Working with nature
Our design concept aimed to work with nature to create a lasting solution that could be retrofitted with minimal disruption into the busy public highways in Marylebone. We designed nine bioretention areas (rain gardens) with a combined surface area of 129m2. These were installed at existing gully locations with new connections to the main sewer added only where absolutely necessary to minimise disruption to the carriageway.
The rain gardens are alongside footways and feature kerbs with strategically located inlets, allowing water from the highway to pool and then filter through specially engineered soil, including an aggregate drainage layer at the base of each garden. This aggregate layer features a perforated pipe to carry overflow water to the existing sewer system. The kerb inlets use silt traps to prevent unwanted materials from being drawn into the gardens, preventing the majority of pollutants entering the sewer and therefore improving the overall water quality downstream.
Importantly, the rain gardens feature a variety of trees, grasses and herbaceous perennials to absorb water, as well as filter pollution particulates. The plants, including flowering species attractive to pollinators, enrich local biodiversity and generally enhance Marylebone’s streetscape.
Multiple social and environmental benefits
Following the installation of the rain gardens, WCC monitored their impact closely for an initial period stretching from late 2019 to early 2020 and reported that there were no floods during storm events in the surrounding streets. A combination of the planting and silt traps are also serving to improve the quality of the water entering the sewer.
Alongside these core water management improvements, the rain gardens have delivered multiple benefits. Pedestrian safety has been enhanced because the gardens act as a buffer between pavements and highways and on the approach to new and enhanced zebra crossings.
Furthermore, the gardens have resulted in a net biodiversity gain, and an improvement in local air quality - a key aim of the Marylebone LEN. We were able to use the project as a chance to install additional cycle parking facilities to promote sustainable transport methods, and further promote a healthier environment (in total the project involved 5,300m2 of physical works).
Installing the gardens also presented a valuable opportunity to enhance the streetscape through the use of high quality materials including yorkstone paving flags and granite kerbs, as well as through the attractiveness of the gardens themselves.
We also seized the opportunity to contribute to community education on urban flooding and water management issues by installing information boards adjacent to rain gardens.
As a result of all the benefits the rain gardens have delivered, WCC is now assessing whether further rain gardens can be retrofitted into other busy streets in central London.