Multiple benefits from transforming urban grey to green
Zac Tudor, Principal Landscape Architect at Sheffield City Council, describes some of the many benefits of transforming a dual carriageway into a multi-functional green corridor.
What prompted you to make changes?
In 2007 the Castlegate and Wicker areas of the city centre experienced devastating floods. Action to reduce surface water flooding was urgently required, particularly as extreme weather conditions are anticipated to be more frequent as a result of climate change. An opportunity arose when 1.5k of former ring road became largely surplus to traffic requirements.
What work did you do?
A bleak dual carriageway has been transformed in a bold and innovative horticultural experiment which also contributes to flood protection and supports urban regeneration.
We created a truly multi-functional project, aiming to increase the efficiency of the highway network whilst creating a green healthy corridor for urban biodiversity and people. Alongside this, the routes create several long distance walking and cycling routes.
Who was involved in this work?
Grey to Green was led by the Council’s award-winning Landscape Team. We sought advice on the planting from Professor Nigel Dunnett of Sheffield University, an internationally acknowledged expert on urban greening, and one of the designers of the successful meadows in the Olympic Park in East London. The works were then delivered by Sheffield City Council’s strategic highways partner Amey.
The projects were jointly funded by the City Council, the Sheffield City Region and the European Regional Development Fund.
What challenges did you have to overcome?
The first phase was completed in 2017, and a second was completed during 2020 - despite Covid, which increased the challenges in delivery.
Many of the design features were still new to highway designers and contractors, so more time had to be taken working through these details on site to make sure the correct solutions and approach were achieved.
What are the benefits of this work for nature and people?
Our Grey to Green project contributes to flood protection and supports urban regeneration. The space has been turned into a green corridor with extensive areas of colourful, but low maintenance planting increasing biodiversity in the city centre. Plants are chosen to be resilient to prolonged drought and occasional flood, as well as to be beneficial to wildlife, and there has been a visible increase in pollinators in the city centre.
This work shows what urban streets and space can become. For example, from protecting pedestrians from air pollution though multi-layered planting, achieving urban cooling through increased tree planting, treating contaminated water, containment of potential micro plastics in the bio swales and promoting health and wellbeing.
The project has been widely acclaimed and enjoyed by local people, particularly local residents, city centre workers and cyclists who have a safe and aesthetically enjoyable journey and experience the benefits of biodiversity.
How does Grey to Green reduce the risk of flooding?
These new planting beds provide porous areas known as Sustainable Urban Drainage, or SUDS for short. They hold back surface water in extreme weather events, because flows are diverted away from the sewers in intense storms.
SuDS principles are used to manage surface water with it being captured, treated at source, controlled and conveyed on or near the surface providing an opportunity for water to be part of the landscape. The scheme manages flows from the new paved pedestrian/cycle surfaces as well as half of the highway (service depths made it difficult to re-profile whole highway). Runoff is collected via simple over edge flush kerbs into a swale running the length of the scheme.
The Infoworks (ICM) modelling showed the overall scheme could contain a 60 minute, 1 in 30 year event with discharge from the whole scheme to the river reduced from 47.3 litres per second to 9 litres per second. A 1 in 100 year, 60 minute event would start to overtop the weirs but modelling shows it would nevertheless reduce discharge rates from 69.6 litres per second to 9.2 litres per second.
In other words, the work we have done reduces the likelihood of damaging floods and will reduce the volume and rate of water flow when flood protection barriers are breached. In combination with other flood protection work, it has meant that Sheffield has escaped further flooding events in the area since, despite severe rain conditions.
Does Grey to Green reduce the impact of increasing temperatures?
Heat island effects are currently being monitored. Early signs are showing positive reductions in the ambient temperatures for the surrounding area, in comparison with the almost one hundred percentage coverage of bitmac surface which was in place prior to the scheme starting (Source: current PhD study).
What other benefits arose from this work?
Our biggest success has been that people are wanting to use these spaces and are talking about and celebrating the achievements both locally together with the wider interest.
Grey to Green attracted new investment and considerable interest from around the world.
Professor Nigel Dunnett, University of Sheffield told us, “I can’t think of anywhere else in the world that has brought these things together, has this level of horticultural and aesthetic sophistication with a scheme which is doing so many ecological things, which is benefiting the city economically as well.”