Flow Controls and Blue Green Roofs

Bob Bray, a SuDS designer for over 20 years, discusses rainwater storage on blue green roofs using controlled flow outfalls as a simple, cost-effective, easy-to-manage and environmentally beneficial solution to today’s SuDS requirements.

The first ‘green roofs’ were constructed in the early 20th century in Germany. These simple living roofs, with a 100mm sand substrate covering a bitumen waterproofing layer, are still effective and are now conserved as a valuable wildlife and environmental asset.

The modern, commercial green roof, however, has been developed to remove water quickly through a shallow substrate – sometimes as little as 50mm – using drainage boards below the soil profile and gravel edges to the roof. These are also used as fire breaks. This approach follows the conventional drainage perception that water should be removed from impermeable surfaces as quickly as possible. This practice contributes to the problem of urban flooding and pipe drainage surcharge.

Controflow SUDS04001

Controflow roof outlets have accessible double-chamber construction

SuDS Requirements for Developments

The need to collect, clean and store water as close as possible to where it falls as rain defines sustainable drainage systems (SuDS). These are now a requirement for many developments, approved and often adopted by local authorities. SuDS are essential in the fight against flooding and pollution – particularly with overloaded sewers, urbanisation and climate change.

In urban areas in particular, roofs may comprise more than 90% of hard surfaces on developments. SuDS and current planning conditions for development require runoff from roofs and other impermeable surfaces to be released at ‘greenfield rates of runoff’, usually 3-8L/sec/hectare. Runoff must also be stored to control the 1 in 100 year return period rainfall event (with an allowance for climate change) – commonly 80 -110mm of water – from the area contributing runoff.

At present the response to these new design requirements is often to propose tank storage below ground. This is expensive, generates waste spoil and provides few additional benefits to development.

Simple Solutions with Flow Controls

A waterproofed living roof surface with a free draining vegetated soil offers an untapped opportunity to meet these current requirements. By simply incorporating a Controflow flow control unit at the outlet, rainfall is held for a short period of time at roof level. The benefits of this ‘blue green roof’ to the internal environment of the building and potential habitat enhancement at roof level are augmented by the management of rainfall and the increasing impacts of climate change. In essence, the blue green roof behaves in a similar way to a natural landscape.

Collecting rainfall over the whole area of the roof and holding it for a short time enables pipe conveyance of water from the roof at a controlled rate to a watercourse or sewer, without overwhelming the finite capacity of the pipe network. In the unlikely event of exceptional rainfall occurring, in excess of the design storm, then a high-level overflow within the Controflow flow control unit comes into play. In addition, Controflow overflow outlet units can be provided around the roof to match the downpipe capacity.

Controflow SUDS04001 Application Diagram

Controflow vertical roof outlet with flow control and overflow on blue green roof

Rain falls on the free draining soil of the blue green roof – usually with 30% void ratio storage – until it is fully wetted and then water is stored in the soil voids. For a 100mm deep soil this storage can be about 30mm of water. Once the soil profile is saturated, water begins to accumulate at the surface within the vegetation, usually maintained at about 100mm high. 70mm of storage will normally accommodate sufficient water to meet full storage requirements.

Lower Costs – Easier Management

Storage at the surface is not only cheaper than box storage below the ground but also ensures soil and plants are fully hydrated ready for any future dry periods. Surface water is held within the vegetation, reducing the risk of spray in windy weather. Water flows slowly through the soil profile, trapping any debris at the surface and is collected laterally in the gravel borders at the edges of the roof. The gravel borders provide a straightforward drainage route from the roof, continuing the filtering process as water makes its way to the outlet.

In order to achieve the low flow rates necessary to reflect ‘greenfield runoff, a specially designed flow control is necessary at the outlet. Here, the Controflow blue green roof flow control unit has been developed using the principle of the ‘protected orifice’ to enable low flow rates to be discharged from a roof outlet. The clean water filtered by soil and the gravel profiles flows slowly to the ‘double chamber’ unit. Water passes through 6mm protective slots in the outer chamber wall and then through a specifically-sized orifice into the inner chamber.

‘Controflow’ blue green roof flow control (with overflow) and overflow-only units are available to fit either over a vertical drain outlet or into a horizontal outlet in a roof parapet. Access to both the slotted outer chamber and inner orifice chamber allows for inspection and cleaning. An integral overflow prevents flooding of the roof surface. These products can also be used to control water storage on ordinary blue roofs.