Background and industry context

Green infrastructure

Natural England ^[1] describes green infrastructure as ‘…a network of natural spaces designed to deliver benefits for people and the planet. This includes, more good quality parks and greenspaces, green routes and corridors like footpaths, cycle lanes, rivers and canals, more nature in towns and cities like wildlife meadows, street trees, ponds and green roofs.’

Green infrastructure (GI) is an essential aspect of all landscapes across the UK. Preserving and supporting GI in the design of High Speed Two (HS2) Phase One (London to West Midlands) is critical to meet the HS2 Environmental Minimum Requirements ^[2], and in the pursuit of technical excellence, learning, and innovation.

The primary aim of this paper is to illustrate methods that prevent the long-term fragmentation of green spaces, using excellent environmental design including green bridges and viaduct-integrated landscapes, through a series of HS2 policies, standards and case studies. HS2, as a nationally significant infrastructure project, has ensured that through its design development GI along the route has been successfully and innovatively integrated within the scheme.

Approach

The concept of green infrastructure preservation

GI preservation embodies a forward-thinking approach to urban and rural planning, recognizing the critical role that natural elements play in sustaining healthy communities and ecosystems. Unlike traditional infrastructure, which focuses primarily on built structures, GI preservation emphasises the conservation and enhancement of natural features like parks, forests, wetlands, and green spaces within the built environment. This concept revolves around the idea that nature is an integral part of a resilient and sustainable community. Preserving and managing GI plays a role in mitigating the impacts of urbanisation, climate change, and biodiversity.

Furthermore, GI preservation acknowledges the complex benefits these natural assets provide. From acting as carbon sinks to reducing the urban heat island effect, these green spaces positively influence the well-being of the environment and people. These assets supply areas for recreation and education, habitats for wildlife and provide environmental services such as flood defence or absorption of air pollution. These natural and semi-natural areas are identified as part of the basic ‘infrastructure’ that enables us to live as we do.

GI is explored within the following HS2 technical documents and initiatives which inform the design of the scheme.

The HS2 Design Vision ^[3]: This document outlines the approach to Design along the HS2 route, with the following key points highlighting the critical importance of Landscape and Environment to HS2:

• The Design Vision sets out the role that design can play in making High Speed Two a catalyst for growth across Britain. To do this we are creating many new places and spaces, such as stations, terminals and rain environments, as well as restoring and growing natural environments. This principle requires that we consider regeneration, identity and environment.

• HS2 has a positive environmental rationale – that high-speed rail is the most efficient way to transport people between cities. Building and maintaining the network will nevertheless create adverse environmental impacts. HS2 will use design to help deliver imaginative, appropriate and environmentally sensitive solutions.

The HS2 Landscape Design Approach ^[4]: HS2 Ltd.’s Landscape Design Approach outlines the projects approach to Landscape Design along the HS2 route, with the following points highlighting where GI is underlined as a pivotal requirement within the delivery of the landscape designs:

• HS2 landscape proposals present a vital opportunity to contribute to sustainable development and regenerate our natural environment. A landscape-led design approach to HS2 ensures that all design elements work together, seeking multiple functions and benefits and delivering the HS2 Design Vision ^[3].

• The route passes through diverse regions and a deep understanding of the landscape, and its symbiotic relationship to the other engineering and environmental disciplines, is essential. This will help to guide and manage how the existing natural environment and new civil engineering structures can have a positive relationship with the landscapes which interact with the route. The scale, importance and delivery of the integrated landscapes make it central to the success of HS2.

• Landscape architects have the ability to deliver outcomes that work for their specific site as well as the wider context. Proposals can contribute to the ambition for a ‘Green Corridor’. This is a network of spaces and assets relating to green, blue and grey infrastructure that run along the HS2 route. Some of these benefits will be delivered through the works carried out by HS2 whilst others will be realised through the support of HS2 funds.

The HS2 Phase One Information Paper E11: “Green Infrastructure and The Green Corridor” ^[5] outlines the concept of a green corridor that will be developed through the construction, mitigation and implementation of the scheme. The information paper introduces the GI concept.

The vision is to create a Green Corridor ^[6] along the spine of England that connects people and nature, safeguarding the environment for future generations, and to create a railway that integrates into the character of the surrounding landscape and is an enhancement catalyst for the environment surrounding it. The green corridor aims to achieve this by mitigating the negative impacts of constructing the railway by creating habitats, green spaces and accesses that connect to the surrounding landscape. Through the provision of additional funds such as local community and environment funds, it also aims to help stakeholders adjacent to the route realise their own GI projects that have benefit for both nature and local communities.

