Why is the City of Ballarat undertaking this project now?
The Ballarat Strategy and Council Plan 2017-21 identify a need for the City of Ballarat to better understand the effects of flooding on properties and introduce appropriate planning controls so that residents and businesses are informed and better equipped to manage flooding when it occurs. This will help to minimise the impacts of flooding on the community through improved planning and development decisions. The information will also be used to help plan and prioritise infrastructure improvements, flood mitigation and flood emergency management in the future.
What is the City of Ballarat doing to reduce flooding risks?
Flooding is a natural hazard in Victoria’s river systems and natural and constructed drainage systems.
Understanding flood behavior enables us to assess the likely impacts and costs of flooding. It also enables us to assess the benefits of different options for managing the community’s exposure to flood risk. There are no quick fixes in reducing the damage caused by flooding. Two centuries of development on flood plains and low-lying areas mean that legacy issues will remain for a very long time.
To assist in our understanding of the flood risk in Ballaratwe have undertaken extensive flood modelling of the city using the latest land and rainfall data and computer technology. We are using this data to introduce overlays into our planning scheme to reduce the risk in flood-prone areas and to investigate a range of mitigation options in the smaller catchments (drainage systems). These options include detention systems, pipe augmentation projects and constructing open spaces that introduce Water Sensitive Urban Design and Integrated Water Management that include mitigation strategies. Each option will be prioritised and funding will be sought in future budgets.
For areas that are affected by inundation caused by stormwater drainage networks reaching capacity, we have an on-going pro-active and planned maintenance program of drainage pit cleaning to reduce nuisance flooding in high risk areas.
Each year, the City of Ballarat spends around $475,000 on drainage maintenance and a further $1 million on drainage renewals and minor upgrade works.
The City of Ballarat works closely with our relevant Catchment Management Authorities, the State Emergency Services, VicRoads (Regional Roads Victoria), Department of Environment, Land, Water and Planning and other government agencies on our flood prevention, response, recovery and mitigation activities.
What is a flood investigation?
A flood investigation generally aims to determine the extent, depth and velocity of floodwater along a specific waterway or series of waterways. The investigation includes several steps to determine this information, these include:
Determining rainfall depths and event durations – these can be for historic and ‘design’ events(design events have a set probability). Typically, modelling would be undertaken for a range of events. This includes small events that occur every 5-10 years on average, up to events that are only likely to happen once every 100 or 200 years. These events are described in a yearly percentage likelihood (Average Exceedance Probability – AEP). For example, a 1% AEP event would have a one percent likelihood of occurring in any given year, this would be the same as once in a 100-year period. The likelihood of a flood event is directly related to the amount of rainfall and the time for which rainfall might occur. The amount of rainfall and time for which it will occur is determined by the Bureau of Meteorology based on historic rainfalls across Australia.
Building a runoff routing model – a runoff routing model determines the proportion of rainfall which becomes runoff and generates flow into the waterway. The model includes a delineation of the catchment area, drainage lines, rainfall depths and durations, infiltration losses and calibration parameters which can be adjusted for each catchment.
Building a hydraulic model – a hydraulic model determines depths, velocities and water levels along the subject waterway. The model comprises of survey data representing physical features, this typically includes topography, culverts, bridges, roads and drains. It also includes a ‘roughness’ parameter to represent vegetation or obstacles in the waterway which can’t be included as physical barriers to flow.
Draft mapping – the model results are mapped showing the maximum water level, depth or velocity over the duration of the event. This may not occur at the same time all over the catchment, but the mapping shows it like this rather than a single moment in time.
Community engagement – community input is sought to confirm the accuracy of the mapping. This ensures the maps are producing a realistic representation of flooding. It also ensures the community are aware of their flood risk, improving flood awareness.
Final modelling and mapping – the flood modelling and mapping is modified in accordance with community feedback. The final maps are then produced.
What is a floodplain?
A floodplain is the area of land which becomes inundated by a 1% AEP event. This is the common distinction made by floodplain authorities in Victoria as setout in the Victorian Floods Review. It is also mentioned within the planning scheme for the delineation of the Land Subject to Inundation Overlay, which enables the City of Ballarat to control development.
How is flood risk determined? And, what is flood hazard?
Flood risk is determined by the frequency of inundation, along with the 1% AEP flood depth, velocity and hazard. If an area gets inundated frequently or has a high flood depth in a 1% AEP event, it is considered to have a high flood risk.
