Nature Based SuDS Guidance Document

INTRODUCTION

The integration of NbSuDS into the system of a development can deliver better solutions environmentally, socially, and economically by developing interrelated design solutions that a traditional approach to drainage would not consider. This section explores what these benefits are and how they can be realized.

NbSuDS offers an alternative solution and demonstrates not only that surface or stormwater can be cleaned, reduced, or slowed down before going back into the overall drainage infrastructure network, but that it can also achieve many other social, environmental, and economic benefits while doing so. NbSuDS can help humanity adapt to some of the challenges we face. They can help communities increase their resilience to the future effects of climate change by mitigating the effects of increasing temperatures and more extreme rainfall. They can also help efforts to reverse biodiversity loss by providing resilient habitats using planting that is robust to the changing climate.

“In aspiring to solve one crisis, we have the opportunity to solve many more and deliver multiple benefits for little or no additional cost.”
CIWEM A Place for SuDS

There is inevitably an overlap between social, environmental, and economic benefits, as often what is good for one may also benefit another. This holistic value is intrinsic to the very core of NbSuDS, which is multi-functional at its heart and should be recognized, valued, and celebrated.

Some of the roadblocks to NbSuDS, both actual and perceived, can be easily addressed when all parties recognize the value of the end product. Different schemes will have different drivers or requirements, and the nature of the site constraints and conditions will fundamentally influence the benefits that can be achieved and to what extent. To maximize the benefits of NbSuDS within a scheme, it is important to consider it at the outset of a project, integrating it into the fabric of the scheme rather than having it as an afterthought. Holistic multidisciplinary thinking between Civil Engineers, Landscape Architects, and Ecologists at the start of a project is much more likely to lead to better outcomes which can maximize the multiple benefits outlined in Table 3.

Environmental | Social | Economic

• Reducing energy consumption | Noise reduction | Land-value uplift
• Reducing surface water flood risk | Heat stress reduction | Reduced flood risk
• Reducing sewer flood risk | Reducing air pollution | Productivity improvements
• Reducing spills from combined sewer overflows (CSOs) | Improved mental health and wellbeing | Increased footfall and retail activity
• Improving water quality downstream | Improved physical health and wellbeing | Green job creation
• Increasing biodiversity | Increased social interaction | Enabling development
• Replenishing groundwater | Increased physical activity | Lower water treatment costs
• Urban heat island (UHI) effect reduction | Educational resource | Improve longevity of surrounding assets
• Carbon sequestration | Reduced crime | Reduced infrastructure costs
• Traffic calming | Payment for NbSuDS

Table 3: Summary of the Benefits of NbSuDS

ENVIRONMENTAL BENEFITS

The sustainable management of surface water runoff using NbSuDS systems, which mimic natural drainage processes, can provide numerous environmental benefits, supporting broader biodiversity and local amenity aspirations in addition to the more traditionally referenced benefits associated with flooding and water quality.

• Reducing surface water flood risk: A loss of natural vegetation in the region has significantly increased the volume and rate at which surface water runs off, thereby increasing the risks of flooding if it is not managed. NbSuDS can be used to manage it by attenuating and/or infiltrating surface water at the source from surrounding impermeable surfaces, reducing the volume or flow rate passed downstream. This reduction in volume and flow rate are the reasons they can reduce sewer flood risk. Similarly, where there is a connection to a combined sewer, these same reasons can help contribute to reducing combined sewer overflows (CSOs) spills to rivers and thus helping improve river quality.

GREENER GRANGETOWN

Following the installation of raingardens at Greener Grangetown, the energy use of the local Marl pumping station was reduced by 17% compared to pre-construction, despite the area receiving double the rainfall volume in the monitored year.

• Improving water quality: When surface water runs over hard surfaces, it collects sediment, litter, and pollutants including oils, grits, metals, fertilizers, pesticides, animal wastes, salts, and pathogens. NbSuDS can improve the quality of water through settlement, filtration, absorption, and the breaking down of pollutants in surface water by vegetation, soil, and the sun. This can result in a reduction of pollutants of up to 90%.
• Increasing biodiversity: NbSuDS can provide a variety of habitat niches, increase the native flora, and increase the structural diversity within the local environment, providing opportunities for fauna to flourish. They provide the ideal opportunity to bring wildlife-friendly green spaces into our towns and cities and link these with existing habitats to create blue and green corridors.
• Replenishing groundwater: Infiltrating systems are designed to allow surface water they collect to drain into subsurface soils and recharge groundwater supplies, where suitable soil conditions exist. The monetary value of this recharged water has been estimated to be worth millions of dollars in some US states.
• Urban heat island (UHI) effect reduction: The presence of green spaces which NbSuDS can provide can mitigate the effects of UHI, which help with climate adaptation. Permanent water bodies that can be provided by some NbSuDS interventions (e.g., ponds and wetlands) can lead to even more effective cooling. Temperature differences associated with trees alone can be as much as 8°C when compared to areas without trees and green roofs can reduce the temperature of the outdoor environment by as much as 42%. This can lead to reduced energy consumption associated with air conditioning.
• Energy consumption can also be reduced by removing surface water from the drainage network, so that energy expended at pumping stations and treatment works is reduced by reducing the volumes they have to deal with.
• Carbon sequestration: Both the plants and the soil associated with NbSuDS can sequester and store carbon. This sequestration of carbon will vary depending on the nature and design of the system used.

