Housebuilder’s New Vision for SuDS

A new homes development in Hampshire by Grainger plc has established a blueprint for a pioneering Sustainable Drainage Systems (SuDS) infrastructure on large mixed-use developments which is providing a best-practice benchmark for developers.

From when it was first talked about, the new 2550-home, 521-acre development at Berewood near Waterlooville was going to strive for the highest environmental standards.    The new mixed-use community, first established back in 1988 by the Hampshire County Structure Plan, would provide housing, as well as 100,000 sq metres of employment land and associated community facilities including shops, a medical centre, a park and a primary school.

Lead developer Grainger plc came on board in 1998 with a vision to ensure the site had as little impact on the natural environment as possible and a goal to eventually deliver a net benefit to the local biosphere.  There was a commitment to understanding and using best practice SuDS techniques.

Fifteen years on,  Berewood is fulfilling its early promise, with a visionary SuDS infrastructure constructed and the phased development beginning to be rolled out.   The strategic surface water drainage scheme for the site incorporates above-ground open-space landscaped SuDS features that are enabled and controlled by engineered SuDs devices – 9 Hydro-Brake® Flow Controls and 8 Downstream® Defender Advanced Hydrodynamic Vortex Separators.

Already, the main approach to the site is taking shape, with its interlinked cascade of SuDs ponds, swales and ditches set to provide an inviting landscaped aspect to the development and invite potential homebuyers to join the new community.

A mix of SuDs techniques have been imaginatively combined to deliver efficient water volume and quality control on a site with challenging drainage conditions.  Importantly, the future proof design achieves exemplary water quality standards over and above any current regulatory requirements to fully protect the local environment.

John Beresford, development director at Grainger plc said:  “We have aimed to create a sustainable community in every possible way at Berewood, and SuDs is a fantastic example of how a drainage system can be extremely efficient while also having minimal negative impact on the natural environment. Its sustainability makes it the perfect choice of drainage system for a development such as this, at this scale.”

Partnership

Ensuring SuDS success for a development of the scale of Berewood required effective partnership from the outset.   Set across the boundaries of Winchester City Council and Havant Borough Council, the site falls within the West of Waterlooville Major Development Area (MDA).

A close working relationship between all stakeholders was established early in the planning process.  A vision for a SuDS infrastructure for the site was integral to the Masterplan and a SuDS Technical Group was established early on with the two developers for the MDA, Grainger and Taylor Wimpey.  The partnership also closely involved the Environment Agency who took a strong lead from the early stages to establish SuDs objectives for the site.

The SuDS Technical Group met regularly to develop the SuDs principles and to plan for maintenance and adoption of the proposed SuDs measures within the scheme.

Working closely with the University of Portsmouth, the Environment Agency also established a monitoring programme for both water flows and quality from the site, as well as the ecology of the area.  Extensive research undertaken by the university has provided an unprecedented level of data against which to benchmark future performance of the SuDS on the site.

Overall design of the SuDS volume and water quality controlled was designed by consultant engineers Mayer Brown.

Site Conditions

The site has relatively challenging conditions and shows what can be achieved with SuDS.  –

The soil profile is a predominantly clay with poor capacity for infiltration and prolonged wet weather can lead to saturation in places.   So planning and positioning SuDS measures had to account for a high potential for natural surface flow, both from existing drainage features and eventual increased runoff from hard surfaces.    The ultimate discharge consent from the site is 5.06 litres/sec/HA for a 1 in 100year scale.  The site discharges principally to the River Wallington on the northern boundary of the site and the North Purbrook Stream to the South West, which also enters the River Wallington.

Paul Stewart, Associate at Mayer Brown, was closely involved in the SuDS design for the site:

“Taking careful account of the natural topography, we designed an infrastructure and landscaping with a mix of vegetated channels, retention basins, wetlands, ponds and swales.  All components play a part in achieving the water flow and quality requirements, but the overall water flow is attenuated  via Hydro-Brake® Flow Controls including at the last outlet before discharge into the River Wallington.

