Green-tech supplies custom bioretention soil to experimental city centre rain garden

The urban rain garden at Upper Pitt Street, is an exemplar project (100% externally funded through the EU URBAN GreenUP project - www.urbangreenup.eu), showcasing how landscape design combined with SuDS principles can be successfully retrofitted across our cityscapes.

Green considerations:

As part of any urban regeneration, it is vital that consideration is given to the green infrastructure, either by making sure we don’t lose what was there, or ideally, improving and increasing it. There are few arguments these days that don’t support additional green infrastructure, any way we can get it. The benefits are many and indisputable: the cooling effect from trees in particular, both from the shade and the moisture they give off through transpiration; the filtering of pollution and carbon sequestration via the leaves and the substrates; and last but certainly not least, the attenuation of rainwater, slowing down the flow of storm water into our already overburdened drainage system. Let’s not forget the potential for added biodiversity; the added shrubs, trees or wildflowers providing vital habitats to bees, insects, and other pollinators.

But if we look more closely at water attenuation; two of the best and most cost-effective ways of dealing with rainwater and runoff are green roofs, or at ground level, rain gardens. Rain gardens are one of the leading types of Sustainable Drainage Systems (SuDS), put in place to try to mimic water run-off as closely as possible to that in the countryside.

Very much like bio-swales, rain gardens are designed to take water runoff from impermeable surfaces such as roads and pavements. They filter contaminants and debris out such as silt and vehicular contaminants washed off the road surface by rainfall, whilst at the same time holding onto the excess water and slowing down the release, which in turn reduces the pressure on the storm drains. They are effective in removing up 90% of pollutants, and up to 80% of sediments from this surface water runoff.

Liverpool’s urban rain garden is designed to partially attenuate water within Polypipe’s ‘Permavoid’ underground storage crates. This stored water provides passive irrigation to the vegetation above, as well as storm water attenuation.

Rain gardens will usually feature plants that can withstand waterlogging up to 48 hours at a time, as most swales will drain over a period of 12 – 48 hours. They can also feature large rocks and pebbles which also help in the filtration process. In the case of the urban rain garden in Liverpool, plants were also chosen for their drought resilience, to suit the specially blended free-draining nature of the substrate below.

Green-tech involvement:

Green-tech has been manufacturing soils since 2004 under the brand name Green-tree. These soils are widely regarded to be amongst the best in the country, suitable for a wide range of standard and specialist landscaping purposes.

With the increasing popularity and demand, in many cases as part of the planning conditions, for green roofs and rain gardens, Green-tree offers a wide range of lightweight green roof and podium substrates, as well as rain garden and bioretention soils, all using our ever-popular British Standard manufactured soil as a base.

In this instance, as Liverpool had no previous experience using rain gardens as a “green solution”, it was proposed to create a test area to demonstrate what could be achieved.

Working in partnership with the Sports Turf Research Institute (STRI Ltd), Green-tech supplied three different bioretention blends for testing:

Blend 1 is the standard Green-tree bioretention mix, featuring screened silica sand, green compost and blown clay particles. This provides sufficient permeability to avoid waterlogging, while at the same time providing enough nutrients and organic matter for the shrubs and other vegetation.

Blend 2 is Green-tree’s High Performance bioretention soil, which has been improved to allow a greater percolation rate, whilst maximising the ability to sequester pollutants from the surface runoff.

Blend 3 is an experimental custom soil containing brick fragments, that in theory should aid permeability, whilst improving filtration. It will be interesting to see how this performs against the more traditional mixes.

The performance of each bed will be monitored by Liverpool City Council, in collaboration with the University of Liverpool and The Merseyside Forest organisation.

The various testing parameters include:

Water flow during storm events and the velocity of discharge.

Using moisture probes to monitor both moisture and temperature levels between the beds.

How the different soils affect: vegetation, pollinators, water quality (and pH levels) and air quality.

After eighteen months, all measured areas had shown improvement. Water discharge had decreased and quality increased. Ambient air temperature decreased and quality improved, and there was a noticeable increase in pollinators.