Shopping around for the best SuDS

The design of supermarket car parks and other hard surfacing needs re-conceptualising with SuDS in mind, argues ACO Technologies research director Martin Fairley

Sustainable drainage has been a central consideration in building and infrastructure design for some years now. Its principal aims are neatly captured in the much referred to Venn diagram depicting quantity, quality, amenity and bio-diversity.

It should come as no surprise then that such aspects are the concern of the forthcoming National Standards for Sustainable Drainage – the draft of which is at public consultation and is a requirement in order to proceed with the implementation of Schedule 3 of the Flood & Water Management Act 2010. Though sustainable drainage system (SuDS) requirements have featured significantly in planning policy statements, building regulation guidance and, more recently, in the Code for Sustainable Homes, their uptake has been limited by concerns that relate to ownership, cost, and maintenance. It is generally agreed that further evidence relating to cost and performance, either initial or on-going, will serve the cause well.

Whilst the principal objectives of SuDS centre on managing stormwater volumes and quality in a way that enhances the environment, realising these ideals requires that the case is made in a convincing way to developers. Within this broad body of influencers, much attention has focused on housing, with subsequent consideration of planning for, designing, constructing and ultimately adoption of SuDS .

Retail’s impact
Significant development, though, relates to other commercial ventures – not least the many substantial areas dedicated to parking and vehicle movement in retail and on business parks and industrial sites. Taking the former category, of the reported 18.6m square metres of grocery retail property alone, 60% of that area is the result of just 10% of store numbers indicating the impact of supermarkets, superstores and hypermarkets [source:].

For this stakeholder group the arguments ultimately relate to the maximisation of shareholder value. Initial and on-going costs are one aspect, but equally important is the capacity to generate revenue, which directly translates to car park space availability. Losing car park spaces is not at all palatable to the country’s economic giants.

It is not difficult to see how the concerns of the SuDS movement arise: traditional drainage relied on moving surface water quickly off the surface into an underground network of pipes, generally combining both surface and foul flows. The shift in weather patterns and changes in land use, coupled with increasing use of the sewerage infrastructure, has led to surcharge into water bodies and flooding.
SuDS seek to manage water at, or near the surface, ideally at source, and through the use of mainly natural features. A variety of traditional SuDS techniques have been applied over the years, together with parallel guidance development from CIRIA, the construction industry research and information association.

Techniques often feature topographical depressions, such as swales and basins, used to convey, hold and treat water by arresting flow velocity, providing volume for storage within the feature, enabling infiltration, sedimentation, and enhancing other natural pollution mitigating processes such as biodegradation.

Holistic design
The national standards will require that such holistic considerations are undertaken far earlier in the project lifecycle, and may need to involve a wider set of design disciplines than is currently the case.
Techniques used in series, starting at source, should be designed to arrest flow and mitigate pollutants successively. It follows that the maxim of managing water at or near the surface is fundamental if the treatment train concept is to be employed.

However the set of traditional techniques often rely on depressions with shallow sides and falls, with subsequent impact on land take – especially where the volume is required to manage in excess of a one-in-100-year event. It can be argued that such land area devoted to volume control is easily implemented in modern housing schemes, for other commercial ventures though, perhaps less so.

Attenuating and treating the large volumes generated on a modern supermarket car park economically requires traditional SuDS to be supplemented by enabling technologies such as linear drainage channel and geocellular storage units. Channel drainage has found widespread use in a wide spectrum of applications.

Key to its success is the capacity to convey at a high level to outlet, this in itself has delivered economic advantage by removing large quantities of the underground pipe network. In the past variations on the linear channel theme have seen the use of ‘build in fall’ channels, designed in reality to assist hydraulically rather than offer self-cleansing.

As the technology has incrementally progressed though, more has been made of the economic virtue – conveyance over longer lengths, serving larger areas has become the norm. Consequently channel capacities have increased and it is now common to find moderately large linear channels with outlet positions that allow surface water conveyance across entire car parks.

Such long lengths no longer benefit from ‘built in fall’ arrangements, with two significant advantages resulting:

1) Surface water is conveyed at near surface level

2) Flow velocities within the channel are relatively low over the majority of the channel length

It thus becomes possible to integrate linear channel systems with soft landscaped surface SuDS features with a high degree of precision in controlling the conveyance rate.

Mitigating pollution
Re-conceptualising existing, well proven surface water management techniques such as linear drainage channels as SuDS-enabling technologies has important consequences for sustainable drainage design for supermarkets. The draft national standards for SuDS require that, in many ‘supermarket’ cases, two treatment stages are needed to mitigate pollution in run-off.

With the use of channels, these stages can be integrated at convenient locations onsite, ideally managing water at the highest relative level. However the integration benefits become greater when geocellular storage is used.

In this case, the large quantities of water from very infrequent events are managed in a most efficient manner in high-void, subsurface, structural components. Additional flow controls can of course be used in tandem.

Throughout such a scheme, evaporation, Infiltration and evapotranspiration losses can be leveraged at any level in the component chain, though critically, this application has advantages where infiltration potential is limited. This methodology of ‘decoupling’ quantity and quality management ostensibly flies in the face of the original SuDS ideal of multi-functionality – of three things from one.

A more realistic analysis might conclude that such arrangements are part and parcel of the increasingly rich and diverse palette of tools available to the practitioner, provided conceptually limiting terminology does not prevent use. The focus should move from distinguishing at component level (green, grey, hard, soft) to facilitating at system level.

To do so may require further consideration and clarification of treatment volumes – should quantity and quality be more distinguished? To illustrate the potential of such an approach, and provide a framework for debate, ACO has engaged prominent designers to develop hypothetical designs based around re-design of real sites to show how SuDS could be employed with little or no impact on car parking space. Three schemes have entailed, the first two illustrate channel-to-SuDS-to-storage.

A further scheme indicates the potential of the retrofit agenda. All three are being audited for compliance to the draft national standards by a competent authority.

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