The pipe drains that are laid along many of the municipality’s natural drainage paths are designed to handle the more frequently occurring rainfall up to and including 20% AEP (also referred to as 1 in 5 year ARI events).

Our design standards are based on these more common events rather than less frequent storms such as 1% AEP events because:

  • gutters and downpipes on residential and commercial buildings are only built to cope with more frequent rainfall, so they overflow in the event of heavier rainfall, meaning the excess water will bypass our pipe drains and become overland flow.
  • to cope with 1% AEP events, kerb and channel drains would need to be replaced with swales (a low area of land, either natural or man-made) and drainage pits approximately 2-4 times larger than their existing sizes. 
  • installing larger stormwater drainage pits and swale drains would severely impact vehicle access to properties and significantly reduce available street parking.
  • for public safety, drainage pits would need to be protected by grates, which would be susceptible to blockage by debris during extreme weather events, reducing their effectiveness.
  • installing larger drainage systems cannot totally remove the risk of flooding in the areas identified by this study.
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