This may seem a somewhat irrelevant subject for those of us living in the hills around the ACT, where there’s a far greater risk of bushfire than flood, but it’s been a relatively quiet month on the Council front and little else worthy of discussion has surfaced, if you’ll pardon the pun. Flood planning also involves a raft (there we go again) of acronyms that might benefit from some explanation.
Two items that have come before Council in the past month or so are the Floodplain Risk Management Plans for Braidwood and Queanbeyan. While the Bungendore Floodplain Risk Management Plan was presented to the Palerang Council back in 2015, the related subject of inundation due to overland flows has also been very much on the agenda in Bungendore of late.
The current approach to flood planning has its genesis in the 1955 floods in the Hunter Valley. Management has progressed since then from the implementation of targeted mitigation measures to the present, more uniform [across the State] strategic exercise. Typically, flood planning now focuses on potential impacts on residential areas, although a range of social, economic, cultural and environmental factors come into play in balancing the relative risks and benefits of allowing ongoing development on a floodplain.
Flood planning controls are currently applied through a Development Control Plan (DCP) in accordance with the Flood Planning Map within a Local Environmental Plan (LEP). The Flood Planning Areas (FPAs) identified on this map are those, invariably adjacent to a watercourse, that lie below the relevant Flood Planning Level (FPL).
Prior to the introduction of the NSW Flood Prone Land Policy in 1984, the adopted FPL was simply derived from the highest recorded flood in the relevant location. These days, sophisticated flood modelling software is used to predict the impact of flood events and FPLs are based on the calculation of Annual Exceedance Probabilities (AEPs). An AEP, expressed as a percentage, is a measure of the probability of a particular flood event occurring, or being exceeded, in any one year. A 1% AEP event is thus one that has a 1-in-100 chance of occurring in any one year, although it should be noted that the fact that such an event might occur, on average, once every 100 years does not mean that two or more successive events cannot occur within a much shorter period. Nonetheless, a 1% AEP event is more serious than a 2% AEP (1-in-50) event, which is more serious than a 5% AEP (1-in-20) event, and so on.
Interestingly, the ACT was one of the first jurisdictions in Australia to adopt the 1% AEP for determination of the FPL, following the Woden Valley flood in 1971, which had an AEP of around 1% and where seven lives were lost. The subsequent adoption of the 1% AEP nationally followed a series of major floods, with 1% – 2% AEP, in the ‘70s. For anyone who can remember them, the 1974 and 1976 events in Queanbeyan reached levels slightly lower (less serious) than a 2% AEP event, while the 2010 floods in Captains Flat and Queanbeyan reached a slightly higher level (more serious) than a 5% AEP event.
Floods in Wollongong in 1984, Nyngan in 1990, Coffs Harbour in 1996, and Wollongong again in 1998 all exceeded the 1% AEP.
While the FPL is now broadly based on the 1% AEP, it usually includes some additional freeboard, to provide allowance for events that might exceed the 1% AEP. The appropriate amount of freeboard is usually determined by considering, amongst other things, the level that might be reached by the Probable Maximum Flood (PMF), which is derived by modelling the impact of the Probable Maximum Precipitation (PMP) in the area in question. This freeboard is typically around 0.5 m, but can be more, with the result that the FPL, which is effectively the minimum floor level specified for dwellings constructed in the FPA, is typically 0.5 m above the 1% AEP.
So, to summarise, we have DCP controls that apply to FPAs identified in an LEP and determined by FPLs that are based on the 1% AEP together with the PMF that’s derived from the PMP. Got it?