Blog

Paul Shaffer, CIRIA

CIRIA’s been promoting sustainable drainage for nearly 20 years based on the strong belief and growing compelling evidence base that SuDS are the best way to manage flood risk, water quality and create better places. We also continue to unapologetically strive to extract the greatest benefit from SuDS delivery – why just settle for compliance with planning requirements and future regulations?

To progress beyond local flood risk management and deliver broader benefits often requires effective engagement and innovation with the integration of various disciplines as part of the design team.

It is with this background and the context of the delays in the delivery of the National Standards for SuDS that towards the end of 2013 some of us got quite excited by a recognised large-scale housing developer stating that have embraced SuDS in the delivery of a housing development. A flurry of emails between CIRIA and the developer quickly ensued, the SuDS trainers visited the site and further discussion followed at a meeting to discuss their aspirations for the site, the SuDS scheme and potential constraints they were working with. This was done so we could better understand the thought process that influenced the overall design and specific decisions.

So in mid-May we met with the developer and had an enlightening and constructive discussion about the scheme – this blog captures the main points.

It was immediately evident that there had been early and open engagement between the developer and the local authority, who also to their credit, were doing their utmost to facilitate what could be a groundbreaking scheme.

As part of the pre-application discussion natural flows paths were identified through the site and these were used to identify the conveyance routes, which in turn influenced the site layout. Not the normal approach to drainage design I hear you say…

The scheme includes swales and permeable pavements to collect and convey flow, with the temporary storage of runoff provided by a basin situated before final discharge.

We were encouraged by the developer’s enthusiasm for SuDS delivery and partnership working with the local authority to get the scheme adopted. All of which were a breath of fresh air, when for the last decade or so we’ve been confronted with somewhat misplaced negativity and risk aversion.

However, the most encouraging aspect, particularly in this time of austerity, is that the developer calculated that the SuDS option was 16% less expensive than the conventional drained approach! Which is comforting in view of Defra’s recent letter (22 May 2014) on the National Standards expressing concern about the impact of SuDS on development.

Overcoming constraints

There were a few of the elements of the design which raised debate and discussion:

Swales

These were quite deep (600mm) and had a filter drain at the base of the channel rather than the conventional grassed swale base. When discussed with the scheme designer, it was apparent that this approach was adopted due to the perceived requirements to take roof water drainage into the ground and the collection of runoff from the main spine road through a conventional gullypot and pipe system.

The inclusion of the piped filter drain as part of the swale construction was also considered essential to avoid the need for multiple swale inlets (as the downpipes would be entering at below ground level). Further thought (and guidance maybe) is required on managing runoff from housing plots, allowing runoff to be carried at the surface are required, which would have avoided the requirements for the headwall inlets.

The challenge of keeping swales shallow was further complicated by the requirement to achieve minimum cover for pipe crossings under roads. This is another design aspect that SuDS designers have to overcome on a regular basis. Whilst approaches of shallow conveyance through the road structure are viable from an engineering perspective, many of the constraints are institutional with the requirement to comply with the DMRB and local Highways Standards.

Landscaping value

The use of Section 38 of the Highways Act for adoption and concerns about funding maintenance requirements placed a significant constraint on the landscaping and aesthetic ambition of the site. There was a request from the local authority to keep maintenance on a par with that required for highway verges. The responsibility for SuDS maintenance (in terms of LA functions) and opportunities for combining synergies with other landscaping requirements on developments is likely to require further consideration by SABS once they’re up and running.

Detention basin 

The design of the detention basin was explored through discussion. It was designed to provide the storage for the entire site, with none of the swales or permeable paving providing storage within the site. Storing and managing flows throughout the site in upstream SuDS components would significantly reduce the size and volume requirements at the basin. Following comments from the earlier site visit, the developer was also already considering improving the landscaping of the basin to enhance its amenity value.

Detailing

There were some other aspects of detailing that were discussed, particularly the (arguably obtrusive) headwalls which were influenced based on guidance that the local authority was comfortable with.  Again barriers are required to be broken down with a move away from the standard headwall detail that we are used to seeing for conventional drainage design.

Next steps and views on constraints

There was significant interest by the developer and their designer in approaches to overcoming challenges and constraints. Many of these constraints can be overcome with improved engagement and move away from standard detailing for drainage features such as heavily engineered headwalls which are no longer appropriate in a SuDS context.

It’s early days in the journey for this developer, but they have showed willing to press ahead and have already engaged with a landscape architect to provide a more integrated approach to SuDS delivery for their next site. However, this can really only be achieved through the right engagement at the right time – ie early in the land acquisition process.

The developer is keen that lessons learnt from this scheme informs the design of future schemes. Fundamentally it has been demonstrated that SuDS for housing schemes are not impossible and schemes can be mainstreamed to the rest of the larger house developers contrary to the belief of others in the industry.

With some further evolution of design the SuDS being delivered will make a greater contribution to the value of future developments and potentially save more money when compared to traditional drainage schemes.

Barring, the usual challenges around adoption and allocating responsibilities for maintenance it would be interesting to know what you think will be the potential constraints for developers in mainstreaming SuDS and how these can be overcome.