From Grey to Green: Istanbul’s Water Life Valleys

Istanbul has combined large-scale infrastructure with ambitious nature-based solutions (NbS) to strengthen water resilience. Beyond the Bosphorus Water Tunnel, the Menel Sytem and expanding distant reservoires to secure water supply and distribution, the Istanbul Water and Sewerage Administration (ISKI) rehabilitated over 80 km of streams in just 6 years, creating “Life Valley” green corridors that reduce flooding, recharge water, and provide public recreation. These efforts are complemented by basin afforestation, stricter catchment protection, and new rainwater harvesting regulations for large buildings. Together, they secure supply and improve urban livability

ISKI’s NbS interventions were delivered alongside traditional infrastructure, with a clear sequencing that ensured both ecological and engineering outcomes:

1. Stream Rehabilitation: Over 80 km of degraded urban streams were cleaned, reshaped, and naturalized in only 6 years. Concrete channels were replaced with vegetated banks, restoring ecological flows and creating green stormwater buffers.
2. Life Valleys: These rehabilitated streams were transformed into linear parks, integrating water retention with community amenities. Valleys serve as floodplains, biodiversity corridors, and recreational green spaces.
3. Catchment Protection: ISKI led basin protection through 230,000 tree plantings, soil stabilization, and the removal of illegal housing threatening water quality.
4. Rainwater Harvesting Regulation: Since 2021, all new buildings larger than 1,000 m² must include harvesting systems. By 2024, 1,243 systems had been approved, embedding NbS into building design.
5. Integration with Grey Infrastructure: NbS measures work in tandem with engineered systems like the Bosphorus Tunnel and treatment plants, forming a hybrid grey-green network.
6. Community Engagement: Life Valleys were designed as public parks, ensuring that citizens saw tangible daily benefits from projects that also secure water resilience.

Implementation proved that NbS can be embedded at megacity scale, delivering ecological, social, and technical benefits together.

Partnerships combined technical expertise, municipal capacity, and community ownership:

• Istanbul Water and Sewerage Administration - ISKI: Lead planner and implementer of NbS measures.

• Istanbul Metropolitan Municipality: Integrated Life Valleys into urban planning.
• Engineering & construction firms: Delivered stream rehabilitation and harvesting systems.

Local communities & NGOs: Engaged in Life Valley design and stewardship

NbS made water management visible for residents:

• Public campaigns built confidence in water quality.
• Life Valleys created green corridors that residents could see and use, linking flood protection with recreation.
• Education promoted conservation, catchment protection, and rainwater harvesting.

Visible NbS projects showcased ISKI’s modern approach and built public trust.

Istanbul’s NbS were financed as part of its broader water security investments:

• Major Infrastructure Backbone: The Bosphorus Water Tunnel alone cost approx. 2 billion TL (€1 billion at the time). Further Melen phases and reservoir expansions added several billion TL in total.
• Nature-Based Solutions: Stream rehabilitation and Life Valleys were implemented within municipal budgets, as part of a wider water investment program estimated at €1–2 billion since the early 2000s.
• International Support: Japanese loans supported early Melen phases, easing the burden on local budgets.
• Operational Revenues: ISKI, as a self-financing utility, funds NbS maintenance through water tariffs and service fees.
• Resources: Approx. 8,000 ISKI staff plus contractors and ecologists managed design, planting, and rehabilitation works.

By embedding NbS within multi-billion TL infrastructure investments, Istanbul ensured green measures were treated as core infrastructure, not add-ons.

The NbS program delivered broad, multi-sectoral results:

• Flood risk reduction: 80 km of rehabilitated streams and 52 km² of green space buffer runoff and reduce flood impacts.
• Catchment resilience: 230,000 trees improved infiltration, reduced erosion, and safeguarded reservoirs.
• Mainstreaming of NbS: Over 1,200 rainwater harvesting systems embedded in new buildings.
• Urban livability: Valleys doubled as recreation areas, with cycle paths and shaded routes integrated into water projects.
• Environmental gains: Healthier ecosystems, cleaner water, and restored biodiversity in degraded areas.
• Public trust: By linking visible green spaces to resilience, ISKI improved citizens’ confidence in municipal water management.

Istanbul demonstrated that NbS can scale up alongside grey infrastructure, securing resilience and improving quality of life in one of the world’s largest megacities.

Despite notable successes, Istanbul faced several barriers in implementing NbS at megacity scale.

• Illegal construction: Ongoing encroachment into catchments undermined basin protection.
• High costs: Stream rehabilitation and integration with grey infrastructure required sustained financing.
• Network losses: Water distribution inefficiencies persisted due to sprawling networks.
• Climate variability: Prolonged droughts and flood events continue to stress the system.
• Governance complexity: Coordinating across municipal, regional, and community levels sometimes slowed implementation.

While NbS proved effective, their success depends on tackling urban governance and ensuring enforcement in sensitive basins, alongside continuous investment.

Istanbul’s Life Valleys and basin protection efforts show that NbS can be mainstreamed into megacity-scale water strategies. Stream rehabilitation, afforestation, and rainwater harvesting regulations are replicable measures for cities facing urban sprawl, degraded basins, and climate risks.

Istanbul demonstrates that NbS are not side projects but essential infrastructure. When embedded in long-term investment programs and tied to visible public benefits, they can transform citizen trust and resilience. The case offers a blueprint for other large cities: hybrid grey-green systems that balance engineering with ecological restoration to secure water futures

• ISKI Official Website
• Revolve Media – Istanbul City of Water
• Istanbul Metropolitan Municipality – Infrastructure Projects

Fatma ÖZCAN MUT
Istanbul Water and Sewerage Administration (ISKI)
www.iski.istanbul