The landscape design of HS2, through the guidance of the above documents, aims to enhance GI and create a green corridor which not only avoids severance and fragmentation, but that actively enhances biodiversity through considered context led design and through ecological connectivity.

Avoiding severance: the fragmentation challenge

One of the challenges in GI preservation is the risk of fragmentation. This section will investigate the consequences of fragmentation, explaining its adverse effects on biodiversity, ecosystem services, and community connectivity.

The HS2 route between London and Birmingham will total 225 km (140 miles) in length, and will be made up of over 150 bridges, over 50 viaducts, over 110 embankments and over 70 cuttings (see figure 1). HS2 Ltd is committed to mitigating the environmental impacts of the project and will deliver many miles of ecological and landscape habitat alongside the new railway. These will comprise approximately 3,500ha of habitats including up to 7,000,000 trees and over 375km of hedgerows (see figure 2). The project has already delivered over 100 ecological habitat sites which include over 180 wildlife ponds, and over 165 barn owl boxes; all of which were delivered in advance of the main construction works.

The challenge of fragmentation within the landscape poses a significant threat to the effectiveness of GI, with far-reaching consequences for biodiversity, ecosystems, landscape character, and community connectivity. Fragmentation occurs when green spaces are severed or divided, disrupting the continuity of landscapes and habitats. Understanding the adverse effects of this is essential for sustainable landscape planning and design.

Biodiversity is particularly vulnerable to fragmentation. Disconnected green spaces limit the movement of species, leading to isolated populations that are more susceptible to genetic bottlenecks and extinction risks. This loss of diversity can have increasing effects, affecting the resilience and adaptability of ecosystems. Ecosystems, too, suffer from the effects of fragmentation. Disrupted connections between habitats impede the flow of essential nutrients, water, and species, disrupting ecological processes. This fragmentation can result in altered ecosystem dynamics, affecting everything from plant pollination to predator-prey relationships.

The impacts of fragmentation and biodiversity loss have led to a decline of our wildlife according to the “State of Nature Report” 2023 ^[7]. “The UK, like most other countries worldwide, has experienced a significant loss of biodiversity”. “As a result, the UK is now one of the most nature-depleted countries on Earth”. “Across the UK, the species studied have declined on average by 19% since 1970”.

The UK Government’s 25 Year Environment Plan ^[8] emphasises improving peoples access to green spaces by promoting the creation of more, and also better-quality, green spaces. It aims to enhance the well-being of communities through initiatives that connect people with nature, fostering healthier and more sustainable living environments. The plan underlines the importance of preserving and explaining GI to the benefit of both people and the environment. The character of landscapes is tied to their continuity, and fragmentation poses a threat to this coherence. Disjointed green spaces often lack the visual and functional integration that contributes to the distinctive character of an area. This loss of landscape character not only diminishes aesthetic value but also weakens the sense of place and identity associated with a region. In terms of community connectivity, fragmented GI hinders the development of cohesive and accessible public spaces. Isolated pockets of greenery may be less utilised, reducing opportunities for recreational activities and social interactions. This lack of connectivity can adversely affect the well-being and sense of community among residents.

The importance of GI in HS2 Ltd.’s approach reflects the government’s environmental aims, as set out in the following documents:

• The Government’s Natural Environment White Paper ^[9] refers to the role of planning in protecting and improving the natural environment and facilitating coherent and resilient ecological networks that reflect the value of natural systems and advocates that green spaces should be factored into the development of all communities.

• The Government’s Biodiversity Strategy for England ^[10]builds on the Natural Environment White Paper, advocating the reduction of environmental pressures from planning and development, by taking a strategic approach to planning for nature and retaining the protection and improvement of the natural environment as core objectives of the planning system.

• The National Planning Policy Framework ^[11] supports the objectives of the Natural Environment White Paper, stressing a proactive and strategic approach to planning for the natural environment, noting that planning policies and decisions should both contribute to, and enhance, the natural and local environment.

Addressing the challenge of fragmentation requires strategic planning that emphasises connectivity and which creates a network of green spaces that seamlessly weave through urban and rural landscapes. This approach not only safeguards biodiversity and ecosystems but also enhances landscape character and fosters community connectivity for a more sustainable and resilient future.

It is anticipated that, without interventions, the HS2 Scheme would result in cumulative significant effects as well as location-specific impacts due to fragmentation and the severance of habitats. The barrier effect caused by such linear infrastructure would have a direct negative impact on the movement of species across the landscape. This subdivision of habitats would divide species into two smaller and therefore less stable and viable populations. The cumulative effect of the HS2 proposals, without mitigation, along with other existing infrastructure, coupled with the presence of urban areas and agricultural landscapes, would increase the fragmentation of habitats and the isolation of species. Those species that do endeavour to cross the HS2 route will experience a higher level of mortality with the collision risks associated with trains and the rail infrastructure, such as overhead power cables. The likelihood of collision would be increased where the route passes through existing commuting and migration routes for species that have been used for long periods of time, such as established bat flight paths between roosts and foraging habitats.

