Sustainable urban drainage systems (SUDS) have shown a high potential to manage urban stormwater from a holistic perspective: managing water quantity and quality in decentralised schemes as well as delivering multiple additional benefits for citizens and urban biodiversity. The main goal of HOFIDRAIN is to solve scientific and technical knowledge gaps related to the performance of filtering sections for sustainable stormwater management. The research focuses on the structural and environmental improvement of filtering sections as well as in their efficiency assessment at city scale. Thus, the main original contribution of HOFIDRAIN will be a toolkit for public administrations and practitioners to promote a general framework for the development of specific regulations and to develop a pre-normative proposal of typified permeable sections. This framework will ensure that all necessary processes of urban regeneration for adaptation to climate change threats related to stormwater management will be conducted incorporating sustainability and smart governance criteria.

HOFIDRAIN consists of three subprojects: ENGODRAIN, developed by Universitat Politècnica de València (UPV), MELODRAIN, developed by Universidad de Cantabria (UC) and POREDRAIN, developed by Universidade da Coruña (UDC).

The experimental characterization of filtering sections as part of several SUDS types (permeable pavements, greenroofs) is one of the main goal of HOFIDRAIN. UC will lead the characterization of filtering sections from a mechanical perspective for their use in urban streets and building roofs. The challenge focuses on increasing the lifespan of these infrastructures as well as their resilience towards climate change. UPV will complete the analysis leading the environmental aspects of the research. The focus is now on specific gaps still unresolved like the long-term behaviour of these SUDS (water quality performance, clogging), their potential to harvest water for reuse or the development of parsimonious mathematical models. UDC will analyse the filtering sections response at a higher scale, complementing the scope of the experimental procedures developed by UC and UPV. UDC will lead the analysis of the impact of filtering sections on the hydrology and the mobilized pollution within these SUDS techniques, comparing the results against conventional impervious pavements.

The second main goal of HOFIDRAIN is to develop characterization programs and smart governance tools for SUDS management at city scale. UDC leads in Spain the monitoring techniques of urban drainage systems. Within the project, UDC aims at developing a comprehensive methodology to asses SUDS efficiency from quantity and quality perspectives. UPV contributes with its experience in smart governance strategies applied to the sustainable urban drainage to catalyse the transition towards a SUDS-based drainage. The project aims at promoting a general framework for the development of specific regulations and a pre-normative proposal of typified permeable sections, taking into account the environmental and structural aspects analysed by the whole project and the management strategies developed by UC, UDC and UPV. UC develops decision support tools for an optimal location of SUDS at the city scale. The aim is to identify vulnerable areas to urbanization and climate change for an optimal location of filtering sections to mitigating those threats and to maximizing the retrofitting opportunities.

Sustainable urban drainage systems (SUDS) are technologies widely adopted in northern and English-speaking countries. SUDS consider an alternative and supplementary focus on conventional urban drainage strategies. Urban filtering sections (green roofs, permeable pavements) are SUDS aiming at source runoff control. Efficiency of filtering sections for a sustainable runoff management has been proved for the last 30 years. However, as for all types of SUDS, there is still some reticence to using them in southern countries with more temperate and dry climate. One of the most important barriers is the long-term uncertainty on clogging. Moreover, pollutants (suspended solids, heavy metals, phosphorus, nitrogen, oils and hydrocarbons) present in urban runoff managed by permeable surfaces significantly contribute to the clogging process and can represent some hazard for infiltrated waters. For this reason, it is necessary to go ahead with this research, to set up, in a reliable way, the life span of these systems and the smart governance mechanisms to catalyse their presence in urban drainage schemes.

Within the framework of the coordinated project HOFIDRAIN, sub-project ENGODRAIN (Environmental characterization of filtering sections and smart governance tools for a sustainable urban drainage at city scale) will focus on to main issues. First, to better understand the knowledge gaps still existing in the environmental behaviour of filtering sections; then, to make progress with smart governance strategies to catalyse the transition towards a more SUDS-based stormwater management.

ENGODRAIN will study, in the long term, the changes in filtered water quality and the effect of clogging in the loss of hydraulic conductivity of the permeable surface under different precipitation and contamination scenarios. In addition, the project will evaluate the feasibility of using permeable pavements as water storage systems for reuse in the urban environment. An important gap is to study the problems associated with the changes in filtered water pH due to the washing of the pavement and its possible consequences in other water quality parameters. Improving the efficiency of these SUDS is also a challenge, by incorporating reactive substances in the composition of permeable pavements. Finally, ENGODRAIN will develop, calibrate and validate a parsimonious mathematical model for permeable surfaces that allows simulating different rainfall scenarios, obtaining results both of quantity and water quality.