BIODIVERSITY NET GAIN

Well-designed NbSuDS can positively contribute towards a project's BNG delivery by providing more habitat units than other urban habitats. Traditional grey landscaping (roads, pavement, etc.) often found within urban environments provides zero habitat units regardless of area, so NbSuDS can provide a substantial contribution to the BNG of urban projects onsite, minimizing or avoiding the need to buy habitat units from external habitat banks. An indication of the maximum amount of habitat units which could be gained is provided, with the proposed habitat within the Statutory Biodiversity Metric shown in brackets.

To maximize the habitat units created and therefore the BNG benefit of NbSuDS, the design and maintenance of NbSuDS should aim to incorporate the following:

• Vegetation structure should be varied, providing opportunities for fauna to live, eat, and breed. Sand piles, stones, and logs can be utilized to create additional micro-habitats.
• Planting should be a mix of native species, appropriate to the conditions within the SuDS; this should include aquatic and marginal plants.

Large scale NbSuDS

Providing a high number of habitat units and maximum benefit to biodiversity.

Medium to Large scale NbSuDS

Interventions as an alternative to ornamental planting or amenity grassland.

Small scale NbSuDS

Interventions as an alternative to hardstanding.

• Infiltration and Detention Basins – 8.40 habitat units delivered per hectare (other neutral grassland).

• Ponds & Wetlands – 10.09 habitat units delivered per hectare (ponds (priority habitat)).

• Bioswales – 3.61 habitat units delivered per hectare (bioswale).

• Raingardens – 5.02 habitat units delivered per hectare (rain garden).

• Green & Brown Roofs – 5.63 habitat units delivered per hectare for a (biodiverse green roof).

• Rainwater Downpipe/Planters – 1.93 habitat units delivered per hectare for (ground-level planters).

• Bioretention Tree Pits - 2.80 habitat units delivered per hectare for (urban trees).

• Absence of invasive non-native plant species (listed on Schedule 9 of WCA) and others which are to the detriment of native wildlife.

• Ponds and wetlands should aim to replicate natural habitats, with good water quality, and should not be stocked with fish. Water levels should be allowed to fluctuate naturally, but water should be present throughout the year (unless drought conditions exist at the peak of summer).

BNG can be maximized further through increasing connectivity of semi-natural habitats and the creation of green corridors as well as linking to local conservation priorities (likely to be listed within LNRS). Where increased connectivity can be demonstrated it is possible to achieve a 10% increase in the created habitat units within this area. This can be increased further to a 15% increase if it can be demonstrated that created habitats within the area are formally identified within local conservation priorities or where this aligns with measures within the LNRS.

SOCIAL, HEALTH AND WELLBEING BENEFITS

NbSuDS solutions can offer more opportunities than ‘grey’ infrastructure to contribute positively to human health and well-being, sharing many of the benefits of green space, which has been proven to have a positive effect on health, quality of life, and amenity for people. Additional social benefits can be realized where local communities are involved in the process of developing and maintaining NbSuDS, contributing to a sense of ownership and community.

• Noise reduction: Roadside vegetation can decrease road traffic noise and the perception of noise. NbSuDS habitats can generate natural sounds, such as birdsong or trickling water, which can significantly reduce the perception of road traffic noise.
• Heat stress reduction: The Urban Heat Island effect increased heat-related mortality by approximately 50% in London during the 2003 heatwave, which was attributed to over 2000 excess deaths. NbSuDS can be used to help reduce the Urban Heat Island effect. The 2022 heatwaves were associated with a total of 2,985 excess deaths in England and heat exposure is estimated to cost the UK economy £260–300 million per year.
• Reducing air pollution: Several studies into the effect of vegetation on air pollution have concluded that correctly designed vegetation can enhance air quality, thereby improving people’s health and wellbeing, with street trees and lower-level roadside planting particularly effective. It is estimated that a 1 µg/m3 reduction in fine particulate air pollution in England could prevent around 50,900 cases of coronary heart disease, 16,500 strokes, 9,300 cases of asthma, and 4,200 lung cancers over an 18-year period.
• Improved mental health and wellbeing: Even a short exposure to the natural environment can have benefits in terms of stress reduction, proven in multiple studies. Higher biodiversity and the presence of water are associated with a greater positive effect. NbSuDS can contribute to the provision of attractive places for increased social interaction, such as between neighbors, which can help promote a sense of community. When communities are involved in the design, installation, or maintenance processes, this can improve participants’ health and wellbeing.
• Improved physical health and wellbeing: The recovery of a group of patients with a view through a window of the natural environment was proven to be faster and required less medication than a group of patients with a view only of a brick wall. Other studies have since linked exposure to the natural environment with a reduced heart rate, decreased blood pressure, and reduced stress hormone levels. Similarly, studies have found that the outdoor environment has an influence on levels of physical activity, and higher quality green spaces are associated with increased physical activity.

