Federation Villages Floodplain Risk Management Study and Plan
FPLs for typical residential development would generally be based on the 1% AEP event plus an appropriate freeboard. Justification for the use of the 1% AEP event, and discussion on the determination of appropriate freeboard is provided below.
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As a guide, Table K1 from the NSW Floodplain Development Manual (Reference 5) has been reproduced (Table 24), and indicates the likelihood of the occurrence of an event in an average lifetime. The data indicates that there is a 50% chance of a 1 in 100 Annual Exceedance Probability (1% AEP) event occurring at least once in a 70 year period. Given this potential, it is reasonable from a risk management perspective to consider adopting of the 1% AEP flood event as the basis for the FPL. Given the social issues associated with a flood event, and the nontangible effects such as stress and trauma, it is appropriate to limit the exposure of people to floods.
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Note that there still remains a 30% chance of exposure to at least one flood of a 1 in 200 (0.5%) AEP magnitude over a 70 year period. This gives rise to the consideration of the adoption of a rarer flood event (such as the PMF) as the flood planning level for some types of more vulnerable development. The Flood Prone Land Package acknowledges this risk and prompts Councils to consider the suitability of critical or vulnerable facilities within the floodplain (even if outside the Flood Planning Area) (see Section 6.4.2). Additionally, it is appropriate to consider a more frequent risk of exposure such as the 5% AEP for recreational and non urban uses, such as recreation facilities. Table 24: Likelihood of given design events occurring in a period of 70 years Probability of Experiencing At Least One Event in 70 Years (%) 99.9
Probability of Experiencing At Least Two Events in 70 Years (%) 99.3
1 in 20 (5%)
97.0
86.4
75.3
40.8
1 in 100 (1%)
50.3
15.6
1 in 200 (0.5%)
29.5
4.9
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1 in 50 (2%)
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Size of Flood (Chance of Occurrence in Any Year) ARI/(AEP) 1 in 10 (10%)
6.4.1.2.
Freeboard Selection
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As noted above, the Flood Planning Level is typically derived from a design flood event (usually the 1% AEP) plus a freeboard allowance. The freeboard can be considered as a compulsory ‘safety factor’ used to provide reasonable certainty that the reduced flood risk exposure provided by selection of a particular flood as the basis of an FPL, is actually provided given the following factors: Uncertainty in estimating flood levels The determination of design flood levels comprises a number of factors and parameters, each containing a degree of uncertainty. These factors may include: • • •
How well the theoretical ARI-Discharge curve fits known flood events, and if it has changed since an historic event; Availability of detailed survey and other topographic data; Reliability of historical flood data; and
118048: R211211_FederationVillagesFRMSP.docx: 11 December 2021
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