Enabling Communities to Thrive - Designing Effective Flood Mitigation Strategies in New Developments

Enabling Communities to Thrive - Designing Effective Flood Mitigation Strategies in New Developments


The NSW Department of Planning, Industry and Environment recently exhibited a state-wide Flood Prone Land Package and the new flood planning controls that came into effect earlier this month. Calibre has expertise in developments within flood prone areas and can explain how these controls could impact your future developments.

Developments within Flood Plain Areas

We’ve all recently seen the devastating affects which regional flooding can have across the community. These flood impacts can not only severely damage properties and infrastructure but can also cost lives.

Whenever a major flood event occurs, we understandably see a strong community reaction. “I’ve never seen it flood before that development went in” is a common reaction.

Whilst in some instances this may be correct, there is a certainly a fair degree of misconception. In fact, our industry (State Government, Councils and Consultants) collectively works very hard at ensuring new developments have no flood impacts upon the community.

When done correctly, proposed measures (such as detention basins, water quality devices and compensatory cut / fill) will ensure that any surrounding properties are not further impacted by flooding.

Flood Storage is a key issue

Developments within floodplain areas need to adhere to strict regulations to ensure that they do not impose additional flooding on others. One of these key principles is to not reduce the “Flood Storage”.

The devastating floods in March across the Hawkesbury - Nepean Floodplain shows why this is so important. The Hawkesbury – Nepean catchment is one of the worst flood affected catchments in Australia, with flooding impacting upon thousands of houses during extreme rainfall events.

The Hawkesbury – Nepean Floodplain is significant in size but is restricted by small “choke points” at Wallacia, Castlereagh and Sackville. During periods of heavy rain, the floodwaters cannot escape fast enough, and we see widespread flooding across the floodplain. This phenomenon is often referred to as the “Bathtub Effect”.

Given the nature of this flood behaviour, it is therefore critical to ensure that proposed developments will not reduce the amount of “flood storage”. This is achieved by providing compensatory “cut” to offset any “filling” (Refer Table 1). When done correctly, this ensures that the bathtub does not fill up any further. The positive effects of this strategy are that flood storage is moved from areas high in the floodplain to a level lower in the floodplain. This provides greater flood storage for more frequent floods (say up to the 10% Annual Exceedance Probability) than currently exists, and potentially reduces flood levels for those flood events.

Flood storage compensation and efficient detention basin designs are effective measure to delivery safe communities in flood floodplains.

Table 1: Cut & Fill Approach

What does Calibre do?

Our team has extensive experience in Flood Studies. We undertake state of the art flood modelling (1D/2D hydraulic modelling software) and provide clear / easy to understand flood maps and reports.

How do we support development? Our Flood Studies provide a clear comparison between “Existing” and “Proposed” Scenarios to demonstrate that flood levels are not increased. We pride ourselves on getting the best possible development for our clients and communities - without affecting others.

The flood studies are utilised to set the development levels and floor levels to meet the Flood Planning Level (FPL), typically 1% Annual Exceedance Probability (AEP) plus a level of freeboard.

Flood evacuation strategies is a service offered to support developments in floodplains, ensuring future communities have a means to evacuate during a flood. Our flood modelling results are also incredibly useful for the emergency services such as the SES (and Councils). The mapping can be used to inform regional flood evacuation routes, strategies and preparedness. Importantly, this helps to reduce the risk to property and life – which is a benefit to all of us.

A typical flood depth map is shown below, from such results areas of flood conveyance, flood storage and flood fringe can be identified, and the developable land optimised through an adequate flood mitigation strategy.

For any enquiries please contact Troy Eyles, Principal Engineer
email Troy.Eyles@calibregroup.com

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