The Rockhampton Flood Crisis – The Fitzroy River Barrage

Posted on Sun 01/02/2011 by

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THE ROLE OF ENGINEERING IN A FLOOD.

The current flood crisis here in Rockhampton where I live has now developed into a waiting game. The rail link in from the South was the first to close more than a week back now. The main highway in from the South has been cut off at the outskirts of the city. The airport was closed on Saturday, New Years Day. Virtually all the Supermarkets across the city have run out of food. Flood waters rise on the Southern side of the River, and the main flood peak is expected on Tuesday now and is expected to be 9.4 metres, or 30 feet 7 inches, making this flood the worst since the huge flood of 1918. In 1918, that major flood, that once in a Century flood, would have caused nowhere near the devestation that this one has, because the City of Rockhampton has become such a large city, now with a population of around 75,000 people. That 1918 flood needs to be kept in mind here as I try and explain something not many people would have thought about.

Luckily for me, and believe me, nothing is lucky in a flood of this proportion, I live on the North side of the River, and while I am only two miles from the River, the land where our house sits is ever so slightly higher than where the major flooding is occurring, even though the land looks so relatively flat.

Keeping that in mind, it makes you think. If the water covers such a vast area on just one side of the River, why does it not spread out on the other side of that River, the North side where we live, because what is coming is virtual wall of water, as the 9 Rivers in this, the major Fitzroy River Basin, are all in major flood, the first time this has happened in recorded history, even when referring back to that 1918 flood.

We moved here in August, and really, considering the impacts of floods is one thing that does not even enter your mind, although in my case, I actually did think about it. Having the facility of Google Earth is a wonderful thing, because you can just type in the address you are considering moving to, and then go visit it on the map. As it scrolled in, I noticed the River there and calculated it to be barely 2 miles from where we were considering moving to.  I thought that was relatively close, because looking at it on a map like this you get no idea of the the topographical nature of what you are looking at. That ended my thoughts on flooding at that time, barely a fleeting thought in passing.

Now that I’m actually here, and now that I’m actually in that flood situation, I have had cause to go and look at that map again. I have also looked at some wonderful information from the local regional Council regarding flood impacts at different heights. The predicted flood peak is still scheduled for Tuesday and is still expected to be 9.4 Metres. The scenarios from the Council show inundation for different heights and the worst case scenario shows a map at a height of 9.5 metres, and we are still not going to get any water close to us, although it does get to within quarter of a mile or so, and that is the very edge.

That again caused me to start thinking. Why so bad on one side of the River and not the other. Take that into consideration also with what has been called an approaching wall of water around three miles wide. This is approaching from the Fairbairn Dam, (Lake Maraboon) which flows into the Nogoa which flows into the McKenzie which then joins with the Dawson and becomes the Fitzroy River.

Again, Google Earth came in handy. I sat here at my computer and followed the River back upstream, and then those other rivers as well, but in the main the McKenzie back to the Nogoa and then back to Lake Maraboon. That exercise of itself was an interesting one because while the confluence of the two main rivers where they become the Fitzroy is only about 80 miles from the City as the crow flies, the River itself is almost 450 miles long, as it snakes about in every direction of the Compass on its way to the Pacific Ocean.

That caused me to look more closely at the map in the area around Rockhampton, and wonder why it doesn’t spread out on both sides, if such a huge and wide volume of water is coming down the system.

With all images at this page, if you click on them, they will open in a new and larger window.

At the right here is a map of the City showing the River flowing from the top left to the bottom right through the City.

Now see that big loop at the top left where the River changes direction back North again. The river, at its normal height will follow the trough it has carved out over all these years. However, with this flood event, what has happened is that the rising River has burst its banks at the bottom of that loop, and is spreading South through all that low lying area now in its flow. You can see the airport on the outskirts of the city, at the left there where the small blue aeroplane is. The inundation in that area has now reached the airport and it was closed almost two days back now. Most of the populated area you can see to the right of the airport as you see it here, is still out of the water with some areas being inundated. That water is spreading away far off to the left of this map, and to the South where it has almost reached the bottom of screen where that A4 sign is.

Now go back to the loop at the top of the map where the River changes direction back to the North and then loops around to the South again. Follow the course of the river until it reaches the outskirts of the City where there is a small white line across the river with two small blue markers.

That is the Fitzroy River Barrage, and I’ll come back to this later here.

Keep following the course of the River through the City and not where it again loops to the North. See the Word ‘Rockhampton’ across the river in that city area. Draw an imaginary line from the letter ‘R’ slightly through the city and then following the outskirts down to where you see the three way road junction marked as A1 in the green icon. All that area is also under water as well. It all flows away to the lower areas in the general flow of the water. That dark green area directly above the loop is also under water back up to the horse racing complex you can see there just under the City name there.

The main city area is (relatively) high and dry, and it’s just the Southern suburb of Depot Hill which is being inundated the worst in the populated areas of the City.

That water will naturally flow away to the South at both of those major loops there as it is being pushed along at close to 10 to 12 MPH, and ever so slowly creeps its way across that land there, in effect quite a large area.

Now keep that in mind because that is what is happening along the whole length of the river as it snakes its way along its length, even while only a small number of miles as the crow flies.

ENGINEERING.

I mentioned in the Heading about the role of engineering, and this refers to the Barrage I mentioned above that I said I would get back to. I have two images that best show what I mean by this.

