Flooding Hazard Mapping in Floodplain Areas Affected by Piping Breaches in the Po River, Italy M. Mazzoleni 1 ; B. Bacchi 2 ; S. Barontini, Ph.D. 3 ; G. Di Baldassarre, Ph.D. 4 ; M. Pilotti, Ph.D. 5 ; and R. Ranzi, Ph.D. 6 Abstract: In recent years, flood-related risk has been increasing worldwide, being inundations among the natural disasters which induce the maximum damage in terms of economic losses. In the research reported in this paper, a methodology to map the flooding residual hazard due to levee failure events induced by piping in embankments protecting flood-prone areas is proposed. Ensemble simulations are used to account for uncertainties in location, geometry, and time-evolution of the levee breaches. Probabilistic flooding-hazard maps are generated combining the results of 192 inundation scenarios, simulated by using one-dimensional (1D) and two-dimensional (2D) hydrodynamic models. The methodology is applied considering 96 different locations and sizes of breaches occurred along a 23-km reach protected by the right levee of the Po River, the right levee of the Taro River, and the left leveeof the Parma River, which delimit a 100-km 2 study area. The influence of obstacles to the flood propagation and consequent hazard-mapping was investigated, taking into account several standard criteria to map the flooding hazard in different European countries. The results obtained from the research reported in this paper provide an example of a rational method to map the residual hazard in areas protected against flooding due to embankment-overtopping, taking into account uncertainties due to breach location, levee fragility, and the presence of topographic obstacles. All these details should be taken into account in land-planning and flood-risk management. DOI: 10.1061/(ASCE)HE.1943-5584.0000840. © 2014 American Society of Civil Engineers. Author keywords: Flooding-hazard mapping; Po River floodplain; Levee failure; Hydraulic risk analysis; Residual risk. Introduction The impact of floods and inundations has increased worldwide in the last decade and a huge amount of damage has been caused in Europe, as documented by Wilby et al. (2008) and European Envi- ronment Agency (2005). The increased flood damages are partly attributable to augmented exposure of lowland areas and partly to river-management activities. Structural measures (e.g., building, heightening, or reinforcing the levees) to let the design flood being conveyed with a given safety freeboard reduce the flooding hazard and (theoretically) the risk. At a practical level, these struc- tural measures can induce the well-known levee paradox on the floodprone area (Vis et al. 2003; Burton and Cutter 2008; Castellarin et al. 2011; Ludy and Kondolf 2012), which is related to the change in the hazard perception related to levee-strengthening. The lower the hazard perception, the higher the urbanization in the floodprone areas and exposure to the flooding hazard. The induced damage by a flood event, the intensity of which is greater than the design flood, will be greater than before the structural defence enhancement. Accordingly, levee failure can significantly increase flooding risk, as demonstrated, for instance, by the failure of the Mississipi River embankment system during the catastrophic 2005 Katrina event (ASCE 2007). Implicitly accounting for the ef- fects of the levee paradox and widespread anthropization of most of the territories of the European countries, European Union (EU) di- rective 2007/60/EC (European Parliament Council 2007; Mostert and Junier 2009) stresses the importance of nonstructural methods to mitigate flooding risk. A key aspect of this approach is the draw- ing, by member states, of flooding hazard and risk maps for poten- tial flooding areas. Maps should be drawn in case of three different flood events with (1) low, (2) medium, (i.e., likely return period ≥100 years), and (3) high probability of occurrence (de Moel et al. 2009). A little flexibility is given to each member state in the def- inition of the return period. According to Italian law, for instance, Decree Law n.49/2010 defines high-probability events as those with return period between 20 and 50 years, medium probability as those occurring once every 100 or 200 years, on average, and low probability as those with a return period up to 500 years. The current legislative framework motivated the development of several studies aiming at proposing new methodologies to address the evaluation of flooding-hazard maps. Assessing the flooding hazard is in fact affected by various sources of uncertainties (Ranzi et al. 2012; Fig. 1), which are related to a cascade of hydrological, hydraulic, and geotechnical uncertainties. Accordingly, assessing the safety of a floodprone area only by means of the levee- overtopping threshold can underestimate the real hazard of the area. 1 Ph.D. Student, Dipartimento di Ingegneria Civile, Architettura, Territorio, Ambiente e di Matematica, Univ. degli Studi di Brescia, Via Branze 43, 25123 Brescia, Italy. E-mail: maurizio.mazzoleni@ing.unibs.it 2 Professor, Dipartimento di Ingegneria Civile, Architettura, Territorio, Ambiente e di Matematica, Univ. degli Studi di Brescia, Via Branze 43, 25123 Brescia, Italy. E-mail: bacchi@ing.unibs.it 3 Assistant Professor, Dipartimento di Ingegneria Civile, Architettura, Territorio, Ambiente e di Matematica, Univ. degli Studi di Brescia, Via Branze 43, 25123 Brescia, Italy. E-mail: barontin@ing.unibs.it 4 Professor, Dept. of Integrated Water Systems and Governance, UNESCO-IHE Institute for Water Education, Westvest 7, P.O. Box 3015, 2601 DA, Delft, Netherlands. E-mail: G.DiBaldassarre@unesco-ihe.org 5 Professor, Dipartimento di Ingegneria Civile, Architettura, Territorio, Ambiente e di Matematica, Univ. degli Studi di Brescia, Via Branze 43, 25123 Brescia, Italy. E-mail: pilotti@ing.unibs.it 6 Professor, Dipartimento di Ingegneria Civile, Architettura, Territorio, Ambiente e di Matematica, Univ. degli Studi di Brescia, Via Branze 43, 25123 Brescia, Italy (corresponding author). E-mail: ranzi@ing.unibs.it Note. This manuscript was submitted on September 17, 2012; approved on May 3, 2013; published online on May 6, 2013. Discussion period open until September 1, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Hydrologic En- gineering, Vol. 19, No. 4, April 1, 2014. © ASCE, ISSN 1084-0699/2014/ 4-717-731/$25.00. JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / APRIL 2014 / 717 J. Hydrol. Eng. 2014.19:717-731. 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