亚开行-绘制尼泊尔马兰奇河的灾害地图：加强加德满都水安全的集水区（英）-2023.11-116页_10mb

Summary of Report: Mapping Hazards in Nepal's Melamchi River Catchment

Introduction and Background

This report analyzes hazards in Nepal's Melamchi River catchment following the 2021 floods that damaged critical water infrastructure serving Kathmandu Valley. The study aims to enhance water security by identifying hazard triggers, assessing impacts, and proposing mitigation measures amid climate change risks.

Key Findings

• Event Analysis: The 2021 floods resulted from cascading hazards, including landslides from the 2015 Gorkha earthquake, glacial lake outburst floods (GLOFs), and heavy rainfall/cloudbursts. Flood peak discharge (up to 2,700 m³/s) exceeded the original design (600 m³/s for a 10,000-year event) by magnitudes of 4.5-7 times.
• New Design Basis: The Probable Maximum Outburst Flood (PMOF) at 3,500 m³/s is recommended to incorporate multi-hazard risks beyond conventional rainfall-based methods.
• Climate Change Impacts: Projections show a likely rise in temperature (2.6-4.3°C by 2100) and precipitation (13-29%), increasing risks of glacial retreat and permafrost thaw. Future discharge may rise by 7-36%.
• Critical Hazards: Identified high-risk areas include Bhemathan plain erosion, Melamchi Ghyang landslide, glacial lakes, and debris flow-prone zones.

Mitigation Strategies

• Short-Term: Implement landslide monitoring systems (e.g., inclinometers, remote sensing), debris clearance, and early warning systems (EWS) for timely responses.
• Medium/Long-Term: Redesign headworks for resilience (including buffer storage for droughts), stabilize river morphology, and adopt integrated river basin management (IRBM).
• Early Warning and Monitoring: Enhance real-time hydro-meteorological data collection, drone surveys, and community-based disaster risk reduction (DRM) for remote areas.

Climate Change and Future Risks

Climate models indicate significantly elevated temperatures and precipitation, with monsoon floods and drought risks increasing. Glacial lake growth and permafrost degradation exacerbate multi-hazard risks, demanding adaptive strategies.

Conclusions and Recommendations

• Repurpose/rebuild the Melamchi Water Supply Project (MWSP) headworks using PMOF criteria to ensure resilience against extreme events.
• Scale up regional collaboration, knowledge sharing, and early warning systems.
• Emphasize sustainable development through integrated river basin management (IRBM) and community engagement.