Safety Aspects of the Design of Roadways as Floodways
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report no. UWRAA 69
Urban drainage systems are often designed under the so called “major/minor” system. According to this concept, the underground drainage system is designed to handle a relatively common flood, typically of five year return period, while the overflow resulting from less frequent flood events is carried by surface drainage systems. The surface drainage system may include specially designated flood ways and urban streets.
The study presented in this report has focussed in particular on the development of safety criteria for children and for vehicles on flooded streets. It has included a review of overseas and local practice, stability calculations for both cars and children and the writing of a computer program to assess the safety of existing roads and of proposed designs.
The appropriate criterion is determined to be the value of the product of flow velocity and depth. However, the study found that this product at the point of instability is not constant, but varies with depth, and that this variation must be taken into account for design purposes. Furthermore, the study indicates that the value of the product at the threshold of stability is significantly lower than that previously published, especially for the case of young children.
The report prescribes a basis for assessing the safety of roadways as flood ways and recommends that, if the implicit safety criteria cannot be met, consideration be given to clear sign-posting to warn of potential dangers.