Sustainable urban drainage systems (SUDS) were almost unknown in Spain two decades ago; today, urban drainage in the country is transitioning towards a more sustainable and regenerative management in a global context where green policies are gaining prominence. This research establishes a diagnosis of SUDS in Spain and examines the extent to which the country is moving towards the new paradigm in three dimensions: (a) the governance and social perception of the community, (b) the regulative background, and (c) the implementation and the technical performance of SUDS. The diagnosis identifies barriers that hinder the change. Then, we define the challenges that Spain has to face to overcome obstacles that delay the transition. Barriers to the governance sphere are related to the lack of involvement, knowledge, and organisational responsibilities. Within the regulative framework, the absence of national standards hinders the general implementation at the national scale, although few regional and local authorities are taking steps in the right direction with their own regulations. From the technical perspective, SUDS performance within the Spanish context was determined, although some shortcomings are still to be investigated. Despite the slowdown caused by the hard recession periods and the more recent political instability, SUDS implementation in Spain is today a fact, and the country is close to reaching the stabilisation stage.

This paper presents a dataset obtained from hydraulic and sediment transport experiments performed in a full-scale urban drainage physical model of 36 m². The study seeks to accurately measure sediment mobilization through the different parts of the model (surface, gully pots and pipe system), also obtaining a precise characterization of water flow and using realistic rainfall simulator to ensure the transferability of the results. Three different rain intensities and five sediment granulometries were tested in 6 hydraulic and 23 wash-off and sediment transport experiments. The following experimental data were produced: surface elevations and 2D runoff velocities measured by visualization techniques; surface and in-pipe water depths; flow discharges, total suspended solids concentrations and particle size distribution at the entrance of the gully pots and at the pipe system outlet; and sediment mass balances. This data is optimal for developing and validating wash-off and sediment transport formulations in urban drainage models, towards better treatment and management techniques for minimizing the impact of urban surface pollutants on the environments of towns and cities.

Rainfall simulators are useful tools for controlling the main variables that govern natural rainfall. In this study, a new drop-forming rainfall simulator, which consists of pressure-compensating dripper grids above a horizontal mesh that breaks and distributes raindrops, was developed to be applied in wash-off experiments in a large-scale physical model of 36 m². The mesh typology and size, and its distance to drippers, were established through a calibration where rain uniformity and distributions of raindrop sizes and velocities were compared with local natural rainfall. Finally, the rain properties of the final solution were measured for the three rain intensities that the rainfall simulator is able to generate (30, 50 and 80 mm/h), obtaining almost uniform rainfalls with uniformity coefficients of 81%, 89% and 91%, respectively. This, together with the very suitable raindrop size distribution obtained, and the raindrop velocities of around 87.5% of the terminal velocity for the mean raindrop diameter, makes the proposed solution optimal for wash-off studies, where rain properties are key in the detachment of particles. In addition, the flexibility seen in controlling rain characteristics increases the value of the proposed design in that it is adaptable to a wide range of studies.

A Computational Fluid Dynamics (CFD) model was developed to analyze the open-channel flow in a new set of egg-shaped pipes for small combined sewer systems. The egg-shaped cross-section was selected after studying several geometries under different flow conditions. Once the egg-shaped cross-section was defined, a real-scale physical model was built and a series of partial-full flow experiments were performed in order to validate the numerical simulations. Furthermore, the numerical velocity distributions were compared with an experimental formulation for analytic geometries, with comparison results indicating a satisfactory concordance. After the hydraulic performance of the egg-shaped pipe was analyzed, the numerical model was used to compare the average velocity and shear stress against an equivalent area circular pipe under low flow conditions. The proposed egg shape showed a better flow performance up to a filling ratio of h/H = 0.25.