The 2021 ‘National Planning Policy Framework’ (NPPF) for England lays out what local planning authorities (LPAs) will require, when setting local policies and also considering planning applications for all developments. The NPPF states that: ‘Planning policies and decisions should ensure that new streets are tree-lined and that opportunities are taken to incorporate trees elsewhere in developments’. This is backed up by other recent government and local guidance such as the ‘National Model Design Code’ which says: ‘All schemes will be expected to follow national policy by achieving a 10% net gain in biodiversity. All new streets should include street trees’.

Maintenance Requirement

However, measures need to be put in place to nurture and allow trees to mature, generally for decades, enabling them to actually deliver their real potential – including net carbon storage, urban cooling through shading and evapotranspiration, biodiversity and public wellbeing. So, the NPPF also requires that: ‘appropriate measures are in place to secure the long-term maintenance of newly-planted trees, and that existing trees are retained wherever possible’.

Local planning authorities now need to incorporate long-term tree maintenance measures in their planning consents and a straightforward spatial solution, such as permeable paving, offers a holistic multifunctional solution. Urban trees and paving have traditionally been seen as in conflict. But this is not the case with concrete block permeable paving, a key sustainable drainage (SuDS) technique to reduce flood risk and make cities more liveable. Concrete block permeable paving (CBPP), offers unique opportunities to collect, attenuate and treat rainwater runoff, removing pollutants for a gradual supply of clean water irrigating green infrastructure.

Irrigation and Gaseous Exchange

Unlike conventional impermeable hard landscape materials, CBPP allows the same pattern of run-off transfer to the ground as natural vegetation, allowing water to reach tree and shrub roots, despite providing an attractive hard surface above. In addition to irrigation directly from rain, with CBPP there is scope for water storage and rainwater capture away from trees and their umbrella canopies for gradual conveyance laterally to trees – essential during the summer.

In addition, CBPP enables air to reach roots and poisonous CO2 to escape from them. The favourable environment created for tree roots avoids pavement surface disruption from upward root growth and facilitates natural growth into lower levels for stability in high winds and longevity. CBPP can be used within new-build, or regeneration schemes with new tree planting and also for ‘rescuing' mature trees. It can be used in conjunction with raingardens/bioretention, tree-pits or proprietary systems avoiding air pipes, reservoirs or other structures for irrigation and gaseous exchange.

Beneficial Relationship

A recent Interpave case study (available via www.paving.org.uk) explored the 20-year beneficial relationship between CBPP and trees at the Martlesham Park and Ride scheme. Here, concrete block permeable paving has operated efficiently with minimal maintenance amongst extensive tree planting without root disruption or other issues.

The benefits of CBPP for trees while retaining accessibility are recognised by the current Code of Practice for accessibility in the external environment, BS 8300-1:2018, which states that: ‘Tree grilles should be avoided. Smooth or paved permeable surfaces should be used wherever practicable’. Permeable paving can be laid level and still avoids puddles without the need for drainage gulleys. It provides a safe, firm, pot-hole free surface for everyone – including wheelchair users and people pushing prams. Two decades of experience in the UK demonstrate the long-term performance of CBPP with minimal, if any, maintenance.

Low Intervention Retrofit

Another important innovation – particularly for regeneration – is the retrofitting of CBPP as an overlay to existing, conventional road bases or other hard surfaces. These thin overlays create attractive, safe and sustainable shared-surfaces. They can be particularly effective when used to supply a gradual flow of clean water horizontally into raingardens or bioretention areas with trees and other green infrastructure, via simple slot inlet/outlets, to store water for irrigation and biodiversity, as well as SuDS. Such low-intervention techniques enable transformation of the public realm in response to the raft of recent active travel, low traffic and open space initiatives.

This approach is exemplified in Bridget Joyce Square, London, an important regeneration project designed by Robert Bray Associates (RBA) in conjunction with McCloy Consulting for the London Borough of Hammersmith and Fulham, which won the top prize at the 2017 Landscape Institute Awards. Here, a typical, adopted asphalt street and adjacent parking areas were transformed for community use with CBPP overlay shared surfaces and tree-planted raingarden basins – an exemplar for future urban landscapes. The basins provide water storage for SuDS to reduce overloading existing drains (in the absence of the CBPP sub-base), as well as for irrigation.

Well-established Green Infrastructure

Interpave revisited the project in August 2021, around 5 years after completion, and noted that trees and other green infrastructure were healthy, substantial and particularly well-established. The permeable paving is also performing well and, it is understood from local sources, experienced no problems during recent extreme summer storms, despite extensive flooding nearby.

RBA founder, SuDS expert and landscape architect Bob Bray commented: ‘All the plants have grown really well. Birches are particularly sensitive to drought and urban heat island effect but they have thrived here and the vegetation has remained green all summer. The critical thing seems to be that even small rainfall events are captured by the permeable paving in summer and with larger events concentrated in the basins’.

For project case studies and guidance on all aspects of permeable paving and SuDS visit the website below.