# Evaluation of Different Simulation Methods for Analyzing Flood Scenarios in the Danube Delta
## 3 Materials and Methods
The content of the digital model of the land supports the design of the flood protection lines for the settlements, and at the same time creating a useful tool for analyzing the dynamics of hydro-geo-morphological phenomena, which are very dynamic in the study area. The correlation between these phenomena and land use can only be made on such a model. The hydro-geo-morphological units were mapped using a LIDAR based DTM [41]. This DTM was obtained from LIDAR measurements performed within the project \"development of a high resolution digital cartographic support necessary for the implementation of plans, strategies and management schemes in the Danube Delta Biosphere Reserve\"--CARTODD [42]. The high resolution digital cartographic framework developed in this project comprises several components: the DTM, the digital elevation model (DEM), elevation classes (EC), and ortho photomaps (OFM). These components represent the foundation layer for approaching the two methods in this study. They were developed by processing primary cartographic data, obtained by modern methods (data collection by LIDAR method), and the high accuracy of ortho photomaps (2.5 MP) allowed detailed visualization of the areas of interest. The development of the numerical field model (NFM) with the LIDAR method was based on the detection of the echo/retro diffusion of a laser pulse, and the measurement of the round-trip flight time of the aircraft-scanned object (e.g., a land surface). The flight parameters were chosen to achieve the required precision objectives, in altitude on the Z axis of 5 cm, and the necessary and sufficient density of points (i.e., average 4-5 pts/m\({}^{2}\)), to correctly detect the dams, taking into account the random distribution of the point cloud. The whole process of hydraulic modeling and GIS processing in flood scenarios analysis depends on the accuracy of the digital terrain model. The interpolation method for minor riverbeds used in this study was inverse distance weighting (IDW). This method is a deterministic spatial interpolation model, based on an assumption that the value at an unsampled point can be approximated by a weighted average of observed values within a circular search neighbourhood [43; 44; 45]. The DTM resolution for the minor riverbed was computed with a similar resolution as the LIDAR DTM (0.5 m \(\times\) 0.5 m), but it was resampled up to 1 m, which allowed an easier input of data in the HEC-RAS and Global Mapper software. The main channel vertical accuracy was 0.10 m.  
In order to obtain a complex digital terrain model, the area comprising the minor riverbed of the digital model was cut and replaced with a new model of the minor riverbed updated by top-bathymetric measurements. For the new digital model of the minor riverbed, top-bathymetric measurements were performed in the study area (along the entire length of the Sulina branch, from Coastal Sfantu Gheorghe to the mouth at the Black Sea) and downstream of the Sulina branch (along the entire length of the Tulcea branch, from Coastal Chilia to Coastal Sfantu Gheorghe), resulting in 2 measured sections. These data were collected with the help of state-of-the-art equipment provided by the Danube Delta National Institute for Research and Development. The equipment used to perform the topo-bathymetric measurements was a SonTek ADCP RiverSurveyor M9 (SONTEK Company, San Diego, CA, USA) which used \(3\times 3\) transducers, each with a different orientation. The topo-bathymetric measurements were performed between the 16 and 27 of March 2020, in conditions of low water levels of the Danube river, recorded at the Tulcea-port hydrometric station, namely 0.52-0.67 m. The two calibration sections in which the hydrological measurements were performed are presented in Figure 1c (S1-S2 first river part and S2-S3 second river part).