The Fitzroy River Barrage With The Water At Normal Height Level

This first image here at left shows the Barrage with the river at its normal height. I explained the Barrage in my first post, but here’s a short precis of that. The river at Rockhampton, even while 25 miles from its mouth at the Pacific Ocean, is still tidal, so the salt water backs up into the river with the tides. Fresh water flows down the river. So as a constant supply of fresh water is maintained for supply to the City, the salt cannot be allowed to mix with the fresh. So the Barrage was constructed across the River, not a dam per se, but just a means to keep the salt out of the fresh. Concrete Pylons were constructed, and in the gaps between them large steel gates were installed.

In the fully down position, they hold back a level of water that will ensure a constant supply of fresh water backs up behind those gates, and from there the water goes to a treatment plant, and if the tides are greater than normal, they hold the salt water from flowing back upstream into that fresh water.

So, why a Barrage of this nature you are thinking. Why not just a dam of some sort, and that would serve the same purpose. Excess water from upstream would just flow over the top of the dam anyway, so why go to all this trouble of constructing something of this nature when a small dam would be the simpler option.

The river has always flooded over the years, but even considering that, a dam would still be the obvious option, because this design of a Barrage is basically just that, a dam across the river, only one with a movable dam wall, the gates that go up and down, and they are mainly in the down position for most of the time. So, even if the decision was for the Barrage over a dam, then why make it so tall, and not just have it to the height of a bit extra than the height of the gates themselves with the gates fully up, because any overflow will just go over the top of it anyway.

Enter engineering.

The top of the Barrage is in fact quite higher than the surrounding land on either side of the river, so at this height, it actually seems incongruous when you observe the structure at the normal height of the river, as shown in the first image.

Okay, consider a dam, which is a completely blocked in structure across the river. If the river floods as consistently as it has over the years, then the problem of the weight of the water and its speed while in flood becomes a problem, as does any major debris which comes down with the flood waters. So, there is always the possibility that with consistent flooding then a dam, even a small one might be breached, creating even further problems.

Close Up of One Section Of The Fitzroy River Barrage

Now look at the second image shown here at the right.

This shows a recent image of the Barrage that I took with the water level at 8.4 metres. As in all floods, all 18 gates are in the fully up position.

Note how in this position, the top of that structure virtually is now a dam, only, instead of being attached to the ground giving it extra strength, this dam sits on top of concrete poles, making it less stable in effect, considering the volume, the weight, and the speed of the flow of water, and that the water might also be carrying large pieces of debris that will smash into this ‘wall’ and possibly weaken it. It stands to reason that no matter what height those pylons are, then it still becomes a large blockage to the water, only now less inherently strong.

Now, note the height of the water here in that image. The distance between the top of the water now, and the bottom of those gates is around 3 metres or close to 10 feet.

The worst flood peak to have ever come to Rockhampton is the 1918 peak of 10.4 Metres. Even if the river were to get to that height, there would still be a gap between the top of the water and the bottom of the gates of around 3 feet. Those extensive and detailed flood plain maps of the city and surrounding areas also show that even if the water is in fact of a greater height than that, it will have all flowed away across the land prior to that big loop on the map, and very little if any will follow the normal course of the river back to the North, past the Barrage and into the city where the flood marker is.

So, when the Barrage was originally engineered, all this was taken into consideration.

As I mentioned in the first Post on the flood crisis at this link, the flood marker is measured from the low water mark with the gauge at around 4 metres. The tide rises, and as the river is tidal, then it would have been a relatively simple calculation to make the top of the base level of the Barrage higher than what the level would be at the high tide mark plus a little to account for king tides, which are higher again that the normal high tide mark. That can be seen at the image of the Barrage with the water at its normal level. From that base at the Barrage, then, with the gates down, that water level of fresh water on the upstream side of the Barrage is sufficient to provide a good supply of fresh water for the City. The gaps at each side of the Barrage that do not have gates now act as the overflow, so the level is kept at a fairly regular height with those gates fully down. As the River moves into a rising phase, as in even moderate flooding, then the gates are raised, because the flow of water from upstream effectively means that any salt cannot back up against that flow. From that high tide mark then, the base of the Barrage structure, then knowing the height of the one in a century flood, that of the 1918 flood peak, they then constructed the structure so that during a flood of this proportion, with the gates fully up, then there would still be a gap between the water level and the bottom of the gates.

We look at a structure like this, with the river at its normal height, and wonder why it is so tall. Then we see it during a flood, and rarely, if at all consider why it is perhaps the only thing sticking out of the water while there is flood all around it.

This is a structure that has been been very carefully, and very cleverly engineered, something that we all take for granted, especially at times like this.

Again, this may seem to be a Post of a technical nature while the true trauma of a flood of this proportion is felt by many people around me. This does not mean I am ignoring the plight of the many. Actually being here in the midst of this disaster makes me one of them in a way.

FLOOD CRISIS LATEST NEWS.

The latest news says that the marker at the Riverslea Station close to where the Fitzroy starts at the confluence of the McKenzie and the Dawson has stopped rising on early Sunday morning and is stable at the current level, which augurs well for Rockhampton. With the water flowing at around 10 to 12 MPH, then there is nothing further adding to that level from upstream in those other 8 rivers. The current peak travelling down the Fitzroy will still take close on two days to reach us, so that predicted peak on Tuesday sounds more and more like being correct.

Right now, it’s just a waiting game, and if the real truth is told, then the impact is still to come, not in the fact that so many people will have had their houses inundated, but that the distinct possibility exists that the city will be completely cut off for anything up to 14 days or more, and food especially will have started to run into even shorter supply than what there is now.

As I mentioned in my earlier Post at this link, this is a disaster in slow motion, and one of mammoth proportions for a city of 75,000 people.