Delivering Innovation

The scope of works saw the site split into four phases areas along Marylebone High Street, Paddington Street and New Cavendish Street. Across these four phases nine bioretention areas known as ‘rain gardens’ were then retrofitted into the busy public highway, totalling 5,300m2 of physical works which drain an estimated catchment area of 7,000m2 through a total rain garden surface area of 129m2.

The site already featured traditional gully connections draining large sections of highway and pavements, with the worst gully draining over 1,600m2. Therefore, the bioretention systems were installed at the existing gully locations with new connections to the main sewer where needed in order to minimise disruption and avoid the cost and risks involved with deep excavations.

Challenges and Solutions

As the location featured a live high street, with significant pedestrian and vehicular traffic including bus routes, as well as a large number of utilities and basements present, the team were presented with several challenges. Therefore, to combat these some of the rain gardens had to be installed shallower than intended and lined with an impermeable membrane to protect services and basements.

Outcomes

The project was successfully delivered and has provided an improvement to the sewer water quality, as well as the area’s flood drainage and air quality as a result. As this was the first use of rain gardens within the council area, Westminster City Council are monitoring them with the view to determining whether further rain gardens can be implemented across central London.