4. Flood Assessment for Property Development


Since the early 2000’s, overland flow has become an increasingly important constraint which has affected large scale developers, Mum and Dad investor/renovators, engineers, architects and certifiers.  This blog hopes to discuss a little more about the issue and how to understand the challenges and solutions in a Brisbane based setting.

Where does the issue originate?  As Brisbane developed in the 20th century, the attitude of conquering the land was used as a framework for the development of new blocks.  Once the trees were cleared, roads were laid out in a rectangular fashion to provide an acceptable street frontage with a deep back yard (Figure 1).  This allowed many residents to grow their own food in home gardens, keep chickens and enjoy the large rear spaces.

Figure 1 - Typical Land Development in the 20th Century

Figure 1 - Typical Land Development in the 20th Century

 Little thought was given to the topography of the land and how it might ultimately affect future generations.  Creeks were filled in, piped and roads constructed where they once flowed.  It was acceptable to put a lid on the drain (which used to be a creek) to ensure the water drained away quickly and didn’t affect the newly constructed house.  Often there were deep, dry gullies across the rear of the properties and little thought was given to where stormwater would flow.  The majority of houses were constructed on stumps and surface water during heavy rain would flow beneath them, occasionally affecting laundries and carports but doing little harm.  The houses were naturally flood resistant and resilient and heavy rain was accommodated as a part of life.

Fast forward to the year 2018 and the style of Brisbane living is very different.  Houses are mainly built as slab on ground construction (Figure 2).  Stormwater entering them can cause tens of thousands of dollars of damage.  Space is at a premium and the large lots are being split and built on with a second dwelling.  Privacy and security mean that many chain link fences (which allowed surface water to flow through) have been replaced by impervious brick, concrete, steel and wooden fencing.

Figure 2 - Typical House densities in 2018 in Brisbane

Figure 2 - Typical House densities in 2018 in Brisbane

Despite this change in living styles and housing, nature hasn’t changed.  Severe storms still produce heavy rain and runoff which exceeds the underground pipe capacity.  With no obvious flow path, stormwater collects in low areas and pushes through fences, sheds, houses and cars to lower ground.  This additional water is called “Overland Flow”.  It is distinguished due to its flooding from the top of the catchment down (i.e. it floods you from the upstream property) rather than from the creek up (where water escapes a creek and flows through an adjacent property).

Overland flow is now a clear part of flood assessments for new allotments, roads, houses and building extensions.  Although Brisbane City Council has many creek flood models to provide predicted flood levels in these waterways, they don’t yet have an accepted overland flow model for the city.  Therefore, they require each new allotment to complete its own Overland Flow study to predict the peak water levels on the block, set minimum habitable floor levels and assess the impacts of any new works on neighbouring properties.

The most common statement I hear from property owners is “I’ve been here 50 years and my property has never flooded”.  However, the engineers completing the flood assessment are required to consider storm events with severe intensities that might only be seen twice a century in parts of the city.  When these actual events occur in Brisbane, severe damage is often the result.  Most recently we saw a storm of this intensity in Graceville, resulting in flooding through yards, carports and houses.

Fortunately, technology has allowed engineers to model these events quite precisely.  Two dimensional hydraulic modelling software is now commonplace.  Figure 3 shows typical modelling output (depth and velocity) and results.  In fact, it would not be best practice to use one-dimensional hydraulic modelling in many or most of the required flood assessments now.

Figure 3 - Two Dimensional Modelling of Overland Flow

Figure 3 - Two Dimensional Modelling of Overland Flow

 As a rule of thumb, overland flow generally is between 0.15 and 0.5 m deep.  It is rarely deeper than that but 0.5 m deep water which is moving fast will destroy a shed or fence easily and wash away a car.

For slab on ground house construction, overland flow will often require a house pad to be filled significantly.  Minimum habitable and non-habitable floor levels are 0.5 m and 0.3 m above the peak overland flow water levels respectively.  This can cause issues when garages require steep driveways to access them.

Where house are raised to allow overland flow to pass under them, Brisbane City Council has stipulated a minimum clearance beneath the house.  For example, if the depth of overland flow is less than 0.6 m deep and the velocity*depth product is less than 0.6 m2/s, the lowest floor level is to be 1.5 m above the highest ground elevation in an undercroft area (BCC City Plan 2014).  If it is deeper than that or has a higher velocity*depth product, the minimum clearance is 2.5 m.

It can be seen that overland flow can be disruptive to development layouts and floor levels.  In my opinion, once an overland flow flag is found on a property, the owners should endeavor to complete a study to determine the depths and extent as soon as possible.  This will allow an integrated design which maximizes the liveability of the proposed design while ensures the site will provide flood immunity and no adverse impacts to neighbours.

Any Overland Flow study needs to be undertaken by a Registered Professional Engineer of Queensland (RPEQ).  A list of these can be found on the Board of Professional Engineers of Queensland, https://www.bpeq.qld.gov.au/.

 Staff from MRG Water Consulting Pty Ltd have completed over 500 of these studies and can assist in answering any questions or issues that arise.  We strive to help all our clients to realize their goals and make the process as painless as possible!

Mark Gibson
Director, (Principal Engineer)
BEng (Civil) MIWM RPEQ FIEAust CPEng NER APEC Engineer IntPE(Aus)
2A/18 Torbey Street, Sunnybank Hills, Qld 4109
Ph: 07 3345 3434 Mob: 0418 569 362

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