Calibration of coastal hydrogeological models for the analysis of groundwater-induced flooding: from instrumentation to model reliability

We determine the sensitivity of groundwater-induced flooding forecast to different modelling strategies (evolving model structure and quantity/nature of data). We test three calibration strategies on shallow coastal aquifers:

• we use the river network as a proxy of aquifer seepage and determine uniform hydraulic conductivity and porosity. Despite limiting assumptions of surface/groundwater connections, this strategy could be broadly deployed with rapid advances in temporal and spatial river mapping at regional to national scales.
• we calibrate uniform conductivity and porosity on hourly piezometric data time-series close to the seashore, which are controlled by both tide and inland aquifer recharge fluctuations. We investigate the limitations of the uniform assumptions and demonstrate the interests of coastal and inland forcing as complementary sources of information.
• we introduce spatially variable conductivities and porosities and use additional inland piezometers. Hydraulic parameters are mapped to the main geological structures of the studied area.

We analyse the results of these 3 competing strategies on the hydraulic parameters and, in a more prospective way, on the spatial distribution of vulnerabilities to groundwater fluctuations. We perform our analysis within the “Rivages Normands 2100” research project, in several sites of Western Normandy (France). In this area, low-lying coastal areas and neighbouring lands are prone to groundwater table increase, leading to flooding of buried networks and building foundations. The site of Saint-Germain-sur-Ay (66 km² coastal watershed) is equipped with 6 piezometers from which we collected 1.5 year-long hourly time series of groundwater level. It includes a low-elevation coastal area made up of sands and a continental area made up of schists.  The modelling approaches allowed us to simulate groundwater levels up to 2100, and to analyse the associated evolution of vulnerability. Simulations are performed using Modflow on a daily timestep, with a 75 m spatial resolution.

Results obtained from the 3 models consistently show that vulnerable areas are mostly clustered close to the shoreline. We also show that, in the studied watershed, the first calibration strategy using the river network data leads to long-term simulations close to the second calibration strategy using the piezometric data of the coastal area. Integration of extra piezometric data from the continental area in the third strategy provides more reliable simulations. This analysis is progressively deployed to other sites and extended to cost-benefit analyses including the costs of site instrumentation and the benefits of forecast reliability. Integration of flood zones aerial photography during extreme events is being implemented to the first modelling strategy, as well as wetland mapping.