Unit 1 - Essays - Causes of River Floods UPDATED

Question 1

With the aid of an example, assess the relative importance of the factors that caused a recent river flood event.

Answer 1

1. Heavy Monsoon Rainfall and Himalayan Snowmelt (Short-Term Natural Causes)
   Point:
   The flood was mainly triggered by excessive monsoon rainfall and Himalayan snowmelt, which increased river discharge beyond capacity.
   Evidence:
   Bangladesh received 60% more rainfall than average between July and September 1998.
   Cherrapunji, India (upstream) recorded 2,245 mm of rainfall in July alone, leading to rapid river swelling.
   The Brahmaputra and Ganges rivers carry water from India, Nepal, and China, and their levels rose dramatically.
   Himalayan glaciers melted due to high temperatures, increasing water levels further.
   Development:
   Since Bangladesh is at the bottom of the river system, all excess water from upstream ended up in its rivers, causing them to overflow their banks.
   The sheer volume of water entering the country in a short period meant that normal drainage processes could not handle the situation.
   Temporal variation: Rainfall was continuous for three months, leading to prolonged flooding.
   Evaluation:
   This was the main trigger of the flood. However, without other worsening factors, the flood might have been less severe.
   The spatial impact was large because the rivers are connected across multiple countries, making the flood inevitable in Bangladesh.
2. Bangladesh’s Low-Lying Topography and River System (Why the Flood Spread So Much)
   Point:
   Bangladesh’s geography made it highly vulnerable to flooding, increasing the extent and duration of the disaster.
   Evidence:
   80% of Bangladesh’s land lies in floodplains, meaning it is naturally exposed to river overflow.
   Most of the country is below 10 meters above sea level, making it easy for water to spread.
   In 1998, over 1,000,000 km² of land was underwater, affecting both rural and urban areas.
   The rivers slowly drain into the Bay of Bengal, meaning floodwaters remained in place for a long time.
   Development:
   Since the land is so flat, the floodwaters did not drain away quickly, prolonging the disaster.
   Cities vs. rural areas: Cities like Dhaka suffered waterlogging due to poor drainage, while rural areas had widespread crop destruction.
   Scale variation: The entire country was affected, but some regions experienced deeper flooding than others.
   Evaluation:
   The geographical location of Bangladesh made the flood worse.
   Even with normal rainfall, Bangladesh is naturally prone to flooding, so this factor was crucial in shaping the disaster.
3. Human Factors: Deforestation and Rapid Urbanization (How People Made It Worse)
   Point:
   Human activities exacerbated the flood by reducing water absorption and blocking natural drainage.
   Evidence:
   Deforestation in the Himalayas (especially Nepal and northern India) increased surface runoff into rivers.
   Without trees, rainfall flowed directly into rivers instead of being absorbed by the soil.
   Urban growth in Dhaka: The population grew from 3.5 million in 1980 to nearly 10 million in 1998.
   70% of Dhaka’s land was flood-prone, but urban expansion continued in these areas.
   Development:
   More surface runoff meant rivers filled up faster, making floods occur more quickly and severely.
   Unplanned urbanization blocked natural waterways, meaning the water had nowhere to go.
   Cities had longer recovery times due to poor drainage infrastructure.
   Evaluation:
   This factor made the disaster worse but did not directly cause the flood.
   If deforestation had not occurred, rivers might have risen more slowly, giving people more time to prepare.
4. Weak Flood Protection and Poor Disaster Response (Government Failures)
   Point:
   Bangladesh had insufficient flood defenses and disaster management, worsening the impact.
   Evidence:
   Embankments along the Ganges and Brahmaputra were breached in multiple places.
   The Flood Action Plan (FAP) focused on embankments but ignored long-term flood control solutions.
   30 million people were displaced because there was no effective early warning system.
   The economic loss was $2.8 billion, and over 1,000 people died.
   Development:
   Failure of embankments allowed water to spread rapidly into villages and cities.
   The government was slow to respond, leaving people without food, clean water, or medical help for weeks.
   Spatial variation: Some areas had stronger flood defenses, so damage varied from place to place.
   Evaluation:
   The government’s failure did not cause the flood, but it made recovery much harder.
   A stronger flood response could have reduced deaths and economic losses.

Conclusion
The most important factor was the heavy rainfall and Himalayan snowmelt, as this was the direct trigger of the flood.
However, Bangladesh’s geography (low-lying land) made the situation far worse by increasing the spread and duration of the flood.
Human factors like deforestation and urbanization made the flood more severe, showing that long-term environmental damage played a key role.
The government’s weak flood protection meant that the disaster caused more suffering than necessary.
Final judgement: While natural factors caused the flood, human failures significantly increased its severity and long-term impact.

