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City of Riga: Stormwater management in urban environment

12 August, 2020 - 14:17

As the consequence of intensive urban development built-up areas are densifying and land becomes more and more covered with sealed, impervious surfaces. Urban settlements – cities and towns – are facing common challenge of increasing stormwater floods, degradation of natural resources and infrastructure caused by climate change and densifying urban areas. Urban development tends to remove the vegetation that intercepts, slows and returns rainfall to the air through evaporation and transpiration. These changes not only increase stormwater runoff, but also accelerate the rate at which runoff flows across the land.


Further, the trend towards densifying urban areas and increasing impervious surfaces reduces the amount of water that infiltrates into the soil and groundwater, thus reducing the amount of water that recharges aquifers and feeds stream flow during periods of dry weather.

Current stormwater management practices consider just one unique solution, pipes. Either combined or separate, sewer systems are designed for maximum capacities based on scenarios up to 10-year storms. However, the pipe solution, also referred as conventional stormwater drainage, has been proved insufficient to manage all impacts related to urban runoff. Conventional stormwater drainage systems such as pipes-based sewer systems, gutters and lined channels are designed to quickly carry runoff to rivers and streams. It considerably impairs the biodiversity, e.g., the the variety of life in urban environment – the variety of species, balance between species and the variety of ecosystems.

Combined sewer system: a universal “one size fits all” unidisciplinary solution:

Source: CSO diagram, US, EPA

Separate sewer system: a universal “one size fits all” unidisciplinary solution:

Source: CSO diagram, US, EPA

These pressures are forcing urban planners and stormwater specialists to develop cost-effective and holistic strategies dealing with greater volume and velocity of stormwater. Potential solutions require going beyond a narrow focus on a single problem to undertaking a comprehensive water resource protection strategy: recognizing the characteristics of specific water resources, understanding the relevant impacts, and tailoring a comprehensive array of tools to individual situations. A combined use of multiple stormwater management solutions is not just possible, but also advisable to increase resilience to extreme weather and climate events. This is possible with an integrated stormwater management approach.

Integrated stormwater management

Integrated Storm Water Management (ISWM) developed by the iWater is a comprehensive approach to stormwater management. Instead of a narrow focus on a single problem, the ISWM undertakes a holistic stormwater management approach: studying the characteristics of specific sites and areas, understanding the relevant impacts, and tailoring a comprehensive array of tools to individual situations. Success requires the integration of the ISWM system into the urban development processes of the city at all levels, from urban planning to operation and maintenance.

In all iWater project partner cities – Helsinki, Turku, Riga, Jelgava, Tartu, Gävle and Söderhamn – solutions have been elaborated how to improve urban planning by integrating stormwater management into all urban planning stages as well as infrastructure construction, operation, and maintenance processes. Each partner city developed its own tailor-made solution – a city-specific stormwater management strategy, programme, or plan as well as each partner city established an institutional model for stormwater management based on a strong commitment of all involved stakeholders, coordinated cross-sectorial cooperation and balanced allocation of functions and responsibilities among municipal authorities and other local stakeholders, such as landowners, property developers, and other.

From the implementation of the iWater project it has been concluded that with an ISWM system a city can:

  • achieve set goals for water quality protection and flood mitigation to protect the natural and built environment;
  • design stormwater drainage solutions for not just the worst-case scenario, but also for average and minimal events to minimize the impact of stormwater on neighbouring lands;
  • determine what solutions and infrastructures together with their interconnections are required to manage the stormwater runoff that results from different storms events; and
  • ensure that stormwater is treated as a resource that enhances our cities, rather than treat it as waste that needs to be removed through underground storm sewers.

Additional information on ISWM.

Stormwater planning tools

Urban development reduces the amount of green areas in cities. However, green areas of different size play a vital role in the adaptation to climate change for example by reducing the risk of flooding, air pollution and urban heat islands of built environments. The significance of green surfaces in the adaptation to climate change is highlighted as the city structure becomes denser. Urban planners need specific tools to ensure that the impacts of climate change are considered in urban development.

In the iWater project the Green Area Factor tool, previously developed by the City of Helsinki, has been updated to Green Factor, optimizing it for stormwater management by bringing stormwater elements a greater weight as well as improving the usability of the tool. The tool has been localized and adopted in all other partner cities: Turku, Riga, Jelgava, Tartu, Gävle and Söderhamn for further use at these municipalities applying a city-specific approach and taking into consideration local planning context, and finally incorporated in the land-use planning process.

Green Factor is a practical and user-friendly excel-based tool for urban planning. The main purpose of Green Factor is to reduce the impact of building on the urban environment and its ecosystem by maintaining sufficient green infrastructures and improving the quality of green areas. It ensures sufficient green infrastructure when building new lots in a dense urban environment. The Green Factor is calculated as the ratio of the scored green area to lot area. The target level for the lot can be achieved flexibly by the garden designer by selecting some of the 39 green elements, such as planted and maintained vegetation or various run-off water solutions, when designing the lot. The Green Factor can, for example, be included in the zoning regulations or used for granting concessions during a construction permit application process.

From the implementation of the iWater project it has been concluded that with the Green Factor tool a city can ensure the planning of efficient, resilient, climate-proof urban environment through:

  • maintaining sufficient levels of ecosystem services, and
  • enhancing the quality of remaining vegetation.

Moreover, it has been witnessed that urban planners find it easy to use the Green Factor, this tool has been evaluated by urban planners as very helpful since it is guiding urban design process as well as it encourages the use of vegetation in the urban environment – courtyards and public spaces.

Additional information on Green Factor.

Cross-sectorial collaboration

Cities are facing a problem of dispersed stormwater management responsibility. In turn, integrated stormwater management requires structured processes and people who know their responsibilities and work together towards common goals in accordance with a certain plan.

To respond to this challenge, iWater partner cities established local multi-sectorial consultative stormwater management groups. These groups were having an active and very important role in supporting development of a city-specific integrated stormwater management strategy, programme, or plan, in localizing and adopting stormwater planning tools as well as in taking part in the project capacity building activities.

Further, these local stormwater groups were, currently are, and in the future will be involved in stormwater management work. iWater partner cities admitted that it is of highest importance to continue the work of established local stormwater groups and, by elaborating their integrated stormwater management strategies, programmes or plans, the cities have taken a long-term commitment to continue their stormwater management work.


iWater project partner cities admitted that one of the main achievements of this project was the shift to new approach to stormwater management, based on improved knowledge and skills of local urban planners, better understanding of the stormwater management concepts and own needs, improved internal and external collaboration, agreements on common stormwater management goals in partner cities, new approaches, tools and many other project outcomes.

As another success of the project partner cities emphasized the iWater-initiated stakeholders’ involvement in cities’ stormwater planning and management processes, in finding common understanding and agreeing on common goals. Stakeholders (urban planners and other civil servants from municipal structures, services providers, infrastructure construction companies, experts, NGOs) were actively involved in urban planning processes and, in result of the project implementation, their understanding of the benefits integrated stormwater management and sustainable stormwater solutions was remarkably improved.


Nika Kotoviča, Urban Planning Expert
City Development Department, Riga City Council

Good to know

iWater project – EU Interreg Central Baltic programme 2014-2020 project “Integrated Stormwater Management” is one of the partner projects of the BSR WATER platform.

Website of iWater project