In the project RisUrSim (Risk management in Urban areas - Simulation and optimization), supported by BMBF (Germany) and the EUREKA project (European Union), a simulation and planning tool has been developed by an international, interdisciplinary consortium of mathematiciens, computer scientists, urban water management scientists, local communities and insurance companies from Germany, Italy and Norway. The unique features of this system are the bi-directional coupling of the flows of the surface flow to the sewage system the high spatial resolution, which allows for the assignment of different hydrological properties to areas belonging to each conduit (in this way the hydrodynamically active parts of the surface are very accurately modelled) Numerical results can be merged with available infrastructure data of the area of interest such that conlusions concerning potential damage can be directly drawn from simulations. The   final report is available here.

Considerable damage is caused every year in urban areas by heavy rainfall owing to overloaded sewage systems. Nevertheless, it is not feasible to avoid flooding completely because it is not economical to dimension constructions according to such extreme events, and a compromise between flood protection and longterm costs has to be found.

The legislator defines the standards for adequate infrastructure flood safety in a comprehensive body of regulations. Compliance with these regulations calls for simulations of water flow on the surface and in the sewage system for different scenarios during the design process of drainage systems. In the framework of the BMBF/Eureka project RisUrSim, ITWM in collaboration with the Technical University of Kaiserslautern has developed a computer model that allows to provide a proof of conformity with EN 752 - in contrast with conventional methods, which can only simulate the sewage system alone.

The crucial innovtion consists in the hydrodynamical computation of the surface drainage based on the shallow water equations and the subsequent bi-directional coupling with existing solvers for the sewage system.

In a preliminary step the necessary data have to be processed. Data processing consists in partitioning the computational domain in subdomains where the surface flow has to modeled hydrologically, and subdomains where drainage is computed hydrologically in the conventional way. The second group encompasses surfaces with homogeneous properties, or surfaces without detailed topographical information feeding water to adjacent areas without back coupling; in general, house premises and buildings are concerned here. A sensible description of the surface for hydrodynamical modelling in urban areas has to include height information in the form of point measurements of suffcient density such that a subsequent cross-linking yields polygons which realistically represent houses, walls, streets, pavements, etc. The processing of these data is handled by geographical information systems (GIS).