Regarding smart governance strategies, ENGODRAIN will evaluate barriers and opportunities that SUDS represent in the urban ecosystem, as well as actions that, from the social perspective, can be carried out to catalyse the transition towards a SUDS-based drainage. The project will evaluate the current situation regarding guidance and legislation to identify gaps in this dimension that hinder the development of SUDS and to promote a general framework for the development of specific regulations. Finally, ENGODRAIN will develop a pre-normative proposal of typified permeable sections, taking into account the environmental and structural aspects analysed in the coordinated project.

MELODRAIN subproject has two main activities integrated in the HOFIDRAIN coordinated project. The first activity starts with the design of a new porous asphalt mixture improved with additives for streets. In this task, the improvement of the material most commonly used as surface layer in the cities all over the world will be studied, considering several additives like rubber, virgin butadiene-styrene and aramid fibers the main ones. The study will find out what is their effect on the mechanical performance of asphalt mixtures, analysing the best method to add them based on their characteristics and their optimal percentage, knowing the mixing times, the manufacture temperature, and if it is necessary, modify the compaction process. The second task will be the development of a new test of control and validation of porous mixtures for streets. The performance of the improved porous mixture developed in the previous task will be compared with the performance of conventional mixtures, developing a new additional test specific for its use in streets. Up to date, one of the most used tests to design porous mixtures in the world is the Cantabro test of particle loss. However, it can be improved studying the response of the porous mixtures in front of the tangential efforts that the rolling of the vehicles produces in the cities. Finally, the first activity will be complete with the study of the properties of the green roofs to improve urban resilience. The roofs do not stand any traffic, but they are not multifunctional urban surfaces directly exposed to meteorological harshnesses, being the vegetated roofs of special interest since they are filtering sections with multiple points in common with the pervious pavements. In this task in lab the main construction materials will be studied in the different layers of the vegetated roofs, standing out the separating layer, the draining layer and the antiroot barrier.

The second part of the subproject starts with the identification of urban risk-prone areas, designing a set of tools to identify vulnerable areas in terms of urbanization and Climate Change, taking into account both the susceptibility to experience them and their potential impacts. To this end, Geographic Information Systems (GIS) will be applied to process open-data and characterize aspects related to the resilience of cities, such as flooding phenomena, urban warming and air pollution. After that, location of suitable areas for implementing filtering systems is fundamental with the aim of palliating the vulnerability of the critical areas identified previously. The implementation of filtering systems in the form of permeable pavements and green roofs will be proposed to reduce runoff accumulation, regulate temperature and contribute to improving air quality in urban areas. This location process will be carried out according to siting restrictions. The last task of this subproject will be the selection of alternatives and supported decision-making oriented to invest in urban regeneration. This task will consist of undertaking a comparative evaluation of the alternatives, enabling the determination of the most suitable combination of permeable pavements and/or green roofs according to resource availability. This selection process will be carried out considering the Sustainable Development Goals (SDGs) and the Urban Ecosystem Services (UES).

The focus POREDRAIN project is the analysis and the development of methodologies to evaluate pollution removal and hydrological efficiency of two different filtering sections: porous pavements and green roofs. To achieve this objective, the Water and Environmental Engineering Research Team (GEAMA) from the University of A Coruña has two singular facilities for the analysis of rainfall generation, filtration and pollution removal by means of large-scale rainfall simulators. The first facility is a physically-based street reach model (1:1 scale, about 36 squared meters) for the analysis of retrofitting a conventional impervious pavement with a porous filtering section. The analysis of green roofs will be performed in a block scale model of two street reaches and four roof blocks (1:4 scale, about 100 squared meters). The test will we conducted with conventional and green roofs. With this approach, the main contribution land uses (roads and roofs) to stormwater flows and pollution of consolidated urban areas will be analyzed.

To validate the efficiency of filtering section in both hydrological (runoff peak and volume reduction) and pollution removal aspects, a systematic and comprehensive methodology adapted to Spanish climatic and geographical context will be developed. This methodology will be complemented with a technical guide, a statistical software tool and a database with open access to academics, public organisms and practitioners of sustainable urban drainage sector, allowing the use of a toolkit for the assessment of the efficiency of the filtering sections. POREDRAIN projects activities complement the rest of actions of HOFIDRAIN coordinated projects. Final results will ensure that all the necessary sustainability and intelligent governance criteria for implementing filtering sections will be applied to the processes of urban regeneration and adaptation to climate change included in the field of sustainable management of urban runoff through the application of this kind of Green Infrastructures.