Key points

  • Raising habitable floor level heights can improve the flood resilience of habitable rooms in a dwelling.
  • Flood-compatible construction materials should be used below 1% annual exceedance probability flood levels.

In March 2021, Pitt Town Bottoms in the Hawkesbury local government area suffered severe flooding following significant rainfall, which resulted in substantial property damage and loss.

Rural residential properties (zoned RU4) are located on the eastern banks of the Hawkesbury River along Pitt Town Bottoms Road.

Under the 1997 Hawkesbury Nepean Flood Strategy, a 1% annual exceedance probability flood level for the area was set at 17.3 m (above the Australian Height Datum). However, many existing dwellings and allotments in areas such as Pitt Town Bottoms are located well below this level and have dwelling entitlements associated with them.

One dwelling in the area had a habitable floor level of 11.6 m. In the March 2021 flood, it experienced over-floor flooding of approximately 1.3 m. A peak flood level of approximately 13 m was recorded at Windsor Bridge (upstream of the site). As shown in Figure 1, all the contents within the dwelling were damaged by flooding. Historical aerial photography indicates there have been buildings at this location since the 1960s – flooding of the dwelling would likely have occurred previously.

  • A yellow house with green windows between a construction site and a shed. There is furniture, storage buckets and garbage bags strewn in front of the house.
    Credit: Rhelm, 2021.
    Figure 1: View of the damage to the old dwelling.
  • A yellow house with green windows between a construction site and a shed. There is furniture, storage buckets and garbage bags strewn in front of the house.
    Credit: Rhelm, 2021.
    Figure 2: The damaged dwelling in comparison to the new raised dwelling when it was under construction.

The council approved a more flood-resilient dwelling and the modifications were approved in early 2021. The key requirements for flood resilience of the new dwelling which were incorporated in the consent conditions include:

  • a habitable floor level set substantially higher than the current dwelling – the March 2021 flood did not reach the habitable floor level (Figure 2)
  • flood-compatible materials for the whole building – it is set entirely below the 1% annual exceedance probability flood level.