Case Study: Early Stakeholder Engagement

Cambrian North Swale opening © Arup

COMMUNITY OPENING AT CAMBRIAN NORTH

Early stakeholder engagement was a key part of this project and the design incorporated measures to address concerns raised by residents. Upon completion, a public opening day was held which encouraged community spirit and celebrated the success of the project.

• Educational Resource: NbSuDS can be used as an educational resource to highlight how surface water is managed and treated, increasing awareness surrounding flooding and the water cycle to name a few.
• Reduced Crime: Urban areas with high levels of ‘greenness’ have lower crime rates than those without. In one study, neighbourhoods where vacant lots have been converted into small parks and community green spaces were associated with reduced crime when compared to neighbourhoods with unimproved vacant lots.
• Traffic Calming: Well-designed NbSuDS in the highway can be used to encourage lower speeds and safer driver behavior, which can create a safer and more welcoming pedestrian environment.
• Improved Amenity: Well-designed NbSuDS can be used to make an area more visually attractive and provide more amenity value for the people who live, work, and visit these places.

ECONOMIC BENEFITS

There is mounting evidence that highlights how NbSuDS can positively contribute to economic outcomes through several ways which are often also linked to improved social and environmental outputs. NbSuDS have demonstrated financial advantages due to a reduction in initial capital expense and lower ongoing operational expenses.

• Land-value uplift: The presence of NbSuDS enabled street trees and green spaces can provide increases in nearby property and rental values, by making new developments more attractive to prospective buyers and renters.
• Reduced costs associated with flooding: NbSuDS can contribute to reducing the costs associated with surface water flooding, which is estimated to cost England £1.3bn to £2.2bn per year, 29% of which falls to businesses.
• Increased footfall and retail activity: Alongside attracting visitors, well-planned green public realm improvements can also lead to customers being more likely to spend on products by 10-50% and can induce increases in commercial trading rates by up to 40%.
• Green job creation: Through the design, construction, and maintenance of NbSuDS, green employment opportunities are created, preparing a more environmentally aware workforce for the future.
• Enabling development: NbSuDS can be used to unlock land that would otherwise be unavailable for development due to a lack of sewer capacity. Developments that create places where people want to live and work can be more attractive and can sell for more.
• Lower water treatment costs: NbSuDS, especially those that infiltrate or use water on site, can reduce the amount of water being conveyed to wastewater treatment facilities, thereby reducing treatment costs.
• Improved longevity of surrounding assets: Features like green roofs can prolong the life of other assets by protecting building components from the elements.
• Reduced infrastructure costs: NbSuDS can be designed to reduce the capital cost of drainage for a site when considered from the start as part of the whole design and thus cost. It can also reduce costs associated with traditional drainage infrastructure over its lifespan by reducing the need for infrastructure expansion and extending its life expectancies.

Payments for NbSuDS:

Payments for services such as mitigating surface water runoff, reducing nutrients entering watercourses, and avoiding costs of water treatment are an emerging market approach to finance nature-based solutions. Currently, this includes:

• Reduction in site owner drainage bills for sites that install SuDS (offered by only 4 out of the 11 water utilities in England and Wales currently).
• Outcome payments from water utilities for reductions in surface water runoff, pressure on the wastewater network, CSO’s, and treatment systems (subject to evidence and agreements with the utility company).
• Payments from Regional Flood and Coastal Committees for flood risk reduction benefits (DEFRA flood defense grant or local levy).
• DEFRA grants for SuDS at schools with risk of flooding.

The Benefit of SuDS Tool (BeST) developed by the UK Construction Industry Research and Information Association (CIRIA) can be used to appraise some of these wider benefits. The CIRIA tool follows a value transfer approach which utilizes information from other similar projects nationally, or from the UK. All of the original studies use methods that survey the population and ask them to reveal their preferences, or assign monetary values to outcomes (such as willingness-to-pay, willingness-to-accept, and hedonic pricing methods) to capture values for amenity, air quality improvements, biodiversity and ecology gain, and broader and wellbeing health improvements for society. The BeST tool is available at CIRIA BeST.