Green retention, seemingly small and yet great

Gdansk has been working for several years to avoid being green on the subject of green retention. How does he manage to do that?

Water – little and much

A manifestation of climate change is the prolonged periods of rainlessness interspersed with short, intense downpours – the so-called “rainstorms. rainfall from storms. A good example is the rainfall distributions that occurred in Gdansk in 2022. The annual values represent approx. 75% of the average precipitation of the 1991 – 2020 multi-year period in the eastern part of Gdansk and approx. 70% in the western part. This allows the entire year to be described as dry on the border of very dry (according to the criterion of Kaczorowska, 1962). In the warm half of 2022. Measuring stations recorded 5 days with IV degree precipitation. Although the entire year is described as dry, the three summer rainfall episodes (August 15, 16 and 20) are among the nine largest in the past five years.

Worth knowing: heavy rainfall – the Khomich scale is used to determine its degree. Level IV on this scale is rainfall that exceeds the standard design values of the stormwater drainage system and most often results in perturbations in the operation of the city’s drainage system. Grade V is critical rainfall, and VI is the heaviest heavy rains.

Water pouring from the sky in a rapid stream is a threat to the city’s functioning. Streets become impassable, public transportation stops working, and access to hospitals and other critical points becomes severely hampered. On the other hand, lack of water also significantly reduces the quality of life. Greenery dries up, water blooms (lack of oxygen), and people feel uneasy. Recognizing the relationship between nature and the needs of a well-functioning city, gave Gdansk’s authorities the first signal to introduce systemic green retention.

GREEN LIGHT FOR GREEN RETENTION

In the case of Gdansk, in addition to climate change, there are additional flood risk factors, such as the specific terrain, similar to mountain towns (the difference between the highest and lowest points is 180 m.). Gdansk is particularly vulnerable to the effects of extreme weather events intensifying under climate change, so it was decided that the traditional underground network and retention tanks were not enough. Activities began with a citizen panel. The result of the meeting between officials and residents – with the common goal of identifying needs and working out how best to manage water – was m. in. acquiescence to the realization of small retention facilities, supplementing the existing network of retention reservoirs.

“The idea is to direct rainwater first to green areas, and only the excess into the sewer system,” explains Ryszard Gajewski, president of Gdansk Water, the city company responsible for flood protection. “With this approach, the terrain of Gdansk ceases to be only a risk factor and becomes a potential. We take advantage of them because depressions and terrain depressions are natural objects of small urban retention,” he adds.

GARDENS FASHION FOR WATER

In a short period of time, Gdansk has mastered the art of designing rain gardens so that they can become dispersed small retention basins. Their main task is to collect and use rainwater from nearby areas, including paved surfaces. The best source for them is water from roofs and sidewalks, from home driveways and parking lots. They are both small retention reservoirs and natural filters. They retain not only rainwater, but also the pollutants it carries (such as heavy metals and nitrogen and phosphorus compounds).

A kind of fashion has developed in Gdansk for rain gardens, which are emblematic of a broader, systemic rainwater management policy. “From the very beginning of our activities, we were keen to achieve a real change in the perception and treatment of rainwater and integrate all urban processes in order to achieve a satisfactory large-scale effect. Demonstration rain gardens have become a kind of lever that has accelerated a change in investment planning or operation of existing green spaces. The key has become thinking about rainwater and finding a place for it in the urban fabric already at the planning and later design stage,” explains Gajewski.

In the interest of flood safety, Gdansk has some of the most stringent rules on rainwater management for investment activities. Already in land use plans are written special requirements, regarding rainwater retention, which are detailed in the technical conditions issued.

As a result, investors are obliged to take care of proper water drainage, and buildings in Gdansk are increasingly using small retention. “We encourage, to use the benefits of green retention. In 2022 alone. We have agreed on designs for surface retention, which will be able to store nearly 130,000^{m3 of} water,” says Ryszard Gajewski.

NOT ONLY GARDENS ARE GOOD FOR WATER

Adapting to climate change means not only new investments, but also adapting existing infrastructure to dynamic conditions. “We are now redirecting rainwater, which was previously taken up entirely by the underground stormwater drainage network through street inlets, to green areas,” explains Ryszard Gajewski. Thanks to properly lowered and profiled curbs, some of the rainwater from dozens of Gdansk streets flows into grass basins, relieving the pressure on the traditional network. “If we don’t retain the water at the point of precipitation, there will be rapid runoff into the sewer system. Capacity will be exceeded and manhole knockouts will occur. Directing water to the greenery flattens out the waves of high inflow. This allows the sewage system to cope better with its excess. The facilities of the so-called. Microretentions work well in both extreme rains and periods of drought. They take up excess water during downpours and store it for times of rainfall shortage,” explains Gajewski.