Unveiling Vulnerability: Flood Susceptibility Mapping Along the Saptakoshi River

Flooding is no stranger to the Saptakoshi floodplain. Known historically as the “Sorrow of Bihar,” this region suffers recurrent devastation despite structural interventions like the Koshi Barrage. But understanding where and why floods occur is key to transforming that narrative. That’s why I developed a Flood Susceptibility Map spanning Chatara to the Koshi Barrage, covering seven municipalities across Sunsari, Udayapur, and Saptari districts.

🧪 The Science Behind the Map

Using Geographic Information Systems (GIS) and a Multi-Criteria Decision Making (MCDM) framework—specifically the Analytical Hierarchy Process (AHP)—I combined environmental, hydrological, and land-use data to build a nuanced flood risk model.

Flood-conditioning factors included:

• Rainfall intensity
• Distance to river channels
• Elevation and slope
• Drainage density
• Land use/land cover (LULC)

These parameters were weighted through AHP, validated for consistency, and overlaid to produce a spatial risk index. The final output? A flood susceptibility map, categorized into five risk classes from Very Low to Very High.

📍 What the Map Reveals

🔴 High and Very High Risk Zones

• Concentrated near the Saptakoshi River and its distributaries
• Dominated by flat terrain, low elevation, and dense settlement
• Includes Barahachhetra, Belaka, and Hanumannagar Kankalini where over 70% of land falls under moderate to very high flood risk

🟠 Moderate Risk Zones

• Transitional belts around peripheral wards and tributary margins
• Vulnerable during extreme monsoons or embankment failures

🟢 Low Risk Zones

• Found in elevated foothills near Chatara and forested sections of the Koshi Tappu Conservation area
• Characterized by natural drainage and vegetative buffers

🧩 Beyond the Map: Exposure & Vulnerability

Through spatial overlays, I assessed:

• Population Exposure: Thousands reside in high-risk zones without adequate flood shelters
• Built-Up Areas: Urban sprawl and impervious surfaces exacerbate runoff
• Agricultural Lands: Croplands are repeatedly inundated, affecting food security
• Infrastructure Hotspots: Vital roads, bridges, and utilities lie within flood footprints

🧭 Why This Matters

Stakeholder	Action This Map Enables
Local Governments	Prioritize high-risk wards for flood preparedness
Disaster Agencies	Design early-warning systems and evacuation routes
Urban Planners	Regulate land use and avoid construction in flood-prone zones
Environmental NGOs	Educate communities and advocate for risk-sensitive planning

🛠️ What’s Next?

To enhance future planning, this study recommends:

• High-resolution data updates for real-time risk tracking
• Scenario-based hydrodynamic modeling to predict breaches and flash floods
• Sensitivity analysis of flood weights to ensure model robustness
• Public dashboard rollout for community access and decision-making

🌄 Conclusion

The Saptakoshi floodplain is complex—but not unknowable. Through analytical mapping and inclusive planning, we can shift from reactive flood relief to proactive resilience. This map isn’t just lines and zones—it’s a call to rethink how we coexist with water, climate, and risk.