1.4 The Human Impact

Question 1

How can deforestation affect the drainage basin?

Answer 1

• Evapotranspiration decreases; lack of trees = les water to be evaporated into atmosphere via transpiration, so there I more water on/n ground to flow into a river
• Infiltration rates decrease: although less plant material blocking water from infiltrating into the soil, tree roots play vital part in allowing water to infiltrate. provide natural gaps within soil to allow after to enter and percolate into groundwater tires. Without trees, ground become less permeable, increasing overland flow
• Interception decreases- precipitation that would have been intercepted instead falls straight into the ground either infiltrates the soil of flow as overland flow
• *Channel flow increases**: more overland flow - increased river discharge

Question 2

How can afforestation affect the drainage basin?

Answer 2

• opposite afffect of deforestation
• more interception, less overland flow, more transpiration
• decrease channel flow, causing lower river levels

Question 3

How does urbanization affect the drainage basin?

Answer 3

• usually causes larger flow into the river channel
• roads/building have impermeable surfaces and likely to have drains
• reduce infiltration rates but increase overland flow
  -deforestation and lack of trees/greenery decreases transpiration, which eaves more water in catchment area

Question 4

How does water abstraction affect the drainage basin?

Answer 4

• ifabstraction rate higher than the replenishing rate, causes a deficit of water
• water levels can deplete, bth on surface and groundwater levels
  -can affect channel flow and discharge, may cause more deposition
• water table may fall when groundwater overexploited meaning there is less groundwater seepage into above ground water stores, which may cause above-ground water levels to fall

Question 5

How does water storage affect the drainag basin?

Answer 5

• dams directly stop and control channel flow
• reservoirs behind dams can raise the water table underground and can affect soil saturation if not properly Magee
• may lead to a lowering of the water table in other areas as the water that would normally be flowing in other areas is instead being stored in the reservoir
• reservoirs usually very large, meaning lot of water can enter the atmosphere through evaporation and increase could formations in the area, affecting precipitation

Question 6

What are the causes of flooding?

Answer 6

Heavy rainfall: cause large amount of overland flow
Prolonged rainfall: oil to be so saturated can no longer infiltrate and flows as overland flow
Impermeable surfaces
Melting snow/glaciers increasing channel flow

Question 7

What are environmental impacts of flooding?

Answer 7

• water/sediment leave the river bank and spill onto floodplain, can waterlog plants and put wildlife in danger
• floods can transport materials (sediment) and deposit them in clean water supplies

Question 8

What are the social impacts of flooding?

Answer 8

• can destroy infrastructure and homes
• cause homelessness, emotional trauma, poverty, disease and injury/death

Question 9

What are economical impact of flooding?

Answer 9

• businesses forced forced to close from flood damage
• money spent on flood defenses, insurance pay out and clean up

Question 10

What are the social, economic and environmental facts of the Bangladesh floods 2004?

Answer 10

Social:
2.6 million homes destroyed
30,000 km of roads destroyed
30 million homeless
600 dead

Economic:
$2.2 billion damage
80,000 hectares of agricultural land flooded

Environmental:
Soil erosion
Water logging
Water-logging

Question 11

What were the social, economic and environmental impact of the somerset floods?

Answer 11

Social:
600 homes evacuated
16 farms evacuated
Many had to live in temporary accommodation
Several villages cut off due to roads flooded
Power supplies cut off

Economic:
Damage cost over £10 million
14,000 hectares flooded for 3-4 weeks
Insurance costs increased

Environmental:
- huge amounts of debris had to be cleared
- floodwater heavily contaminated with sewage and pollution
- soil damaged for nearly 3 months
-in some s took 2 years to restore soil

Question 12

What is prediction of flood risk and how is it used?

Answer 12

• technology made it possible to calculate risk of river flooding before flooding is an imminent risk
• probability of a flood of a certain size can be predicted based on past flooding records (Flood recurrence interval)
• floods can be modelled using software and other equipment, to predict how different circumstance would affect flooding
  e.g slope, topography, precipitation rates, soil moisture content

Question 13

What are Flood Recurrence Intervals and how are they used?

Answer 13

• method of presenting the probability of a flood of a given size in an area
• recurrence intervals are in years, e.g 100 year flood
• 100 year flood means that there is a 1% chance of a flood of this size occurring ay given year
• higher the recurrence interval, less likely it is for a flood of that size to occur
• the higher the recurrence interval the large the flood will be if it occurs

Question 14

How forecasts and warnings used in river flooding management?

Answer 14

• flood forecasting uses precipitation forecasts to estimate the level of flooding risk in an area
• Stream flow data and models of stream flow routes can calculate the areas most at risk when heavy precipitation is forecasted, meaning warnings can be issued days before

Question 15

What is hard engineering?

Answer 15

physically altering a river and its channel through building structure or using machinery

Question 16

How are dams used to protect against rivers?

