Cairns
Notes 27-Feb-2024
I was booked to see a government member about aspects of the Jasper flooding event and in particular the Caravonica Flood Channel when QLD cabinet meets in Cairns this week. However, I was advised last night that no time is available so I've decided to make my talking points and work notes publicly available.
As other people I met at community meetings (Yorkeys, Holloways, Machans) will be making representations for those areas, I thought I would look at the mountain range edge near where I live. I chose to look at the Caravonica State School flooding event because it appears to be a clear cut and concise example where a council planning approval has changed flood flow directions and, in my opinion, increased the loss and damage to state government staff and government owned facilities. I chose the Cairns West Arterial Road because it is state government owned infrastructure that appears to have affected the flood channel upstream from the school including in some badly affected areas in Caravonica i.e. to this non-lawyer, we may have a piece of state government infrastructure associated with an increase in flooding depth.
Note: I am currently a candidate in the local government campaign and I have tried to keep campaigning out of this web page. For my campaign website, see phillipmusumecifordivision8 and in particular sections on land use and planning. The request by all resident associations for a temporary development pause in the Barron River delta area is imminently sensible, as is their request that a model of the River Delta area be part of future planning. It seems to me that a model is also necessary for identifying areas that require mitigation effort and confirming if mitigation is feasible.
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Section 1 outlines a logical progression of my 2 talking points plus a question, and sections 2-4 are related material for further reading. I am not an expert in this area and I do not have access to any appropriate modelling tools. Any suggestions for mitigation are totally untested but you can quickly find them by searching for "mitig".
1 Caravonica Main Flood Channel
This development on raised ground in a flood channel shows the 135m wide channel squeezed into only 30m of culverts (pan down to see more of Caravonica and Kamerunga Villas). For an annotated snapshot showing distances, click here. When water is in the flood channel, it is usually moving north and later heads east towards the ocean.
A common understanding in Caravonica is that this raised ground development restricted flood flows i.e. water backed up until it reached the height of the embankment road on the east, and then the flood water flowed around the raised ground barrier by going east over the Cairns West Arterial Road and then back west into the school.
The school is prepared for water levels rising in the flood channel and flowing east so it has a small levee bank. However, no one expected that flood flow patterns would be changed and that water would flow westwards into the school from the road. As far as anyone can tell, this is the first time that westward flow has occurred. (If you are wondering why the water flowed back westwards, you can get ideas with the Google Earth view that can show culverts near the Smithfield bypass intersection - see link below - but keep reading here for now)
What was the least that council could have done? They should have maintained more of the flood channel capacity (as "required" in the Cairns Plan) e.g. by adjusting the shape or area of the raised land.
Future:
The extent of upstream and downstream effects will only be identified if there is an independent review of the Jasper event as a whole by experts in the field, and if a hydrology model is employed to test ideas of cause and effect. If this is done, then mitigation options can be tested e.g. changes to the school car park so that the flood channel passage is widened to be much larger than 30m.
The Cairns Regional Council has approved development in a flood channel. It is the state government that pays for increased flood damage in state schools. This suggests that the state government has an interest in mitigation to save money. To reduce future damages, the state government might also consider what mechanisms to put in the planning act to prevent councils from approving development that puts neighbouring property (including state schools) at increased risk of damage.
Click to inspect the road (pan to the right to inspect sand mines to the east of the road and note the large levee banks at the south). Zoom in and then pan along the road to verify that no culverts are present.
The lack of culverts means that water cannot escape east until it rises as high as the road embankment. As noted above, there is a restriction for flood channel flows north and we have seen that lead to flood flows over the road. This suggests more problems for Caravonica including Kamerunga Villas and Lake Placid, because flood waters need to reach the road embankment height before escape.
One approach to flood plains is to preserve them as much as possible so that flood waters can move away quickly. A chaotic approach is to allow each developer to raise their land higher than the neighbour, which just pushes water onto existing properties. The original Cairns City Council decided to avoid any "race to high land" by treating the river delta flood plain in a special way with a dedicated management control plan (see the CCC 1997 plan below).
The state government also has special instruction for soil conservation on flood plains, which is to some extent relevant here as the Barron River delta contains sugarcane farm land. See Chapter 10 - Land management on flood plains - click on the download icon and then look for section 10.6.5 Infrastructure on flood plains, which mentions how roads should be kept low to minimise erosion and allow flood plains to disperse flood water in the usual way. If you search for "Darling Downs", you will come across an example where the height of roads was reduced to less than 200mm to allow better escape of flood water (Figure 10.20).
