Flood Management Glossary
Probable Maximum Flood (PMF)
The PMF is the largest flood that could conceivably occur, typically estimated from probable maximum precipitation
coupled
with the worst flood producing catchment conditions. The PMF extent defines the floodplain and incorporates all flood
prone land. The PMF is a very rare and improbable flood. Despite this, a number of historical floods in Australia have
approached the magnitude of a PMF. Every property potentially inundated by a PMF will have some flood risk, even if it
is very small. Under the State Government’s Floodplain Development Manual (2005), councils must consider all flood
risks, when managing floodplains.
100 Year Flood Event
A 100 year flood is the flood that will occur or be exceeded on average once every 100 years. It has a probability of 1%
of occurring in any given year. If you have experienced a 100 year flood, it is still quite possible for you to experience
another event of similar magnitude within your lifetime, as a large flood events occur randomly. Some parts of Australia
have received a 2 or 3 100 year floods within a few years of one another. On average, if you live to be 70 years old, you
have a better than even chance of experiencing a 100 year flood.
Catchment
The land area where stormwater runoff caused from rainfall drains to a single location and discharges to another body of
water, such as a lake, estuary or ocean. It always relates to an area above a specific location.
Flash Flooding
Flooding which is sudden and unexpected. It is often caused by sudden local or nearby heavy rainfall. It is often defined
as flooding which peaks within six hours of the causative rain. All catchments with the Shellharbour Local Government
Area may experience this type of flooding.
NSW Flood Prone Land Policy
The primary objective of the NSW Flood Prone Land Policy is to reduce the impact of flooding and flood liability on
individual owners and occupiers of flood prone property, and to reduce private and public losses resulting from floods,
utilizing ecologically positive methods wherever possible.
Floodplain Development Manual
The Floodplain Development Manual supports the NSW Government’s Flood Prone Land Policy in providing for the
development of sustainable strategies for managing human occupation and use of the floodplain considering risk
management principles based upon a hierarchy of avoidance, minimization (using planning controls) and mitigation
works.
Floodplain Risk Management Plan
A management plan developed in accordance with the principles and guidelines of the Floodplain Development Manual
that identifies how particular areas of flood prone land are to be used and managed to achieve defined objectives. It
usually includes both written and diagrammatic information.
Local Overland Flooding
Inundation by local runoff rather than overbank discharge from a stream, river, estuary, lake or dam (NSW Floodplain
Development Manual 2005).
Mainstream Flooding
Inundation of normally dry land occurring when water overflows the natural or artificial banks of a stream, river, estuary,
lake or dam (NSW Floodplain Development Manual 2005).
AHD
Australian Height Datum. This is the standard datum that most flood levels are measured from. Its value is approximately
equivalent to mean sea level.