Methodology to Combine Topography and Bathymetry Data Sets for Hydrodynamic Simulations: Case of Tagus River

AbstractRecent techniques for acquiring elevation, such as the advanced synthetic aperture radar (SAR) interferometric techniques, enable the creation of very detailed models in a short time. However, because of its incapacity to penetrate the water, the collection of bathymetric information in water-covered areas must be performed with other techniques. Thus, data compatibility is a key factor for two-dimensional (2D) hydrodynamic simulations because a single elevation model is required in general. Such operation is often challenging, because it is not simply obtainable by merging the data sets or replacing the main river channel information with the bathymetry. This study presents a geographic information system (GIS)–based methodology to merge the bathymetry and topography of a river and validates it with 2D flood event simulations. Therefore, the floodplain topography data sets, including emerged fluvial features and profile-shaped bathymetric surveys covering only limited sections of the river channel, were combined to produce a single elevation model. The methodology is based on the extraction of the riverbanks on a digital elevation model (DEM) to locate the thalweg and two ancillary lines, which support an interpolation of cross-section data. The case study is the middle section of the Tagus River in Portugal, for which distinct data sets of bathymetry (namely, cross sections) and a DEM obtained by the advanced interferometric techniques were available. A segment of flooded/unflooded areas obtained from a SAR image of a previous flood event was used as a basis to compare the flooded area resulting from two hydrodynamic simulations. The disagreement in flooded area extension was reduced from 12% difference to 1% using the proposed combined DEM. Water levels, measured at a hydrometric station, were also used for assessment. A significant improvement was also found using the combined DEM: the maximum water level differences decreased from 218.7 to 24.4 cm.