Building in High-Risk Flood Areas: The Particularly Complex Case of a Head Office in Nîmes City, France

A major French company was planning to build its new regional head office on a specific site in the city of Nimes, in the South of France. Having experienced one of the most dramatic flooding event in 1988, with over 400 mm of rain in 6 hours causing almost 3 m of water in the streets and the death of 11 people, most of the city of Nimes is now considered a high risk flood area because of its drainage limitation.

Under these conditions, and in order to continue the development of the town, the French authorities have authorized new constructions, but under very strict conditions. In particular, a specific hydraulic study must be carried out to (i) analyze in detail the context and behavior of flooding in the area in its current state, using all existing information (ii) demonstrate that the planned construction will have no hydraulic impact on the surrounding area, i.e.it will not increase the risk of flooding for the neighborhood, (iii) define the security procedure to keep people working in the building safe during the flooding event.

To achieve these objectives, a particularly accurate hydrological and 1D-2D hydraulic model was successfully developed with PCSWMM and used to :(i) model the current state, (ii) determine the impact of the architect’s initial plan (iii) propose specific adjustments to the plan to ensure that construction has no hydraulic impact (iv) define the post construction flooding, enabling the most appropriate security measures to be defined.

The model developed is particularly detailed and advanced because (i) it covers an area of over 100 ha, (ii) the mesh has been adapted to the geographical context, (iii) existing buildings and pipes were taken into account, (iv) it represents all existing complex hydraulic components (v) it has been calibrated on the basis of various sources of information.

Finally, it should also be noted that the model developed was a key element (i) in defining the most appropriate constructions (without the hydraulic model, it would have been impossible to define the solution that would have no impact on the surrounding area) and (ii) in ensuring that all stakeholders accepted the project since they could clearly underststand it.

Thanks to this study, the authorities agreed to the construction of the new regional head office, which is now successfully up and running.

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