Reconnection strategies: the solutions

Implementing reconnection strategies for landscapes severed by roads and railways involves innovative solutions to preserve and enhance the natural environment combining new and existing green assets.

There are various approaches to reconnection that have been explored and developed through the HS2 design process. One approach is burying the route, which minimises visual and noise impact and allows continuity of green assets and habitat connectivity. Tunnels, whether traditional or cut-and-cover, such as Burton Green Tunnel ^[12] offer ways to traverse the landscape without disrupting its surface, maintaining ecological connectivity. Another option is bridging over landscape features via aesthetically designed structures allowing for unimpeded transportation and ecological continuity across the landscape.

Green bridges – bridging habitats and enhancing connectivity

Green bridges, also sometimes known as wildlife crossings are structures designed to facilitate the movement of flora and fauna across barriers like highways or rivers.

Green bridges reduce significant ecological impacts caused by severance, enabling the movement of wildlife and preventing genetic isolation. They play a crucial role in supporting the ecological balance of ecosystems by allowing species to access vital resources, such as feeding or breeding grounds, on either side of the scheme (see figure 3). These structures also address safety concerns for both humans and animals, reducing the risk of collisions between wildlife and vehicles. In addition to wildlife benefits, green bridges contribute to overall landscape connectivity and decrease visual impacts, creating a more aesthetically pleasing corridor for members of the public.

The HS2 Phase One Information Paper E15: “Green Bridges” ^[13] explains what green bridges are, how they will contribute to mitigating the environmental impacts of the proposed scheme, and the way they will be integrated into the surrounding landscape. It categorises green bridge types based on their principal purpose:

• Type 1 – Green bridges for Bechstein’s bats.
• Type 2 – Green bridges for important populations of scarce bat species (other than Bechstein’s bats) and high value assemblages of bats.
• Type 3A – Green bridges for landscape and habitat connectivity and/or the dispersal and passage of wildlife (with a single vegetated zone).
• Type 3B – Green bridges for landscape and habitat connectivity and/or the dispersal and passage of wildlife (with a double vegetated zone).
• Type 4 – Other types of green bridge with site-specific functionality.

Considerations which have informed how green bridges have been incorporated into HS2 designs to encourage ecological connectivity include:

• Invitational areas: Locate plants that are attractive to target species at the entrances to the bridge (e.g. fruit producing shrubs) so that the target animal species ‘discover’ the crossing and use it.

• Habitat structure and vegetation guides: It is important that there is diversity in planting to accommodate a wide range of species including UK natives, with a combination of shelter, and open habitats. Corridors of vegetation (e.g. hedgerows) which lead animal species to the bridge, and which continue across the bridge, will deliver animals safely to the other side.  Bridges with taller linear tree planting features leading up to and across the bridge, appear to be more intensely used by bats than those bridges with scattered features.

• Width: Existing green bridge examples across Europe range hugely from 8 – 140 m in width. A recommended width in Europe is 40 – 50 m. However, this can be reduced for England as they are not required to accommodate larger mammals such as bears. The A21 Lamberhurst bypass Scotney Green Bridge ^[14], a celebrated UK green bridge example, is 30 m at the narrowest point and increases in width at the edges.  This bridge accommodates an access road to Scotney Castle, and most species have been shown to be using the bridge (including hazel dormouse, which is a European Protected Species).

• Noise and light reduction: Studies found that bridges with earth mounds or screens to reduce noise (particularly sudden noises such as a train passing) and with no lighting installations were used more intensively by animal species.

• Location: Green bridges should be installed where existing high-quality habitats or existing wildlife corridors are severed. By having more than one green bridge along the scheme, and linking severed habitats, species will be more likely to locate and use a green bridge.

These principles are outlined in case study 1 Calvert Green Overbridges within Buckinghamshire. These green overbridges mitigate the impacts of HS2 on an important bat population associated with constituent woodlands of the former Bernwood Forest, including Bechstein’s bat (Myotis bechsteinii) which is very rare in the UK, and classified as near threatened at the European level.

Viaducts: maintaining landscape flow

Viaducts are often considered by designers as barriers to connectivity, but they can be transformed into valuable assets in connecting landscapes.

