The Stockport Interchange development is a pivotal component of Stockport’s extensive £1 billion town centre rejuvenation initiative and represents one of the largest singular projects beyond London. The interchange is a collaborative effort between Stockport Council, the Stockport Mayoral Development Corporation, Transport for Greater Manchester (TfGM), the Greater Manchester Combined Authority, Homes England, The Manchester City of Trees Charity and development partner CityRise. EPG and The Environment Partnership (TEP) were originally brought into the project with the purpose of enhancing the biodiversity proposals, reducing the water usage and consumption, reducing the risk of flood risk on the development and downstream, and enhancing the overall sustainability of the designs.
EPG and TEP’s original areas of review was split into five distinct proposals and put forward at RIBA stage 2:
- The residential building
- The park
- The bus interchange apron
- The Mersey frontage
- Building interface on Daw Bank and Swaine Street
The residential building
EPG proposed a bio-solar roof that combined an extensive green roof with a shallow blue roof drainage board. The roof would be designed to manage the 100-year storm event, with an additional 40% added to peak flows for climate change and the discharge from the roof restricted to greenfield runoff flows. The planting scheme suggested on the roof was to be low maintenance, with the types of plants specifically chosen to enhance the biodiversity and attract invertebrates to the area. The benefit of this is that attenuation does not need to sit beneath the building between the piles. Also, by including a blue roof and solar panels, the efficiency of the solar panels is enhanced as evaporation associated with the vegetation can cool the air surrounding the PV panels.
The park
EPG proposed a blue roof with a passive irrigation system. Further detail regarding the park is included later in this document. In addition, EPG suggested a hydraulic SuDS pavement for the bus interchange apron. A hydraulic pavement follows similar principles to pervious pavements with a porous sub-base providing the attenuation for the 100-year storm event. An additional 40% was added to peak flow for climate change, but with impervious surfacing. Using oil and silt interception and collection channels adjoined to shallow diffuser units enables the water to be collected and diffused at a shallow horizon into the pervious sub-base. The benefit of this is that attenuation sits within the first 600mm of road build-up, this allows the removal of the deep attenuation tank and petrol interceptor.
The combination of the proposals enabled the removal of the originally proposed attenuation tank and petrol interceptor. These items were planned to sit around 1-1.5m beneath the bus apron, however within this area there is high rockhead and potential caves within the rock. Abstraction of the deep tanks and interceptors removed the requirement of carrying out a dig and excavate in rock which could potentially clash with cave features. In addition to this, the deep tanks were below the flood levels of the River Mersey and therefore would be hydraulically locked during peak storm events. This meant that runoff would need to be pumped into the United Utilities combined sewer adjacent to the development. By using blue roofs, SuDS and hydraulic pavements, the runoff was discharged directly into the River Mersey above any flood levels, ensuring full disconnection from the United Utilities system.
Following an engagement meeting with the clients and stakeholders, the EPG proposals for the park were taken forward to full construction. EPG was notated to a specialist design by Willmott Dixon, the main contractor, and worked through the NEC Risk Register to ensure that all contractual risks were covered.
This resulted in EPG being responsible for:
- The design to Stockport Council Planning standards and the standard set out within ‘The SuDS Manual’ of the Permavoid blue roof layer undertaking the hydraulic design to ensure that the tanks could manage the 100-year storm event, plus the additional 40% for climate change with the discharge restricted to green field runoff rate. Structural calculations to CIRIA 680 & C737 were undertaken to ensure that the Permavoid cells could manage the proposed trafficking received on the roof.
- The design of the restricted outlets and the overflow outlets for the blue roof design with hydraulic calculations.
- The structural and hydraulic design and calculations for the pervious pavements on the park including interactions with movement joints.
- Full waterproofing design of the park including movement joints, penetrations and the link with the residential building and the Wellington Road Bridge which is a Grade II Historic England listed building.
- Full build up design of the lightweight fill and substrates, liaising with the landscape architect to ensure that the build-ups proposed were linked with their planting schemes and ensure that amenity and biodiversity is maximised.
All design work included full general arrangements, sections, details, specifications, calculations and planning, and RIBA stage reports. EPG was fully cognitive of the CDM requirements, and all potential risks were designed out where practicable. Where these risks could not be designed out, they were minimised as much as feasible, and the Design Risk Register/Assessment (DRA) was produced.
EPG offered to Willmott Dixon and the development clients a £10m 12-year collateral warranty for the works undertaken by EPG on this project. EPG has a robust check, review approved to British Standards BS1192:2007, BS EN ISO 19650-1:2018 & BS EN ISO 19650-2:2018. For this project, EPG and all other consultants worked off a single document control management system ensuring BIM level 2 compliance.
All the work conducted by EPG was undertaken and delivered in a timely fashion and before any deadlines. Regular site visits meant any changes were picked up and addressed when required, feedback was given and being part of a multi-disciplinary team ensured that the rooftop park was finished to a very high specification, on budget and on time.
