Evaluating the multiple benefits of a Blue-Green Vision for urban surface water management

A Blue-Green City aims to recreate a naturally-oriented water cycle while contributing to the amenity of the city by bringing water management and green infrastructure together. The Blue-Green approach is more than a stormwater management strategy aimed at improving water quality and providing flood risk benefits. It can also provide important ecosystem services, socio-cultural benefits and adaptability to future (uncertain) changes in climate and landuse. However, quantitative evaluation of the benefits, their spatial distribution and co-dependencies are not well understood. The Blue-Green Cities Research Consortium has adopted an interdisciplinary approach to quantitatively evaluate the benefits of Blue-Green infrastructure (BGI) and their relative significance. A new ArcGIS evaluation tool has been developed which can identify the spatial distribution of different benefits and normalise benefits onto a uniform scale. This allows the local impact of multiple benefit types, benefit dependencies and dis-benefits to be directly compared, helping decision makers to co-optimise the benefits from the outset of project planning. The tool was successfully piloted in 2014 in Portland, Oregon, a city with a Blue-Green Vision and extensive investment in green infrastructure, primarily to help reduce the number of combined sewer overflows and improve water quality. This paper also reports on the application of the benefit evaluation tool in Newcastle (UK). Here, hydrodynamic models have been developed to simulate pluvial flood inundation and the movement of water through BGI. An overland flow model has been integrated with the subsurface drainage network to handle discontinuous free surface and pressurised flows. This allows the simulation of mixed flows in pipes and realistic modelling of sewer outflow events. A hypothetical future is presented for a residential area of Newcastle where all pavements and back-alleyways have permeable paving and all gardens are greenspace. Modelling shows that the BGI provides temporary storage and helps alleviate the burden on the subsurface system. The Blue-Green Vision for Newcastle was developed by the Learning and Action Alliance (LAA), an open arrangement where participants create a joint understanding of a problem and its possible solutions based on rational criticism and discussion. The LAA encourages cooperation between a diverse range of stakeholders from different disciplines and backgrounds, including local authorities, major landowners, water companies, academia and environmental groups, and represents a novel approach to facilitate the negotiation of a Blue-Green Vision that addresses strategic objectives, public realm improvements and, not least, the management of urban surface water.