In this issue



   - What’s to Come



3D Flood Visualisation

   - New Video




-  Undercroft Clearance



Frequently Asked Question

   - 1D vs 2D modelling



Wrap Up

   - See you next month


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Hello everyone and welcome to the August edition of “Keep Things Flowing”, the monthly newsletter presented by Storm Water Consulting.

In this month’s edition Darren presents our latest 3D flood visualisation video, we present information on undercroft clearance requirements within Brisbane City Council and we answer another frequently asked question.


3D Flood Visualisation

When I was at QIT in the mid 80’s I remember my hydraulics lecturer teaching us about the concept of a 1 in 100 year flood.  For reasons unknown (even to himself), a 1 in 100 year flood level was referred to as a “Q100 Flood Level”. 

In my student mind I filed this fact away:

Q100 flood = lots and lots of water.

I did not know at the time that I would spend my professional career trying to visualise such an event.  Now, nearly 30 years later, with the aid of computer animation, visualising such an event has suddenly become a whole lot easier.

Please enjoy the following video to gain an appreciation for the difference between a flood similar to a DFL event and a flood similar to an RFL event.

Click the Video to Play!


Undercroft Clearance Requirements

A common issue that we encounter during the analysis and design process is that a lot of developers are unaware of the regulations regarding undercroft clearances for development within Brisbane City Council.

The release of the Flood Overlay Code in July 2014 brought about new design criteria for undercroft areas within Brisbane. These new design criteria were aimed at maintaining the hydraulic characteristics of the flood affected development area, reducing the risk of debris blockage and reducing the risk of impact to other properties.

When a development is proposed to be constructed on a suspended floor (or slab) the minimum undercroft criteria presented in Table 1.0 below are required to be met to comply with the acceptable outcome criteria of the Flood Overlay Code. 


Table 1.0 – Extract from Flood Overlay Code

Flooding Source

Minimum Clearance Requirement

Overland Flow - where the velocity-depth  product <0.6m2/s and the flow depth is < 0.6m in a 2% AEP flood event

Lowest flood level is to be 1.5m above the ground elevation in the undercroft area

Overland Flow - where the velocity-depth  product > 0.6m2/s and the flow depth is > 0.6m in a 2% AEP flood event

Lowest flood level is to be 2.5m above the ground elevation in the undercroft area

Creek/Waterway – Flood Planning Areas 1, 2, 3 and 4.

Lowest flood level is to be 2.5m above the ground elevation in the undercroft area


In the event that the proposed development is not able to achieve the criteria in Table 1.0 above, the performance criteria of the Flood Overlay Code must be addressed. Often a flood risk assessment is required to be undertaken to satisfactorily show compliance with the performance outcome criteria. Depending on the hydraulic characteristics of the flooded area, the performance outcome criteria may not be adequately addressed. In this circumstance a redesign of the development may be required.

If you are planning to build in a flood prone area or are unsure of the potential flooding in the area please contact our office and we would be pleased to answer any questions you have.




I’ve recently submitted a Stormwater Report as part of a development application and the Council have issued an Information Request in response. The Information Request stated that the hydraulic modelling presented in the Stormwater Report was undertaken using 1D modelling software, which is not considered appropriate. The Council has asked that the hydraulic modelling be redone using a 2D modelling software package. What is the difference between the two software types and is the modelling required to be redone as Council have asked?


The difference between 1D and 2D hydraulic modelling software packages is that the 2D package has the ability to model water moving in multiple directions at the same time. In a 2D modelling package flow can move in the x, y planes at the same time, whereas a in a 1D modelling package the flow is restricted a single plane. Basically a 2D model has the ability to choose the direction of flow, based on the ground elevations whereas a 1D model is reliant upon the modeller to specify the flow direction.

When the direction of flow is known, either a 1D or 2D software package is adequate for modelling purposes. When the direction of flow is unknown then a 2D model would be better suited. The fact that Council have asked for a 2D model does not necessarily mean that the original 1D model was inadequate. A review of the modelling methodology and discussions with the Council would be an appropriate course of action given the request.


Wrap Up

We hope you enjoyed this edition of Keep Things Flowing. Feedback on articles presented is always welcomed and for further information on any of the articles presented please don’t hesitate to contact our office.

As always, Keep Things Flowing!

The Storm Team