Gdańsk's Stormwater Retention Model — Open Reservoirs and Rain Gardens After the 2001 and 2016 Floods

After the 2001 flood, Gdańsk's municipal water company Gdańskie Wody built more than 50 open retention reservoirs along the city's streams, plus several rainwater pumping stations. Published analyses report that, thanks to works in the Radunia channel basin, no emergency situations were observed there during the extreme rainfall of 14 July 2016 (roughly 129–160 mm in about 16 hours in the Strzyża catchment), although other catchments still saw hundreds of fire-brigade call-outs.
From 2018 the company added a second layer: rain gardens and other nature-based solutions (about a dozen rain gardens by 2022). A peer-reviewed Gdańsk case study (Kasprzyk et al., Science of the Total Environment, 2022) found that the proposed rain gardens could reduce runoff volume by 18.1–30.2% and peak flows by 17.9–32.0% at catchment level, and individual bioretention units can cut runoff by as much as 97–98% depending on the event and catchment. Note these catchment figures are modelled estimates, not long-term measured outcomes.
Policy backs the practice: new housing estates are to rely on nature-based retention rather than conventional rainwater drainage, and any drainage project must provide retention before a building permit is issued. The Baltic Sea Region 'Urban Surface Retention' good-practice sheet (Interreg IWAMA and BSR WATER projects) gives indicative costs of about €150 per m³ of retention for swales and €250–500 per m³ for rain gardens in existing urban space, roughly 50% less in new development.
Limitations: the reservoirs are conventional grey-green engineering rather than payments for ecosystem services, carbon and biodiversity benefits are not quantified, and long-term monitored performance data for the rain gardens remain limited.

Read the full analysis: https://www.balticwaterhub.net/good-practice/urban-surface-retention-system