GCSE Revision - Rivers - Flooding

A river floods when the discharge is too large for the channel to hold - it bursts its banks

Flood Hydrographs

Hydrographs are used to show the relationships between precipitation and the amount of water in the river - discharge.

Hydrographs are used to predict whether a river is likely to flood.

Discharge is measured in Cumecs (cubic metres of water per second).

Lag Time is the delay between maximum precipitation and peak river discharge.

Short Lag Times are caused by steep slopes, impermeable rock, sparse vegetation and a small drainage basin.

Long Lag Times are caused by gentle slopes, permeable rock, dense vegetation and a large drainage basin

The Causes of Flooding

  • Precipitation - Heavy rainfall over a long period results in saturated soil and surface run-off

  • Flash Flood - Hot dry areas where the land is baked hard causes flooding if there is an intense burst of heavy rainfall - water cannot infiltrate and results in rapid run-off.

  • Snowmelt - When temperatures rise and snow melts the stored precipitation is released as run off as infiltration will be low as the ground is still frozen.

  • Deforestation - When trees are cut down this reduces interception, transpiration and utilisation by the trees. This results in increased run-off

  • Urbanisation - When land is urbanised vegetation is removed and the land is covered in concrete and tarmac. Storm drains are used to transfer the water from the surface of the land to the river which increases the chance of flash floods.

It is important to know about general causes and effects of flooding from above and below but it is also very important to revise your case studies - One should be from an MEDC and one from an LEDC - For case study information these two just click on the two images.

Bangladesh

Mississippi
 

Buildings and property washed away or damaged by water and mud

Crops ruined and farmland saturated for months afterwards preventing new planting

Economic impacts on individuals, industries, insurance companies and governments

People and Animals can drown in fast flowing water

IMPACTS OF FLOODING

Positive impacts of flooding include - deposition of fertile silt, washing away of pollutants and repleneshing groundwater

Transport can be interrupted - airports can be closed, road and rail networks submerged and river traffic closed down.

Sewage contaminates drinking water supply and causes diseases such as cholera & typhoid

 

Flood Control

Hard Strategies

  • Dams - built in the upper river valley and are designed to store water and therefore control the discharge of the river.

  • Levees - This increases the height of the river banks and therefore the river can contain more water.

  • Straightening Meanders - This increases the speed of the river to remove water from affected areas.

  • Spillways - These are overflow channels which allow rivers to flood areas of unused land or areas which do not cause much damage.

  • Afforestation - This is the planting of trees which increases interception, evapotranspiration and reduces run off.

Soft Strategies

  • Flood Warning Systems -This enables people time to remove possessions and evacuate areas.

  • Sandbagging - This attempts to flood proof homes and buildings as a last resort.

  • Insurance - This spreads the cost of the flood damage.

  • Flood Plain Zoning - This tries to organise the flood defences in such a way that land that is near the river and often floods is not built on. This could be used for farming. The areas that rarely get flooded therefore would be used for houses, transport and industry.

Links - Bangladesh Case Study
         - Mississipi Case study
         - DVD - Footage of Mississippi
Common Questions
         - Describe the causes of flooding
         - Referring to an example explain the effects of flooding
         - Outline the main differences between soft and hard engineering
         - From an example you have studied explain if the flood control scheme was
           successful