Retention roofs act as urban sponges

In view of rapid urbanisation and climate change, towns and cities worldwide need to manage water-related risks and make changes in the interests of sustainability and resilience. In this case, urban resilience means, in particular, adapting to the climate. In terms of extreme weather events, it will be vital to modify the infrastructure to solve water-based problems, such as shortages or flooding, which chiefly arise due to urbanisation across the world. To do so, there must be fundamental changes in the way urban environments manage water. An international approach to solving this problem is the sponge city philosophy.

The sponge city philosophy

The sponge city philosophy is an urban planning approach whose core aim is to restore the natural water cycle in urban areas. Its goal is to enhance the infiltration, evapotranspiration, collection and reuse of rainwater in towns and cities.

The sponge city’s underlying concept is to view rainwater as a valuable resource and to leave it in its natural cycle for the most part. The idea is for the city to absorb and store rainwater like a sponge and provide access to it locally where required. Which entails the following benefits:

  • Local evaporation and infiltration of the rainwater puts less pressure on the sewers.
  • Flood damage is decreased through delayed and reduced runoff.
  • The natural water cycle is encouraged.
  • Evaporation of the rainwater benefits the urban climate and binds dust.
  • During dry periods, any rainwater stored can be used for local irrigation of urban green spaces instead of drinking water.
  • Groundwater recharge is encouraged.
  • Rainwater is purified via the soil through infiltration.

Goals of the sponge city philosophy

The sponge city’s primary goal is to make cities more resilient to climate change by creating an urban water cycle, allowing rainwater to evaporate and infiltrate. In addition to this primary goal, there are five other key ones.

  1. Local water retention
    Over the past few years, local, heavy rainfall has often caused severe flooding and serious damage. The purpose of the sponge city is to decrease and slow down peak flows. This puts less pressure on urban sewers and cuts the risk of flooding during heavy rainfall drastically.
  2. Improvement in urban water quality
    Due to the rapid pace of urbanisation, drinking water security in lots of towns and cities is in jeopardy. Which is why reducing water pollution is one of the sponge city’s most important goals. In a sponge city, the rainwater is filtered and cleaned via the soil or the substate on green roofs.
  3. Re-establishing the natural water cycle
    In order to re-establish a natural water cycle, local infiltration and evaporation must take place. The evaporation performance of intensive green roofs is much higher than that of extensive ones. Local storage of the rainwater is also one of the sponge city’s most key objectives.
  4. Reducing water consumption
    Due to the fast growth of urban populations, towns and cities have been confronted with numerous water crises over the past few years. The sponge city concept involves using rainwater in different ways and dispenses with the previous concept of rapidly draining water and retains it as a resource instead.
  5. Improving the urban microclimate
    Overheating of urban areas is one of the best-known urban climate phenomena. Therefore, improving the urban climate through evaporative cooling of various elements of the sponge city is a major goal.

Blue/green infrastructure in the sponge city

A growing number of publications shows that solutions based on nature, in particular enhancing the blue/green infrastructure, are sustainable methods of creating water-resilient towns and cities.

Blue/green infrastructure is gaining recognition as a complementary method of urban flood risk management, alongside the grey infrastructure (sewers) in many towns and cities. This is because it provides a whole host of benefits alongside its main one, which is to provide flood protection. The advantages include improving water and air quality, increasing local climate resilience, maintaining the natural water cycle, boosting urban dwellers’ well-being and improving biodiversity in towns and cities.

In the sponge city, the blue/green infrastructure can help to collect, store, and reuse urban rainwater sustainably. Due to various sponge city elements, much of the water can slowly evaporate through natural processes and therefore counteract the rising heat in built-on areas. Unsealed and greened areas with a high level of evapotranspiration are ideal in this respect.

Green roofs, trees, rainwater harvesting cisterns and the inclusion of permeable road surfaces are the most important and well-known urban blue/green solutions in terms of sustainable rainwater management. Above all, it’s retention roofs that help to store water and reduce the impact of stormwater runoff locally.

The retention roof – the heart of the sponge city

Green roofs have been used to control rainwater for decades because they retain rainwater and delay peak flows, which cuts the risk of flooding. An even greater contribution is made in this case by retention roofs, which are designed to delay runoff considerably and have exceptional water storage capacity.

Retention roofs cut peak flows because they retain and store rainwater and increase the evaporation performance of roofs. Consequently, the risk of city-centre flooding decreases and the urban water supply is more like a natural one.

Shetty et al. (2022) investigated the impact of green roofs on urban rainwater runoff. Their findings showed that the retention rate of a standard green roof is already just under 100% at low precipitation. During heavy rainfall, a normal green roof can retain approx. 65.2 % of the water. This percentage rises to 75.6 % if the rainwater is stored too.

In addition to providing flood protection, retention roofs also offer other benefits. For instance, they improve the urban microclimate, biodiversity, rainwater management and people’s well-being. Another huge advantage is that it can be used as a planning tool for existing infrastructure.

Fig. 1: Retention roof water cycle; our own graphic 2022

Shetty, Nandan; Wang, Mark; Elliott, Robert; Culligan, Patricia (2022): Examining How a Smart Rainwater Harvesting System Connected to a Green Roof Can Improve Urban Stormwater Management. In: Water 14 (14), p. 2216. Retrieved here: Link