Cool Towns, Margate
Margate in Kent, on the Isle of Thanet in the south of England, is a historic seaside resort. Becoming popular during the Victorian times, this town exemplifies the typical coastal urban realm – with a mixed demographic, but an ageing population. In ancient times, the Isle of Thanet was truly an island, surrounded by the sea and an inland channel; built on chalk with silty clay topsoil, but the channel silted up in the Middle Ages and is now part of the mainland.
Over the past few years, there have been multiple flooding issues in Margate. The Wastewater Treatment Plant dealing with the sewage from Margate has been overwhelmed in storm conditions every year, with sewage spills affecting the blue flag beaches around the Margate area, which has been expensive and damaging for the whole economy of the area.
In addition to the stormwater issues, Margate has suffered from the increasing summer temperatures; this is a feature of climate change, longer drier and hotter summers. Whilst it may seem attractive to have hotter summers for a seaside location, these higher temperatures can affect people's health, the quality of life of residents, as well as air quality.
Kent County Council applied to the EU Interreg Cool Towns project for funding to pilot a scheme to use nature-based solutions to mitigate both stormwater and the heat island effect during the summer. Explaining that Margate is at risk of impacts of climate change, the town was selected as one of the towns across the EU to take part in this exercise. GreenBlue Urban are a partner with the EU Interreg 2 Seas program, and WSP worked on the technical design. The installation was carried out by Duke Civil Contractors of Kent.
George V Park was identified as an area which could provide SuDS for the surrounding area by the LLFA. Stantec Design we’re appointed to review initial concepts to see if SuDS could be retro fitted. The outline design looked to disconnect all existing gullies from the combined sewer to reduce flooding within the downstream catchment. Through the detailed design George V Park was sized to intercept and retain surface water runoff from George V Avenue and the three adjacent streets.
The park itself now take flows from the surrounding gullies into bio-retention swales which then convey flow through a series of detention basins which in turn discharge to ground via soakaways. As such the scheme has been able to remove a significant of existing catchment from entering the combined sewer. 40 trees were planted in this area by local community groups to help with water attenuation.
Nearby streets were identified as being sensitive to impacts of climate change – in July 2019 the local medical practice had to close for 3 days due to being too hot. With initial concepts again from Stantec Design It was decided to implement a program of street tree planting, including some trees with additional soil and water provision for monitoring.
In Maynard Avenue and Garrard Avenue, over 30 trees were planted, of which 11 trees were installed in full GreenBlue Urban ArborFlow systems. The trees were a mixture of Field maples (Acer Campestra Louisa Red Shine) and Maidenhair (Ginkgo Biloba) species, The chalk geology enables infiltration, which means that stormwater coming off the roads and hard surfacing enters the tree pits via drop kerb, over a profiled paving area to break the force of the incoming water, down a swale and into the tree pit below. The water then percolates through the bio-remediation soil, down through a drainage stone and into the chalk base.
Each of the eleven locations include 12 cubic meters of bioretention soil with a pore space of more than 20% to capture and attenuate 2,400 litres of runoff within the soil profile, reducing peak flow outputs from the tree pit allowing the surface water to be discharged into the chalk subsoil in the site, completely removing the water from the surface water drainage network.
GreenBlue Urban have taken the lead to ensure these trees are being monitored for not only their ability to attenuate the stormwater but also for their ability to evapo-transpire. When trees have an available water supply to the rooting volume, water can be taken up by the tree and transpired through the canopy, cooling the air. This solution can be very effective, as has been shown in cities across the world, and will be measured and monitored by the University of Kent, taking the trees in standard tree pits as a datum, and therefore seeing how much better trees in the enhanced pits are.
It is well known that 90% of all the water that trees absorb from the ground is evapo-transpired. The question is whether this can be maintained for longer if water is held in the soil with carefully mixed soils.
Many lessons have been learnt from this project. There are many varied and sometimes surprising reasons why residents may not want street tree planting; there are below ground constraints that may not show on utility mapping; correct specification of drainage stone and soils is critical, proper maintenance is required, including topping up of soils where initial settlement may occur, and irrigation is necessary during dry periods for at least three years.
KCC states:- The ArborFlow suite of products integrate together to provide a complete system which effectively allows us to integrate healthy and thriving trees for future generations which in turn provide so many other benefits for stormwater management and cooling to mention only two!
The Cool Towns initiative has enabled Margate to be at the forefront of urban climate change mitigation, and over the next few years will doubtless prove that urban greening is an effective way of providing a sustainable and resilient urban future. This program also supports the local initiatives seeking additional biodiversity, green space, carbon reduction and improved air quality.