A regeneration project improving biodiversity, economy, and water management in a disconnected part of the city.

Introduction

An invitation from the Society of Garden Designers (SGD) to visit the innovative Grey to Green scheme was a chance to see one of the most groundbreaking planting projects in the country. An enterprise that aims to improve so many aspects of the city from biodiversity and flood defence to local economy and infrastructure. On an aptly rainy day, I went to see how the planting is holding up to the onset of Autumn.

A rainy visit to the Grey to Green project

History

The Castlegate area in the city of Sheffield was, as the name suggests, the site of Sheffield Castle, built by the Normans some time after 1066. The site was considered an important position of influence due to the confluence of two rivers, the Don and the Sheaf.

These fast-flowing rivers helped drive the city through the industrial revolution leading to Sheffield’s brewing and tool manufacturing industries.

In the 1800s, the Victorians created huge underground culverts to divert and give a subterranean passage to the Sheaf and other rivers. This was for a variety of reasons such as allowing more room for industry, to cover the unpleasant-smelling polluted water, and to create storm drains.

Fast forwarding to 2007, an unprecedented volume of rainfall and flooding hit the UK, with Sheffield particularly badly hit. Such was the intensity of rainfall, the Don was overwhelmed resulting in widespread damage to homes and businesses, leading to the loss of two lives. The financial cost of the flooding was estimated at £50m even before replacing lost infrastructure was taken into consideration. One of the reasons cited for the vulnerability to flooding is the large expanses of paved surfaces that funnel water into the drainage system rather than absorbing it.

The Issues

Castlegate, prior to the project, was a forgotten area, riddled with a mass of redundant roads and complex junctions, physically disconnected from the rest of the city. This, combined with the fact that the area was ‘a poor physical environment’ as described by the European Nurseryman Association (ENA), meant that people stayed away with nowhere to sit and relax. This led to investment also overlooking the area, leading to it economically underperforming relative to other parts of the city. This is all in addition to the problem with flooding.

The Grey to Green scheme, funded by Sheffield City Council, is an ecological and economic development strategy reacting to three central issues:

The need to regenerate the Castlegate area, reconnecting the previously cut off site to the wider city;
To reuse the expanse of redundant roads;
Reaction to the threat of flooding such as that in 2007.

Map of the Grey to Green project
*Grey to Green Sheffield (2023) available at: https://www.greytogreen.org.uk/background*

Grey to Green Innovation

The response to these issues was to replace 1.3km of tarmacked roads with a linear corridor of green planted space with Sustainable urban Drainage Systems (SuDS) at the forefront of the innovation. The green infrastructure also employed cycleways, pedestrianised walkways and seating to encourage pedestrians and cyclists to use and visit the area.

Before and after the project was constructed — Before and after construction
*Grey to Green Sheffield (2023) available at: https://www.greytogreen.org.uk/background*

The SuDS approach to planting utilises rain gardens and bioswales to mimic the way in which rainwater finds its way back to rivers in nature: ‘cleanly, slowly, sustainably’ [Grey to Green Sheffield 2023]. The planting beds (shown below) work as a rain garden, with a series of connected sunken beds that entrap surface water and its pollutants. Each section is connected with an outlet allowing water to move down the chain of beds as volume increases. The surface water is absorbed slowly into the ground and by the multi-layered planting. It is estimated that 24,000 bathtubs of water each year are prevented from entered the sewage treatment works as a result of these beds. The rainwater is then returned to the river cleansed of pollutants, oils and microplastics, much like it would in a natural environment, rather than adding volume to the sewers.

The rain garden showing overflow outlets (before planting matured)
*Grey to Green Sheffield (2023) available at: https://www.greytogreen.org.uk/innovation*

Overflow outlet in rain garden — Overflow outlet

The rain garden planting slows down the surface run-off, absorbing some of the volume before it is moved on, whereas bioswales are designed to absorb larger amounts of water. They are shallow depressions and have a porous underside, to allow for water absorption. Like in the Grey to Green scheme shown below, the bioswale is densely planted.

Grey-to-green-bioswale — Densely planted bioswale in the Grey to Green project

The difference in approach from the Victorians to the present day couldn’t be more stark. Where they went to huge engineering and financial efforts to cover the river using it to remove the waste from the city, the modern approach is to see the rainfall as a resource that can ‘irrigate our green networks, keep our city cool and build awareness of natural processes.’ The scheme acknowledges the water and celebrates it.

