Is Green the New Grey? If Not, Why Not?

  • Approximately USD 6.7 trillion global investments are needed to achieve the water-related SDG targets by 2030. As things stand, these investments will be dominated by traditional “grey” infrastructure
  • Currently, Nature Based “green” Solutions represent only 5% of global investments in water resources
  • Future investments in Nature Based Solutions can enhance water security but constraints need to be overcome to facilitate wide-spread implementation

Grey Infrastructure

Grey, engineered water infrastructure (e.g. dams, irrigation systems, dykes etc) is a cornerstone of socio-economic development and poverty alleviation. Countries without adequate infrastructure are typically amongst the world’s poorest, “held hostage” to the natural vagaries of climate and water. Inability to manage variability in rainfall and river flows is a significant impediment to economic growth, and despite indigenous knowledge and locally devised adaptation strategies, often undermines livelihoods and peoples’ well-being

Meeting future demand for food, water and energy requires significant investment in infrastructure. It is estimated that future global investments in the water sector need to be between US$ 6.7 trillion by 2030 and US$ 22.6 trillion by 2050 if the water-related SDGs are to be achieved. It is anticipated that much of this investment will go into the construction of traditional grey infrastructure.  

Notwithstanding the societal benefits and contributions to national economies, poor people have too often paid the price of past grey infrastructure development. Insufficient attention to environmental impacts and the needs of the poor has resulted in losses of biodiversity and degradation of ecosystem services. Combined with failure to fully compensate people forced to relocate as a consequence of large infrastructure projects this has resulted in suffering and additional development barriers for more than 500 million people worldwide. Impacts are so pervasive that some economists have questioned the rationale for large grey infrastructure, arguing that if environmental and social costs are properly accounted for, many large infrastructure investments contribute more to economic fragility than economic growth.   

Nature Based Solutions

Lake Tana Shoreline, Ethiopia

Nature based, green solutions for water, are being promoted by the UN, the World Bank and others, to complement and, in some instances, replace grey infrastructure. Nature Based Solutions (NBS) for water take many forms but are all inspired and supported by nature and use, or mimic, natural processes to improve the supply and quality of water and/or reduce the impact of water disasters (e.g. floods and droughts).  In 2018, the United Nations World Water Development Report concluded that NBS can enhance water security by improving water availability and water quality whilst simultaneously reducing water-related risks and generating a range of additional social, economic and environmental benefits in both rural and urban environments.

In Colombo, Sri Lanka, wetlands within the city and peri-urban areas were, until recently, being rapidly degraded though pollution and infilling for the construction of buildings. This degradation of wetlands was exacerbating flooding. In 2010, flood damage costed the city more than US$ 50 million and investigations were initiated to find ways to mitigate flood damage. Now, the wetlands are protected and are being rehabilitated for a range of benefits, including flood protection. Computer simulations have shown that during intensive rainfall events the wetlands are able to store tens of millions of cubic metres of water, thereby protecting many parts of the city and reducing economic losses and flood risks for many hundreds of thousands of people.

NBS also offer important responses for climate change mitigation and adaptation. For example, in Ethiopia, “exclosures” are areas in which grazing, woodcutting and agricultural activities are prohibited to promote the natural regeneration of degraded landscapes. Such exclosures can bring many benefits, reducing erosion and improving soil and vegetation, thereby increasing carbon sequestration. In places, they have also improved groundwater recharge, promoting spring and stream flows and increasing dry season water availability for communities.   

Why NBS are not widespread

NBS are not a panacea. They are difficult to finance and implement, and uptake remains limited. Globally just 5% of current investments in water resources is in NBS. There are many reasons why investment in NBS remains low but lack of awareness and knowledge are paramount. Questions have been raised about the reliability and cost-effectiveness of NBS in comparison to grey infrastructure solutions as well as their resilience to climate change.

Frequently, a dearth of numerical information is a constraint to encompassing NBS in water resource planning. For example, whilst an engineer can easily quantify the impacts of grey infrastructure, such as a dam or an embankment, on river flows, the inherent complexity of most natural systems means that it is rarely possible to quantify the impact of NBS in the same way. For example, the way a forest influences river flow depends on a wide range of biophysical factors including topography, climate, soil and geology not just within the forest itself but also the landscape in which it lies. Furthermore, the impact will typically change over time, for example, between wet and dry seasons, as soil moisture and vegetation characteristics change. The absence of both quantitative information and widely accepted methods to calculate impacts, make it difficult for engineers and others to incorporate NBS in water resources planning.  

Other constraints to NBS include the fact that traditional cost-benefit approaches often fail to adequately incorporate the wide range of co-benefits accruing from NBS, many of which are difficult to express in monetary terms. Moreover, in comparison to grey infrastructure, most NBS require participation of far more stakeholders as well as the engagement of non-traditional partners, both of which can be time-consuming and costly, requiring new skills from water resource practitioners. Finally, enabling conditions and policies for financing and implementing NBS are often missing. In contrast to grey infrastructure it is difficult to own, and hence manage, green infrastructure.  

The future

Stream – Ethiopia

It is clear that future investments in water need to build on the lessons of the past. Future investments must be more resilient to climate change and other impacts, higher performing and more cost effective in terms of benefit streams to societies, environments and economies. NBS for water have huge potential and a critical role to play but unless the constraints outlined above can be overcome, grey interventions will continue to be the default in the future. Fully integrating NBS, alongside grey infrastructure, in water resource planning and management requires an improved evidence base incorporated within a systems thinking approach, accounting for multiple ecosystem services and recognizing trade-offs from the perspectives of all, not just some, stakeholders.


Matthew is a principal researcher with the International Water Management Institute, specializing in water resources and wetland and hydro-ecological studies. He is currently leader of the research group Sustainable Water Infrastructure and Ecosystems.

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