A flood is defined as the covering of normally dry land by water that has escaped or been released from the normal confines of a lake, river, creek or other natural watercourse, a reservoir, canal or dam.
WHAT IS A FLASH FLOOD? - Flash flooding is localised flooding that occurs when heavy rain cannot drain away quicker than it falls. A flash flood is defined by the speed of flooding, not the source or location of flooding. Flash flooding is typically caused by short duration storms over a localised area or catchment. The Bureau of Meteorology describes flash flooding as "Flooding occurring within about six hours of rain, usually the result of intense local rain and characterised by rapid rises in water-levels." reference
A local example of a flash flood is the "supercell" thunderstorm that hit Woden in January 1971 where the Canberra Times reported rainfalls up to 100mm in 1 hour were recorded by private rain gauges in the suburbs of Farrer and Torrens." reference
WHAT IS FLOOD RISK? - Flood risk includes both the probability of a flood occurring and the consequences if a flood occurs. The consequences of a flood are in turn affected by the number of people and properties exposed to floodwater and the vulnerability of these people and properties. For example, a river might burst its banks regularly, but if this flooding occurs in an isolated area where there are no people or infrastructure, then the flood risk is considered to be low. Similarly, a river might flood very rarely, but if many people and properties are located near this river and they live in dwellings that are vulnerable to floodwater damage, then the flood risk will be higher.
HOW PRONE IS CANBERRA TO FLOODS? - Canberra planning has always taken into account the need to avoid development in flood prone areas. Since the 1970s planning for new urban development in the ACT has kept development above the 1% Annual Exceedance Probability (AEP) flood level. The local storm water system is designed to cope with the 1% AEP storm flows through a combination of piped flows and overland flows. However, no areas are completely immune to flooding. Floods greater than the 1% AEP are possible, and extremely intense local rainfall can cause localised flash flooding.
WHAT IS A 1% AEP FLOOD? - The 1% AEP flood is a theoretical flood that is estimated to have has a 1% chance of being equalled or exceeded in any year. For example, if you experienced a 1% AEP flood last year, the chance of experiencing a similar magnitude flood this year is still 1%, regardless of when the previous 1% AEP flood was experienced. The 1% probability is calculated using computer modelling, historic rainfall and runoff records and a range of other assumptions. The value of the 1% AEP is an estimate that will change as the climate changes and as more historic rainfall and flooding information is gathered over time that might change assumptions used in the modelling and estimations.
WHAT DOES ACT FLOOD DATA SHOW? - The flood data map shows an estimate of the areas likely to be flooded during a 1% AEP flood - also previously known as the 100 year flood line. The ACT flood map shows flooding extents for riverine flooding only i.e. flooding from named watercourses such as rivers and creeks.
WHEN IS ACT FLOOD DATA BEING RELEASED? - The ACT flood data show the 1% AEP flood for the Molonglo River from Yass Road downstream to the Lake Burley Griffin surrounds and further downstream to below Coppins Crossing. There is a program to update flood studies over the next three years for creeks and some major stormwater channels within and adjacent to urban areas. Once these studies are completed, the 1% AEP flood extents will be made available on the ACT Government's ACTMAPi website.
The ACT Government is providing this flood data for information purposes only. This data is derived from the best available modelling of the catchments and watercourses. The ACT Government cannot and does not guarante