Rain Garden



Project summary

  • Urban design
  • SuDs
  • Ecology and habitat creation
  • Sustainable water management

The aim of the Liverpool Rain Garden was to form an educational blueprint of what can be achieved within urban centres for the benefits of water management and quality, sustainability, biodiversity, community health and wellbeing.

The project utilised expertise from across the STRI Group, with design from STRI and our environmental company EPG, and build from our construction division Carrick Construction. The rain garden was implemented along the pavement of an existing road at Upper Pitt Street, near the Baltic Quarter, transforming the stark and somewhat uninspiring street into a miniature green oasis.

Funding came through the EU URBAN GreenUP project with additional support from STRI Group, WSP, and Polypipe, in the interest of SuDS research and innovation.


  • Retrofit a green infrastructure within Liverpool City Centre as part of an EU funded incentive
  • Create, protect, and enhance biodiversity
  • Increase water retention and create betterment of discharge rates
  • Create an attractive sense of place with an educational story
  • To incorporate monitoring systems, to prove the SuDS concept
  • Be cost-effective and repeatable for utilitarian landscapes such as city centres, or residential areas where there is less funding for SuDS retrofits


STRI designed the rain garden in line with the brief laid out by Liverpool City Council, and Polypipe, as part of the EU’s URBAN GreenUP project to increase the sustainability of cities through innovative nature-based solutions.

The garden was designed to intercept rain and surface water run-off from the surrounding road and pathways through water inlets along the length of the new kerb. Some water is used immediately by the plants, and some is held in the soil substrate. During heavy rainfall, excess water drains through the soil substrate into Permavoid attenuation units below. The units allow a significant volume of stormwater to be stored underground where it is re-used in dry periods through passive irrigation, being transported up through the cell unit via absorbent capillary cones and spread over the top surface of the unit by a highly absorbent geotextile.

Designed to be slightly sloping, the garden is divided into three sections, or beds. These beds are connected via underground pipes, and each contains a flow restrictor, allowing a measured amount of water to be retained in each, before discharging at a slower rate into the next. This creates a SuDS treatment ‘train’. At the end of the treatment train, any excess water flows into the existing road gully system at a slower rate providing an approximate 50% betterment on previous discharge rates.

Working in partnership with Green-tech, STRI group developed different bioretention blends for each of the three garden beds. By varying the mixes, it is possible to measure how the different substrates perform in terms of moisture retention, temperature regulation, pollutant sequestration, as well as visibly demonstrate the effects at surface level, through the differences in plant success.

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.


The plants used were specifically selected due to their ability to survive extreme conditions, and self-seed, allowing the garden to have evergreen all year round whilst also improving the natural habitat.

A flow meter was installed to measure the level and velocity of water flowing through the garden, so the total discharge volume could be calculated. The meter transmits data to cloud-based software and allows comparisons to be made between the anticipated discharge within the catchment area (pre-rain garden) and the actual discharge (post-rain garden). Data from nearby weather stations can also be gathered to determine the level of attenuation the rain garden is giving, allowing the actual performance to be compared against that calculated as part of the design process.


  • The rain garden was completed in June 2022, transforming Upper Pitt Street, and raising the profile of retrofit SuDS designs
  • Showing signs of improving the habitat with numerous pollinators visiting the garden, including ladybirds, bees, and butterflies
  • Data collected so far has shown that longer duration, less intense rainfall events have been entirely attenuated by the rain garden
  • The plants are establishing well and landscape maintenance visits during the initial installation period have been minimal, instead relying on the resilience of the species selected and the passive irrigation from water harvested underground. This reduces water use and the frequency of maintenance visits required, making for a more sustainable long-term solution, for a modest maintenance budget
  • More outcomes from this project will be known in 2023, as we continue to collect and monitor data.

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