Concrete Floodway Design Apart from bridges and culverts, a flood crossing provides for flow over a road at a specific location
under conditions determined by the designer. Flood crossings can be broadly divided into two types:
• Causeway – a roadway across a watercourse or across tidal water, specifically designed to
resist submergence.
• Floodway – a roadway across a shallow depression subject to flooding, specifically designed to
overtop and constructed to resist the damaging effects of overtopping.
From these definitions, a floodway is a special case of causeway.
Floodways are provided, generally, under the following circumstances:
• Where flow across the road is infrequent or of short duration.
• Where traffic volumes and serviceability requirements are low, and the cost of a bridge or major
culvert structure is not justified.
• In conjunction with a bridge or culvert, where the bridge or culvert is designed to pass a lesser
flood than the total waterway design flood, and the floodway provides relief during large flows.
The bridge or culvert may be designed not to be overtopped, or to be submerged as part of the
floodway (with the associated structural implications).
