733 River Flow 2014 – Schleiss et al. (Eds) © 2014 Taylor & Francis Group, London, ISBN 978-1-138-02674-2 Representing skewed bridge crossing on 1-D and 2-D flood propagation models: Compared analysis in practical studies P. Costabile & F. Macchione LAMPIT, Laboratorio di Modellistica Numerica per la Protezione Idraulica del Territorio, Department of Environmental and Chemical Engineering, University of Calabria, Italy G. Petaccia & L. Natale Department of Civil Engineering and Architecture, University of Pavia, Italy ABSTRACT: This paper deals with the application of flood propagation models in rivers with skewed bridge crossings. In such situations, the application of 1-D models is not trivial because of the uncertainty related to the narrowing induced by bridge piers within the cross-sections. In order to study this problem, this paper investigates 1-D and 2-D state-of-the-arte unsteady flow models that have been applied to the situations characterized by bridges or a sequence of skewed bridge crossing. The availability of terrestrial and airborne LIDAR and the comparison between 1-D and 2-D results, allowed practical suggestions to optimize the 1-D results in the above mentioned situations. cross-sections with very small spacing if desired, providing a more accurate depiction of the river geometry (Pramanik et al. 2010). Recent advances in LIDAR surveying techniques have reduced the costs of obtaining Digital Elevation Model (DEM). Moreover, the availability of LIDAR data makes the use of 2-D modelling for flood inunda- tion more convenient than before (Gallegos et al. 2009; Ernst et al. 2010). Despite studies addressing the optimal cross-sectional spacing in 1-D models of flood propagation (see, for example, Castel- larin et al. 2009), particular care and experience are required in identifying the most useful sections for the hydraulic computations. This aspect has to be managed carefully when applied to the correct representation of the interaction between bridges and varying flow regimes. Research related to the representation of hydraulic structures (i.e. bridges) in the hydraulic modelling, either using 1-D or 2-D hydraulic models, is a topic of increasing interest in the literature (see, for example, Pappenberger et al. 2006, Cook & Merwade 2009, Fewtrell et al. 2011, Brandimarte & Woldeyes 2013, Hailemariam et al. 2014). However, there are very few practical studies that discuss, in practical situations, the effects induced by bridges on the longitudinal and transversal variation of the maximum water surfaces and flow regimes aimed at flood hazard assessments. For example, Costabile et al. (2014b) discussed two situations in which bridges cross rivers orthogonal to the principle flow direction, located in almost rectilinear river reaches confined 1 INTRODUCTION In producing flood inundation maps, especially in urbanized areas, the use of one-dimensional numerical models may be inadequate because they may not represent all the features of the flow con- ditions constrained by infrastructure that occur in real cases. However 1-D models are still very popular due to their reduced computational time, their ease of implementation, the reduced need for topographic data and lower level discretization (Macchione & Viggiani 2004, Petaccia et al. 2013; Costabile et al. 2014a). In order to take into account two-dimensional features of flow within given cross sections, terms representing the momentum exchange between the main channel and the flood- plain are often added to 1-D unsteady flow model- ling investigations (Cao et al. 2006; Costabile and Macchione 2012). Despite this, strong limitations exist in representing water surface profile using 1-D models where water surfaces are assumed horizon- tal across section, and due to the inability in detect- ing transverse changes of flow regime. Moreover, the accuracy of flow simulations significantly depends on the choice of the sequence of cross- sections describing the river reach. In the past, the river geometry was described using few surveyed cross-sections, so numerical sections, obtained by their interpolations, were added to complete the computational domain. Nowadays, the use of techniques for the automatic extraction of cross- sections from DEMs allows the modeller to obtain