrance and Te Aumiti (French Pass) that are published in this Almanac.

Cook Strait: The tidal streams in and around Cook Strait are unreliable and mariners are warned to exercise every precaution when navigating in the vicinity. The streams often run in one direction for eight to 10 hours, but cases have been reported of them going for 18 hours or more. When the streams have been running in one direction for an extended period, it has been found that the opposite stream is much weaker or, on some occasions, hardly noticeable. The maximum rates shown on the chart which are normally attained during spring tide conditions are also liable to be encountered at any other time. In the vicinity of Karori Rock and Cape Terawhiti rates of up to 7 knots are frequently experienced, but as a rule do not last for more than about an hour. Small vessels are warned to keep well clear of tide rips as they may lose steerage way and may, in extreme cases, capsize.

Te Aumiti (French Pass): Mariners are cautioned against taking a vessel through the Pass against the stream, as not only does a vessel sometimes not answer the helm owing to the stream eddies acting on the bow, but there is danger of meeting a vessel coming from the opposite direction which is not visible in sufficient time to prevent collision. Powered vessels of moderate size can, with prudence, pass through at slack water or with the tidal stream and in doing so save 15 miles of distance and avoid the heavy cross sea that is frequently met northward of Rangitoto ki te Tonga (D’Urville Island) and Stephens Island. Local knowledge is highly desirable.

The flood and ebb streams attain rates of 5 to 7 knots and do not set directly through the narrow channel but across it. The flood stream sets in a south-westerly direction as far as the narrows and then along the shore between Channel and Rock Cod Points. The ebb stream flows in the opposite direction. See the tidal arrows shown on the chart (NZ 6151). Slack water lasts about 20 minutes. The extraordinary irregularity of the sea bottom, together with the narrowness of the channel, creates many eddies in this area. Care must be taken to prevent a vessel being swung round on to Collinet Point by the eddies.

Tory Channel Entrance: Tidal streams can be very strong through the east entrance to Tory Channel, reaching rates of 7 knots as indicated on chart NZ 6154. Tide rips form along the western approaches to the entrance and eddies form around Okukari Bay. Some cross channel shear is present but is generally weak relative to normal channel flows.

Vessels approaching the entrance from Cook Strait against an east-going tidal stream will find that the stream at the entrance normally sets SSE onto the starboard bow. When abreast of West Head Light the stream sets onto the port bow. Care must be taken to account for the effect of the streams. Without local knowledge, low-powered vessels should not use the entrance at spring tides, whether entering or leaving Tory Channel.