Natural Hazards and Disasters — NCERT Solutions

Given concept: Conditions necessary for cyclone formation.

Answer:
The basic requirements for the formation of a cyclone are:

1. Warm ocean water (sea surface temperature of at least 26–27°C) to provide heat and moisture energy.
2. High humidity in the lower and middle troposphere to sustain convection.
3. Atmospheric instability — warm, moist air must rise rapidly.
4. Coriolis force — sufficient distance from the equator (at least 5° latitude) so that the Coriolis effect can cause the air to rotate.
5. Low vertical wind shear — winds at different altitudes should not vary greatly in speed or direction, so the cyclone structure is not disrupted.
6. A pre-existing weather disturbance (low pressure area) near the ocean surface to initiate the process.

Given concept: Regional differences in flood characteristics.

Answer:
Floods in Eastern India (Assam, Bihar, West Bengal, Odisha):

• Caused by heavy monsoon rainfall and overflow of rivers like the Brahmaputra, Ganga, and Mahanadi.
• Rivers carry heavy silt load from the Himalayas, raising riverbeds and causing frequent overflow.
• Floods are widespread, prolonged, and affect large populations every year.
• Aggravated by cyclones from the Bay of Bengal in coastal areas.

Floods in Western India (Rajasthan, Gujarat):

• Occur rarely and are caused by sudden, intense rainfall in an otherwise arid/semi-arid region.
• The dry, hard soil has low infiltration capacity, so water runs off quickly causing flash floods.
• Rivers are seasonal (ephemeral) and floods are short-lived but intense.
• Less frequent but can be devastating due to lack of preparedness.

Given concept: Causes of drought in Central and Western India.

Answer:
Central and Western India (Rajasthan, Gujarat, Madhya Pradesh, Maharashtra's Vidarbha region) experience more droughts due to the following reasons:

1. Low and erratic rainfall — these regions receive less than 750 mm of annual rainfall, which is insufficient and unreliable.
2. Arid and semi-arid climate — high temperatures and low humidity lead to high evaporation rates.
3. Poor water retention — sandy and rocky soils have low water-holding capacity.
4. Sparse vegetation — lack of forest cover reduces moisture recycling and increases runoff.
5. Dependence on monsoon — the southwest monsoon is often weak or delayed in these regions.
6. Lack of perennial rivers — most rivers are seasonal, so water availability is limited during dry months.

These combined factors make Central and Western India highly drought-prone.

Landslide-Prone Regions of India:

Landslides are common in the following regions:

1. Himalayan Region — Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, and the hill states of the Northeast (Assam, Meghalaya, Mizoram, Nagaland). The young, fragile geology, steep slopes, heavy rainfall, and seismic activity make this region highly vulnerable.
2. Western Ghats and Nilgiris — Kerala, Karnataka, Tamil Nadu, and Maharashtra. Heavy rainfall during the southwest monsoon triggers landslides.
3. North-Eastern Hill Ranges — Arunachal Pradesh, Manipur, Tripura. Steep terrain and heavy rainfall are the main causes.
4. Vindhya and Satpura Ranges — Less frequent but occur during intense rainfall.

Mitigation Measures:

1. Afforestation — Planting trees on slopes to bind the soil and reduce surface runoff.
2. Retaining walls and check dams — Construction of engineering structures to hold back loose material on slopes.
3. Proper drainage systems — Diverting surface water away from slopes to reduce saturation.
4. Restriction on construction — Avoiding construction of roads, buildings, and dams on unstable slopes.
5. Early warning systems — Installing sensors and monitoring systems to detect slope movement and warn communities.
6. Land use regulation — Preventing deforestation, overgrazing, and unplanned urbanisation in hilly areas.
7. Community awareness — Educating local communities about landslide risks and evacuation procedures.
8. Slope stabilisation — Using techniques like rock bolting, wire mesh, and terracing to stabilise slopes.

Vulnerability:
Vulnerability refers to the degree of susceptibility of a community, region, or system to the damaging effects of a natural hazard. It depends on factors such as population density, economic conditions, infrastructure, preparedness, and the physical characteristics of the region. A highly vulnerable region is one where people lack the capacity to anticipate, cope with, resist, and recover from the impact of a disaster.

Drought Vulnerability Zones of India:

Mitigation Measures:

1. Rainwater harvesting — Collecting and storing rainwater through rooftop systems, check dams, and tanks.
2. Watershed management — Conserving water in catchment areas through afforestation and soil conservation.
3. Irrigation development — Expanding canal, drip, and sprinkler irrigation to reduce dependence on rainfall.
4. Drought-resistant crops — Promoting cultivation of millets, pulses, and other drought-tolerant varieties.
5. Interlinking of rivers — Transferring water from surplus river basins to deficit regions.
6. Groundwater recharge — Constructing percolation tanks and recharge wells.
7. Crop insurance — Providing financial safety nets to farmers affected by drought.
8. Awareness and education — Training farmers in water conservation and efficient water use.

Concept: Human-induced (anthropogenic) causes of disasters.

Answer:
Developmental activities can become the cause of disasters under the following circumstances:

1. Construction of large dams: Large dams like Tehri Dam and Sardar Sarovar alter the natural flow of rivers, increase the risk of reservoir-induced seismicity (earthquakes), and can cause catastrophic floods if they fail. They also displace large populations and submerge forests.

2. Deforestation: Clearing forests for agriculture, urbanisation, or industry removes the protective vegetation cover, leading to increased soil erosion, landslides, flash floods, and desertification.

3. Unplanned urbanisation: Construction of buildings and roads on floodplains, coastal areas, or unstable slopes increases vulnerability to floods, cyclones, and landslides.

4. Mining activities: Excessive mining destabilises the ground, causes land subsidence, and can trigger earthquakes and landslides.

5. Overexploitation of groundwater: Excessive extraction of groundwater leads to land subsidence and increases drought vulnerability.

6. Industrial activities: Emission of greenhouse gases contributes to climate change, which intensifies extreme weather events like cyclones, droughts, and floods.

7. Road construction in hilly areas: Cutting into slopes for road construction destabilises hillsides and increases the risk of landslides.

Conclusion: When developmental activities are carried out without proper environmental impact assessment and without respecting the natural carrying capacity of the ecosystem, they can trigger or intensify natural disasters. Sustainable development that balances human needs with environmental protection is essential to minimise such risks.