Nature-Based Infrastructure
Are natural systems the ideal form of reparation for a ravaged environment? Turenscape’s ‘sponge city’ concept has thoroughly transformed the health and amenity of Haikou’s Meishe River. The positive flow-on effects have been considerable.
Fresh water that courses through a river sourced from rainwater or melting snow can often be so precariously subjected to pollution on the way to the sea that the word ‘fresh’ will be a misnomer. The pollutants can stem from erosion, uncontrolled urban run-off, non-existent sewage treatment, and industrial and agricultural waste discharge, and they are compounded by municipal inaction or oversight on drainage management. They become the source of water-borne diseases and degraded aquatic eco-systems.
The problem can be so acute that in some countries of Asia, congested rivers are viewed as liabilities rather than a sought-after asset, and residential developments turn their backs toward them. Dense developments also exacerbate the speed of runoff, raising the prospect of flooding.
In China, the scale of the problems is exponentially multiplied because of concentrated urbanisation. Seventy-five per cent of the surface water is reportedly polluted in China, caused mainly by non-point-source (meaning diffused) pollution, including urban runoff. Yet, the city of Haikou in southern China offers positive lessons on how a mistreated river can be nursed back to health by multi-agency efforts.
In the past four decades, Haikou’s population grew tenfold from quarter of a million to 2.3 million, with little investment in its natural and infrastructural drainage system. As a result, the 23-kilometre Meishe River that runs through it became a ‘sewage dump’, shamed on national TV. Interim measures taken by the city – which included building walls to control floods and tides, dredging to deepen the riverbed, locking off polluting tributaries, et cetera – served only to aggravate the problem, denting its repute as a tropical tourist city.
In 2016, the Haikou municipality decided to tackle the problem holistically and appointed Beijing-based Turenscape as the landscape architect to lead the mission of remediating Meishe River. The project also included designing a major park (the 80-hectare Fengxiang Park) and a linear corridor of 13 kilometres running through densely built areas.
Sharing his idea of the ‘sponge city’, Dr Yu Kongjian, founder and Principal Designer of Turenscape, explains that its practice of slowing down the flow of stormwater into rivers is founded on ancient Chinese farming wisdom in regulating water flow with paddy fields and ponds. Rivers are then not suddenly inundated such that they overflow their banks during torrential downpours and high tides. Instead of efficient runs formed by modern concrete drains channelling water into artificial embankments, the flow rate is restrained through courses over dykes, ponds, swales, terraces and wetlands. These natural obstacles not only allow water to be slowed, filtered and sieved, but to be cleaned by the soil, flora and microorganisms as well. The ‘sponge’ will both absorb impurities and slow the flow of water.
Collectively, these measures improve the quality of the water, enhance biodiversity and beautify the environment. Concurrently, the rejuvenated riverbanks are turned into people-friendly leisure destinations.
In Haikou, Turenscape adopted three strategies. Firstly, by planning an ecological infrastructure based on terrain and hydrology, they created what is in effect a giant green sponge to separate stormwater from wastewater. It encompasses the river and all its tributaries, its new wetland, and all built green spaces. The green sponge also incorporates a network of pedestrian walkways.
Secondly, Turenscape transformed ‘grey’ into ‘green’. Existing concrete flood-barrier walls were replaced with eco-friendly riverbanks. The waterway was undammed and reconnected to the sea to allow for tidal movements again, and shores were rehabilitated with mangroves. Continuous elevated pedestrian paths made these benefits accessible to the public at large.
Thirdly, the team integrated a terraced wetland park as the heart of the project. Mimicking natural wetlands, a series of interconnected terraces (constructed wetlands) were built along the riverbank to replace the existing concrete flood walls and a garbage dump. These terraces were designed as water-cleansing facilities; pre-treated sewage water is cascaded down over sealed channels to open courses and progressively rid of odour.
Two types of water flow through the wetlands to be cleansed: non-point-source polluted water runoff that carries nutrients, and sewage from the local urban villages that have yet to access the centralised sewage treatment system. In the former case, the constructed wetlands can clean 6,000 tonnes of urban runoff daily to swimmable quality. For the latter, tests have shown that 3,500 tonnes of domestic sewage is being cleansed to the same quality every day. To achieve this, mobile pre-treatment equipment has been installed at the wetland inlet to remove the smell and pathogens that may cause a public health risk, before the grey water is channelled into the open wetland. The biomass from the wetlands is harvested and regularly decomposed into fertilisers for use in the landscape.
The cleaned river has brought back the fish and birds, re-established the mangrove and tropical fruit trees, and reclaimed human patronage. In 2018, the Ramsar Convention added Haikou as one of the 18 new Wetland Cities of International Importance. For the designers, it reaffirms what the empirical evidence has shown – that the nature-based solutions showcased in this project could also have great value elsewhere.