2017年-世界发展银行全球_Return_on_Investment_in_Green_Urban_Development___Amelioration_of_Flood_Risk_in_the_Msimbazi_River_Catchment_Dar_Es_Salaam_Tanazania_162页_85mb

Summary of "Promoting Green Urban Development in Africa: Enhancing the Relationship between Urbanization, Environmental Assets and Ecosystem Services"

Case Study: Return on Investment in Green Urban Development – Amelioration of Flood Risk in the Msimbazi River Catchment, Dar es Salaam, Tanzania

Core Content

This study, commissioned by the World Bank and co-funded by The Nature Conservancy, evaluates the potential costs and benefits of implementing green urban development (GUD) measures to reduce flood risk in the Msimbazi River Catchment in Dar es Salaam, Tanzania. The research focuses on the integration of natural and built infrastructure to address urban flooding, which is a critical issue in the city due to unplanned settlements, poor drainage, and pollution.

Main Objectives

• Assess the potential of green urban development to mitigate flood risk in the Msimbazi River Catchment.
• Evaluate the economic and social benefits of various stormwater management interventions.
• Compare the return on investment (ROI), net present value (NPV), and internal rate of return (IRR) of different scenarios.

Key Findings

Flood Risk in Dar es Salaam

• The Msimbazi River Catchment is one of the most flood-prone areas in the city, with frequent and severe flooding during the rainy season.
• The catchment is highly polluted, with pollutant levels in the river mouth exceeding safe levels for human contact by up to 1000 times.
• Unplanned residential development on floodplains and riverbanks has worsened the situation, increasing stormwater runoff and reducing the floodplain’s ability to absorb water.

Green Urban Development Measures

• GUD involves the protection, restoration, and enhancement of natural systems such as forests, riparian buffers, and floodplains.
• These measures not only reduce flood risk but also provide co-benefits like improved water quality, recreational spaces, and biodiversity conservation.
• The study identifies several feasible interventions, including:
  • Swales to improve drainage in flood-prone areas.
  • Reforestation in the Pugu Forest Reserve.
  • Enhanced riparian and floodplain areas to store and slow down water flow.
  • Wetland parks and recessed gardens for both flood mitigation and urban green space.
  • Community-based river cleaning projects to address solid waste pollution.

Cost and Benefit Analysis

• The total initial investment cost for the GUD interventions was estimated at $40 million, with annual maintenance costs of $1.6 million.
• Resettlement costs for households in the 60m river reserve areas were estimated at $44 million, significantly increasing the total cost to $84.135 million.
• The study evaluated five scenarios to determine the most effective flood risk reduction strategies, ranging from removing buildings from flood-prone areas to implementing a combination of GUD and conventional storage measures.

Key Scenarios and Their Impact

• Scenario 3 (GUD + riparian setbacks) offers the highest reduction in expected annual losses (EAL) at -51%, though it requires the highest investment.
• Scenario 1 is the most cost-effective, with an ROI of 21% and a payback period of 7–19 years.
• The combination of GUD and detention basins (Scenarios 4 and 5) provides a balance between cost and effectiveness, though they are more expensive than Scenario 1.

Methodology

• Flood risk was assessed using three phases: hazard, exposure, and vulnerability.
• Hydraulic models (e.g., FLO2D) were used to simulate flood depth and velocity.
• Fragility functions were applied to estimate the vulnerability of different building types (informal masonry, formal masonry, reinforced concrete frame).
• ROI analysis was conducted using a 6% discount rate, comparing the benefits (reduction in EAL) with the costs of implementation.

Recommendations

• Riparian setbacks and floodplain restoration are essential for reducing exposure and flood risk.
• Green infrastructure should be integrated with conventional engineering solutions to maximize co-benefits.
• Community-based initiatives, such as river cleaning, are important for maintaining water quality and reducing pollution-related risks.
• Resettlement of informal settlements in flood-prone areas is necessary for effective GUD implementation, but must be carefully managed to avoid encouraging future settlement in protected areas.

Conclusion

Green urban development offers a sustainable and cost-effective approach to reducing flood risk in Dar es Salaam. The study highlights the importance of balancing ecological, structural, and non-structural measures to achieve long-term environmental and economic benefits. While the initial investment is substantial, the long-term cost savings and co-benefits justify the implementation of these measures as part of a broader strategy for sustainable urban development in African cities.