Under the LIFE Urban Rainfall project, led by the Municipality of Hegyvidék, Budapest is working hard to develop a new, modern and comprehensive rainfall strategy, which it hopes will serve as a long-term guide to managing rainwater and turning the city into a sponge city. What is a sponge city? It is a fair question. On the one hand, the concept of a sponge city is a response to the problems caused by extreme rainfall due to climate change, and on the other hand, it is a modern reinterpretation of past solutions. One of the problems of cities is that, although rainfall does not vary from year to year, its distribution is becoming more extreme. Long periods of drought are followed by short periods of heavy rainfall, leading to flooding, waterlogged basements and similar events that are not at all desirable in cities. The sponge city wants a solution for both extremes, so its principle is not to discharge water through the sewer system, but to absorb it like a sponge, retain it as long as possible and use it later, even during droughts.

To enrich Budapest’s rainfall strategy with experiences and solutions that can be integrated into the existing urban fabric – infrastructures, networks, areas – the Urban Rainfall project team visited Vienna on its third study trip. Vienna’s climatic conditions are very similar to those of Budapest. The average temperature and, most importantly, the rainfall are similar, and rainwater management is also a priority, but in many respects implementation is ahead of Budapest.

The first and most important lesson is the importance of coordinated stormwater management regulation. One of Budapest’s difficulties in this respect is the dual administration, with its 23 districts operating separately from the Municipality of Budapest, which makes it difficult to manage issues such as stormwater. It is well known that rainwater does not stop at the district boundary just because the area five centimetres away already belongs to another municipality. One of Vienna’s strengths is precisely the coordinated effort of the different authorities. The Vienna Building Authority, the Vienna Sewerage Authority, the Municipal Garden Office, the Vienna Water Authority, the District Planning Authority and even the Roads Management Organisation are all helping to turn Vienna into a sponge city as soon as possible. Here are just some of the efforts Vienna is making in its commitment to becoming a sponge city:

  • For new construction, or when converting or renovating existing areas, at least 15% of the total surface area must remain unpaved.

  • Green roofs are mandatory for all new buildings.

  • The building authority carries out its own assessment of each area and determines what percentage of the rainfall that falls on it must be treated by the landowner and how much can be discharged. This will depend mainly on the hydrological characteristics of the area (soil type, water retention capacity, existing green infrastructure, etc.). In principle, water can only be discharged from an area to the sewer if it cannot be drained or treated locally by any means.

  • Trees should be planted along roads at regulated intervals.

  • In other places, they are trying to create separate catchments to collect rainwater from the roofs of buildings and channel it into nearby parks and green infrastructure.

With these and a series of similar rules, Vienna is trying to improve its rainwater management. Of course, a few things are worth noting. Firstly, the entire length of Vienna’s sewer network would be two and a half times the length of the Budapest sewer network. In other words, managing the sewer network in Budapest is a much more complex task. Another help for Vienna is that the regulator and the control body are “under one roof”. In Budapest, for example, green roofs and stormwater management are mandatory in many districts, but the municipalities do not have the power or the means to control and enforce them, and often these regulations are not enforced. It is also true that this nicely coordinated effort is actually being implemented in new build neighbourhoods. But there is still work to be done in the old, established parts of Vienna. However, Vienna is also working on a ‘Sponge City Light’ concept that could be applied to the inner districts. Most of the talk here is about breaking up the pavements.

One of the current innovations – still under development – in the inner districts is a self-activating, sensor-operated kerbside brick with small doors. Believe it or not, small islands surrounded by tree-lined curbs are still used to channel rainwater into the curbside by a small sliding door on one of the curb bricks that is manually closed every fall and manually opened every spring. The idea of the sliding gate is to protect the trees by blocking water runoff from the sidewalk or driveway in the winter when the roads are salted, but letting it in during the spring and summer when there is no salting. But this also means that someone has to be sent out to the green spaces every seasonal change to open and close the small sliding gates. Vienna is now trying to modernise this labour-intensive task in three steps. The first step is to create a curbside door with a temperature sensor and a solar-powered automatic system that opens and closes the door depending on the temperature outside. When the temperature drops below 8 degrees, the door closes. If it rises above 8 degrees, it opens. This saves opening the door. However, whether the green area and the trees in it need watering at the time should still be determined by visual inspection. This is why a new version of the pump brick is being developed, which will also measure soil moisture and send a signal to the maintenance company whether or not the tree needs watering. This will also make irrigation much more efficient. The forthcoming rainfall strategy could give a huge boost to the transformation of Budapest into a sponge city. It is important to highlight that a local rainfall management action plan is also being prepared in the district, also as part of the Urban Rainfall project. This complex strategic document will identify specific locations in the district, based on runoff modelling in the Hegyvidék, climate models showing expected weather changes and a vulnerability assessment currently under preparation, which will take into account the green areas of the mountainous areas, the built environment and the population, where rainfall can be captured, silted up and stored to mitigate the damage caused by sudden rainfall and the problems of water shortages during periods of drought.