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Rainwater retention

Urbanisation and industrialisation have led to reduced vegetative cover and decreased water storage in the subsurface, as well as the concentration and accumulation of surface runoff in sewage systems due to reduced infiltration into the soil [1] [2] [5]. Until now, no universally applicable urban flood management solution has been developed. However, hybrid measures that combine nature-based solutions (NBS) with conventional engineering solutions/grey infrastructure have been identified as the optimal mix of security provided by grey infrastructure with the multiple co-benefits of nature-based solutions [3]. 


The typical engineering solutions are rainwater systems collecting rainwater via drainage pipes, channelled to the urban rainwater system, and further to the water utilities to be purified. In older cities, rainwater is typically collected in the same water systems as sewage water, which creates large volumes of water for the local water utilities to purify.  


There are a number of nature-based solutions for rainwater retention [4]: 



Green roofs are associated with three main positive effects: 1) cooling and evapotranspiration that leads to lower local temperatures, 2) storing capacity of the excess water, and 3) sunlight absorption.  



Green roofs can be categorised as follows:  


Intensive green roofs have both ecological and aesthetic requirements, as they are usually designed also for recreational purposes. The growth media is relatively thick and deeper than in extensive green roofs. Typical plants for intensive green roofs are trees, shrubs and perennials. Intensive green roofs need regular irrigation and fertilization. The water storage capacity is 30-160 l/m2. 


Extensive green roofs are light weight systems, typically characterized by minimum maintenance and management. Typical plants for extensive green roofs are low growing, rapidly spreading and shallow-rooting. Extensive green roofs are defined as a roof for which the capacity to store water is at least 25 l/m2, typically between 20-50 l/m2 . 


Smart roofs represent an extension of conventional green roofs because the system is equipped with a drainage system under the vegetation layer. The drainage layer retains storm water.  


Constructed wet roofs connect green roofs and constructed wetlands for domestic wastewater (grey water) treatment. Wet roofs are most commonly planted evenly with wetland or marsh plants, but can also have height differences. In addition, constructed wet roofs retain storm water for some time, and gradually release rainwater and reduce the overall runoff. Constructed wet roofs have positive impacts on the microclimate and cool down the local climate. 


VERTICAL GREENING in building integrated solutions 

Vertical greening in building integrated solutions can be either facade-bound or ground-based. These solutions have lower capacity of water retention compared to green roofs. Since pure vertical soil is not found in nature, nearly all types of vertical greening need maintenance [4].  




Bioswales are vegetated linear and low-sloped pits constructed near buildings or roads to reduce flood risk after heavy rains. Bioswales absorb, store, and convey surface water runoff.  




Most rainwater retention solutions are already available on the market or very close to commercial deployment. However, it should be noted that many of the solutions are climate-dependent. Solutions that consider long-term durability and viability in northern climate zones tend to be in the demonstration phase.  


Examples of solutions available on the market or close to commercial deployment: 


  • Intensive and extensive green roofs in moderate climate zones can be found in demonstrations and commercial products in e.g., UNaLab. The solutions in UNaLab include also northern climate solutions. Moderate and hot climate solutions can be found e.g., in URBAN GreenUP and in CLEVER Cities. 

  • Facade-bound vertical greening is implemented in e.g., GROW GREEN. Ground-based vertical greening is constructed e.g., in URBAN GreenUP

  • Bioswales are implemented in CLEVER Cities and in UNaLab in the northern climate zone, and in the moderate climate zones in GROW GREEN and in UNaLab


Examples of small-scale demonstration and piloting: 


  • UNaLab handbook gives technical description and layout for demonstration of smart roofs and constructed wet roofs [4]. 

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