Floodplain Delineation of the Ranillas Meander in Zaragoza Using GUAD-2D

by Francisco Olivera, (M.ASCE),
Martín Rodriguez,
Javier Murillo,



Document Type: Proceeding Paper

Part of: World Environmental and Water Resources Congress 2008: Ahupua'A

Abstract: After the selection of Zaragoza (Spain) as the host city of the International Expo 2008, the Gobierno de Aragón (i.e., the local government) conducted a detailed study of the flood risk in the surroundings of the Ranillas meander, north-west of downtown Zaragoza, which is the area where the expo facilities were to be built. This study added to an existing one conducted by the Confederación Hidrográfica del Ebro (i.e., the corresponding river authority), which characterized the status of the Ebro River between Miranda del Ebro and Zaragoza, a 325-km river stretch. The study presented here was conducted to assess the effect of the new buildings — some of which are being built partially obstructing the main channel — on the floodplain width along the 5.5 km of the Ranillas meander. The GUAD-2D flow model was used to delineate the inundated area for the 10-, 25-, 50-, 100- and 500-year flood events. Flood plain extents for the 10- and 25-year floods were available from the study by the Confederación Hidrográfica del Ebro and corroborated our results that the floodplain was constrained within the river channel for such low return period event . GUAD-2D is a distributed two-dimensional shallow-water unsteady flow model jointly developed by INCLAM S.A. and the Universidad de Zaragoza. The model averages the Navier-Stokes equations over depth to obtain mass- and momentum-conservation equations. The calculation module is based on an upwind finite volume method and uses Roe's approximate Riemann solver. The terrain topography was defined by an elevation grid that resulted from the combination of a LIDAR digital elevation model(DEM) with a horizontal resolution of two meters and vertical accuracy of 0.15 m, a digital surface model that included building (and other man-made structure) and vegetation information, and 14 river cross sections along the meander. Likewise, a Manning's n grid was developed to describe the terrain surface roughness. For each event, a flood hydrograph was entered as a boundary condition at the most upstream end of the stream segment. The peak flows for the 50-, 100- and 500-year events were 3,800 m³/s, 4,300 m³/s and 5,100 m³/s, respectively, according to studies conducted by the Spanish Centro de Estudios y Experimentación de Obras Públicas. Additionally, the stage-discharge curve of the flow gauge located at the La Almozara bridge (the downstream end of the meander segment), developed by the Confederación Hidrográfica del Ebro, was applied. The simulations showed that the effect of the new buildings was minor downstream of the constructions, but created a backwater along the meander that increased the flooding of part of the city of Zaragoza. Overall, GUAD-2D proved to be a very powerful tool for simulating shallow flow over large areas with complex topography and provided valuable insight on the effect of altering the hydraulic conditions of the Ranillas meander on the risk of flooding in Zaragoza urban areas.

Subject Headings: Rivers and streams | Floods | Structural models | Model accuracy | Hydrologic models | Flood plains | Urban areas | Risk management | Spain | Europe

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