Question 2

“Assess the view that rainfall intensity is the most important factor in causing a river to flood.”

Answer 2

1. Rainfall Intensity as a Cause of Flooding
   Point: Intense rainfall can overwhelm a river system, leading to rapid flooding when infiltration capacity is exceeded.

Bangladesh 1998:
Monsoon season brought exceptionally heavy rain; 300mm in a single day in some regions.
Total monsoon rainfall 30% higher than usual, increasing river discharge significantly.
Large-scale river flooding lasted for over three months, affecting two-thirds of the country.

Boscastle 2004:
75mm of rain in just two hours; peak rainfall intensity reached 300mm/hr.
The rivers Valency and Jordan were overwhelmed due to the sheer volume of water in such a short time.
Flash flood occurred with only a 30-minute lag time, causing severe damage.

Development:
Rainfall intensity matters because infiltration has limits; once exceeded, water flows as runoff into rivers.
In both cases, intense rainfall was the immediate cause of the flood, but it is crucial to assess whether this was the most important factor.

2. Drainage Basin Characteristics and Land’s Ability to Absorb Water
   Point: The physical features of the drainage basin affect how quickly rainwater enters the river.

Boscastle:
Steep-sided valley caused rapid runoff into rivers.
Impermeable slate rock meant little infiltration, leading to a short lag time.
Small drainage basin increased the flood’s intensity.

Bangladesh:
Low-lying land (two-thirds of the country below 5m above sea level) meant floodwaters had nowhere to drain.
Three major rivers (Ganges, Brahmaputra, Meghna) converge, meaning that rain from upstream areas also contributed to flooding.
The Brahmaputra basin covers 580,000 km², leading to water accumulation over weeks, unlike Boscastle’s short-term flood.

Development:
In Boscastle, the small, steep catchment caused an intense but short-lived flood.
In Bangladesh, the vast basin meant floodwaters remained for months.
This shows that the drainage basin’s characteristics determine both the scale and duration of flooding, not just rainfall intensity.

3. Human Activity’s Role in Worsening Flooding
   Point: Urbanization, deforestation, and poor infrastructure increase flood risk.

Bangladesh:
Deforestation in the Himalayas (forest cover down by 50% since 1950) reduced interception, increasing runoff.
Rapid urbanization in Dhaka (70% of land covered in concrete) prevented infiltration, worsening flooding.

Boscastle:
Bridge structure acted as a dam, blocking water and worsening backflow.
Urban expansion into floodplain areas increased damage and vulnerability.

Development:
Human activities reduced the land’s ability to absorb or slow down floodwaters.
In Bangladesh, deforestation increased runoff, while urbanization meant water had nowhere to go.
In Boscastle, poorly designed infrastructure contributed to the severity of flooding.
This suggests that while rainfall intensity may initiate flooding, human changes to the landscape greatly worsen the impacts.

4. The Influence of Long-Term Climatic and Seasonal Patterns
   Point: Some areas are naturally prone to flooding due to seasonal rainfall patterns and antecedent conditions.

Bangladesh:
Monsoon season (June-October) delivers 80% of annual rainfall, so rivers are already high before extreme events occur.
Himalayan snowmelt in late spring increases river discharge before monsoon rains even begin.
The 1998 floods lasted over three months because of continuous rainfall.

Boscastle:
August 2004 flood followed weeks of heavy rain; soil was already 60% saturated, meaning it could not absorb more water.

Development:
Even if an intense rainfall event happens, flooding is more likely if the ground is already saturated or rivers are already full.
Bangladesh’s long-term monsoon and snowmelt meant the flooding was inevitable, even if the rainfall intensity had been lower.
Boscastle’s wet antecedent conditions made the flood worse than it otherwise would have been.
This suggests that pre-existing conditions and seasonal variations are just as important as the intensity of rainfall.

Conclusion
Rainfall intensity is important because it determines how much water enters a river system at a given time.
However, the characteristics of the drainage basin control how severe and long-lasting a flood will be.
Human activities have made many areas more vulnerable to flooding by increasing surface runoff and reducing natural drainage.
Seasonal and climatic factors mean that some regions, like Bangladesh, are already predisposed to flooding before an extreme rainfall event even happens.
Final judgement: While intense rainfall may be the trigger for a flood, it is not the most important factor overall. The combination of basin characteristics, human activities, and long-term climatic conditions ultimately determines how severe and prolonged flooding will be.