Flood events were selected by analysing the average daily discharge recorded at Nice station
TESTED PARAMETER: DRYING DEPTH (DD) VARIATION INTERVAL: FROM 0.05 TO 0.2
4. Results and discussions
The process to build a 2D free surface flow model with Mike 21FM software tool, requires an accurate methodology with several steps. Firstly, the uncertainties sources of the input data were identified and quantified. To do this, topographic and hydrological (discharges in upstream of the Lower Var valley) data were analysed before building the hydraulic model. Some inaccuracies were highlighted:
- bathymetry and areas around bridges which cross the river are interpolated, - crest levels of dikes are lightly higher than the reality,
- discharges used as upstream boundary conditions present an uncertainties of 15% because of water level measurement and conversion into discharge.
Therefore, the 2D hydraulic model is totally dependent on input data and it induces that the model accuracy will firstly increase with the quality of input data. The most uncertain input data are topographic data related to DEM. Moreover, shaping meshes and model geometry also influences results.
To improve the knowledge of uncertainties about input data, the Global Sensitivity Analysis should be applied. This approach would allow to rank the sources of uncertainties when the 2D hydraulic model is built. [Abily and al., 2016] have already applied the GSA in the Lower Var valley especially for urbanized areas. In this research, the GSA method was not applied. Hence, extending this method on the whole Lower Var valley will improve knowledge about input data uncertainties for 2D hydraulic modelling.
Statistical approaches to evaluate the model have shown that the 2D free surface flow model is efficient for different seasons. The hydrodynamic process of the Var River depends on the climate variations, and three main periods have been identified: drought, spring, and winter flood periods. Regarding the results of each season, the model is able to simulate the water level close to the reality with some uncertainties, especially in the case of base flow during spring flood events. Firstly, the drought periods are well represented with only a difference of 5% between observed and simulated data. In addition, the water level has been underestimated, which is in the direction with the extensive dry weather. At the weir locations, the hydro-power plants functioning is not taken into account in the model. Secondly, the water level trend of a flood event is correctly modelled as well as the intensity of the peak value. From Carros to Nice, the wave is propagated in 1 hour 30 minutes in reality against 1 hour 10 minutes in the model. Comparing simulated and observed water levels, the wave is delayed 30 minutes at Carros station, and 10 minutes at Nice station. The delay noticed at Carros station is due to the upstream boundary condition that is extrapolated from Carros station. At Nice; the arrival of simulated wave is reasonable.
The wave propagation is correctly simulated. Nevertheless, simulation of initial water level related to the base flow in spring flood events appears as a difficulty for the model. Indeed, the simulated water level before the rising stage and after the recession process presents 18% of difference with the observed
out the parameter, which will improve the base water level for a spring flood event. At this stage, the only thing to do is to generate an initial condition with a base flow of 100 m³/s before simulating a spring flood event. Finally, the flash flood events in the Lower Var valley are correctly simulated by the model.
Buildings were not included in the model, especially in the Var River close to the weirs. This could influence underestimation of water level by the 2D hydraulic modelling.
The use of Mike 21FM has shown some difficulties to build the model. Basically, this software tool was developed for coastal issues and not for river management issues. This complicates the definition of boundary conditions and the flooding/drying cell process. Indeed, to avoid numerical instabilities a huge basin was created in downstream part of the model domain, and several flooding/drying thresholds were tested. Furthermore, the mesh generation reveals some designing constraints: boundary conditions, definition of different mesh types, file extension to interpolate the mesh.
The final 2D free surface flow model presents limitations and can be improved in the future. Firstly, to validate the model, selection of events was an essential step. Then, the accuracy of the discharge appears as a limitation. Indeed, the main hypothesis is the application of the flow rate recorded at Carros station 7 km in upstream. To deal with this issue, the discharge applied on the upstream boundary condition will be extracted from the hydrological model developed with Mike SHE software. Secondly, due to the initial objective, this model was not developed for urban flood management, and then the geometry was not designed with the idea to represent an overflow in urban areas. Then, one of the limitations of the 2D hydraulic model is the water propagation after overflowing river. Indeed, the mesh resolution is not refined in streets and the buildings are not implemented. To address the flood risk management in Nice, after the overflowing river, the mesh has to be designed with buildings and higher mesh resolution at some places. However, the model was improved (see Chapter IV) to correctly model the dikes overflow:
piles of bridges were added and the mesh was refined close to the dikes. Thirdly, in this configuration, the model does not consider the exchanges with the aquifer, thus the coupling aspect between 2D free surface flow model and groundwater flow model has been developed in this way. Finally, the mesh setting causes some troubles with the sediment transport simulation, especially at weir locations. In this configuration, the weirs are not included as structures, and then are removed during a flood event by using sediment transport simulation. The software tool used is not able to fix a layer, as weir, in the sediment transport module. Hence, the sediment transport modelling with this model is actually limited and has to be improved by fixing a layer at weir locations. This methodology can be developed with other software tools (Telemac 2D or Iber).
As a conclusion, the user of this 2D free surface flow model has to keep in mind limitations which were identified along the process to build the model: the input data uncertainties, the modeller hypothesis, and the limits related to the calibration/validation methodology. Some suggestions were given to improve the model in respect with the existing data. Actually, this model is able to reproduce the
hydrodynamics behaviour of the Lower Var River for different seasons and can be applied to different scenarios (see Chapter IV).