Managing Outflow and Green Roof Detention Systems: Enhancing Water Management

As urban populations soar, cities are facing unprecedented challenges. As they become more and more densely populated systems, they are increasingly marked by a host of daily social, energy, logistic, waste, traffic, and environmental challenges, all unfolding atop solid, impervious surfaces. This reality will force cities to recognize and nurture their sponge function. The concept of sponge cities targets the planning of blue (water storage-based), green (vegetation-based), and grey (smart building-based) infrastructures that store and reuse water runoff caused by heavy storms in order to reduce flooding. Cities have so far tackled this with a variety of approaches and rates of success, but measures must also be taken at the regional and national levels, in addition to the urban.

Image 1: Urbanscape green roof on the Karolinska Hospital, Sweden.

Every city is its own challenges, but none has a magic wand to wave them away.

Let’s take a closer look at a case in Germany, specifically the city of Berlin, with a current metropolitan population of more than 3.5 million inhabitants. The Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (BMUB) was already issuing programmes highlighting the role of green roofs and facades for healthy urban populations several years ago, while several initiatives were launched at the city level. The courtyard greening programme ran from 1983 to the end of 1995, during which period 1,643 projects were approved, with 740,000 m2 of courtyards and facades, and 65,000 m2 of roof areas greened in the process. As early as 1990, ecological requirements were defined in the guidelines on public funding for social housing, stipulating that resource conservation and environmental compatibility should be considered in building projects. Eligible for funding were vegetation concepts for greening facades and roofs, as well as other specific ecological open space concepts and their implementation. Since 2019, the “Green Roof PLUS” funding programme, formerly known as “1,000 Green Roofs”, has been promoting the planting of vegetation (> 100 m2) on existing buildings, particularly in high-density urban quarters. As there is currently no legal obligation to add green roofs to existing buildings, public funding is a very important measure here (Environmental Atlas Berlin, 2020). This case shows that there is a need for systematic approach, which takes decades – but remember, even Rome wasn't built in a day!

Image 2: Green roofs in a residential area (https://worldgreeninfrastructurenetwork.org/eu-parliament-green-roof-climate-change/)

Berlin’s green roofs have been a common and popular research topic throughout the decades. There is an interesting recent paper from Koehler (2023), which describes the green roof evolution from a temporal perspective. The author acknowledges that, “looking back over green roof development… we see a shift from a simple and environmental ecological feature to a multi-purpose solution with several benefits.”

Potsdamer Platz, Berlin: a district with water-sensitive urban design

Potsdamer Platz is historic square, reborn since the fall of the Berlin Wall. The revitalized 1.3-hectare area sits at the intersection of the city’s 5 most bustling streets, forming a star-shaped square. It encompasses 19 buildings, 10 streets, 2 car parks, and 3 underground car parks. The water-sensitive urban design, building-integrated water recycling systems, and industrial regeneration have come to represent an iconic symbol of designing “Blue and Green” urban waterscapes using rainwater where it falls. This includes Potsdamer Platz’s 30,000 m2 of green roofs.

The Potsdamer Platz features stormwater retention systems designed to minimize burden on Berlin’s existing water infrastructure, including its many green roofs. The stormwater management, harvesting, and recycling system is fed by the city’s 21″ of annual rainwater. The green roofs retain and then release water to the large, on-site buffer pond, which has 5 underground storage tanks. Cisterns provide water for the 3-acre artificial Piano Lake, and approximately 4 million gallons of water are circulated though filtration beds once every 3 days, as the planted biotopes filter and circulate the water. The excess overflow is used as grey water for flushing toilets, irrigation, and fire systems. The cleansed water flows into a beautiful large piazza for visitors to enjoy. In all, Potsdamer Platz manages an estimated 23,000 m3 /yr of potable water with its combined stormwater storage of 13,500 m3, and its cooling capacity reduces summer temperatures by approximately 2 °C, resulting in savings on energy (Greenroofs.com, 2024).

Image 3. Potsdamer Platz rainwater management: @Atelier Dreiseitl/Ramboll

Storm water issues tackled with green roofs

Green roofs are an effective architectural tool for managing storm water because they can retain a significant amount of water and their drainage function helps reduce and slow runoff before it hits the ground.

In Germany, green roof regulations are described in the FLL guidelines. These guidelines list the requirements for planning a green project and set metrics regarding runoff management.

One of these metrics is the runoff coefficient C, which measures the runoff time delay. The dimensionless indicator is the ratio between the runoff into the drainage system and the actual precipitation level at the same spot. If rainwater can seep away or evaporate due to the roof’s surface characteristics, the runoff coefficient is less than 1.0. DIN EN 12056 initially specifies the runoff coefficient C for all roof surfaces with this value.

For green roofs and gravelled roofs, reduced runoff coefficients from DIN 1986-100 and the FLL guideline apply, depending on the roof design, as follows (FLL-Dachbegrünungsrichtlinien 2018, Werten bei Dachneigung bis zu 5°):

• With a green roof height of 6 to 10 cm, Cs = 0.5
• With a green roof height of 10 to 15 cm, Cs = 0.4
• With a green roof height of 15 to 25 cm, Cs = 0.3
• With a green roof height of 25 to 50 cm, Cs = 0.2
• With a green roof height of above 50 cm, Cs = 0.1

Knauf Insulation Urbanscape is always committed to overachieving on requirements. Our new Detention C green roofs (https://info.urbanscape-architecture.com/hubfs/Documents%20-%20ENGLISH/Urbanscape-Detention-GR-EN_012024.pdf) smash the standards, providing for a drastically reduced runoff coefficient, even with their low height.

Every city and every project still has its own challenges and requirements for storm water management. Some projects depend upon a specific runoff delay time, and in such cases, project specifics must be calculated. Such project-specific runoff calculations are made possible by our proPET tool. Don’t hesitate to contact us for more information (urbanscape@knaufinsulation.com ).

Literature:

Environmental Atlas Berlin, 2020. Green roofs 2020. https://www.berlin.de/umweltatlas/en/land-use/green-roofs/2020/introduction/

FLL-Dachbegrünungsrichtlinien 2018. Werten bei Dachneigung bis zu 5°

Greenroofs.com, 2024. Potsdamer Platz. https://www.greenroofs.com/projects/potsdamer-platz/

Koehler, M., 2023. Extensive green roof study on a retrofit building in Berlin, Germany: Report after 34 years of monitoring. Journal of Living Architecture. Volume 10, Number 1, Pages 1-12.