Why leaky paving is set to do well in Wales

It's over four years since the UK government chose to abandon dedicated requirements for sustainable drainage systems (SuDs) on new developments in England using the Flood and Water Management Act 2010. Instead, a government statement and the National Planning Policy Framework simply requires SuDs on new developments of 10 or more dwellings (and similar scale commercial developments) built since 6 April 2015.

However, on 7 January 2019 the Welsh government implemented schedule 3 of the Act, which makes detailed provisions for SuDs. These include setting up a SuDs approving body (SAB) in each local or unitary authority alongside the local flood authority duty.

SAB approval will be required before SuDs can be built on new and redeveloped sites, except those less than 100m² in area, and the drainage will need to conform to the new Welsh SuDs standards. The SAB will then be required to adopt and maintain approved SuDs that serve more than one property.

Surface technology

The Welsh SuDs standards state, ‘Surface based sustainable drainage components…are generally simpler and easier to operate, monitor and maintain’. Concrete block permeable paving is uniquely placed to meet these requirements as an attractive, surface-based, multifunctional SuDs technique. It also addresses both flooding and pollution issues by storing and cleaning water runoff at source.

A SuDs system based on concrete block permeable paving eliminates pipework, gulleys and manholes and so costs less than conventional paving and drainage. It can also accept runoff from other impermeable paving and roofs, giving further savings, and can simply infiltrate rainwater falling on it straight into permeable ground.

Alternatively, permeable paving can collect and store water on site during storms, before gradual discharge to other SuDs features along the ‘management train’, to surface water or combined sewers, or to watercourses. Water discharged to open SuDs features on the surface can then be used safely to enhance landscape design and biodiversity.

The principle can be optimised for greater efficiency by considering distinct storage ‘compartments’ of concrete block permeable paving incorporating straightforward orifice flow-control devices. Water storage is then strategically deployed around a development within the permeable paving compartments, avoiding additional land take for water storage, treatment or conveyance. This technique is particularly helpful on high-density urban sites where space is at a premium.

Latest guidance

UK experience of concrete block permeable paving over the past 20 years has proved its long-term performance with minimal, if any, maintenance. This is reflected in the latest edition of UK trade association Interpave’s free guidance on the topic, which aims to ensure that concrete block permeable paving delivers predictable and robust solutions, and minimises cost, maintenance and adoption issues.

The guidance covers the use of concrete block permeable paving as a key SuDs technique and shares methodologies with The SuDs Manual. It also recognises European and British standards, and encourages the use of pavement construction materials that are widely available. It deals with all aspects and stages of design, construction and operation.

The structural and hydraulic engineering design procedures have been comprehensively updated and new, straightforward maintenance and reinstatement guidance added.

This article is based on the authors’ briefing article in the latest issue (172 CE2) of the ICE Civil Engineering journal.