Green and blue corridor in Skanste neighbourhood of Riga city
Green and blue corridor in the Skanste neighbourhood near Riga historic centre is a multifunctional green and blue infrastructure servicing the area of total size around 100 ha and offering multiple functions/benefits: stormwater runoff attenuation before restricted discharge into sewer network, groundwater recharge, stormwater treatment by plants and soil, landscaping and recreation function, space for snow storage during winter, neighbourhood microclimate regulation, biodiversity provision, quality public space and recreation.
The green and blue corridor is implemented as a cascade of bioswale, vegetated canal with a gabion support wall, creek and dry river in the park and a series of ponds, before limited discharge into the stormwater sewer system.
Solution to which problem
Skanste neighbourhood is a rapidly developing mixed use area near Riga city centre with rapidly increasing stormwater runoff and stormwater management infrastructure with limited capacity. The green infrastructure corridor is implemented as an alternative to a costly conventional solution involving construction of stormwater sewers and reconstruction of a cascade of stormwater pumping stations and pressure mains.
Technical conditions
Skanste neighbourhood used to be a mix of logistics and low-quality greenfield areas that is now being rapidly sealed. It has unfavourable geological conditions (swampy sands), low vertical elevation that makes it flood-prone and high ground water level. The conventional stormwater management system, consisting of a cascade of pumping stations and pressurised pipes, is not prepared for the increased stormwater runoff.
Construction and construction supervision may be a challenge, as this is the first Latvian system of such scale. The scale and complexity of the system may also be a challenge during maintenance.
Implementation
The first idea about the green and infrastructure in Skanste area emerged in 2012, when the area was chosen as one of the feasibility study sites in the project (D)rain for Life, aiming at promoting sustainable stormwater management systems and financed from the Estonia-Latvia cross-border cooperation programme. The feasibility study established several options for the stormwater management system, including more conventional and more sustainable options. Considering similar investment and operational costs, but greater total benefits and a reserve for resilience, Option2 (Ditch system with three medium water bodies), offering larger provision of green infrastructure in public space, had been recommended.
Following the feasibility study, between 2014 and 2017, the Skanste area local plan has been ordered by the Riga City Municipality and prepared by urban planning experts bureau “Grupa93” Ltd. in extensive consultation with the area’s stakeholders. In parallel, landscaping and surface water runoff management concept for the Skanste area had been prepared. The local plan and the landscaping concept further developed the and detailed the feasibility study’s recommended option, turning ditches into landscaped canals, adding a creek-like canal in a park and turning three medium-sized water bodies into a larger one downstream: see below
In 2017-2019, technical design of the system was carried out, as a part of the first phase Skanste area revitalisation project, including design and construction of street network and park zone, designed with the stormwater corridor as a backbone. The technical design further detailed the green and blue corridor concept, adding significant recreation functionality and providing high-level detail, including detailed dynamic hydrological modelling of the system, including water level analysis in different rain events, including provision for the runoff from the entire development of the area. In May 2020, construction works of the revitalisation project started.
The system services the area of around 100 hectares. Stormwater enters the system through sheet runoff or through rain sewer connections from the surrounding land plots and streets and runs through a cascading system of swales, canals, ponds. The system is designed with the permanent water level at 0.7m LAS, which corresponds to the baseline groundwater level. At the downstream of the system, a limited discharge structure is designed with the maximum limited discharge to the city sewer network. The technical solution allows lowering the water level up to -2.5m LAS, to prepare large retention capacity in case of a large rain event or for maintenance purposes.
The stormwater is retained in the system and treated by plants. The system is designed so that it is attractive and functional both in dry season, during everyday rain, in rain events with lower probability as well as extreme rain. The system is designed to function well and protect against flooding even in the event of a 200-years rain and with restricted runoff from the surrounding land plots – up to 500-years rain.
Result
It is a bit too early to talk about the results as the construction of the system has just started. However, several interim conclusions can be drawn about the process and technical design of the system:
- It was very beneficial to have analysed different options for the green and blue corridor prior to planning and to refine them at the planning stage;
- The multi-functional green-and-blue corridor, having roughly the same costs, offers much greater benefits compared with the traditional sewer systems: water attenuation before restricted discharge into sewer network, groundwater level control, stormwater treatment by plants and soil, landscaping and recreation function, space for snow storage during winter, neighbourhood microclimate regulation, biodiversity provision, quality public space and recreation;
- Having generous provision of the storage volume in the public space gives a greater flexibility as it allows both unlimited discharge from properties presently as well as limiting discharge in future, in order to use the green-and-blue corridor as emergency storage for extreme rain event in a wider area.