Authors' Accepted Manuscript Title: Importance of River Bank and Floodplain Slopes on the Accuracy of Flood Inundation Mapping

Effective flood assessment and management depend on accurate models of flood events, which in turn are strongly affected by the quality of digital elevation models (DEMs). In this study, HEC-RAS was used to route one specific water discharge through the main channel of the Eskilstuna River, Sweden. DEMs with various resolutions and accuracies were used to model the inundation. The results showed a strong positive relationship between the quality of the DEM and the extent of the inundation. However, even DEMs with the highest resolution produced inaccuracies. In another case study, the Testebo River, the model settings could be calibrated, thanks to a surveyed old inundation event. However, even with the calibration efforts, the resulting inundation extents showed varying degrees of deviation from the surveyed flood boundaries. Therefore, it becomes clear that not only does the resolution of the DEM impact the quality of the results; also, the floodplain slope perpendicular to the river flow will impact the modelling accuracy. Flatter areas exhibited the greatest predictive uncertainties regardless of the DEM's resolution. For perfectly flat areas, uncertainty becomes infinite. while using high resolution data, with or without the presence of reference data. With the main focus on the quality of the DEM, this paper will compare the results from two previous studies conducted over Eskilstuna and Testebo Rivers, Sweden, to show ambiguities in the flood maps produced.