Effective and innovative landscape design underneath a viaduct plays a crucial role in preserving ecological connectivity by seamlessly linking landscapes and habitats. By integrating thoughtful design principals such as continuous hedgerows and linking areas of woodland and habitat within the landscape design, the areas beneath the viaducts establish ecological corridors which act as natural pathways connecting any fragmented landscapes. This approach aims to create harmonious environments that facilitate the movement of species and biodiversity connectivity, which are neither affected nor constrained by the structures that pass over them.

Many of the HS2 viaducts traverse water courses and their associated flood zone areas. Rivers as dynamic components of landscape play a pivotal role in connectivity both ecologically and within landscape character. Riparian zones, along the river serve as transition areas connecting the aquatic and riparian environments. The protection and restoration of these zones underneath the viaducts are key contributors in maintaining the overall landscape connectivity. Efforts have been made to ensure that viaduct structures span well designed river corridors not only sustain aquatic ecosystems but also influence the terrestrial habitats. See case study 2 Edgcote Viaduct, and case study 3 Balsall Common Viaduct, for future evidence of this approach.

Conclusions

The below points summarise the key approaches that have been set out in this paper in relation to the design approach to connectivity of GI:

• GI definition: The paper provides a comprehensive definition of GI, emphasizing its role in ecological balance, well-being, and urban resilience.
• HS2 design strategies: It outlines the design strategies employed by HS2, including the use of green bridges and viaducts to prevent the fragmentation of green spaces and promote reconnection with the environment.
• Government alignment: The document highlights HS2’s alignment with the UK government’s environmental aims and policies, as outlined in various documents like the Natural Environment White Paper ^[9] and Biodiversity Strategy for England ^[9].
• Challenges of fragmentation: The article addresses the challenges associated with GI fragmentation, discussing its adverse effects on biodiversity, ecosystems, and community connectivity.
• Design solutions: It explores design solutions, such as burying the route, green bridges, and viaducts, to counteract landscape fragmentation and preserve ecological connectivity.
• Case studies: The inclusion of case studies, provides practical examples of how HS2 integrates GI preservation into its design.
• Green bridge guidelines: The paper outlines guidelines for green bridge design within the HS2 scheme, emphasizing invitational areas, habitat guides, vegetation structure, width, noise, and light reduction.
• Viaducts as assets: It explores how viaducts, often considered barriers, can be transformed into valuable assets in connecting landscapes.

The main recommendations are to implement the HS2 principles and strategies in preserving GI during the design and implementation stages, emphasizing the importance of ecological balance, biodiversity, and community connectivity.

HS2 Ltd.’s dedicated approach to GI reflects a commitment to balancing the demands of progress with the preservation of natural landscapes. The intricate landscape design strategies employed by HS2 Ltd underline the significance of GI, encompassing diverse elements from field boundaries to viaducts and green bridges.

HS2 Ltd.’s landscape design approach, as outlined in documents like the HS2 Design Vision ^[3], and the HS2 Landscape Design Approach ^[4], underpins the pivotal role GI plays in sustainable development. By weaving existing and new green assets into a cohesive environment, the HS2 project will ensure that where possible its development enhances, rather than severs, the green fabric of the landscape.

Examining the landscape strategies for Edgcote Viaduct (see case study 2) and Balsall Common Viaduct (see case study 3) reveals a multi-faceted approach. From minimising impacts and preserving existing connectivity networks to enhancing habitats and addressing the challenge of fragmentation, these case studies highlight HS2 Ltd.’s commitment to GI preservation.

The HS2 landscape design philosophy serves as a blueprint for future infrastructure projects, emphasizing the importance of preserving, enhancing, and restoring GI.

References

[1] Natural England website: What is Green Infrastructure?

[2] HS2 Environmental Minimum Requirements

[3] HS2 Design Vision

[4] HS2 Landscape Design Approach

[5] HS2 Phase One Information Paper E11 Green Infrastructure and The Green Corridor

[6] Green Corridor Prospectus

[7] State of Nature Report – 2023

[8] The Government’s 25 Year Environment Plan

[9] The Government’s Natural Environment White Paper

[10] The Government’s Biodiversity Strategy for England

[11] The Government’s National Planning Policy Framework

[12] Burton Green Tunnel

[13] HS2 Phase One Information Paper E15 Green Bridges

[14] Green bridges: safer travel for wildlife (including references to the A21 Lamberhurst bypass Scotney Green Bridge)

[15] Natural England: High Speed 2 (HS2) Wildlife Licensing

Supporting materials

Case Study 1 – Calvert Green Overbridges

Case Study 2 – HS2 Edgcote Viaduct

Case Study 3 – HS2 Balsall Common Viaduct