Answer 16

• physically obstruct a rivers natural course, leading to the build up of a reservoir behind a dam if channel flow is large enough
  Pros:
• effective at preventing flooding as river can no longer flow
• can be used to generate electricity

Cons:
- expensive
- displacement of settlements/ disruption of wildlife

Question 17

How is channel straightening used to protect against flooding?

Answer 17

• straightening is where bends and irregularities in the river channel are removed using machinery, leaving a straight channel e.g by cutting off meanders
  Pros:
• can be straightened so water flows faster through the channel, meaning water less likely to overflow onto floodplain and instead moves quickly downstream

Cons:
- issues as water just sent further downstream where it can cause flooding and enhance erosion (as has high velocity)

Question 18

How are levees used to protect against flooding?

Answer 18

• can be reinforced or constructed for flood prevention
• can be natural
• provide raises embankment so water within river channel has further to rise before it can spill out of the channel onto the floodplain

Cons:
- expensive
- can spoil look of rivers

Question 19

How do diversion spillways protect against river flooding?

Answer 19

• is a constructed channel that allows excess water to flow into it when channel is overflowing
• spillway may direct water to further downstream or to a different river
• usually floodgates that control when they are opened/closed

Cons:
- they may direct water to areas that do not flood naturally which can cause environmental damage if spillway floods

Question 20

What are different drainage basin management systems?

Answer 20

Afforestation and treemanagement: decrease surface runoff
- must be managed properly as unstable trees on floodplains can lead to more flooding of trees fall over and block river

• Floodplain land use management:
• limit urbanisation as this can increase risk to homes flooding and decrease infiltration
• sustainable farming practices: (avoid overgrazing and monoculture (one crop))
• some cases flood plains left to naturally restore and farmers given subsidies

Floodplain mapping:
- using river flooding records map areas at risk
- used to make decisions about construction and land use to limit damage to properties

Question 21

Why is wetland conservation used to protect against river flooding?

Answer 21

• wetlands provide environments which floodwater can inundate limiting flooding elsewhere

Strategies to conserve them:
- banning/limiting drainage of wetlands
- monitoring changes (water level changes) so actions may be taken to counteract them
- reintroducing species of plants/animals that have previously been affected
- recreating conditions of wetland e.g in a dry session so ecosystem can continue to be stable until normal water levels restored

Question 22

Why is river bank conservation used to protect against flooding?

Answer 22

• damaged river bank will not hold water as effectively increasing risk of flooding

Strategies:
- protecting/reintroducing vegetation which helps to limit erosion by building a natural barrier
- limiting erosion also reduces sediment in water which means over time less sediment from bank washes into the river and builds up in the river, leaving them more clear for water to flow through
- ensuring no waste on river bank (litter picking) so river channel not blocked

Question 23

How does river restoration help to protect against river flooding?

Answer 23

• process of restoring a river back to its original state before it was altered
• necessary after failed hard engineering techniques, that create more damage to river system and increase flooding further downstream
• example is reintroducing meanders to straightened channels
• often takes place where the land is no longer seen as valuable

Question 24

What were the hard engineering attempts made in Bangladesh to reduce the impact of the flood?

Answer 24

Artificial embankment:
- $6 billion dollar embankments made around their two largest rivers
- 7,555km long, 7m high and a wall built from cobble, stone and earth
- aimed at increasing the discharge of water the river channel can hold, reducing risks of flooding
- increases channel capacity and hydraulic radius but stop floods replenishing fields with nutrients and can stop rainwater escaping into fields

Cons:
- due to lack of workers and economic funding of resources, the embankments are not very strong and great chance that the embankments will erode and eventually collapse

Evaluation:
- provided protection against storm surges and flooding events of moderate severity, they have exacerbated more frequent flooding and promoted potential flooding impacts during the most extreme storm surges

Question 25

What is a soft engineering strategy used in Bangladesh to help reduce impacts from flooding?

Answer 25

Floating schools:
- minimise disruption to child’s every day lives
- teachers and students can continue with daily work/lifestyle
- every floating school is equipped with a classroom, internet access, library and solar-powered computer
- can accommodate 30-35 students

Cons:
- if boat damaged
- funding constraints
- overcrowded classrooms
- monsoon rains

Question 26

What are human causes of flooding in Bangladesh?

Answer 26

Urbanisation:
- growing population rate of 38%
- people install efficient drainage systems and concrete over the land
- rainfall runs off these concrete surfaces quickly and fills the river much quicker than if the natural land was left
- near Dhaka wetlands that were once used for wildlife have been taken over and now used for industrial purposes
- Many of Dhaka’s canals, which are crucial for draining excess water, are blocked by garbage, construction debris, and encroachment

Deforestation:
- forested area reduced by 50% in last 20 years

Question 27

What were the physical causes of flooding in Bangladesh?

Answer 27

Heavy rainfall:
- monsoon season 2,3000 mm per year

Steep land:
- surrounded by Himalayan mountains
- 70% of land mass situated less than 1 metre above sea level

Tributaries:
- near mouth of 3 major rivers: Brahmaputra, Meghna and Ganges
- 700 rivers in total

Impermeable rock:
- Himalayas mainly impermeable rock