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‘Supporting sustainable drainage on waste sites’

Sophie Crossette, principal consultant at Eunomia, examines the importance of drainage systems at waste sites and what site developers should consider when designing drainage systems.


OPINION: Obtaining planning permission for new waste sites has traditionally focused on noise and odour controls, with the aim of reducing the impact on the neighbouring sites and local environment.

However, as the impacts of climate change become more apparent, with more extreme weather events increasing the risk of flood risk and surface water runoff, attention is turning towards the need to design site drainage systems in a way that mitigates against flood risk. What do waste site developers need to consider when designing site drainage systems so that the impact on the environment is minimised?

SuDS

Sophie Crossette, principal consultant at Eunomia

Sustainable Drainage Systems (SuDS) are surface water drainage systems that mimic the natural flow and processes of an area of undeveloped ground to slow the flow of water into the natural environment, promoting infiltration, replicating filtration and delivering water quality benefits, following the Hierarchy of Drainage. They challenge developers to move away from the traditional surface water drainage systems that channel flows into piped mains drainage systems, which then discharge to the sewer network. SuDS use a range of different mechanisms to discharge to the natural environment and provide additional biodiversity gain such as:

· Green roofs (a roof partially or fully covered with vegetation to slow runoff);

· Attenuation ponds (water storage areas that slow the flow of floodwater);

· Swales (shallow vegetated channels with gently sloping sides);

· Bio-retention areas and rain gardens (planted scrapes or verges); and

· Planted rills (artificial planted channels designed to improve water quality).

Legislation

While the basis for the UK legal requirements for SuDS were first introduced in Scotland with The Water Environment and Water Services Act (WEWSA) (2003), the need and demand for SuDS in recent years has increased as a result of a number of severe flooding incidents, with growing recognition that climate change will increase the frequency and intensity of flood events.

Within the UK, the legislation and management responsibilities for national flood risk have been devolved to the relevant regulatory bodies in each nation. This has led to variations in how the standards are being applied across the nations and the application process new or existing sites have to go through to achieve permission for their scheme, with some operators having difficulty reconciling SuDS requirements with their environmental regulator’s requirements for operating waste facilities.

Traditionally, the requirements of the waste site regulators have led to facilities being constructed with a high proportion of the site laid with impermeable surfaces, with minimal ‘natural ground’. This is to ensure that any foul runoff from waste storage areas can be captured and diverted to a sealed drainage system and that the yard areas are constructed in a way that is robust enough to cope with a high volume of heavy goods vehicles accessing the site to both deposit and collect waste materials. At face value neither of these operational and permitting requirements are conducive to SuDS.

Therefore, Eunomia is increasingly being asked by our clients how SuDS can and should be incorporated within their waste transfer sites, depots and household waste recycling centres (HWRCs) to minimise their impact on local flood risk while preventing environmental pollution. This has required us, working in partnership with Stantec, to think creatively when supporting our clients through the conceptual and detailed site design process to maximise the environmental benefits and meet the SuDS requirements without risking non-compliance with their environmental permits.

Requirements

Below are a few examples of measures we have implemented with clients which meet the needs of both waste permitting regulations and SuDS requirements:

· Maximising existing small areas of green space. This has included introducing swales and bio-retention zones within verges on access roads around sites and diverting roof runoff through planted rills to the side or rear of the buildings to slow down runoff and provide water treatment, biodiversity and visual amenity benefits.

· Introducing green or infiltration spaces. This includes considering where staff and visitor parking spaces, traditionally separate from operation areas with a reduced risk of pollution, can be converted to permeable paving systems to allow infiltration through a filtered system and reduce surface water runoff. Green roofs could also be considered to reduce runoff and improve biodiversity, though development costs will be higher.

“We should embrace the opportunity to think creatively about how we can reduce the impact of essential service sites”

· Alternative sub-terrain storage. Where space for attenuation ponds is restricted, developers will often turn to tanks to reduce the discharge rate to drainage systems; but hard tanks do little to support SuDS principles. Smarter thinking is needed and the use of proprietary products, which can provide another train of water quality treatment within the tank, present a better solution. These might also be used in conjunction with onsite water capture for reuse benefits such as site or vehicle wash down.

So, while SuDS do introduce challenges in their integration into new and existing operational waste facilities, they can also present opportunities at the development stage to improve the environmental credentials of a site. As they are a measure that is set to stay, we should embrace the opportunity to think creatively about how we can reduce the impact of essential service sites and start to provide some more tangible environmental benefits.

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