The “major/minor” approach is being adopted as standard urban drainage design practice in many countries. Further to the normal primary drainage design for minor storms, this approach now requires management of inundation during extreme storms of return period fifty years or more. Surface flows then have to be investigated over an urban topography which is highly irregular and initially dry, and the transition from these conditions to major storm flows is difficult to analyse. The proposed modelling solution is based on the full integral hydraulic equations incorporating possible shock fronts. Wetting and drying simulation, careful channel resolution at low flows, and implicit solution methods with flat response properties are also important. Key points are illustrated by case studies.

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