Outlining a master plan framework for the design and assessment of flood mitigation infrastructures across large-scale watersheds

On September 16, 2020, the Hellenic Ministry of Infrastructure assigned to the concessionaire of the Central Greece Motorway E65 the design and construction of supplemental works for the urgent flood protection of areas along the motorway alignment, including the Western Thessaly region (Greece). Considering the damages and losses induced by the Medicane Ianos over the greater Thessaly region the concessionaire, on its own initiative, proclaimed the need for developing a Master Plan for the West Thessaly flood protection. The final area of interest, herein referred to as Western Peneios watershed, occupies approximately 6400 km2, thus constituting a mega-scale hydrological, hydraulic and water management study that poses multiple conceptual and computational challenges. The overall question of the Master Plan is to provide a synthesis of already proposed as well as new projects (dams, embankments, ditches), and prioritize them under a multipurpose prism. The methodological framework is comprised of three axes: (i) a preliminary assessment of specific areas where high risk is expected due to flood phenomena, by utilizing a GIS-based multi-criteria decision analysis approach, (ii) a semi-distributed representation of the rainfall-runoff transformations and the flood routing processes across the entire watershed, and (iii) a coupled 1D/2D hydrodynamic simulation of the flood prone riverine system, also including a highly complex system of artificial channels. The final planning prioritizes the strengthening of flood protection in the study area through the combined influence of a set of large-scale projects, i.e., dikes, multi-purpose dams (permanent reservoirs) and retention basins of controlled inundation (temporary reservoirs). The objective is to sketch a framework for facing similar studies in a holistic manner, while maintaining a high level of computational efficiency and explainability.