In This Issue

 

Welcome

   - In This Issue

 

 

Updated Floodwise Property Reports

-  BCC

 

 

Safe Engineering Practises

-  Hydraulic Engineering

 

 

Wrap Up

   - See you next month

 

 

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www.stormw.com.au 

 

 

Phone:

07 3398 4992

 

Welcome

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

In this month’s issue of Keep Things Flowing, we discuss Brisbane City Council’s latest update to their Floodwise Property Reports and we discuss some safe engineering practises that we implement as hydraulic engineers.

 

BCC Floodwise Property Reports

Earlier this month Brisbane City Council updated their Floodwise Property Reports with information from the 2017 Brisbane River Catchment Flood Study (BRCFS). The Brisbane River Catchment Flood Study project was led by the State Government and was undertaken due to the recommendations from the Queensland Floods Commission of Inquiry following the January 2011 flood event. The Brisbane River Catchment Flood Study covers areas across four different local government areas including Brisbane, Ipswich, Somerset and Lockyer Valley. The study is said to be the most comprehensive flood study ever undertaken in Australia and involved more than 50,000 computer simulations and investigated 13,340 scenarios that influence flooding.

Flood level data from the Brisbane River Catchment Flood Study is now included on Floodwise Property Reports for properties affected by Brisbane River flood water. An example of an updated Floodwise Property Report is presented below.

As shown above, the final row in the table presents flood level data from the Brisbane River Catchment Flood Study.

Currently the 2017 BRCFS flood level data is only for information purposes and is yet to be adopted for application in planning schemes. A timeframe has not been given as to when flood levels in the Floodwise Property Reports will be updated with information from the BRCFS.

 

Safe Engineering Practises

Part of the role of a hydraulic engineer is to ensure that a safe and practical design is provided so that a development site would not create an adverse impact to neighbouring properties or be subject to adverse risk from flooding conditions. This issue can prove difficult at times given financial impacts, design constraints and aesthetic impacts associated with development modifications. We’ve provided a number of examples below in which a development was modified to provide a safe and practical design. Understanding the principles in the examples below could help you with your development.  

Example 1 - Emergency Flow Path Design

Brisbane City Council’s City Plan mapping shows that a property is affecting by overland flow. It is concluded that the underground stormwater pipe system has capacity to convey all of the runoff from the upstream catchment. The developer therefore wants to build over (and block) the overland flow path. Is this acceptable? In the majority of circumstances the answer would be no. Even though the stormwater pipe has capacity to convey all of the runoff, consideration must be given to a number of different scenarios which include:

·         What happens if the stormwater pipes were to block?

·         What happens if the upstream catchment develops and there is more runoff?

·         What happens if runoff from the upstream catchment can’t get into the stormwater pipe?

·         What happens if there is a bigger rainfall event than the pipe was designed for?

All of the scenarios above could result in overland flow through the property. An emergency overland flow path should always be provided. Blocking the flow path in this instance could result in damage to the subject property or damage to neighbouring properties.

 

Example 2 - Evacuation Route

A multi-unit building is proposed to be constructed on a property adjacent to a creek. Flood modelling has been undertaken and the results show that the site is not inundated by flood water from the creek. However, the results also show that the road providing driveway access to the site is inundated by up to 1.0m of flood water. The development is therefore isolated during a flood event. If an emergency situation arose, residents could not evacuate the premise. Increasing the intensity of land use in flood prone areas, particularly areas that cannot be evacuated safely during flood events is poor engineering practise. The majority of Council’s within South East Queensland have now adopted this principle and require trafficable access to be provided to multi-unit developments. In order to provide a safe and practical design, the development could be modified to relocate the driveway access to the property to a trafficable location. This could involve redesigning the development layout or negotiations with neighbouring properties to provide a trafficable access route.

 

Example 3 – On-site Detention

 A development site is located in the upper reaches of a catchment and is not subject to any flood water. Properties downstream of the site are subject to flooding conditions caused by overland flow. It is proposed to alter the site conditions by increasing the impervious site area. Does the site need a detention system? Development of the site without the use of on-site detention could increase the runoff from the property and could increase flood levels on downstream properties. The site would therefore require on-site detention. Providing a detention system would ensure the peak discharge from the site would not be increased.

 

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

 

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