Saturday, 10 December 2016

Where the heck is the Carwoola flood monitoring station?

In my previous post I commented that
  • Point D is approximately the location from which the Bureau of Meteorology (BoM) compile estimates of rainfall for their river flow/flood danger analysis in the Molonglo valley.  Trying to work out exactly where that is to be found is a work in progress.
It is pleasing to report that progress has been made and the task is complete.  Many thanks Jan for showing me the site.  There are many nuances to this as reported below.  

The site is about 650m West (ie downstream) of the low-level crossing on Briars-Sharrow Rd (and about 1200m SSW of the location provided by BoM records).  To prove that I found it, here are a couple of images of the station:
The cylinder in the foreground is the current device, which sends signals by wireless (or satellite, I'm not sure).  The green shed in the background is an older device which used to be connected by landline.

A crucial part of the operation is the pipe at the foot of the cylinder: this is full of gas which is periodically released and the pressure required to do this is used to calculate the height of water above the end of the pipe (which is, helpfully, in the river).  This process means that the cylinder, and associated devices are well above the River level and thus not going to be whupped by any sensible floods.  The need for maintenance is obvious: apart from anything else the supply of gas will need renewing.

This next image focuses on the top of the cylinder showing the rain collector on top and the solar panel which presumably keeps the transmissions happening.
The first nuance relates back to the naming of the site as Carwoola.  I suspect that it is, technically, not in Carwoola but Hoskinstown.  That's because I think the River is the boundary at this point and the North Bank is in the latter locality.

The more interesting nuances relate to where the data from the site appears in BoM "products".  The next image comes from the BoM series South Coast Rainfall and River Conditions.
I have labelled two sites in this.  
  • The Burbong site (#570012), appearing as a turquoise triangle, appears to be used to calculate flood risk and is described by BoM as the "... key gauging station of Molonglo River at Burbong (410705) ...".    At the time of writing this the River level is cited in the river level conditions map as -0.083  - presumably relative to a minor flood level.  It is also intriguing that the reference number for Burbong stated in the linked report is very different to that revealed by clicking the icon on the map.  (On thinking about this a little more, I suspect that the number in the text report refers to some item of equipment in an old device - equivalent to the shed at Carwoola - whereas the actual work is done by some device equivalent to the cylinder, with a similar identifier.  Someone just hasn't bothered updating the title of a graphic and a text reference.)
  • The Carwoola site (#570986) - greyed out - is not used to assess flood risk, but is used in a BoM product Latest River Heights for NSW South West, and gives a current level of 0.7m.  Another member of the Carwoola community has noted that if this level rises to 1.4m then water is over the low-level crossing.  However that is not an official classification.
If one switches to rainfall measurements the Carwoola site is shown as a turquoise dot while the Burbong site disappears (possibly because there was no rainfall there in the previous 24 hours and the consequent grey dot is obscured by the ACT boundary).

So it appears that one has to be very careful in using these data to understand which station is being referred to and what its purpose is.  From my perspective, I generally want to know whether Briars-Sharrow Rd is open and will thus refer to the Carwoola site.

Apart from resolving the question of where the Carwoola station is located (albeit opening several cans of worms about BoM metadata) it was nice to find some native water lilies  (Nymphoides montana) in the river near the pipe.


Tim Bateman said...

I use the rule of thumb that 1.3m means the water is probably lapping at the concrete. It can rise so quickly I saw it go up 40cm within 30 mins last winter. Just my general experience everyone must judge for themselves

Flabmeister said...

Thanks Tim. I agree with your rule of thumb and the caution that the water can rise quickly. There can also be a lag between the time of the rain and the arrival of the flood at particular places. A key point is to avoid the hero pills and not enter floods.