NbS & water management

NBS can contribute to sustainable urban water management by increasing infiltration, enhancing evapotranspiration, providing storage areas for rainwater and removing pollutants. In order to prevent cities from being flooded, rainwater must be effectively discharged from areas where its accumulation can result in harm to humans and damage to infrastructure. Creating artificial water bodies or ecosystems within urban areas, or conserving and enhancing natural ones, can retain and store rainwater and urban run‐off. The aim is to prevent precipitation water from directly flowing into the sewerage system (overcharging the system), thus reducing and delaying flood peaks and allowing controlled discharge. NBS for water retention include creation of natural spaces for temporary water storage (green areas and urban wetlands); improving infiltration (green areas, plants improving infiltration); and enhancing evapotranspiration (trees, green areas, parks). Storing stormwater and grey water can also conserve water for re‐use both on‐site (e.g. for maintenance of green areas) and for distant water needs (Young et al., 2014), thus providing additional water resources and reducing pressure on existing freshwater sources.

Using NBS rather than grey infrastructure for water storage allows for additional infiltration, contributing to the replenishment of ground water resources with potential positive impacts on water availability.

NBS for water storage and stormwater management can be combined with NBS for increasing water quality and water use efficiency by remediating some of the wastewater and urban run‐off stored using phytoremediation, so that it can be reused, released into water bodies or allowed to infiltrate into the ground. Measures of this kind can contribute to reducing the depletion of freshwater resources and thus increase drought resilience.

NBS may have the potential to transform an urban area with an impermeable surface into an urban water body with renewed ecosystems, with water flows and functions which can be integrated into the wider catchment. Nature‐based or combined grey‐green solutions for water management can also provide additional co‐benefits with regards to:

• Urban biodiversity
• Improving the urban environment and living conditions, with benefits for human well‐being and quality of life
• Improvement of air quality, with benefits for human health
• Improving the urban microclimate and reducing the urban heat island effect through the cooling effect of evapotranspiration;
• Climate mitigation. As for all NBS, urban trees and green areas enhance direct carbon sequestration in plants and soils, but the quantities involved may not be significant enough for NBS to be considered an effective means for achieving local GHG reduction targets, especially when irrigation is required for maintaining plants (Baró et al., 2015; Pataki et al., 2011). However, green solutions may produce lower emissions compared to grey solutions (e.g. engineered solutions made from cement and other construction materials) that aim at the same goals.
• Indirect economic benefits (increasing real estate values and tax income for local governments).

Potential water management actions: 

• Renaturing (opening urban waterbodies channels, de‐culverting, vegetation, greening increase waterfronts). 
• Use of vegetation in urban areas (e.g. street trees, grassland, green roofs and facades, infiltration gardens and urban forests). 
• Creation of artificial waterbodies for short term temporal water storage. 
• Creation of new vegetated surface waterbodies (ponds, drains, lakes, bio‐retention cells). 
• Creation of new subsurface waterbodies for water storage. 
• Create areas for temporary flooding along rivers (floodplains) by moving flood protection infrastructures. 
• Restore/create/increase wetlands in river‐basins.