*2.2. Overview of the Hydrodynamic Model*

A 1D/2D urban drainage model of the whole municipality of Barcelona was developed, calibrated, and validated using the hydraulic modelling software Innovyze InfoWorks ICM®and local observation data from different rain events. This model is an update of the model used for the Drainage Master Plan of Barcelona of 2006.

A rainfall-runoff model was used to compute the runoff from buildings that were assumed to directly drain into the 1D hydraulic model representing the drainage network. The 2D model was used to reproduce overland flow on streets, parks, and further areas located at terrain elevation that can be flooded. The 1D and the 2D models continuously interact with each other through model nodes that physically represent surface inlets (like gullies or maintenance holes) to the drainage system. At these nodes, the exchange of water between the 1D and the 2D model was computed using different modelling options available in InfoWorks (nodes can be defined as Inlets 2D, 2D, Gullies 2D) that compute water flows as a function of local water levels and inlet geometries.

A peculiarity of this 1D/2D model is that part of the rainfall is directly applied to the streets and parks represented by the 2D overland flow model, and part to the rainfall-runoff model used for buildings, roofs, and elevated areas. Conventional urban drainage modelling approach generally applies rainfall directly to rainfall-runoff models that compute runoff diverting it into the 1D hydraulic model, and flooding can only occur through a maintenance hole surcharge. In Barcelona, it is believed that urban floods are partly caused by deficient surface drainage capacity due to, for instance, a reduced number of street gullies.

This model includes approximately 2164 km of pipes, 67,967 pipes, 66,158 nodes, 18 detention tanks with a total volume of approximately 461,600 m3, 489 weirs, 22 pumps, 47 sluice gates, and 120 outfalls. The full 1D Saint-Venant equations are used to solve the sewer flow. Rainfall-runoff processes were simulated using a single non-linear reservoir model with routing coefficients that are a function of surface roughness, surface area, ground slope, and catchment width. Initial hydrological losses were simulated to be approximately 1 mm (a little higher in pervious areas and lower in impervious ones).

Continuous losses were only applied to pervious areas and were simulated using the Horton model. The 2D model has an unstructured mesh with 662,071 cells created from a Digital Terrain Model (DTM) with a resolution of 2x2 m<sup>2</sup> obtained by a special combination of a 3-D scanning and laser scanning (LIDAR) provided by the Cartographic and Geological Institute of Catalonia (with an accuracy of 0.15 m for altitudes). The 2D cells have areas in the range of 5–125 m<sup>2</sup> in the urban area and 100–10,000 m<sup>2</sup> in the upstream rural areas.

The 1D/2D model was quantitatively and qualitatively calibrated and validated using data from four different historical rain events. In Barcelona, approximately 25 rainfall gauges and more than 100 water level sensors and flood event videos and photos are available. Calibration and validation details can be found in deliverable 2.2 of the EU RESCCUE project [27].
