Rivers - KQ3 (issues from development of river basins)

Question 1

What are the human activities increasing flooding on the River Severn?

Answer 1

1 - Increased planting of forestry upstream in the Welsh mountains:
- Deciduous trees = higher discharge in winter = less interception as trees lose leaves (only intercept 15-25% of annual interception)
- 45% of land in upper catchment is used as pasture and moorlands = contributes to quick runoff = flooding more likely = after period of high rainfall water reaches river quickly and increases flood risk
2 - Urbanisation :
- Increase in impermeable surfaces = runoff is quicker as doesn’t percolate into soil
- Lack of vegetation reduces interception and evapotranspiration

Question 2

What are the physical factors increasing flooding on the River Severn?

Answer 2

1 - Steep sided valleys:
- River Severn at Plynlimon (Welsh Mountains) passes through steep valleys (area can be above 300m) = runoff is quicker into river
2 - Geology:
- Severn’s catchment is mostly slate and shale = impermeable = rain is not absorbed by rocks = speeds up runoff
3 - Rainfall:
- Central Wales = over 2000mm of rain/year
- Large amount ends up in tributaries before flowing to main river = large volumes come together at confluence in short space of time = amount surpasses river’s capacity
- Majority of tributaries = lower catchment = only a few metres above sea level

Question 3

How does geology increase flooding vulnerability?

Answer 3

Cause
- Porous rocks (chalk and sandstone) store water and release through groundwater flow to rivers
- Non-porous rocks = more flooding
Effect
- Runoff is rapid
- High peak flows
- Short lag times

Question 4

What are lag times?

Answer 4

The time between peak rainfall and peak discharge

- If it is too short the river will not be able to cope with the sudden amount of water = flooding

Question 5

How do slopes increase flooding vulnerability?

Answer 5

Cause
- Steeper slopes = quicker runoff = higher speed = can exceed capacity of river after period of high rainfall = flood
Effect
- Steep slopes = steep rising limbs (on graph = gradient = think quadratic graphs) = steep falling limbs = high peak flow = short lag time

Question 6

How does vegetation increase flooding vulnerability?

Answer 6

Cause
- Interception = high when lots of vegetation = slows movement of water to group = increases evaporation
- When trees are deciduous discharge is higher in a forested basin in winter = less interception
Effect
- Lots of vegetation = gentle rising limbs = gentle falling limb = low peak flow = long lag time

Question 7

How does rain fall increase flooding vulnerability?

Answer 7

Cause
- More rainfall = more flooding (river exceed capacity)
Effect
- Short lag time

Question 8

How does river confluences increase flooding vulnerability?

Answer 8

Cause

- Large volumes of water come together in short time = can exceed rivers capacity

Question 9

What are soft engineering methods used to reduce the risk of flooding in the lower catchment?

Answer 9

• Temporary barriers lines with plastic and sandbags along road
• 500m flood embankment
• Flood gates
• Property modifications
• Prediction and preparation

Question 10

What are hard engineering methods used to reduce the risk of flooding in the lower catchment?

Answer 10

• Creation of new wetlands
• Construction of a wall around the site to reduce the size of the floodplain
• Mythe WTW Flood Alleviation Scheme
• Controlling agricultural pollution from nitrates by further regulation
• Negotiation of improvements to rural sewage disposal (eg Waters Upton)

Question 11

When and where did the River Severn flood?

Answer 11

July 2007; Tewkesbury, Gloucestershire

Question 12

What were the economic impacts of the flood?

Answer 12

• 5000 homes and businesses flooded
• 27,000 domestic insurance claims
• 6800 business claims
• Closed shops = loss of income
• 500 commuters stranded at Gloucester train station = could not get to work
• 10,000 motorists stranded on roads (eg M5 = closed due to landslides)
• £25 million to repair roads
• Farmland flooded = maize, potatoes, hay crops = submerged = loss of income
• Crops containing sewage = destroyed = loss of income
  ADVANTAGE = flooding deposits alluvium onto floodplains = soil was fertile

Question 13

What were the social impacts of the flood?

Answer 13

• Water treatment plant in Tewkesbury flooded = 140,000 houses without water for 15 days
• Electricity sub-station flooded near Gloucester = 50,000 homes without power, 135,000 homes with no water for 17 days, cost £50 million
• Emergency services had difficulties accessing the area to help people in trouble

Question 14

What were the environmental impacts of the flood?

Answer 14

• Small mammals drowned
• Ground nesting birds affected = could not nest = loss of population
• Fish left stranded on floodplain = decline in fish stocks for farmers

Question 15

What were the causes of the flood?

Answer 15

• High rainfall
• 6900kms drains in this area = could not drain
• Lots of tributaries come together (confluences)
• Large catchment area
• Antecedent conditions before heavy rainfall = ground was already saturated = new rainfall could not be stored
• Buildings = impermeable surfaces and increases runoff
• Urbanisation in area of floodplain