MVRDV will work alongside Dutch design studio Studio Makkink & Bey and landscape designer Ben Kuipers to overhaul the 938-metre-long Seoul Station Overpass, creating a new pedestrian pathway boasting 254 species of trees, shrubs and flowers.
Their competition-winning concept, known as the Seoul Skygarden, also involves creating a nursery for cultivating plants for the city's other green spaces, alongside cafes, flower shops, libraries and greenhouses.
"Many viaducts and pedestrian overpasses in Asian cities are purely functional elements but, due to the scale of the Seoul Station Overpass, an opportunity exists to create a unique public space in the heart of Seoul," explained MVRDV, which is also creating a square-shaped garden peninsula to host the World Horticultural Expo in 2022.
The overpass was constructed in the 1970s as a connecting road between Namdaemun market and the west of the city, and it crosses a major railway station. But in 2009 it had to be closed to heavy vehicles after inspections found it to be unsafe.
Rather than demolishing it, a competition was launched inviting architects and designers to submit ideas for its reuse.
The decision follows on from the city's ambition to become one of the world's most eco-friendly cities – a goal revealed by Seoul mayor Oh Se-Hoon in 2008. He believes the metropolis could become "a city within a park".
The old Cheonggyecheon stream – which at one stage was used as a highway – has already been reestablished as a public waterway, and the government has also been looking at ways to tackle air pollution and renovate abandoned buildings.
"The Seoul Skygarden hopes to build on the city's ambition to be greener, more attractive and more user-friendly so as to inspire a process of change for the entire neighbourhood," said MVRDV.
The new route will cut the 25-minute walk around the station to just 11 minutes, and will generate 1.83 times the cost of its renovation and maintenance in economic benefits, according to research by Seoul Institute and the Korea Planners Association.
The arboretum, described by the designers as "a library of plants", is central to the concept and will be organised in accordance with the Korean alphabet to make the park easy to navigate.
The design is also intended to be flexible, allowing it to change or grow in the future. Staircases, lifts and escalators could be added to create more access points, and the designers suggest that satellite gardens could also be installed alongside, "sprouting like branches from the existing structural piers".
The project is due to complete in 2017 and be delivered in collaboration with various local partners, including architecture office DMP, landscape studio KDI and structural engineers Saman and Cross.
It is the latest in a series of proposed landscape projects inspired by the High Line – the popular New York park designed by architects Diller Scofidio + Renfro, landscape architects James Corner Field Operations and planting designer Piet Oudolf.