Concentrated erosion on a plot of wheat (Winter 2021, Planques, Pas-de-Calais)
© BRGM
The need
Developed by BRGM, the WaterSed model simulates runoff and soil erosion on the scale of a plot or a catchment area for a given rainfall event or sequence of rainfall events. It can be used to test scenarios based on the use of vegetative barriers (fascines, hedges, grassy strips or ponds) or scenarios involving changes in land use and/or farming practices, as well as to study the consequences of future climate change scenarios.
In the Canche and Authie catchment area, the management of run-off and soil erosion is the responsibility of the Syndicat mixte Canche et Authie (Symcéa), which works with the EPCIs (inter-municipal public entities). To provide an operational response to this major challenge, Symcéa decided to adopt the WaterSed model, developed by BRGM, in order to:
- improve its knowledge of the hydraulic and sedimentary operation of catchment areas,
- be completely autonomous during the "study" phase of solutions to combat run-off and soil erosion,
- meet the performance requirements set out by the Artois Picardie Water Agency.
The results
The transfer of the WaterSed model to an EPAGE (water development and management body) such as Symcéa was a major first for BRGM. The success of this transfer was made possible by the strong commitment of Symcéa's staff and the creation of a range of teaching aids by BRGM. This collaboration was also an opportunity to develop a set of GIS tools peripheral to the model, enabling it to be implemented more quickly. The transfer was organised around the Lacres-Rollez basin, used as a “test basin” to learn lessons at every stage, from the technical diagnosis of hydraulic and sedimentary operation to the implementation of solutions to combat run-off and soil erosion.
Studies analysed different scenarios to explore the hydraulic and sedimentary efficiency of a wide range of control solutions: vegetative barriers, structural hydraulics, redrawing of plot boundaries and changes to farming practices. Results showed that programmes to manage run-off and soil erosion must implement a full range of techniques in order to drastically reduce these phenomena in the areas at risk. These techniques need to be considered as part of a coherent hydrographic unit and to be positioned along the watercourse as part of a complementary and interlinked approach if they are to have a significant impact on the different spatial scales of the catchment.
Implementing soil conservation techniques at plot level is undoubtedly the most powerful lever for significantly reducing mudflows. However, this will require coordination, along with the technical and financial tools to support local farmers. For BRGM, the emphasis in the future will be on studying these new agricultural practices and integrating them into the WaterSed model.
Sedimentation upstream of a fascine (Winter 2021, Preures, Pas-de-Calais)
© BRGM
Using the results
The WaterSed model was pre-configured by BRGM for the whole of the Canche and Authie catchment area by collecting and formatting data, setting the parameters of the model and calibrating it for application by Symcéa across its area. For Symcéa, the WaterSed model and its peripheral tools now form part of a new methodological framework, whose reach is not only technical, but also political, human and social in that it seeks to involve all stakeholders and use all levers to reduce mudflows in this region.
The partners
- Syndicat mixte Canche et Authie (Symcéa)
- Artois Picardie Water Agency
- Pas-de-Calais Departmental Council
