What is levee?
Some knowledge between you and known flood risk.
LEVEE is Perennial’s free, educational and interactive platform - helping you understand flood behavior, risk and planning in NSW.
LEVELS - How High
EXTENTS - How Far
VELOCITY - How Fast
EXPOSURE - How Safe
EVENTS - How Often
LEVELS - How High EXTENTS - How Far VELOCITY - How Fast EXPOSURE - How Safe EVENTS - How Often
custom built for levee learn.
GIVE our new WIDGETS A TRY below!
LEVEE Learn is now live - with interactive tools and plain English guides that make flood risk easier to understand. LEVEE Map is coming soon, bringing known flood information together in one place
Flood levels, risk and planning
Tap a flood to fill the section and see how high and how far it reaches. The plan view shows the same extent from above. Switch landforms to see why the same rules play out differently on a valley creek and a wide coastal floodplain.
Hazard combines depth and velocity, not depth alone. The deepest, fastest water sits near the watercourse (high hazard, the H5 to H6 range where it is unsafe for people and buildings), easing to low hazard at the fringe. Councils map these as flood risk precincts (high, medium and low), and the precinct that applies to a lot is recorded on its Section 10.7 Planning Certificate. The tightest development controls apply in the high precinct.
About the Flood Planning Level (FPL)
The FPL is the number that shapes a build. It is the 1% AEP flood level plus freeboard (typically 500 mm) for waves, blockages and model uncertainty. Habitable floor levels are generally set at or above it, so the FPL is what determines slab and floor heights, and it is the control most DA conditions turn on. It is a planning level, not a flood you would ever see at that exact height.
Next steps for a specific property
Once you know which flood reaches a property, see how often that happens over the life of a home in the "How likely is it?" view. To confirm the levels and the flood risk precinct for a specific block, a council flood study or the Section 10.7 Planning Certificate is the next step.
Notes and assumptions
The Flood Planning Level is a planning control, not a flood. Freeboard, typically 500 mm, is added above the 1% AEP level to allow for waves, blockages and model uncertainty, and habitable floor levels are generally set at or above it. Council controls vary by flood risk precinct, and Section 10.7 Planning Certificates record which precinct applies to a lot. Levels and extents shown are illustrative, not modelled for any specific site.
How often will you see a flood?
A "1-in-100 year" flood does not arrive on a schedule, it has a 1% chance in any given year. Slide to see how that chance adds up over the time you would own a property within the flood's reach.
The 1% AEP is the benchmark for NSW planning, the Defined Flood Event that flood maps, the Flood Planning Level and development controls are built around.
How flood risk works: likelihood and consequence
Risk is how likely a flood is, multiplied by how bad it would be. Frequent floods are likely but usually shallow and minor. The largest floods are severe but rare. The 1% AEP sits where that balance is managed, which is why it anchors planning.
If a property sits within this flood's extent, there is about a 55% chance it floods at least once over 80 years.
Projected climate, indicative. Applies a representative ARR 4.2 design-rainfall increase of about +20% (middle SSP2-4.5 pathway, ~2090). A more intense storm of the same rarity becomes more frequent, so today's 1% AEP flood shifts to roughly a 1-in-40 event.
The Probable Maximum Flood is roughly three times the flow of the 1% AEP. It is less a real forecast and more a stress test: the model dumps an extreme volume of water to see how the landform responds, where the floodplain finally spills, and how far up the hill stays dry no matter how bad conditions get. The consequences are severe, which is why evacuation routes, on-site refuge, hospitals, schools, evacuation centres and critical utilities are located above it.
These are frequent storm events, happening multiple times a year. Events up to this size are what stormwater infrastructure (gutters, kerbs and pipes) is designed to carry.
The 1% AEP is the benchmark design flood used across NSW. Most flood mapping, the Flood Planning Level and development controls are built around it.
What this means for insurance and property
Flood risk has real costs. Homes that face flooding more often, or sit below the 1% AEP benchmark, generally pay more for home and contents insurance, and in very high-risk areas some insurers limit or exclude flood cover. Flood history can also affect lending and resale. Reducing the chance and depth of flooding, through where and how a home is built, is what keeps a property insurable and its premiums lower. This is a general explanation, not financial or insurance advice.
Next steps for a specific property
Want to know what applies to a specific block? A council flood study, a flood certificate or the Section 10.7 Planning Certificate is the next step. A flood study confirms which flood actually reaches the property, which is what sets the chance shown here.
Notes and assumptions
This assumes the property lies within the selected flood's extent. A property only reached by a rarer flood has a much lower chance: one inside the frequent (10%) extent floods often, while one only inside the PMF extent floods very rarely. The flood levels view shows which flood reaches a given property.
Probabilities assume an independent chance each year and describe how often events of this size occur, not the depth or hazard at any specific property. The PMF has no fixed probability; an annual chance of around 1 in 100,000 is adopted here for illustration. The climate setting is indicative: it applies a single representative design-rainfall increase drawn from the Australian Rainfall and Runoff Version 4.2 Climate Change Considerations for a middle SSP2-4.5 emissions pathway, then translates that intensity increase into a shift in flood frequency through a simplified growth curve. A site-specific assessment would use the duration, location and emissions pathway relevant to the catchment, where projected increases range more widely. Published projections extend to about 2090; the longer exposure periods shown here apply that same design condition across the period rather than projecting beyond 2090. Separately, design rainfall estimates are also refined over time as records lengthen and methods improve; these revisions can move estimates up or down depending on the location, and are not a one-directional trend.
Why filling the floodplain matters
The floodplain stores floodwater like a bathtub. Fill part of the tub and the displaced water has to go somewhere. Slide to fill the floodplain with development and watch what the same flood does.
In the natural floodplain the 1% AEP flood sits below every floor. Each home is built with its floor at the Flood Planning Level, a safety margin above that flood.
Flood level: at or below the original 1% AEPNext steps for a specific property
This is the conceptual picture. A real Flood Impact and Risk Assessment models conveyance and storage for the specific catchment, not a simple bathtub. For a particular block, a council flood study or a flood consultant can confirm what a proposal needs to demonstrate.
Notes and assumptions
Levels shown are illustrative, not modelled. A floodplain behaves like a bathtub: fill removes storage, and the displaced volume has to go somewhere. Each development is small on its own, but the effects are cumulative over time, adding up to higher flood levels across the floodplain. Eventually the same flood rises above the Flood Planning Level that the floors were set to. This is why councils control filling on flood prone land, and why a Flood Impact and Risk Assessment must demonstrate that a proposal does not raise flood levels for others.