Nat. Hazards Earth Syst. Sci., 20, 59–72, 2020 https://doi.org/10.5194/nhess-20-59-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Enhancing the resilience to flooding induced by levee breaches in lowland areas: a methodology based on numerical modelling Alessia Ferrari, Susanna Dazzi, Renato Vacondio, and Paolo Mignosa Department of Engineering and Architecture, University of Parma, Parco Area delle Scienze 181/A, 43124 Parma, Italy Correspondence: Alessia Ferrari ([email protected]) Received: 18 April 2019 – Discussion started: 22 May 2019 Revised: 9 October 2019 – Accepted: 4 December 2019 – Published: 13 January 2020 Abstract. With the aim of improving resilience to flooding occurrence of extreme flood events (Alfieri et al., 2015) and and increasing preparedness to face levee-breach-induced in- the related damage (Dottori et al., 2018) in the future. undations, this paper presents a methodology for creating a Among the possible causes of flooding, levee breaching wide database of numerically simulated flooding scenarios deserves special attention. Due to the well-known “levee ef- due to embankment failures, applicable to any lowland area fect”, structural flood protection systems, such as levees, de- protected by river levees. The analysis of the detailed spa- termine an increase in flood exposure. In fact, the presence tial and temporal flood data obtained from these hypothetical of this hydraulic defence creates a feeling of safety among scenarios is expected to contribute both to the development people living in flood-prone areas, resulting in the growth of of civil protection planning and to immediate actions during settlements and in the reduction of preparedness and hence a possible future flood event (comparable to one of the avail- in the increase in vulnerability in those areas (Di Baldassarre able simulations in the database) for which real-time mod- et al., 2015). As a result, more people are exposed to less fre- elling may not be feasible. The most relevant criteria con- quent but more devastating floods, for which the statistical cerning the choice of mathematical model, grid resolution, frequency is difficult to assess due to the historical changes hydrological conditions, breach parameters and locations are in river systems (Black, 2008). discussed in detail. The proposed methodology, named RE- Moreover, the presence of levees causes a reduced flood SILIENCE, is applied to a 1100 km2 pilot area in northern attenuation, which in turn increases the damage when a Italy. The creation of a wide database for the study area is breach occurs (Heine and Pinter, 2012). Despite all the ef- made possible thanks to the adoption of a GPU-accelerated forts adopted in embankment design, maintenance and moni- shallow-water numerical model which guarantees remark- toring, the residual risk associated with levee-breach flooding able computational efficiency (ratios of physical to compu- in the surrounding areas cannot be neglected, and its evalu- tational time up to 80) even for high-resolution meshes (2.5– ation is hence gaining attention worldwide (Huthoff et al., 5 m) and very large domains (> 1000 km2). 2015; Pinter et al., 2016). Nowadays, mathematical models for flood simulation, which solve physically based equations for hydrodynamics (Teng et al., 2017), represent an essential instrument for flood hazard and risk assessment (e.g. Apel et al., 2004; Qi and Altinakar, 2011), including the residual 1 Introduction risk due to levee breaches (Vorogushyn et al., 2010). Numer- ical modelling can contribute to drawing up flood risk man- Flood events adversely affect communities living in flood- agement plans with prevention and protection measures to prone areas, causing huge damage in terms of economic reduce flood-related damage and enhance resilience (defined losses and human lives. Recent studies identified a rising as the ability of the system to recover from flooding in flood- trend in flood frequency and affected population in the past prone areas; De Bruijn, 2005). The accurate predictions of few decades (Kundzewicz et al., 2013; Paprotny et al., 2018) inundation scenarios provided by numerical simulations can and suggested that global warming will determine a growing Published by Copernicus Publications on behalf of the European Geosciences Union. 60 A. Ferrari et al.: Enhancing the resilience to flooding induced by levee breaches in lowland areas also be useful for assessing civil protection and adaptation adopted for the pilot area are reported. Moreover, a few ex- strategies (Jongman et al., 2018) and emergency planning amples of simulation results are provided, and their possible during flood events (Tarrant et al., 2005; Dulebenets et al., practical use is discussed. 