Combined hydrologic and hydrodynamic simulation of extreme flood events Ianos and Daniel in the Kalentzis river basin, Western Thessaly (Greece)

This study focuses on the event-based simulation of two recent high-impact flood events, namely Ianos (2020) and Daniel (2023), in the Kalentzis river basin, located in Western Thessaly (Greece). The basin covers 528.8 km², while its lower part, extending about half of the total drainage area, features a substantially complex environment, with heavily modified drainage networks, artificial levees, diversions, and interconnected channels discharging through the vicinity of the urban area of Karditsa. The study area is representative of lowland agri-urban floodplains, where the flow dynamics are mainly determined by infrastructure and land use constraints, amplifying flood vulnerability. The modelling approach integrates hydrologic and hydraulic components to reconstruct the spatiotemporal evolution of both events, under realistic boundary conditions and available field evidence. The hydrologic modelling was implemented using HEC-HMS. Input rainfall data were derived from ground stations of different resolution, processed at 15-minute intervals. Curve number and initial abstraction parameters were adapted to reflect the extremely dry antecedent soil moisture conditions at the beginning of the two events. The model outcomes were evaluated based on macroscopic hydrological information from the broader area.

Hydrodynamic simulations were carried out using HEC-RAS 6.6 within a 1D-2D coupled framework. River channels were modelled as 1D elements, while floodplain areas were represented using 2D diffusive wave equations with subgrid terrain discretization, based on high-resolution LiDAR data. Lateral structures and embankments were explicitly included, allowing dynamic simulation of flow exchange, including overtopping and dynamic interactions between channels and floodplains.The model replicates quite successfully the observed spatial patterns, timing, and magnitude of flooding during both events, as corroborated by satellite imagery, field observations, and reported damages. This is strong evidence that the overall framework effectively captures the basin’s flood response under contrasting rainfall conditions, highlighting the sensitivity of runoff generation and flood routing to antecedent wetness, spatial rainfall variability, and topographic complexity. The combined use of calibrated hydrologic and hydraulic models provides a robust basis for reconstructing past flood events with high spatiotemporal resolution, offering insights into dominant flood-producing mechanisms and the limitations of existing protection infrastructure. These results support evidence-based planning and underline the importance of integrated modelling approaches for flood risk management in vulnerable agri-urban catchments.