Flood hazard is a combination of a 1% AEP, flood depth and velocity. A flood hazard is typically used to determine if roads are trafficable or if a property will be isolated by a flood event. The guidelines are set out in Australian Rainfall and Runoff (2016) and adopted by most floodplain management authorities across Victoria, specify the Flood Hazard limit for road trafficability as 0.4 m2/s. There are numerous flood hazard classifications (H1-H6) based on velocity and depth outlined in Australian Rainfall and Runoff (2016).
Will the flood modelling influence the City of Ballarat’s future mitigation works for infrastructure upgrades?
Which waterways are part of the project?
There are 11 waterways that are part of this project, with some tributaries (streams flowing from waterways) counted as one.
The waterways included are:
Canadian Creek and tributaries
Little Bendigo Creek
Hit or Miss Gully
Which waterways are not part of the project?
There were several waterways, including defined waters, drains and gullies not included in the project.
The Winter Creek was not included due to its location on the outskirts of the municipality. It also does not directly impact any densely populated areas or areas identified for future development.
The Union Jack Creek was not included because stormwater drainage mapping in Buninyong is currently being reviewed. The Union Jack Creek has been identified for modelling soon.
What is the difference between this modelling and modelling previously undertaken?
Why does this mapping appear to be affecting more (and less in some areas) properties then previous mapping?
The new mapping varies from that previously undertaken in several ways, including:
Computer software and hardware updates have led to improvements, allowing the area to be better modelled. The new models are likely to be more accurate because of the finer resolution topography and better representation of structures (bridges, culverts etc).
Australian Rainfall and Runoff was updated in 2016 (previous version was 1987).The update included changes to recommended design rainfall depths, losses and temporal patterns. These updates have changed the design model inputs.
There were various flood mapping methodologies used in previous studies. The current project has adopted a standard approach recommended in Australian Rainfall and Runoff (2016).
What rainfall data was used to build the model?
What is a 1% AEP event?
How will the information from the flood study be used?
Can I make a submission about the model?
What are the next steps?
What is a planning scheme?
What is an overlay?
What planning overlays relate to flooding?
Why do the overlays only represent 1% AEP flood events?
The Land Subject to Inundation Overlay (LSIO) is a representation of the 1% AEP flood extent and the Floodway Overlay (FO) is a representation of areas which have a higher flood risk. Areas of FO have a 1% AEP depth of greater than 0.5 m and/or a velocity faster than 1.5 m/s and/or a velocity x depth of greater than 0.4 m2/s.
The 1% AEP flood event is used as a basis for these overlays in Victoria. The 1% AEP event is used for declaring flood levels and flood areas under the Water Act (1989) and setting minimum building floor levels under the Building Act (1993).
What overlays are likely to be introduced as part of the Planning Scheme Amendment?
The Special Building Overlay identifies areas prone to overland flooding. The purpose of this overlay is to set appropriate conditions and floor levels to address any flood risk to developments.
The Land Subject to Inundation Overlay identifies land susceptible to flooding associated with waterways and open drainage systems.
The Floodway Overlay identifies land carrying active flood flows associated with waterways and open drainage systems. This overlay is categorised by depths in excess of one metre.
Some development on properties affected by one of these overlays require a planning permit. Most planning permit applications would be considered by the Corangamite Catchment Management Authority and the City of Ballarat. The application process would seek to ensure, amongst other matters, that any development maintains the free passage and temporary storage of flood waters and the minimisation of risks from flood hazard.
Will the amendment limit the development potential of my property?
Will the amendment affect the value of my property or the cost of insurance?
The property market determines the value of any residential property. Property owners should seek their own valuation advice if they are concerned that flood classification may influence their property value.
The insurance industry has developed its own database of flood risk Australia wide for individual properties, which has regard for both depth of flooding and frequency of flooding. This National Flood database is based on the most up-to-date flood studies, rather than planning controls.
What if my property is covered by a Heritage Overlay or other overlay controls?
The City of Ballarat’s Heritage Policy strongly discourages demolition of existing ‘significant’ or ‘contributory’ graded heritage buildings. Non-contributory buildings covered by a Heritage Overlay may be demolished and replaced, but a planning permit is still required to demolish the building and construct a new one.
Extensions to existing buildings in a Heritage Overlay will require a planning permit. Depending on the extent of any flood related overlay that may cover the property, the floor level of the extension may need to be raised above the flood level (or at the very least be constructed no lower than the existing floor level of the building). Each application is assessed on its merits and a site-specific solution can often be found to balance the competing issues of flood risk and heritage.
When will the project be finished?