“Further SuDS will be installed on a phase by phase basis within catchments, by developers taking on each of the housing and commercial packages and integrated into the existing surface drainage scheme.  We have assumed that in each of the development packages, porous pavements will be used to encourage infiltration where possible

“Controlling the water quality and quantity discharges on site during the building phases is also critical, so the hard engineered features play a vital role during the construction phases, before vegetation of the natural SuDS features is fully established.

“The hydraulics of the whole site are fully modelled in MicroDrainage software package.  In a major storm event the Hydro-Brake® Flow Controls provide essential control of water quantity and flow on the site.”

Key SuDS Features

  • Carriageway edge swales and footway swales along sections of the highway will be adopted by Hampshire Highways.  All other features will eventually be adopted by the local authority SuDS Approving Body.
  • Linear basins with shallow pools at either end to provide habitat biodiversity along the main Broad Street that intersects the site with outfall controlled by Hydro-Brake® Flow Controls.
  • A series of three cascading ponds near to the first junction at the entrance to the site, with marginal planting.  Discharge from the ponds is controlled by Hydro-Brake® Flow Controls, and weir gully overflows with Downstream Defender® treatment devices .
  • Large linear basins at the bottom of the site with micropools, a bog garden and wet woodland area.  Discharge from the basins is controlled by Hydro-Brake® Flow Controls and  Downstream Defender® advanced hydrodynamic vortex separators.  The final discharge into the River Wallington is also controlled by a final Hydro-Brake® Flow Control and Downstream Defender®.
  • Twin detention basins close to the centre of the site to provide attenuation and storage from the traditionally drained road, as well as providing back up storage for SuDS features.  Outfall is controlled by Hydro-Brake® Flow Controls.
  • A cascade of dry swales in the south west corner of the site, controlled by orifice plates on weirs.

“We specified the Downstream Defender® for a number of reasons,” points out Paul Stewart. “The overall objective was that the design offers a reliable and consistently performing long-term technique for trapping silt and floatables, and provides intercept protection for minor hydrocarbon spillages.”

The master SuDS plan has a number of ponds and swales to soak up the sediment as the surface flow drains down the slopes.  The conventional approach would be to place catch pits at the outfall to entrap any sediment that escapes the ponds.  Usually constructed as deep concrete pots, catch pits provide an additional defence when there is high flow or high load.  They can be visually intrusive and the open water surface provides a potential risk to residents.

“The Downstream Defender® delivers  good SuDS practice,” continues Paul. “They are effectively closed and buried, so do not require barriers and are totally compatible with the natural SuDS features; the units enhance the natural silt floatables trapping performance and act as back up protection during construction.

“This is not only important while the vegetation is establishing itself on the banks and on the beds of the features, but also if there is high silt loading during storm events or if construction work is less than perfectly contained.  The construction phases will last many years, so ongoing protection is important, and on a residential site there are bound to be other incidents, whether a mower fuel spillage or a careless disposal of soil.”

“It is intended that the SuDS features will be maintained by a private contractor engaged to maintain the whole site and funded by the residents.  The roadside swales will be maintained by the Hampshire County Council through local taxation,” Paul Stewart added.

The Environment Agency is continuing to monitor the site:  “We supported the development and its SuDS scheme from an early stage in the planning,” comments George Woodward, Project Officer at the Environment Agency. “It was immediately apparent that the Berewood development was an important step forward in SuDS provision for flood risk management, water quality control and biodiversity.

“Here was the first major opportunity in the UK to study the environmental benefits of SuDS schemes before, during and after construction.  To do this we set up a research project in conjunction with the University of Portsmouth and Mayer Brown.  We were aided in this by the positive approach by Grainger plc, who embraced the benefits that SuDS bring.

“We have monitored the ecological and chemical quality of the River Wallington, with the initial evidence showing the effectiveness of SuDS in removing pollutants.”

“We were confident that our design would improve water quality, but have been surprised by the extent.   All the test samples demonstrate that the water leaving the SuDS system is of a higher quality than the watercourse which we are discharging into,” Paul Stewart added.

The initial phase of the Berewood site is due to be fully complete in the Summer of 2013, but further phases will continue for the rest of the decade.

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