Dodge tide

This is a local South Australian term for a neap tide with minimal rise and fall over the course of a day or two. While very 'flat' neaps (see neap tide) occur in a number of locations worldwide, the term 'dodge' is used only in South Australia. Professor Sir Robert Chapman, C.M.G., writing in the Official Yearbook of the Commonwealth of Australia of 1938, stated
"At spring tides the range, due to the semi-diurnal waves, is 2(M2 + S2), and at neaps, if the two are equal, or nearly equal, they practically neutralize one another and cause no rise nor fall at all. This is what happens at Port Adelaide where at this period the recording gauge shows frequently little or nothing in the way of tide, in some cases the level of the water remaining almost constant for a whole day; in other cases one small tide occurs during the day. On each side of this tide is markedly irregular both as regards time and height, and the apparent impossibility of saying when the tide will be at this particular period has presumably gained for it its name 'The Dodger'."

An interval of minimal tidal range at Adelaide is circled in the graph below. The moon entered its first quarter on 7 August 2000.

Dodge tide plot

As an example of how the tides 'dodge', from the high and low waters. The tide predictions for Port Adelaide (Outer Harbor) for the 17th January 2004 ar

0512 1.0
1002 1.2
1521 0.9
2345 1.9

For 18th January

0915 0.8

and for 19th January

0216 1.9
1025 0.5
1730 1.4
2107 1.2

By looking at the time and height differences between highs and lows we can see the effect.

By contrast, at around Springs on 26th of January

0022 0.5
0634 2.3
1300 0.3
1902 2.0

ie: around 6 hours apart

The dodge tides are relatively easy for us to predict now with computers, but imagine in the early days of Port Adelaide, trying to pick a time to sail or arrive on the high tide around Neaps.

To see when a dodge tide is likely to occur in Gulf St Vincent PDF files of annual plots of the predictions for Port Adelaide (Outer Harbor) are presented for 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 and 2014.

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