1. Chief Scientist’s Group report summary

Nature-based solutions (NbS) are increasingly being used to reduce human impacts on river catchments. However, evidence of their effectiveness to deliver multiple benefits in different catchment settings is limited. This project examined the effects of NbS on freshwater environments by synthesising evidence from the scientific literature. Understanding where NbS are most effective will help the Environment Agency, catchment managers and practitioners make more informed decisions about their implementation.

1.1 Background

Many river catchments and waterbodies in England have been modified through land-use change, agricultural intensification, abstraction, and wastewater discharge. While these changes were intended to benefit society, some have damaged freshwater ecosystems and changed water quality, water availability and flood risk. NbS are intended to improve catchment function and to protect, manage and restore ecosystems. Examples include afforestation, constructed wetlands (see Figure 1), and the re-introduction of instream wood. NbS can also enhance the resilience of the water environment to pollution and the impacts of climate change. However, there are also trade-offs: sometimes NbS can have unintended consequences for other parts of the environment. These trade-offs may vary over time or only occur in specific settings. It is important to understand multiple benefits and potential trade-offs of NbS in different contexts.

Figure 1. A series of connected ponds designed to help mitigate diffuse agricultural pollution by capturing and filtering sediment and nutrients from the surrounding farmland.

1.2 Approach

This project involved a review of 135 reports from the scientific literature identifying the benefits and disbenefits of 17 types of NbS in 9 different catchment contexts. Benefits and disbenefits are considered under the environmental themes of low flows, flood risk, nutrient and sediment pollution, water quality, and biodiversity and habitats. Nine broad contexts are used to cover the range of catchment types within England. Where information is available for a given NbS and catchment context, a level of confidence is attributed based on the quantity and quality of evidence and summarised in a ‘look up’ matrix.

1.3 Results

The evidence suggests that NbS largely have positive effects on the environment. For example, NbS were shown to enhance water quality through intercepting, transforming and storing nutrient pollutants in a wide range of settings. However, several studies indicated mixed or negative effects of NbS on water quality. This highlights the importance of selecting the right interventions and implementing them in the right places. In some cases, the effectiveness of NbS varies over time. Hedgerows and ponds can trap nutrients mobilised in storm events but release stored nutrients in subsequent storms. Extreme events and a changing climate may affect the outcomes that can be achieved by NbS. More studies address the potential of NbS for flood risk management than for low flows, biodiversity or habitats. The most studied contexts are mixed agricultural areas and arable-dominated landscapes with lighter soils. Notable knowledge gaps remain in wet upland landscapes and lowland peat environments.

This review is limited to freshwater-related NbS from English catchments (or those in comparable climates), and therefore only covers a selection of the global evidence base. It is acknowledged that there are additional benefits and disbenefits of NbS (for example, climate regulation) beyond those covered by the environmental themes considered in this project.

1.4 Conclusions

NbS have a wide range of benefits, but these depend on the catchment context in which they are implemented, and by ongoing management and maintenance. The evidence matrix and report produced from this project can be used by practitioners to help make decisions about the suitability of NbS for different catchments and to minimise trade-offs and unintended consequences. The matrix can be updated as further information becomes available, thereby providing greater confidence in the knowledge of what benefits NbS can provide.

1.5 Publication details

This summary relates to information from project SC230009, reported in detail in the following output:

• Report: SC230009/R3
• Title: Multiple benefits of nature-based solutions: an evidence synthesis
• Project managers: John Robotham and Judy England, Chief Scientist’s Group

This project was delivered by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: [research@environment-agency.gov.uk] (mailto:research@environment-agency.gov.uk).

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