Hazard map and modelled developments at the scale of a municipality in the Strasbourg Eurométropole and quantification of their theoretical efficiency.
In the framework of the French “PAPI” Directive (Flood Prevention Action Programmes), the Strasbourg Eurométropole has taken on the issue of muddy water flows following the events of 2016, 2017 and 2018 and has launched, in collaboration with BRGM, a risk qualification programme for all the municipalities concerned. As part of this programme, a study of the issues concerned was carried out and a management strategy proposed to reduce the impact of muddy water flows on populations.
The hazard qualification, carried out using the WATERSED sediment transfer module, took into account the runoff water flow lines (calculated on the basis of the DTM – with 1 metre resolution – of the Strasbourg Eurométropole and corrected on the basis of field surveys), the nature of the ground, the most unfavourable type of crop likely to be cultivated on each plot and the related soil parameters, as well as the amount of rainfall. The hazard was thus qualified on the basis of the masses of sediment likely to reach an urban area.
A study of the threatened facilities downstream of the starting areas identified 96 watersheds requiring the installation of structures for managing muddy water. These catchment areas have been prioritised in order to implement measures and allot budgets towards the areas most in need of protection.
Lastly, the installation of soft hydraulic structures (fascines, grassy strips) was simulated on all the watersheds, showing that this would reduce by nearly 80% the quantities of sediment reaching the threatened areas if all the existing structures and those modelled as part of a future management strategy were effectively deployed and in optimal operating condition.
Using the results
On the basis of the results of the study, the Strasbourg Eurométropole started to install the first soft hydraulic structures in spring 2019, before the first rainy episodes likely to result in muddy water spills.
In order to prioritise the actions to be implemented each year, modelling will be carried out each year to identify the catchment areas requiring the installation of protective structures, according to the projected land use and constraints encountered on the ground.
- Strasbourg Eurométropole – Prevention and Environmental Issues Department
- Municipalities of Achenheim, Blaesheim, Breuschwickersheim, Eckwersheim, Hangenbieten, Kolbsheim, Lamperheim, Mittelhausbergen, Mundolsheim, Niederhausbergen, Oberhausbergen, Oberschaeffolsheim, Osthoffen, and Vendenheim
- Alsace Chamber of Agriculture
- GERICHO Group (risk management and history of muddy water flows)
The fight against soil erosion was entrusted to the Strasbourg Eurométropole on 1 January 2018, and this study enables us to engage in discussions with stakeholders based on precise information of direct relevance to each party. We are thus in touch with the realities of individual municipalities and catchment areas and as closely as possible to the scale of individual agricultural plots. Since we wished to combat these phenomena by installing soft hydraulic structures, the simulations carried out by BRGM enable us to take a positive and proactive approach to convince the agricultural community that the efforts made can help to achieve the objectives of protecting people and property while also preserving the value of agricultural land.