Flood modelling simulates how water moves across a floodplain during a flood event, producing the flood levels, extents, depths, and velocities used to inform development controls, infrastructure design, and emergency management.
Hydraulic Modelling Approaches
| Model Type | Description | Typical Application |
|---|---|---|
| 1D model | Represents flow along defined channels/cross-sections | Riverine flood studies, simple floodplains |
| 2D model | Represents flow across a full grid/mesh surface | Complex urban floodplains, overland flow paths |
| 1D/2D linked model | 1D for channels/culverts, 2D for floodplain | Most modern Australian urban flood studies |
Common software packages used in Australian practice include TUFLOW, HEC-RAS, and MIKE FLOOD.
Flood Study Process
- Data collection: survey (LiDAR/DEM), hydraulic structure details (culverts, bridges), historical flood records
- Hydrologic modelling: derive design flow hydrographs (see Hydrology and Catchment Analysis)
- Hydraulic modelling: route hydrographs through the floodplain model
- Calibration/validation: check model against historical flood records where available
- Design event simulation: run the model for the required range of AEP events
- Mapping and reporting: flood extent, depth, velocity, hazard, and hydraulic categorisation maps
Design Flood Events
| Event | Typical Use |
|---|---|
| Frequent events (20% AEP and more frequent) | Minor drainage design, nuisance flooding assessment |
| 1% AEP ("100 year") | Primary planning benchmark: flood planning levels, development controls |
| Probable Maximum Flood (PMF) | Extreme event: emergency management, high-consequence infrastructure (e.g. dams) |
Flood Planning Level (FPL)
$$ FPL = \text{1% AEP flood level} + \text{Freeboard} $$Freeboard (commonly 300–500 mm in Australian planning schemes) provides a safety margin for modelling uncertainty, wave action, and climate change allowance.
Hydraulic Categorisation
Floodplains are typically categorised to inform appropriate land use controls:
| Category | Description | Typical Controls |
|---|---|---|
| Floodway | Conveys significant flood flow: high velocity/depth | Highly restricted development |
| Flood storage | Stores floodwater, contributes little to conveyance | Development generally must not reduce storage |
| Flood fringe | Low hazard, outside floodway/storage | Development generally permitted subject to FPL controls |
Flood Hazard Assessment
Hazard is generally assessed from a combination of depth and velocity, often expressed as a depth-velocity product ($D \times V$), against provisional hazard curves distinguishing:
| Hazard Category | General Description |
|---|---|
| H1–H2 | Relatively safe for people/vehicles |
| H3–H4 | Unsafe for vehicles/children; caution for adults |
| H5–H6 | Unsafe for all people and vehicles; potential structural damage |
Impacts of Development on Flooding
New development must typically demonstrate no worsening of flood behaviour elsewhere:
- No net loss of flood storage volume
- No increase in peak flood level upstream or downstream beyond acceptable limits
- Filling/earthworks within the floodplain generally require offsetting excavation ("cut to fill" balance) or explicit approval
Climate Change Allowance
Current Australian flood studies commonly test a sensitivity scenario incorporating projected rainfall intensity increases, reflecting a warmer climate's effect on short-duration extreme rainfall.
Practical Notes
- Model calibration against a real historical flood event, where data exists, is the single strongest indicator of a flood model's reliability: treat uncalibrated models with more caution, especially in complex urban catchments.
- Freeboard is not a substitute for accurate flood level estimation: it is a safety margin on top of it, and shouldn't be relied on to cover model or data uncertainty.
- "No worsening" assessments must consider both upstream afflux (backwater effects from fill/structures) and downstream impacts from any increase in conveyed flow: checking only one direction is a common gap.