Accuracy of the Digital Terrain Model and Its Impact on the Results of Hydraulic Modelling in Floodplains

The most important input for modelling the water flow in an inundation area is the Digital Terrain Model (DTM). The significance of DTM accuracy increases with activities related to the re-opening of floodplains to rivers, according to the Biodiversity Strategy 2030 issued by the European Committee in 2022. In this study, three Digital Terrain Models were compared: two DTMs (fourth and fifth versions) generated as Czech standards by the State Administration of Land Surveying and Cadastre, and a purpose-built DTM created by the CzechGlobe institute, CAS. A series of hydraulic calculations were carried out combining the three DTMs with the set of discharges corresponding to return periods of 1, 5, 20, and 100 years. The “typical” inundation area on the right bank of the Morava River was chosen to compare the modelling results. DTM inaccuracy affected the hydraulic modelling results primarily when smaller discharges passed the inundation area, mostly due to DTM inaccuracies in local open channels and water-collecting ditches, which are poorly and erroneously depicted when using the less-accurate fourth- and fifth-version DTMs. This study also shows that there was no direct correlation between the locations of DTM inaccuracies and differences in water levels obtained via hydraulic modelling, which dropped with increasing flood discharge. The error in the calculated water level exceeded approximately 0.75 m for Q1 and approximately 0.33 m for Q100. The error depends on the morphology and segmentation of the floodplain, the configuration of the hydraulic model, local changes and human interventions in the area, and the type of DTM, the technology used, and its accuracy and resolution. This study contributes to assessment of the accuracy of hydraulic modelling in flood inundation areas and indicates how DTM accuracy affects hydraulic modelling outcomes.