2019a,b). The paper is organized as follows: in Sect. 2, the RE- For civil protection purposes, real-time numerical mod- SILIENCE project is presented, and the most important fea- elling is the most well-suited solution when dealing with tures and requirements of the methodology are described in large river basins whose flood events last several days, con- detail. Section 3 illustrates the application to the pilot area, sidering that the simulation time (a few hours) is usually together with some examples of the results. The assumptions, much smaller than the physical duration of the flood. More- the advantages and the implications of the methodology are over, hydrologic inputs or water level measurements from discussed in Sect. 4, and concluding remarks are finally out- upstream river sections can be used as boundary conditions lined in Sect. 5. to predict the flood propagation along the river. Conversely, in small–medium river basins with short-lasting floods (less than 1 d), real-time simulations are much more challeng- 2 Flooding scenarios induced by levee breaches: the ing because of the following: (i) they have to be based on RESILIENCE project rainfall–runoff models and weather forecasts, which are char- The RESILIENCE project aims at defining a new method- acterized by high uncertainties, and (ii) their computational ology for mapping flood scenarios due to levee breaches, and physical times are characterized by the same order of which can be helpful for improving preparedness and sup- magnitude. Focusing on levee failures, the real-time predic- porting the development of technical and scientific tools for tion of possible breach locations is very difficult in practice emergency planning and management, consistent with the due to the complexity of the breaching process and to the EU Floods Directive 2007/60/CE. Several breach locations uncertainties in the embankment material characteristics (of- along a leveed river are preliminarily identified, and multi- ten heterogeneous and with unknown local discontinuities), ple discharge hydrographs, characterized by different return especially for small rivers. Considering all these limitations, period, are considered. Each combination of breach position the creation of an offline database of hypothetical flooding and upstream boundary condition corresponds to a simulated scenarios constitutes an alternative solution to real-time fore- flood scenario. In this way, a large database describing differ- casting based on integrated hydrologic–hydraulic modelling. ent hypothetical real levee-breach events in that area is cre- This paper presents a methodology for assessing the flood- ated. The results of these simulations, made available to pub- ing scenarios induced by levee breaches with the purpose of lic administrations, can be fundamental not only for emer- increasing resilience in lowland areas. For a given exposed gency planning but also for taking appropriate actions of civil area, the RESILIENCE project (REsearch on Scenarios of protection in the course of real flood events. Inundation of Lowlands Induced by EmbaNkment Collapses In the following sections, the most important assumptions in Europe) aims at the creation of a wide database of high- of the methodology concerning model selection, spatial res- resolution numerical simulations concerning several hypo- olution, hydrological conditions, breach locations and mod- thetical flood scenarios, each one characterized by a specific elling are discussed thoroughly. Moreover, the most relevant breach location and an upstream discharge hydrograph with simulation outputs and their usefulness for civil protection an assigned return period. While previous studies combined purposes are described. the results of different scenarios in order to create probabilis- tic flood hazard and flood risk maps (Di Baldassarre et al., 2.1 Numerical model 2009; Vorogushyn et al., 2010), in this work breach scenar- ios are not associated with their probability of occurrence. In 2.1.1 Background fact, the focus of this study is not on flood hazard mapping but on the evaluation of flood dynamics, arrival times, and Free-surface flows are traditionally described by means of maximum water depths and velocities, required for the defi- the shallow-water equations (SWEs), i.e. depth-averaged nition of civil protection strategies, which should be equally mass and momentum conservation laws (Toro, 2001), which effective regardless of the event probability, breach failure can be written either in one-dimensional (1-D) or in 2-D mechanism, etc. Accurate simulation results are obtained form. In the past, the high computational effort required to thanks to the adoption of high-resolution meshes and of a ro- perform fully 2-D simulations led to the development of 1-