Hybrid grey and green water infrastructures (HGGI) are combinations of grey, smart and green infrastructures, aimed at producing (climate) resilient water systems, reliably controlling peak flows and/or delivering clean water, sustaining environmental flows, and providing ecosystem, economic and social services. They can therefore be seen as supplying multiple benefits simultaneously.

The implementation level (planning, designing, constructing and maintaining) of HGGI cannot be evaluated by a Technology Readiness Level (TRL) alone; it also needs a symbiosis of multiple dimensions, as distinguished in the Symbiosis Readiness Level (SRL) (Sommer, 2019). The SRL incorporates four dimensions: Technology, Business, Ecology and Management. The use of the SRL as an approach to evaluate priority actions for the further wide-scale implementation of HGGI has resulted in a list of actions per implementation level.

To outgrow the ‘proof of concept’ level, a Technological priority action could be to improve the performance of green infrastructure solutions through the use of artificial intelligence and real-time control, while an Ecological priority action would be to determine which parameters are to be monitored for environmental, social and economic impact assessment, and what tools (e.g., Life cycle analysis, multi-criteria assessment) are to be used to quantify these impacts, the circularity and the resilience. A priority action within the Business dimension could be to develop methods to effectively communicate the pros and cons in order to strengthen citizen support and engagement. In the Management dimension, match-making meetings could be organized to create initiatives on technological developments of hybrid solutions and demonstration projects.

Further growth on the implementation level, towards ‘commercialisation’, could be achieved by priority actions such as: Incentivize the peloton of water managers to make time for exploring and repeat testing new solutions and to learn from the experiences of the front runners. Demonstrate and quantify the costs, benefits and co-benefits of HGGI solutions in order to maximize ecosystemic benefits and minimize costs, spatial and other resource demands. And identify and remove legal and regulatory barriers keeping utilities or businesses from taking up new roles and responsibilities in urban water management.

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