water Article A Flood Inundation Modeling Approach for Urban and Rural Areas in Lake and Large-Scale River Basins George Papaioannou 1,2,3, * , Lampros Vasiliades 2 , Athanasios Loukas 4, * , Angelos Alamanos 5 , Andreas Efstratiadis 6 , Antonios Koukouvinos 6 , Ioannis Tsoukalas 6 and Panagiotis Kossieris 6   Citation: Papaioannou, G.; Vasiliades, L.; Loukas, A.; Alamanos, A.; Efstratiadis, A.; Koukouvinos, A.; Tsoukalas, I.; Kossieris, P. A Flood Inundation Modeling Approach for Urban and Rural Areas in Lake and Large-Scale River Basins. Water 2021, 13, 1264. https://doi.org/10.3390/ w13091264 Academic Editor: Yurui Fan Received: 4 March 2021 Accepted: 26 April 2021 Published: 30 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Hellenic Centre for Marine Research, Institute of Marine Biological Resources and Inland Waters, Anavyssos, 19013 Attiki, Greece 2 Department of Civil Engineering, School of Engineering, University of Thessaly, 38334 Volos, Greece; lvassil@civ.uth.gr 3 Studies Program of the Department of Forestry and Natural Resources Management of the Former Technological Institute of Thessaly, General Department (Larisa), University of Thessaly, 43100 Karditsa, Greece 4 School of Rural and Surveying Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece 5 The Water Forum|Centre for Freshwater and Environmental Studies, Dundalk Institute of Technology, Marshes Upper, Dundalk Co., A91K584 Louth, Ireland; angelos.alamanos@dkit.ie 6 Department of Water Resources and Environmental Engineering, School of Civil Engineering, National Technical University of Athens, 15780 Athens, Greece; andreas@itia.ntua.gr (A.E.); A.Koukouvinos@itia.ntua.gr (A.K.); itsoukal@mail.ntua.gr (I.T.); pkossier@mail.ntua.gr (P.K.) * Correspondence: gpapaioan@hcmr.gr(G.P.); agloukas@topo.auth.gr (A.L.); Tel.: +30-22910-76349 (G.P.); +30-231-099-6103 (A.L.) Abstract: Fluvial floods are one of the primary natural hazards to our society, and the associated flood risk should always be evaluated for present and future conditions. The European Union’s (EU) Floods Directive highlights the importance of flood mapping as a key stage for detecting vulnerable areas, assessing floods’ impacts, and identifying damages and compensation plans. The implementation of the EU Flood Directive in Greece is challenging because of its geophysical and climatic variability and diverse hydrologic and hydraulic conditions. This study addressed this challenge by modeling of design rainfall at the sub-watershed level and subsequent estimation of flood design hydrographs using the Natural Resources Conservation Service (NRCS) Unit Hydrograph Procedure. The HEC- RAS 2D model was used for flood routing, estimation of flood attributes (i.e., water depths and flow velocities), and mapping of inundated areas. The modeling approach was applied at two complex and ungauged representative basins: The Lake Pamvotida basin located in the Epirus Region of the wet Western Greece, and the Pinios River basin located in the Thessaly Region of the drier Central Greece, a basin with a complex dendritic hydrographic system, expanding to more than 1188 river-km. The proposed modeling approach aimed at better estimation and mapping of flood inundation areas including relative uncertainties and providing guidance to professionals and academics. Keywords: EU Floods Directive; flood risk management; extreme rainfall; SCS-CN; 2D hydraulic modeling; HEC-RAS; fluvial floods; ungauged rivers 1. Introduction Natural disasters caused by floods are responsible for the most casualties and eco- nomic loss in Europe and worldwide [1,2]. For example, for the period 1995–2015, floods affected 2.3 billion people and costed 157,000 human lives [3]. In general, there are five distinct types of floods (i.e., flash, fluvial, pluvial, urban, and coastal floods), which may stem from various processes and sources. Flood impacts could be aggravated by human activities and interventions in the natural systems (e.g., deforestation, earthworks, water works, urbanization). Fluvial (river) flooding is the most common type of flood and has Water 2021, 13, 1264. https://doi.org/10.3390/w13091264 https://www.mdpi.com/journal/water