Watersquares: The Elegant Way of Buffering Rainwater in Cities

Topos Vol. 74 2010/11, pp42-47. Florian Boer

Climate change is bringing many problems, one of them being the increase of heavy rainstorms, resulting in sudden amounts of water levels that the city sewerage systems cannot cope with. This is not helped by the fact that the cities are increasing in density, the available spaces is being built up and the “open spaces” are designed to be impermeable, leaving very little infiltration space. This is particularly serious in Holland due to the fact that many of its cities are situated below sea level.To solve this problem the city planners and engineers of Rotterdam have devised a plan to build sufficient water storage within the city and combine it with improving the quality of public space. The city council is also encouraging individuals to create green roofs on their properties to buffer storm water before it reaches the ground. “Here the buffing effect is limited, but the educational aspect is relevant”.

The solution presented here is that of De Urbanisten and Studio Marco Vermeulen, where flood basins are built under public spaces, designed so that they can be used as public open space.

Issues

·Safety. Not only is the increase of heavy rainstorms on their mind and the effects of sudden amounts of water in the city, but also the rise in sea levels.

·Quantity of water. Increasing rainfall increases flooding. At least 80 hectares of extra open water bodies are required in Rotterdam to cope with the drainage of water.

·Quality of water. Existing open water has to be clean, clear and plant-rich. One of their goals is to separate waste water from the relatively clean rainwater by segregating the existing sewerage system.

Scale
The solutions range from large scale technical proposals to more aesthetic small scale interventions.

Program
The solution De Urbanisten and Studio Marco Vermeulen propose is to integrate flood basins in the city under public spaces that people can use and enjoy. Only during heavy rainstorms would this space be flooded, and therefore acting as temporary storm water storage. This way, the money spent on the infrastructure is also used for investing in better public open spaces. The typologies can vary depending on the scale and space available: sunken squares, smart street profiles, water balloons, dams on sloping surfaces.

How
As described in the article by Florian Boer in the TOPOS magazine: The design of the pilot square is divided into two main parts: a sports field and a hilly playfield. The sports field is sunken into the ground by one metre and is surrounded by steps which can also function as a stand for people to sit and watch a game. The hilly playfield is sunken into the ground too and comprises numerous spaces set at different levels, to sit, play and linger. Both parts are enclosed within a green frame of grass and tress which borders the square. Most of the time the watersquare will be dry and used as a recreational space.

Challenges and Opportunities
Only when heavy rainfalls occur does the watersquare change its appearance and function. The rain is collected in streams, brooklets and ponds that the children can play in and around. The water is stored in the square for a couple of hours, and once the city’s water system is back to normal it can be drained. During prolonged downpours, the water levels in the square can rise to the point that they flood the sports field and rest of the square. Hygiene is an important issue. This is why the water is collected into a water chamber first, it is filtered, and then it runs into the square till it can be discharged into the nearest water body. It is not intended for the water to stay in the square for long. It has been calculated that statistically once every two years will the water stay in the square for up to 32 hours. When evacuated, some dirt and debris may be left behind, so it is important for the watersquares to be cleaned after its use as a buffering space. Another possibility that has been contemplated is to use these spaces as an ice rink in winter when it is freezing. Such facilities are usually too expensive to implement in a regular outdoor space.