Urban Surface Retention System

In recent years, urban areas are increasingly exposed to flooding and urban floods, so-called flash floods (unusually heavy rain). The main reason for these recurring situations is the expansion of cities and exchange of permeable areas - e.g. green areas - to impermeable. Additional observed climate changes tend to increase the frequency of extreme phenomena, e.g. long periods of drought or short-term high intensity rainfalls (microbursts). In order to avert that danger, it is therefore necessary to increase the implementation of 'small retention' solutions in urban areas both on the scale of the whole city and the scale of a single plot.

The proposed Urban Surface Retention System (in Polish: System Powierzchniowej RetencjI Miejskiej, SPRIM) assumes retention in many dimensions (from micro on a building scale to small reservoirs in a district) compatible with the urban system, in which all retention levels are equally important and complementary to each other so as to create a multi-level structure and increase the level of urban protection against the effects of climate change.

Actions taken to implement the idea of sustainable rainwater management involve the construction of devices and facilities for the retention, infiltration and treatment of rainwater. Local solutions are used to stop and purify rainwater in the place of rainfall. These include, first of all, bio-retention devices, e.g. rain gardens, green roofs, hydrophyte ponds, but also perforated surfaces supporting the infiltration of rainwater into the ground. SPRIM consists three levels of retention on a city scale.

Solution to which problem

1. Tank-based retention systems - systemic retention on a city-district scale. In the general case, wet, dry or semi-dry tanks retain accumulated water in natural watercourses before its directed to the drainage system below the tank. When the tank is filled to the level of emergency overflow, the water will drain from the tank with an intensity similar to the inflow volume. In this situation, losses below the device should be taken into account. Examples are solutions used in the Gdańsk city.

2. Surface / small urban retention systems - solutions in the scale of the city, district, street or housing based on the development of urbanized areas designated for urban greenery, also called urban retention systems. Water supplies a single system object in a dispersed manner (surface runoff) or cumulated from a small hardened surface (via a drain). Rainwater overflow in the facility drains by emergency overflow or a planned surface runoff path to the next facility. In most cases, rainwater is the main source of evapotranspiration, evaporation and infiltration processes. The system solutions created in this way are built from individual micro-retention objects. Examples are the Gdańsk rain gardens, and the new district ( households assets)  build without any storm water system and only with green surface infrastructure.

3. Microretention - individual elements of urban retention functioning on the scale of a single building or a plot, also called a small urban retention object, e.g. a single rain garden, green roof, green shed (Gdańsk, Poland)


Implementation of emerging outflows and connection with existing storm water infrastructure.


The project aims to increase retention in Gdańsk supporting devices and systems at various levels of retention.

The expected result is a reduction in the number of floods and flash floods in the city, as well as an increase in groundwater supply. In addition, surface runoff should be reduced, which flushes pollution from the catchment area - from streets, roofs, industrial areas or gas stations - and transports them to storm water drains or directly to surface waters. Participation of the inhabitants in planning processes is necessary, social acceptance is very important, it helps that the residents feel as co-hosts of the city.


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