everyone and welcome to the May edition of “Keep Things Flowing”,
the monthly newsletter presented by Storm Water Consulting.
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
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
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.
above, the final row in the table presents flood level data from
the Brisbane River Catchment Flood Study.
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.
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
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:
happens if the stormwater pipes were to block?
happens if the upstream catchment develops and there is more runoff?
happens if runoff from the upstream catchment can’t get into the
happens if there is a bigger rainfall event than the pipe was
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
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
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.
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.
Keep Things Flowing!
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