Martin Fairley Research Director at ACO Water Management explains how SuDS have been utilised to achieve effective surface water management and biodiversity…
A collaboration between Leicester City Council, Asda, ISG and ACO Water Management has enabled a first in sustainable urban drainage (SuDs) within a supermarket development. The integrated drainage solution combines proprietary and vegetative systems to achieve effective surface water management, whilst introducing a level of biodiversity never attained before on a retail development.
The project had strict planning guidelines that required two swale inlets to be incorporated onto the site. Working within these guidelines, Asda’s site feasibility team and ACO worked closely with Chryse Tinsley, the council’s landscape architect, to move away from the more common permeable paving solution, towards a cost-effective engineered system – integrating vegetative elements and proprietary products that effectively manages stormwater on or near the surface.
Sustainable Drainage Systems (SuDS)
SuDS is based on an underlying philosophy designed to counter the evident problems of urbanisation and climate change – where it is becoming increasingly untenable to discharge to an aged, over capacity, centralised sewer treatment infrastructure or to groundwater or a water course. Supermarkets present large, often impermeable areas that shed water to discharge downstream. As such, they are rainwater intensive. In addition, the pollutants introduced from run-off through vehicle operation can have an adverse ecological impact.
SuDS present methodologies and tools to re-examine how surface water run-off is managed – controlling it at site level and effectively decentralising and decoupling any negative components of run-off (volume, rate or quality) from receiving waters and managing on the surface, using natural vegetative and manufactured features and products in combination with modelling, engineering, design and creativity that benefits flora, fauna and those who live nearby.
Surface water management for Asda
The car park gradient was set at 1:60 – falling away from the building towards a site boundary positioned swale feature. A high capacity slot drainage system was installed over an approximate 90m length, serving a catchment of over 4000m2. Specified with shallow drainage inverts of 225mm Ø, the system enables a treatment train to be employed, where surface run-off cascades through a number of treatment components en-route to eventual outfall.
Surface water treatment
The first treatment stage comprises a shallow sub-surface granular stone media, to facilitate filtration and biodegradation as well as usefully slowing the flow of run-off.
With this feature allowing ‘on surface’ car parking, it’s crucial that the aggregate is protected from sediments that might otherwise bind the media. For this, two components are employed for evaluation: a filter device installed in the outlet chambers, designed to filter particulates over 0.5mm; and polystyrene filled sacks that sit in the gully, providing adsorbent properties to which particulates bind.
Water is further converged via four manhole chambers, two of which also serve roof and service yard areas. Three manholes subsequently deliver water to the receiving swale. This necessitated a shallow invert design throughout; making use of the surface gradient of 1:60 – in combination with specifically designed shallow channel outlets.
High performance surface water treatment plant
The site’s petrol filling station is drained via a conventional oil separator to a detention basin to the north of the main swale. To further treat the potentially high pollution load, a high performance surface water treatment plant was installed. The system used effectively removes the majority of sediments and also treats dissolved metals such as copper and zinc. The system uses proprietary filters to remove and contain pollutants providing a point source for maintenance.
The ACO SuDS Swale Inlet unit links the proprietary conveyance drainage systems to the swale, and serves to dissipate some of the flow energy. The aesthetically pleasing solution is manufactured from a high strength sustainable material that offers greater durability. The inlet detail also cuts installation time.
The swale itself receives run-off from the roof, the west car park, east car park, service yard, PFS and access road, and is the final conveyance stage to outfall. Chryse Tinsley worked closely with main contractor ISG to ensure the swale was constructed with appropriate contours. The result is a clear meandering pathway for the runoff.
Chryse Tinsley said, “Once developed, the planting regime will provide further opportunities for wildlife and biodiversity. Planting is expected to flourish over the next 12 months and for those using the store the natural features created will provide ambiance and natural habitat. For nearby domestic premises the trees, plants and water also provide a natural dividing line – quelling noise, whilst also providing a natural visual buffer.”
The scheme design exploits fully the drainage gradient on site and by combining, in sequence, shallow outlet inverts with granular sub-bases, filter strips, separators, detention basins and swales, a treatment train has been devised with many potential benefits. Water quality will improve through filtration, biodegradation, separation and exposure to sunlight.
ACO Water Management
Tel: +44 (0)1462 816666