SafetyAspects of the Design of Roadways as Floodways

ReportNo WSAA 69

November 1993




Urbandrainage systems are often designed under the so called “major/minor” system.According to this concept, the underground drainage system is designed to handlea relatively common flood, typically of five year return period, while theoverflow resulting from less frequent flood events is carried by surfacedrainage systems. The surface drainage system may include specially designatedfloodways and urban streets.


Thestudy presented in this report has focussed in particular on the development ofsafety criteria for children and for vehicles on flooded streets. It hasincluded a review of overseas and local practice, stability calculations forboth cars and children and the writing of a computer program to assess thesafety of existing roads and of proposed designs.


Theappropriate criterion is determined to be the value of the product of flowvelocity and depth. However, the study found that this product at the point ofinstability is not constant, but varies with depth, and that this variationmust be taken into account for design purposes. Furthermore, the studyindicates that the value of the product at the threshold of stability issignificantly lower than that previously published, especially for the case ofyoung children.


Thereport prescribes a basis for assessing the safety of roadways as floodways andrecommends that, if the implicit safety criteria cannot be met, considerationbe given to clear sign-posting to warn of potential dangers.


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