Flooding in Pittwater
Fl
Key terms and definitions
100 Year Flood
Also known as:
• 1% Annual Exceedence Probability Flood (1% AEP);
• 100 Year Average Recurrence Interval flood (100 Year ARI);
• 1-in-100 year flood.
The 100 year flood means you have a 1-in-100 chance that a flood of that size (or larger) occurring in any one year; the term relates to
the amount of rainfall and intensity of the flood event for a specific location. The term 100 year flood is interchangeable and means the
same as the 1% AEP flood, 100 Year ARI flood or 1-in-100 year flood. The 100 year flood doesn’t mean that if it floods one year, it will
definitely not flood for the next 99 years. Neither does it mean that if no flooding has occured for 99 years that it will result in a flood the
following year. Some parts of Australia have received two 100 year floods in one year.
Mainstream Flooding
Mainstream flooding is defined as water that flows over the banks of creeks and lagoons. In Pittwater, the major creek systems form the
primary mainstream floodplain areas. Properties affected by mainstream flooding for the 1% AEP flood event and greater require flood
related development controls, a minimum floor height and a Flood Planning Level (FPL).
Probable Maximum Flood (PMF)
The Probable Maximum Flood (PMF) is the largest flood that could conceivably occur at a particular location. It is usually based on a
theoretical amount of rainfall and is much greater than a 1% AEP flood.
The PMF is used by Councils and the NSW SES in disaster planning and emergencies. There are no development controls imposed on
residential properties for managing the PMF, as the likelihood of such an event occuring is so small that it’s not reasonable for residential
dwellings to accommodate it. The PMF can result in Category 2 development controls for non-residential land uses such as aged care
facilities.
Overland Flow Flooding
Overland flow is defined as surface runoff before it enters a waterway. It is caused by rainfall which flows downhill and concentrates in
low points. Most stormwater systems are designed to remove water from the catchment quickly in small events. Larger events such as
the 1% AEP flood exceed the pipe’s carrying capacity resulting in a build up of water upstream. Council manages overland flow flooding
through the use of Category 3 flood hazard and corresponding development controls.
Flood Levels (m AHD)
In Australia, flood levels are measured relative to mean sea level (Australian Height Datum – AHD). Flood levels, floor levels and ground
levels are generally given in metres AHD (mAHD). For example: if the minimum floor level of your house is 6.7m AHD and the projected
100 year ARI floodwater level is 7.2m AHD, theoretically the floor of your house would be 0.5m under water in a 1% AEP flood event.
Freeboard
Freeboard provides a safety factor for greater protection against different types of flooding. Freeboard describes a factor of safety
expressed in metres above a flood level for flood protective or control works. Freeboard is intended to allow for the uncertainties in design
and construction. Adding a freeboard to a flood level can greatly reduce the risk of a structure flooding.
Useful Links
•
•
NSW SES information on how to be safe during a flood http://www.floodsafe.com.au
Real time local rainfall & water levels http://new.mhl.nsw.gov.au/users/NBFloodWarning/
•
Council’s local flood information http://www.pittwater.nsw.gov.au/environment/natural_hazards/flooding
Flood Planning Level and Minimum Floor Level
Flood Planning Levels (when they apply) are the same as minimum floor levels. All properties affected by flooding have a minimum floor
level specific to that property.
The following figures illustrate how the minimum floor levels are calculated for each flood category (in accordance with the DCP). Please
note the figures are not to scale.
Category 1 is mainstream flooding. The minimum floor level is calculated by adding a 0.5m freeboard to the 1% AEP mainstream flood
level for a particular property.
Category 1 High or Low Hazard
Ground Level
FPL = 1% AEP flood level + 0.5m freeboard
Creek
1% AEP flood level
(100 year flood level)
Category 2 is all flooding up to the PMF with no minimum floor level for residential development. Non residential properties may require
a minimum floor level at or above the PMF.
Category 2
PMF (Category 2) - no development controls for residential land use
Ground Level
Creek
PMF can result from all types of flooding
Category 3 Major is overland flow flooding with a minimum depth of 0.3m. The minimum floor levels for Category 3 Major needs to be
at or above the FPL.
Category 3 Major
Ground Level
FPL = 1% AEP flood level + 0.5m freeboard
1% AEP Major Overland Flow Path + 5m
horizontal buffer
Flood depth greater than 0.3m for major
Category 3 Minor is overland flow flooding with a depth of between 0.15m and 0.3m. The minimum floor level needs to be at or above
the 1% AEP + 0.3m vertical freeboard. The term FPL does not apply to this flood category.
Category 3 Minor
Ground Level
Minimum Floor Level = 1% AEP flood level + 0.3 freeboard
1% AEP flood level - overland flow
Flood depth between 0.15m and 0.3m
For more information on the flood categories and development controls refer to the Pittwater 21 DCP.