Planting Matrix

The planting in the Grey to Green scheme has to work really hard. It has to cope with periods of drought, with high rainfall, have season-long interest, take little maintenance, and of course be beautiful. Nigel Dunnett, Professor of Planting Design at the city’s university and master of plant communities, led the approach.

The planting is multi-layered with specimen trees (Quercus palustris; Gleditsia triacanthos) and multi-stemmed trees (Betula pendula; Cercis siliquastrum) providing top-level structure. A mix of evergreen and deciduous shrubs (Pinus mugo ‘Mops’; Euonymous alatus; Sarcococca hookeriana var. digyna) form a middle level.

Cercis siliquastrum in the Grey to Green scheme — Cercis siliquastrum amongst planting in the Grey to Green corridor

A highly diverse range of perennials were split into those that could deal with wet or waterlogged conditions in the swales and those that were more drought tolerant. In each habitat however, the plants were randomly planted to give a naturalistic effect. Examples of ‘Dry’ plants include Achillea filipendula ‘Coronation Gold’, Origanum ‘Rosenkuppel’, and Salvia nemerosa ‘Caradonna’. ‘Swale’ planting includes Echinacea pallida, Lychnis flos-cuculi, and Eupatorium cannabinum. These perennials combined with a mix of 40,000 bulbs like Eremus x isabellinus ‘Cleopatra’, created a long period of seasonal interest as well as huge variety of textural and floral appeal.

Knitting the planting together throughout the project is the continued presence of grasses. As in many naturalistic planting schemes grasses play a major part, offering long periods of interest particularly in late season. Calamagrostis x acutiflora 'Karl Foerster' stands erect throughout, whilst other grasses like Molinia ‘Poul Petersen’, Stipa gigantea, and Deschampsia cepitosa ‘Goldtau’ also feature.

Low maintenance planting with variety of grasses

Biodiversity

With diverse and beautiful plants comes a huge increase in biodiversity. This sustainable environment provides habitat for insects to feed, live and move around in helping the wider ecosystem all the way up the food chain. Artwork located around the site also provides in-built homes for insects further adding to the benefits of the project.

Already, solitary bees, lacewings and wasps have all been found on site, and astoundingly there has been a 561% increase in biodiversity recorded.

Success

This scheme in the north-eastern part of the city have undoubtedly been a visual and esuccess. The planting is wonderful and has improved the site for people who work and live in this part of the city immeasurably, after all, according to a Sheffield survey before the project took place, 98% of people wanted to see more green in the urban environment. In fact, 20% of locals have redirected their commutes to go through the green corridor.

Tour of a small seating area in the Grey to Green project

The project has had a profound success in flood defence, biodiversity and economic stimulus for investment. Though this scheme is considered to be groundbreaking in this country, it could be said to be almost necessary for similar projects to become commonplace among our cities and urban areas, after all who could argue against it!

References

Dunnett, N. (2023) Grey to Green, available at: https://www.nigeldunnett.com/grey-to-green-2/

European Nurseystock Association (2021) Grey to Green Sheffield (Phase 1), available at: https://uk.thegreencities.eu/best_practices/grey-to-green-sheffield-phase-1/

Gregory, S. (2019) Into the Megatron: We explore Sheffield's secret rivers, available at: https://nowthenmagazine.com/articles/into-the-megatron-we-explore-sheffields-secret-rivers#:~:text=A%20subterranean%20cinema.,the%20tram%20bridge%20above%20it.

Our Media (2023) Gardens Illustrated: What is Grey to Green? The initiative bringing colour to Sheffield city centre, available at: https://www.gardensillustrated.com/gardens/town-and-city/grey-to-green-sheffield

Study Smarter (2023) Sheffield Flooding, available at: https://www.studysmarter.co.uk/explanations/geography/river-landscapes/sheffield-flooding/#:~:text=Extreme%20rainfall%20in%20the%20summer,a%20large%2Dscale%20rescue%20operation.

The University of Sheffield (2023) Designed Ecology, available at https://www.sheffield.ac.uk/landscape/research/ecology

Urban Green Blue Grids for resilient cities (2023) Bioswales, available at: https://urbangreenbluegrids.com/measures/bioswales/

Sheffield: Grey to Green