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Human activities on rivers (Causes of flooding (Prolonged rainfall >…
Human activities on rivers
Causes of flooding
Prolonged rainfall > saturated flood plains > surface run-off
Heavy rainfall > less chance of infiltration > reaches river faster
Relief > steeper > faster
Geology > impermeable rocks > higher chance of surface run off
Vegetation > lots reduces flood risk > deforestation > lack of interception > flows to river
Urban land use > drains and sewers > water directly to river
tarmac and concrete > impermeable
Hydrographs
Peak discharge - max. water held in channel
Peak rainfall - max. amount of rainfall
Lag time - time taken between peak rainfall and discharge
shorter = more likely to flood
Factors of lag time
Geology discharge > impermeable rocks > water cannot drain
through rock layer > overland flow > short lag time > permeable > groundwater flow longer lag time
Soil type > clay saturates easily > rapid overflow > short lag times > dry soils > slow water transfer > long lag times
Slope > steep > fast > shorter lag time
Drainage basin shape > high density w/ more streams > speed water transfer > short lag time
Antecedent conditions > wet before storm > saturated ground > shorter lag time
Vegetation > plants intercept rainfall via transpiration > longer lag time
Rising limb - increase in discharge
Falling limb - decrease in discharge
Base flow - normal discharge of a river
Hard Engineering - building artificial structures which try to control rivers
Dams and reservoirs - traps water and releases it when needed
Advantages
produces electricity via turbines
attracts tourists
Disadvantages
expensive
traps sediment > less water
floods habitats > rots vegetation >
releases methane
disturbs local settlements
River straightening and dredging - speeds and deepens water
Advantages
more water held in channel
reduces flood-risk in built-up areas
Disadvantages
dredging needs frequency
increases flood risk downstream
Embankments - raising river banks
Advantages
cheap/one-off
flood water contained
Disadvantages
looks unnatural
increases flood risk downstream
Flood relief channels - artificial channel going either downstream or to an area where it can be absorbed
Advantages
removes excess water
Disadvantages
expensive
cont. water rise >
channel also floods
Soft Engineering - sustainable and natural approach
Flood warnings and preparation - environmental agency monitors and issues warnings via media
Advantages
people can try to prepare and protect properties, e.g. with sandbags
possessions will be saved > fewer insurance claims
Disadvantages
warnings may be inaccessible
flash floods are too quick for effectiveness
don't physically stop flooding
Floodplain zoning - allowing only certain land uses on the floodplain
Advantages
reduces flood risk for expensive and important buildings
less damage > fewer insurance claims
Disadvantages
existing land uses can't be changed
planners have to decide what type of flood
Case study: the Cumbrian floods 2015
December 2015, Storm Desmond w/ 341.4 millimetres of rain falling over a 24-hour period
Physical causes
Warm air from the mid-Atlantic blown to land
forced upwards by the Cumbrian Mountains
air cools and condenses into heavy rain, stopping over Cumbria
falling rain poured into the River Derwent and River Cocker
confluence at Cockermouth > flooding
Impacts
Social
43,000 homes suffer power cuts
5,200 homes flooded
1 death
40 schools close
Economic
disrupted rail and road
businesses suffered property damage
costed £500 million
govt. provided £50 million for flood victims and businesses
insurance companies to pay £325 million
Environmental
altered rivers from increased erosion
heavy rain washed nutrients from soil
flooded vegetation > habitat loss
Risk reduction
river dredged and self-closing flood barrier due to earlier floods but ineffective
The Environmental Agency alerts for evacuation and reduced economic loss
government pledged £2.3 billion on flood defences