![]() The only way that the accuracy of an estimated flood prediction of this nature can be judged is over a very long time period. However, the flood may not occur within a 200 year period and yet conversely could occur twice over the next 10 years. The 100-year flood is also referred to as the 1% flood, since its annual exceedance probability is 1%. I know that my property has never flooded - why is it shown within the flood plain?Ī one-hundred-year flood is a flood event that has a 1% probability of occurring, or being exceeded in any given year. Therefore the inundation areas illustrated within the Surface Water Maps should not be viewed as exact or precise. As small changes to the height of ground surfaces can markedly change the direction of surface water flows, it is difficult to predict with certainty the exact route along which surface water will flow and pond. The Surface Water Flood Maps illustrate the low lying areas and hollows that are estimated to be prone to flooding from rainfall events with specific return periods. Surface water flooding occurs when the ground is unable to absorb the rainwater, causing it to flow over the surface and fill depressions and low spots in the landscape where local natural and engineered drainage systems are overwhelmed. Surface water does not have a ‘natural’ floodplain as such, particularly in the urban environment where the flow of the surface water is tightly constrained by the built environment. For example, an area that is located at the extreme outer edge of the 10% AEP river floodplain is estimated to have a 10% chance of flooding in any year but other areas that are situated closer to the river may have a greater than 10% chance of flooding in any year. It is important to note that even within an illustrated floodplain some areas are more likely to flood than others. Similarly the area highlighted within the 10% AEP floodplain has a 10% or greater chance of flooding in any year. For example the area highlighted within the 1% AEP floodplain outline for rivers has a 1% or greater chance of flooding in any year. Each of the floodplains highlighted in the maps are associated with a flood event which has a particular chance of occurring. The extents of the floodplains have been estimated using predictive computer modelling techniques. What do the outlines for floodplains and inundations areas show?įloodplains are the relatively flat areas of land adjacent to rivers and sea that are estimated to be subject to periodic coverage by flood water. ![]()
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