We do not have any river delta development management. The current Cairns Plan does have a "Flood and inundation overlay code" but evidence in Caravonica and other nearby beach suburbs shows it is worthless.
Future:
A hydrology model is needed to test ideas on how to mitigate flooding in locations such as Lake Placid and Kamerunga Villas. Without a model to test ideas, my guess is options might include extending a barrier along the edge of the river (perhaps similar to what is near the Lake Placid van park) and/or making sure flood flows to the north and east are not blocked or unnecessarily restricted like near the school.
This section of the Cairns West Arterial Road is about to be doubled to 4 lanes by the state government. This offers us a choice:
Retrofitting culverts by adding 2 new lanes with culverts, switching traffic from the old lanes to the new while retrofitting culverts to the old lanes, and then opening all 4 lanes as a road that allows flood flows in a way that is more compatible with the QLD Government land care; or
Do nothing differently i.e. double what is there now and create a "Cairns West Arterial Weir";
Personal opinion: without access to a proper hydrology model to test ideas, my guess is that culverts would work best for road users and nearby residents. Any mitigation work along the river would also need to be factored into the decision making i.e. the local government and state government would need to work together.
Handling many levels of government: In the case of route 91, we have a state government piece of infrastructure that appears to affect flooding in a local suburb. Some form of development check mechanism is needed but it needs to work across local and state government. As the only alternative route over the river is a national highway, any successful development check on the Barron River delta really needs to work across all levels of government. The ideas in the 1997 Cairns City Council development control plan are very relevant, but they (or an updated version) would need to be enforced no matter which level of government is funding the infrastructure. Ideally, it would connect in with any integrated city planning. A summary is: we need collaboration across levels of government, and we need a proper single Delta development control plan like in 1997.
A final question: This simple example shows different levels of government interacting in ways on the Barron River Delta that increase damage and costs. I do not have the resources to provide examples on the eastern side of the Delta but I understand others may be doing so. Would a single entity be able to stop this mismanagement between governments and reduce costs?
2 Documents for the Barron River Delta
1980 Mulgrave Shire Council and QLD Government Barron River Delta Study
BARRON RIVER DELTA INVESTIGATION by D.A. Robinson, D.J. Cook and J.G. Barff, paper published in Coastal Engineering 1980. This is a paper from the Conference Proceedings https://doi.org/10.9753/icce.v17.54 as published 1980-01-29, but it is not the much larger main report which I understand is available in the JCU Townsville Library.
The work represented by this paper collected real data over a number of years and did build up understanding of sediment transfers and water flows. It also looked at the potential (and history of) river path and river mouth changes. An update of this work can inform any work to manage river mouth and river path location.
Very happy to add a link to any online copy if available.
1997 Cairns City Council Barron River Delta Development Control Plan
Development Control Plan - Barron River Delta (CCC 28 August 1997) (main diagram only)
This control plan requires developers to provide contour maps for the development area in before and after form, so that council can perform its own flood checks. The required resolution is 0.5m but we have access to much higher data these days e.g. LIDAR.
The plan also has a general aim to preserve the operation of the flood plain. It does this by actively preferring that urban development go into areas already featuring urban development i.e. farm land had a better chance of continuing on with its dual role of being a flood plain (the bigger and flatter a flood plain is, the more shallow is the flood water).
Development Control Plan - Barron River Delta - Planning Study (1997) (diagrams only)
This is the analysis published with the development control plan.
These versions were scanned and processed locally. Very happy to add a link to any other online copies if available.
3 Related to the Barron River Delta
EMERGENCY ACTION PLAN — TINAROO FALLS DAM (ID 370), ISSUE: 10.1 — September 2023
This document includes a Sunwater indication of significant flood points on the Barron River Delta, as part of normal safety assessment of a dam failure.
It appears that Sunwater has a model that captures the dynamics of flood flow in the aftermath of a dam failure i.e. there might already be a hydrology model owned by government with an ability to model this area.
See sheet 97 of 132 (drawing 256190, sheet 7 of 9).
4 Related to new Barron River Bridge
The Barron River bridge on the Cairns West Arterial Road is about to be doubled to 4 lanes.
Personal opinion: my guess (without the aid of models) is that this should be done as soon as possible because the only alternative transport route across the river (near the airport) is further down in the delta where the river path itself is much less stable e.g. we have seen a new mouth form at Holloways Beach in December 2023.
Quick link http://tinyurl.com/cairns4878