Green-Grey Infrastructure Solutions

Delivering resilient and robust solutions to water security challenges often requires a combination of “green”
and “grey” solutions. While nature-based solutions (NbS) can help secure clean, reliable water for people and nature and mitigate against water-related climate disasters, while delivering on a number of co-benefits, water resources managers will continue to rely on built or grey infrastructure to deliver water security objectives. Moreover, an existing installed base of conventional grey infrastructure is often present in watersheds, which can be complemented with green solutions to enhance water security impacts and/or reduce costs associated with infrastructure operations and maintenance.

Green infrastructure (also sometimes called natural infrastructure, or engineering with nature) intentionally and strategically preserves, enhances, or restores elements of a natural system, such as forests, agricultural land, floodplains, wetlands, coastal forests (such as mangroves, and riparian areas, among others (Browder et al. 2019).

Grey infrastructure refers to built structures and mechanical equipment, such as reservoirs, embankments, pipes, pumps, water treatment plants and canals. These engineered solutions are embedded within watersheds or coastal ecosystems whose hydrological and environmental attributes profoundly affect the performance of the grey infrastructure (Browder et al. 2019).

While the terms green and grey infrastructure appear to be binary or opposed to one another, the reality is that there is a green-grey spectrum along which these NbS and grey infrastructure options exist. Different combinations of solutions along this spectrum can be assembled in a portfolio fashion to match the project objectives and local conditions regarding political, social, and natural environment conditions.
 

Importantly, watershed conditions will always modify and impact the efficacy of grey infrastructure, and hence holistically considering all options along the spectrum broadens the toolkit available to address water security concerns. The specific kinds of solutions generally depends on (1) project objectives, (2) desired performance and reliability characteristics, (3) relative cost characteristics, (4) local policy and regulations, (5) native ecosystems and existing watershed health, (6) land use patterns and (7) social acceptance of proposed works.

The most cost-effective and sustainable system for water management and delivery of water services likely
requires a thoughtful combination of NbS and grey infrastructure, which necessitates integrating these concepts into water resource planning processes. 

The two pathways for integrating NbS into hybrid green-grey design as part of an integrated water management system include the following:
 

Between these two options, the first option is by far the most common today; however, there is an opportunity to change the way infrastructure is planned, designed and financed to more effectively address water management challenges with intentional integrated planning from the start.

To assist in initial planning, the below table provides examples of complimentary green and grey solutions to common water management issues adapted from Cassin et al., 2020).