Floods & Droughts: Hydrological Instabilities

UPSC Mains PYQs

Urban Flooding (2017): "The frequency of urban flooding in Indian cities has increased manifold over the past few decades." Discuss the causes of urban flooding, distinguishing them from rural flooding. Evaluate the effectiveness of NDMA guidelines on urban flood management in addressing this crisis. (15 Marks, 250 Words)

📊 High-Yield Data & Statistical Fact Sheet

Water Management & Inundation Data:
- Urban Runoff Rate: Built-up concrete surfaces generate 55-60% immediate runoff, compared to under 10% in natural soil landscapes.
- Drainage Bottleneck: Stormwater drains in major Indian cities are historically designed for rain intensities of only 12-20 mm/hour.
- Drought Footprint: 68% of India's cultivated area is vulnerable to meteorological and agricultural droughts in varying degrees.
- Himalayan GLOF Threat: NDMA has prioritized 189 glacial lakes out of 9,000+ in the Himalayas as highly vulnerable to sudden moraine failures.

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1. FLOODS: URBAN VS. RURAL DYNAMICS

Rural Floods: Driven by heavy basin-wide monsoonal precipitation, high river discharge, and low soil absorption capacity. Characterized by wide spatial spread, slower rise times, and prolonged inundation of agricultural crop fields.
Urban Floods: Driven by concrete-induced ground impermeability (asphalt blocks percolation), encroachment of natural urban channels/lakes, and obsolete municipal drain systems. Leads to rapid flash-flooding, traffic grids, and electric shock hazards.
Sponge Cities & NbS: Urban designs integrating bioswales, retention basins, permeable pavements, and restoring floodplains (e.g., Chennai and Bengaluru master plans) to absorb surge waters.

GLOFs occur when unstable moraine-dammed glacial lakes breach due to melting, landslides, or seismic triggers:

Catastrophic Impact: Sudden release of millions of cubic meters of water, causing instant downstream damage to hydropower dams, highways, and villages (e.g., the 2023 South Lhonak Lake disaster in Sikkim).
National GLOF Risk Mitigation Project (NGRMP):
- Early Warning Sensors: Real-time lake-level telemetry and satellite altimetry to track volume swell.
- Controlled Siphoning: Deploying high-discharge pipes to mechanically drain glacial lakes, relieving hydrostatic pressure from moraine walls.

Droughts progress through distinct stages depending on water availability:

Meteorological Drought: Long periods of deficient precipitation below the long-term climatological average.
Hydrological Drought: Depleted surface reservoir storages, dry stream flows, and dropping water tables (monitored by the Central Water Commission).
Agricultural Drought: Loss of soil moisture resulting in crop wilting. Tracked via the National Agricultural Drought Assessment and Monitoring System (NADAMS) using NDVI/NDWI satellite indices.
Strategic Actions: Expanding micro-irrigation (PMKSY - Per Drop More Crop), constructing rural farm ponds (Amrit Sarovars), and utilizing CWC's Spatial Decision Support Systems.

Drought Monitoring Satellite Index: NDVI (Normalized Difference Vegetation Index) / NDWI.
National Drought Tracker System: NADAMS.
Hydrological Storage Monitor: Central Water Commission (CWC).
Glacial Flood Project: National GLOF Risk Mitigation Project (NGRMP).
GLOF Engineering Intervention: Controlled Siphoning.
Urban Flood Design Solution: Sponge Cities (Permeable pavements & bioswales).
Drought Prone Crop Area: 68% of national cultivated land.

Notes updated up to March 2026. Sources: CWC Bulletins, NDMA Guidelines.