Flood modelling and risk assessment

Flood modelling

The following information is provided as guidance for flood and development practitioners.

Model inputs

Regional flood models were updated in response to the availability of improved modelling technology, updated national guidelines and new rainfall data. In 2021, we concluded a joint investigation with the Bureau of Meteorology to resolve anomalies in the local and national rainfall dataset. New model inputs as endorsed by Council in July 2021 (G21.0720.019). They include:

• updated design rainfalls
• new national rainfall-runoff simulation methods
• new modelling software
• updated topography data
• updated land use mapping
• new downstream boundary simulation methods.

Events modelled

The flood models cover all the city's catchments:

• Logan-Albert-Pimpama
• Coomera
• Loders-Biggera
• Nerang
• Tallebudgera-Currumbin
• Coolangatta

This modelling includes flood level, depth, velocity, hazard and inundation-time maps for a range of events, including the probable maximum flood (PMF):

• 20% AEP* – 0.5% AEP
• 5% AEP – 0.05% AEP
• 1% AEP – PMF
• 0.2% AEP

*Annual exceedance probability

Technical flood studies

Council has undertaken detailed technical flood studies to determine the level of flood risk of every property within the City of Gold Coast.

Flood risk assessment

View map

The Queensland State Planning Policy requires a risk assessment be undertaken to inform the provisions of a planning scheme relating to natural hazard risk. An amendment to the Planning Scheme therefore requires the undertaking of a fit for purpose, city-wide flood risk assessment, including preparation of detailed flood studies and models to determine flood behaviour characteristics for a variety of rainfall events and meteorological scenarios.

The flood risk assessment was developed using the new flood modelling data to identify potential hydraulic risk across the city and inform land use planning responses. Hydraulic risk is based on the hydraulic behaviour of floods. It considers the flood depths and velocities for a range of events, from frequent to extremely rare (i.e., 20% AEP, 5% AEP, 1% AEP, 0.5% AEP, 0.2% AEP, 0.05% AEP and PMF^). It also considers the likelihood of each of these events occurring. Using this analysis of potential hydraulic risk, the risk assessment then maps flood risk across the city in 5 risk categories:

• very high
• high
• medium
• low
• maximum regional flood extent.^^

^ probable maximum flood
^^the maximum or outer boundary of the city's floodplains or the PMF

The results of our flood risk assessment have informed the proposed amendment. They also inform emergency and disaster response, asset management planning and renewal, community awareness and customer information. View the flood risk awareness map.

Potential hydraulic risk matrix

The city-wide flood risk assessment calibrated the Gold Coast's regional flood modelling outputs to map potential hydraulic risk across the city by using categories of risk never used before. It considered flood depths and velocities for a range of events, from frequent to extremely rare, and the likelihood of each of these events occurring.

Technical experts can see exactly how the potential hydraulic risk was calculated below.

Reference: The Australian Institute for Disaster Resilience (AIDR) Handbook 7

Planning Scheme flood policy

Maintaining the Gold Coast's current flood resilience is the key policy approach that underpins the proposed Designing for flood major amendment to the City Plan. The new flood maps and the risk assessment help to inform how to continue achieving this. One of the main mechanisms for maintaining the city's flood resilience is regulating building and development. This includes:

• requiring the establishment of finished building floors to above a defined flood level
• minimising the risk of exposure of vulnerable land users to a flood event
• encouraging development to connect to our flood resilient road network
• maintaining flood storage and conveyance
• providing a minimum area of flood resilient land to support intended development.

Defined flood level

One of the primary mechanisms for regulating building and development to improve flood risk resilience is to require building floors be established at a minimum level, the designated or defined flood level (DFL). This is also regulated through the Queensland Building Regulation. We apply various assessment benchmarks that relate to the DFL. Regulating these will continue through the proposed Flood risk overlay code.

A primary policy position is the adoption of the 0.5% AEP for the DFL, representing a departure from the 1% AEP. Several Queensland councils have already moved towards adopting a DFL greater than 1% AEP. This is largely associated with:

1. the new approach to modelling multiple events
2. the new risk-based approach to planning, and/or
3. in response to significant local flood events.

Importantly, for the Gold Coast, the newly modelled 0.5% AEP generally aligns city-wide with the current 1% AEP (modelled in 2016). This is due to the use of 6 new flood model inputs endorsed by Council in 2021 (G21.0720.019). Because the risk assessment results were positive, this change represented the least possible change for the most possible people. To better quantify that, the 1% AEP defined flood event currently covers approximately 39,000 hectares across the city. In future, the adopted 0.5% AEP defined flood event will cover approximately 41,000 hectares. This is an increase in geographic extent of approximately 2000 hectares, representing only a 6% increase in area.

Currently, non-statutory interactive mapping provides flood depth and flood level information based on the 1% AEP event. In future this mapping will also be the 0.5% AEP event. This mapping will be updated regularly to reflect the latest topography data or new digital terrain mapping. As a result, it will also reflect the most up to date, site-specific, DFL information. The benefit of this is that improved ground levels will display in the mapping in a timely way, communicating accurate DFL requirements for development. As with current practice, we will continue to offer the service of confirming the site-specific DFL through a Flood search report. We are currently investigating options for improving this service.