Sunday, 13 May 2012

Australia - The Murray mouth & the effect of the tides

This is a post to provide a convenient repository for charts & images I've linked to in this post on Jennifer Marohasy's blog concerning the Murray-Darling Basin, and the Murray mouth and the barrages in particular. First, a bird's-eye view of the subject under discussion (the full-size pic is BIG btw):

Murray mouth from the NW                                                              Source: Carbon Talks Blog
... and a map of the area of the lakes and the mouth showing the barrages:

The Barrages                                                  Source: Murray-Darling Basin Commission 2005
Another BIG hi-res pic showing the incoming tide and dredging in progress.

Murray mouth - dredging in progress, July 2010                                     Source: Water for Good
Bathymetric (depth) map of Lake Alexandrina
Bathymetry - Lake Alexandrina (May 2009)                      Source: Department of Environment

Bathymetric (depth) map of the mouth and upper Coorong.
Bathymetry - Murray mouth & Upper Coorong (2010)       Source: Department of Environment

Bathymetric (depth) map of the lower Coorong, South Lagoon.
Bathymetric map - Coorong South Lagoon (2010)           Source: Department of Environment

The tides at Goolwa Beach, off the map to the NW (actually from Victor Harbor) during April 2012, reproduced from WillyWeather and combined by me:

.. and sea level at Victor Harbor, 2000-2010 (note the zero trend):

Portland, VIC, though a fair distance (I'll check it out) away to the SE, is IMHO a good proxy for Victor Harbor, and therefore the Murray mouth. Peak tides at Victor Harbor are often higher than at Portland, though the monthly and annual profiles are almost exactly in step. Here's the max. and min. tides for Victor Harbor (read from WillyWeather) for February 2012, chosen because there's an overlap with the latest available hourly data from Portland:

Note the individual tidal range increases from 0.5m on the 1st and 2nd up to 0.9m on the 7th, when there's a stretch of double-tides, with the lesser second tide increasing as the first tide begins to decrease. Two green "trough" (low tide) markers close together indicates two tides over 24-25 hours. The extreme tidal range is 1m, from 0.2m above datum to 1.2m. MSL is 0.65 (calculated) above datum. The resolution here is 0.1m, so the accuracy is +/- 0.5m (5cm). The NTC (BoM) flags Victor Harbor as having a range of 1.25m over a year as shown below:
Tidal range around the Australian coast.                     Source: Bureau of Meteorology
... and a wind rose for Adelaide (nearest location on the selector map here.)

Wind rose for Adelaide, SA                                           Source: Bureau of Meteorology

This shows that for about one third of the time, the wind is from the NE or N, pushing the sea back from the Murray mouth, assuming there's little difference between Adelaide and the mouth.

I'll add a few more charts later to demonstrate the parallels between Portland and Victor Harbor (and therefore the mouth) later, but this hourly chart for Portland gives a good picture of the annual tidal profile (though with slightly lower extreme tidal range). Note the lighter areas indicating single tides, the darker areas double tides, and the annual tidal profile and range. Month numbers on the bottom (x) axis are out-of-step because calendar months have varying numbers of days, though after May they line up quite well.

... and finally, another very large hi-res aerial photo of the mouth, this time at low tide, complete with ubiquitous and never-tiring dredger. Note how far out the waves begin to break over the shallow-sloping sea bed.

Murray mouth at low tide, dredging in progress                                        Source: StudyAdelaide
Any suggestions for content and/or links below, please....


  1. Interesting stuff. I don't really understand why the pattern of two very uneven tides each day around the Southern Australian coastline.

    There is much food for thought on this topic here:!OpenDocument

    Interestingly at the Australian Bureau of Statistics website and written in 1938.

  2. Thanks for the link - most interesting. The pattern has to have a lot to do with the southern island of New Zealand, which acts as a kind of "breakwater", and the Lord Howe Ridge to the W curving slightly W of N, parallel with the QLD coast towards New Guinea. The relatively shallow water over the ridge will have a slowing effect, and SI NZ has a "shadow" or masking effect, with the tidal bulge travelling around the N and S coasts, with some flowing between the islands.

    I'm pretty sure the effect is non-existent on the Queensland coast, becoming more evident further S along the north NSW coast, and it's very clear further S still and along the the S coast towards the Bight.

    The very low tides along the southern part of the S-N running coast of WA has to be due to the bulk of Oz "getting in the way". Water can only run along the S and N coasts, and when the "waves" gets to the extreme W, the N and S waves have to travel towards one another to meet along the W coast. Fixed volume of water spread along the long coast equals low tide height. The N and S waves are of dissimilar amplitude, (N is greater, it's closer to the peaks induced by Moon and Sun), which explains why the low tides are more evident to the extreme SW, rather than the middle of the W coast.

    I think I'll put this one up for discussion, though I already have a fairly distinct idea of the dynamics involved. I'll sort out some hourly charts which show where the effect exists, and where it doesn't, and how clear it is. The exact timing wrt the position of the Moon and Sun will be very relevant to understanding it.

  3. I would like to ask you to share some links to other sources dedicated to this theme just in case you happen to know any.