# Multitemporal SAR Data and 2D Hydrodynamic Model Flood Scenario Dynamics Assessment
## 3 Method
### Water Depth Estimation Combining SAR Images and a Digital Elevation Model  
Although exploitation of river inundation satellite images, especially for operational applications, is mostly restricted to flood extent mapping, there is significant potential for improvement in a three-dimensional characterization of floods (i.e., flood-depth maps) using SAR satellite images. When considering that estimation of water levels plays an essential role in quantifying potential flood event damage so control action can be taken during the emergency phase ([37]), the flood extent limits extracted by the SAR image here were combined with a Digital Elevation Model (DEM) allowing extraction of the terrain elevations inside all relevant limits and, thus, the estimation of elevation difference intervals across the surrounding topography (within the flood limits). The Basilicata Basin Interregional Authority provided the authors with a plano-altimetric survey of the study area using laser-scanning technology and digital aerial photogrammetry. The airborne laser scanner system is an active remote-sensing sensor technique, that couples a laser attached to an aircraft with precise kinematic positioning provided by a differential global positioning system (DGPS) to produce accurate elevation measurements [38]. Data acquired from the Bradano River mouth area was formulated by a Topeye MKII system, which works in full waveform in order to provide a better description of all ground elements. Indeed, it is able to record multiple echoes besides the first and the last pulse ([39; 40]). In area characterized by more complex topography, a flight was made at 600 m with an orthophotos resolution of 5 cm and a laser density of 40 pt/m\({}^{2}\). The resulting laser scanner points cloud had a vertical accuracy of about 0.15 m and horizontal accuracy of 0.30 m. When considering that the increase of DEM resolution corresponds to exponential computational time growth in hydraulic model simulation performance, it was considered appropriate to guarantee high altimetric detail of the hydraulic elements that have most influence on water flow (such as river reach dimension, cross-sections, banks, and levees, etc. ), through manual manipulation rather than using an excessively detailed DEM grid resolution size [41]. Therefore, a square grid of 10 m resolution was used for input of the 2D hydraulic model, while the geometries of the most important hydraulic elements were entered in the grid to maintain good characterization of study area topography. Indeed, resolution should be chosen in relation to model structure and complexity, which always have limitations [42]. For instance, the FLORA-2D model cannot include inertial terms in governing equations, and therefore has low sensitivity to small-scale DEM features, meaning that close field flow processes are smoothed out, even when high-resolution grids are used [43]. Finally, it is important to note that finer grid resolution would require a more complex parameterization of the 2D hydrodynamic model, and, as consequence, it could increase the uncertainty of head losses and roughness [44]. Within this framework, the method for incorporating available topographic data into the model structure is the key to achieving the best compromise between detail (or precisions), maximum expected accuracy (which depends more on available dataset than on grid resolution), and computational efficiency [42]. In our opinion, the use of subgrid treatments, when combined with appropriate upscaling of resistance parameters, would, in many instances, be a suitable approach to making better use of available topographic details, while being consistent with the uncertainty related to model structure and other input data available for model-building and evaluation. Previous studies (e.g., [10]) have derived water levels by computing the differences in elevation between each grid cell and the elevations of the flowpath-connected downslope grid cell where the flow enters the channel.