This contribution describes the state of the art of numerical flood simulation, in particular, the models based upon hydraulic laws. Both theory and practical aspects are discussed. In the introduction, various types of applications of these models are listed with their requirements in terms of robustness, accuracy, speed of operation, and engineering staff time. After introducing the 1D Saint Venant unsteady flow equations and describing the meaning of their terms, the concepts of kinematic and diffusive wave approximations are discussed. These provide better ways of understanding the physical behavior of flood waves and demonstrate the link between hydraulic and hydrologic flood propagation models. Numerical discretization of the unsteady flow equations is introduced with special attention given to the correct and robust modeling of rapidly varying flow, as occurring around hydraulic structures, discontinuities in channel beds, and flood‐generating breaches of river embankments and reservoir dams. Proceeding with the 2D flow description and its numerical discretization principles, the emergence of hybrid 1D/2D models is discussed with their different options of linking model components of flood‐prone areas. The quality of models also relies very much on the availability of detailed and correct data. This contribution briefly introduces the impact of new data collection techniques on numerical flood model schematization.

This paper identifies the philosophy of open architecture as a feasible vision capable of transforming modelling software packages into living products. This vision, invoked within the specific context of software production in the field of flood forecasting within the Environment Agency, promotes the emerging requirements and consensus of users, academics and software producers. In the past, the philosophy of closed architecture dominated the use, development resources and investment in modelling systems by producers and users. As closed architecture encourages the development of monolithic software products with limited scope for innovation by third parties, investments often do not return the value of their full potential. A consensus is emerging that this is no longer tenable. The time is right: for the producers of hydraulic and hydrologic software tools to move from the culture of ‘doing things better’ to ‘doing things better and doing better things’; for users to design their own systems through assembling off-the-shelf software products; and for academics to have a less restrictive environment in which to innovate. The consensus view is rendered viable in a partnering culture undoing many barriers and restructuring many concepts. The paper postulates that software development is a paradigm and shifts through the forming, proliferating, norming and performing stages. This postulate is substantiated by citing evidence for the following associations:The forming stage is associated with the development of early computer programs.The proliferating stage is associated with closed architecture.The norming stage is associated with open architecture to create interoperability.The performing stage is associated with open source to freely share and improve source codes.

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