A methodological approach for flood risk management in urban areas: The Volos city paradigm

A methodological approach based on the implementation of the EU Floods Directive in Greece is developed and presented for flood risk management of urban areas. The flood risk assessment procedure is demonstrated for Volos city of Thessaly, Greece, where frequent flood episodes are observed due to intense storms. A unified deterministic extreme event-based methodology is applied for hydrologic and hydraulic modelling of floods. The hydrologic part is based on semi-distributed application of the HEC-HMS rainfall-runoff model with spatially-distributed design hyetographs. The SCS-CN method is used to estimate effective rainfall and the SCS synthetic unit hydrograph to produce extreme flood hydrographs at subwatershed scale. The hydraulic modelling is based on the propagation of flood hydrographs across the river network and the mapping of inundated areas using the HEC-RAS 2D model with flexible mesh size. Representation of the resistance caused by buildings have been simulated with the local elevation rise method using transformation of the Digital Terrain Model to a Digital Elevation Model. For the adopted design hyetographs upper and lower estimates on water depths, flow velocities and flood inundation areas are estimated taking into account structural and parameter uncertainty of the hydrologic and hydraulic models by varying antecedent soil moisture conditions and roughness coefficient values. The results indicate the uncertainty introduced on flood risk management in urban areas using typical engineering practices.