Placing SuDS at heart of green spaces

PLACING SUDS AT THE HEART OF GREEN COMMUNITY SPACES

TfL’s Precious Birabil lists the myriad benefits of incorporating Sustainable Drainage Systems (SuDS) in highway schemes

Footway build-out and crossing integrated with SuDS rain gardens, wildflower and tree planting in Enfield, Hazelbury School Sustainable Drainage Systems (SuDS) features such as rain gardens or swales are designed to mimic natural processes by slowing and reducing the flow of water into the sewer system. As London continues to grow, climate change, population growth and densification are all contributing to increased levels of surface water runoff. This increases the risk of surface water flooding.

Transforming neglected spaces

SuDS on the highway can take the form of roadside rain gardens in buildouts, trees and low-level planting, which are designed to capture run-off from adjacent hard surfaces, roadside swales in existing green spaces and permeable paving. As well as reducing flooding, SuDS can also help to transform and reactivate neglected places by introducing attractive planting.

Healthy Street schemes provide many opportunities to integrate SuDS into different spaces on the street, for example, by creating rain gardens in pedestrian areas and pavement build-outs, which also act as traffic calming and parking control.

SuDS work by taking surface water flow from impermeable surfaces such as carriageways and footways and transferring them into a permeable area, which allows the water to slowly percolate through the system. This allows controlled flow back into the sewer. The overall effect is to significantly slow and reduce the flow of water into the sewer system, reducing the chances of capacity being exceeded during high rainfall events.

38 Rain gardens

SuDS rain gardens retrofitted into build-outs help to limit parking and manage flood risk next to Hazelbury Primary School, Enfield

Adapting to future threats

Surface water flood risk is one of the biggest climate change risks facing London. Localised surface flooding occurs when the volume of water entering the sewer network exceeds capacity. During prolonged rainfall events, this can result in significant transport disruption and damage to residential and commercial properties. Forecasts show that climate change will result in more intense, high rainfall storm events.

The impact of climate change will disproportionately affect very young and older people as well as people living in poor quality housing and in deprived areas. Many people will find it more difficult to recover from flooding and will suffer more as a result of inequalities in health and transport accessibility.

Integrating SuDS into transport infrastructure can help deliver cost savings in mitigating risks and adapting to future threats. SuDS can also help deliver many benefits that improve quality of life while meeting transport objectives and set out in the Mayor’s Transport Strategy and London Environment Strategy. SuDS can help to deliver improvements in biodiversity as well as contribute to Healthy Streets objectives and contribute to the improved health and transport accessibility for all Londoners.

Co-designing with the community

The pandemic has reinforced the need for peoplefriendly streets and access to flexible and pleasant places that provide opportunities for leisure and social interaction, which help to reduce the impacts of health inequalities and deprivation. Well-planned SuDS can also transform the public realm and contribute to the full range of Healthy Street Indicators. Co-designing with the community and local groups such as Business Improvement Districts (BIDs) can help to encourage buy-in for new schemes. These groups can also take ownership of the SuDS and have a greater role in managing and maintaining their local environment.

Attractive green space has also been shown to help encourage the switch to greener transport modes. Many boroughs are already delivering high quality SuDS schemes through the Liveable Neighbourhoods programme.

Maintenance cost savings

The London Borough of Waltham Forest introduced a range of SuDS rain gardens by reimagining residential rat runs through the creation of filtered permeability and new public spaces using pocket parks and parklets.

Residents and businesses were consulted early on to contribute to the design and ensure ownership of the scheme. The local community also helps to maintain these new spaces, with the council providing support in challenging periods, for example, installing irrigation points.

Overall costs and maintenance associated with SuDS features can be greatly reduced through good design with community and business involvement.

Integrating SuDS into schemes

SuDS should be considered where there is available space on the highway. SuDS are most cost effective when combined with existing works and local improvement schemes.

There are types of locations and assets that present the best opportunities for integrating SuDS while achieving transport objectives. For example, SuDS can be retrofitted into wide footways and underutilised spaces such as the middle of roundabouts in order to provide environmental and transport benefits such as reducing speeds and improving the look and feel of the space.

The opportunity for SuDS can be assessed in the early planning stages by a project team with the right skills and experience including a drainage officer. It is important to understand the flow and routes of water runoff at the street level and impermeable areas from which the runoff can be intercepted and captured. Getting a topographical survey done early on in the scheme development can greatly assist with this.

When designing the system, it is necessary to consider how water will enter and leave the SuDS feature. It is also vital to build in overflows so that water can leave the system during extreme rainfall events. Mapping tools such as The London Green Infrastructure Focus Map1 can help identify areas in greatest need of green infrastructure interventions. This can then target SuDS in areas at risk of surface water flooding.

Urban Design London2 is currently running online training workshops for borough officers in order to raise awareness and help plan and design SuDS on the highway. n Precious Birabil, transport strategy & planning, City Planning, Transport for London

SuDS features can accept and store run-off from impermeable surfaces