North Australian FISH FINDER forums

Thought you might find the attached useful.

Its about places in the NT that have Diurnal and Semi-Diurnal tides.

Diurnal means 2 tides a day (1 high and 1 low).

Semi-Diurnal means 4 tides a day (2 high and 2 low).

NT isn't unique with such a mix of tide types. Qld and WA also have them.

There is a third type called a mixed semi-diurnal tide.

Those are generally found where there is a land mass on the western side of a body of water that has diurnal tides.

The existence of the western land mass interferes with the western movement of the earth's tidal bulge. That in turn brings further local influences into play.

Gulf of Carpentaria is one such area and the eastern side of Cape York Peninsular in Qld is another. A third is the Gulf of Mexico.

How that happens is interesting but not particularly useful.

The basic question about differences in tide times between 2 places go out the window when one place is diurnal (2 tides) and the other is semi-diurnal (4 tides). Like trying to compare apples with oranges.

This list lets u know in advance how many tides per day you'll be experiencing if going somewhere that you've never been before.

In rivers of the western side of the Gulf ( Rose, Roper, MacArthur etc) you have 1 high and 1 low but you also have another influence which translates to a second high or low where the tidal run keeps going in the 1 direction but the tidal level drops or rises!

If somebody wants to know what these strange semi-diurnal tide graphs look like let me know.

Meanwhile here's the NT list.

would love to hear your interpretation of semi-diurnal tides Ronje.

ps. I don't think it'd be news to anyone here about tides running out for longer in rivers or shallow bay areas.

Which tidal boffin came up with all that research, outstanding.

Hi

Its no news to anybody here either about tides draining out for longer in rivers or shallow bay areas. We have a 5 hours in 7 hours out tide here. But its still semi-diurnal. 2 tides a day. Just like places in the NT to be sure.

But that's not what mixed semi-diurnal tides are all about. Its something different again.

DH
Obviously somebody with lots of time on their hands. Want to hazard a guess who came up with the outstanding research?

Here's a fairly simple example of a semi-diurnal tide. 2 a day.

Darwin Harbour over a few days.

Tides building left to right.

Note the 3 little bulges marked A in the tide graph on the left hand side.

Keep them in mind.

Here's a graph of a simple diurnal tide over the same period as the Darwin Harbour one above.

It happens to be from the Gilbert Islands which has classic diurnal tides 1 a day and is a perfect example.

Height varies but that's all. No bulges in the graph shape. Pretty straight-forward.

Here's one of the MacArthur River ( just for Itsinmeblood).

Same time period as Darwin and the Gilbert Islands.

But the shape of the tide graph is completely different.

Note the peaks A,B,C,D. They are varying in height as is the peak immediately preceeding it. Towards the left side, the graph looks like a diurnal tide with a double peak.

Towards the right side, the graph looks like a standard semi-diurnal graph.

The blue and green colours signify tide height NOT direction of flow.

At the lead-up to peak A the tide level is rising. Suddenly it starts to fall but the direction of the current doesn't change. The level starts rising again until finally at A the level starts to drop and the tide direction reverses. After a few days of these "crazy" happenings, the tide starts to behave itself again and acts like a normal diurnal tide.

THAT'S what makes the tides in the Gulf seem crazy. And THAT is what's meant by mixed semi-diurnal tides (not merely predictable changes in simple "in-and-out" periods. Completely different kettle of fish).
"
The external (but predictable) influence that causes these things merely superimposes itself onto the graph and that influence ripples through from left to right causing strange things to happen with levels and flow direction.

Exactly the same thing happens in the Rose, Roper, Town, Limmen rivers along with Karumba etc. The only thing different is the timing at different places.

Remember the bulges on the left hand side of the Darwin graph. I suspect its the same influence but is much less pronounced. Those bulges coincide with low tidal level change. Might expand the length of time on a Darwin Harbour plot and see if there's anything significant that this narrow window might miss.

interesting stuff ronje, not fishing the mcarthur though, coupla hundred k's south...

Can get Bayley Is, Sweers I and Karumba graphs