Example of hazard mapping in the Ajaccio area (mouths of the Gravone and Prunelli rivers)
© BRGM
The need
The Gulf of Ajaccio is characterised by an urbanised rocky coastline interspersed with small beaches, river mouths and wetlands behind the dunes. Historical data relating to storms and recent events show that the Gulf is exposed to different types of coastal flooding: overtopping and overtopping with successive breaking waves.
To support the South Corsican Departmental Directorate for Inland and Marine Areas (DDTM), BRGM used three models in sequence:
- Wavewatch III (WW3), which propagates waves along the entire coastal domain and generates time series of wave height, period and direction with which to force other models,
- SWASH in wave-to-wave profile mode to calculate overtopping flows,
- UHAINA, which calculates coastal flooding from the output of the two previous models.
The results
The coastal flooding hazard was mapped on a scale of 1:5,000 over some fifty kilometres, based on calculation and cross-referencing of wave heights and flow velocities. This innovative approach to characterising the dynamics of these phenomena made it possible to fine-tune the mapping of low-lying areas, previously conducted for the whole island using a 'static' approach. By calculating the flows and injecting the data into UHAINA, we were able to reproduce a historical event (Storm Adrian, 29/10/2018) and to observe that almost the entire perimeter of the gulf is exposed to overtopping with successive breaking waves for a 100-year event.
These modelling studies made it possible to identify more vulnerable sectors (over a larger area with a higher level of risk) in the vicinity of Ajaccio international airport, behind a wide dune belt and close to the mouths of the Gravone and Prunelli rivers, in particular by taking into account overtopping and river flows, as well as the methods used to represent land use.
The atlas of coastal flooding zones previously produced by BRGM provided us with information not only on wave height but also on the areas subject to the mechanical impact of waves and overtopping with successive breaking waves. Based on these results, we carried out dynamic modelling to obtain data on the maximum extent of wave overtopping, as well as the associated heights and speeds. This tool will enable better urban planning and it will also be useful for crisis management.
Numerical modelling strategy based on the use of WW3, UHAINA and SWASH
© BRGM
Using the results
Produced in accordance with ministerial instructions (guide issued in May 2014), these results are intended to be used by the DDTM as part of a Coastal Risk Prevention Plan. A consultation was organised with BRLi, the company studying the risk of river flooding, before selecting the river flows and sea levels in the area of the river mouth. Further, the knowledge developed through this study on coastal flooding phenomena and the associated risk of mechanical impacts can be fed into crisis management tools as well as supporting the work of the government’s Regional Flood Adviser.
The partners
- Southern Corsica Departmental Directorate for Inland and Marine Areas (DDT2A)
