River flooding and management

2007-10-18_152010

Flooding

Causes of flooding

These are the same as factors affecting a storm hydrograph;

  • basin size,
  • precipitation,
  • temperature,
  • land use,
  • geology,
  • soil type,
  • drainage density,
  • tides,
  • urbanisation.

The 1982 River Ouse Flood – Yorkshire (causes):

  1. Initial snow fall from deep depressions.
  2. Subsequent warmer temperatures, heavy rainfall and rapid snow melt.
  3. Smaller rivers unable to cope with amount of water.
  4. Infiltration capacities exceeded resulted in much overland flow.
  5. Increased overland flow in urbanised areas.

Effects of floods

  • Destruction of communications and buildings.
  • Loss of life and property.
  • Drainage problems.
  • Spread of disease.
  • Destruction of crops.
  • Impacts on economy.

The 1982 River Ouse Flood – Yorkshire (impacts):

  • 800 properties flooded.
  • 19 000 hectares of agricultural land under water.
  • Military assistance needed to distribute food and protect property in York, Selby and Boroughbridge.
  • Commercial properties flooded with water to a depth of 1m.

Managing floods

As with coastal protection a variety of measures can be implemented but their cost and effectiveness need to be carefully monitored.

Approach: Aim: Advantages: Disadvantages:
Regulate land use Promote wise use of land, to reduce flooding and limit building Cheap, immediate effect and long term, provide for most suitable use of land Does not prevent floods, not always applied well or to existing land uses
Dams and reservoirs Protect existing land uses, limit flooding, promote recreation Protects existing land uses, controls water flow, allows recreation Costly, Lengthy construction, maintenance needed, wildlife destroyed, communities disrupted, sedimentation.
Land treatment Promote soil & water conservation, reduce flood levels Cheap, attacks flood problem at source Not always relevant, limited effect
Acquire flood plain land for public use Reduce losses from floods Permanent, numerous benefits Costly, sites not always suitable for other uses, land shortage may result
Insure against floods Promote flood regulations Spreads cost of flood losses Can be costly
Install warning systems Warn property owners of threats, allow for evacuation Allows people to adjust to the flood threat Requires education, systems must be carefully maintained

Low flow

Causes of low flow

The opposite of the factors leading to flooding, but in addition population pressures, lead to increased use for settlement, industry, agriculture and recreation.

Effects of low flow

One of the best places to see the effects of low river flow is in California, where demand for water far outstrips supply. Supply is greatest in northern California and the excessive demand for water has led to: forest fires, drought, land subsidence, reduction in farming productivity, limits on domestic water use, and decrease in sediment on flood plain.

Managing low flow

In order to limit the impacts of low flow the following measures may be considered:

  • Impose financial constraints on water supply, such as water meters.
  • Increase education, for example, limit the amount of water used in irrigation to need only in many countries irrigation is very costly and waters food that is surplus to requirements.
  • Develop other surface water storage, such as reservoirs.
  • Encourage more use of groundwater via wells and pumped extraction of water.
  • Impose laws to restrict excessive domestic water use in times of drought.
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