A floodway is either:
a) that part of the floodplain specifically designed to carry flood flows and ideally capable of containing the Defined Flood Event.
b) a section of road designed to be overtopped by floodwater during relatively low average recurrence interval (ARI) floods.
The Queensland State Planning Policy defines a floodway as follows, "those areas of the floodplain where significant discharge of water occurs during the DFE. Floodways are often aligned with naturally defined channels, that even if partially blocked would cause a significant redistribution of flood flow, or a significant increase in flood level. What constitutes a floodway may vary from one floodplain to another or part of a floodplain to another. Floodways will normally be identified as part of a floodplain management study or flood study where their importance in the overall behaviour of flood flows can be properly taken into account. Where a study to determine floodways using local criteria has not been undertaken, a floodway (for the purposes of the SPP) shall be an area where, at the DFE, the floodwater has:
- a velocity-depth product of 0.3 square metres per second or greater, OR
- a velocity of 1 meter per second or greater"
Floodways need to be kept clear of debris and blockages. If floodways are blocked, the floodflows will redistribute over or around the blockage, which can result in some nearby areas receiving deeper and swifter floodwaters.