River management

River Processes

Sediment systems

Sediment in aYou should note the following:

  • Sand is easily transported at lower velocities.
  • More velocity is needed to pick up material than to carry it in suspension.
  • In times of highest discharge, velocity increases, as does erosion.
  • The division between Transportation and deposition is small. This means that only a small decrease in velocity leads to sedimentation.
  • Competence is the maximum size of material a river can transport.
  • Capacity is the total load actually transported.

River load

Suspended sediment load:

This is carried with the body of the current. It can consist of suspended bed material, which is fine to medium sands, which have come from the riverbed. The material such as silts and clays is light and can be held in suspension.

Bedload:

Can be either exogenetic or endogenetic, and moves by sliding, saltating, or rolling. It is larger than suspended sediment load.

 river comes from a variety of sources. It may be from outside the river (exogenetic) that includes, mass movement, rill and gully erosion and sheet wash.

Dissolved load:

This is held in solution and can come from erosion, pollution, mineral springs and chemical weathering.

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  • Created by: Emily
  • Created on: 19-03-12 20:44

Erosion

Four main types exist:

1. Corrasion

When a river picks up material and then rubs it against its bed and banks. Erosion occurs by the process of abrasion, and is most effective during times of flood. It is the main method of both vertical and horizontal erosion.

2. Attrition

Boulders collide with one another as they move down the river, and can break into smaller pieces. Over time rocks become more rounded in appearance.

3. Hydraulic action

The force of water that hits river banks, and then pushes water into cracks. Air becomes compressed, pressure increased and the riverbank may, in time collapse.

4. Solution/corrosion

A continual process, which is as a result of the chemical composition of the water.

Transportation

Once friction has been overcome, transportation of material in a river begins. There are three main processes of transportation:

1. Suspension

Very small particles of clay and silt are carried in suspension. The larger the amount of turbulence and velocity the more particles that can be picked up. Material held in suspension usually accounts for the greatest part of the load of a river.

2. Solution

Water in a river, contains acids. Where the bedrock is easily dissolved running water will remove material in solution. It is a minor process except in areas of limestone.

3. Bedload

If particles are very large and not picked up by the current they will move in one of two ways:

Saltation: pebbles sand and gravel are lifted up by the current and bounced along the bed.

Traction: larger boulders rolling or sliding along the riverbed. Only experienced in times of great flood.

Deposition

This happens when the velocity of a river is reduced. Then the main factors leading to deposition are shown in the table below:

A Low rainfall reducing precipitation B A river entering the sea or a lake, reducing velocity C Water becoming shallower D Increase in load E River overflows its banks, depositing material on the flood plain

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River channel landfor

V-Shaped valleys and interlocking spurs

In the upper part of a river, vertical erosion is common creating a steep sided river valley. Interlocking spurs form as the river has to take a winding course due to the highland areas that jut out.


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River channel landforms

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V-Shaped valleys and interlocking spurs

In the upper part of a river, vertical erosion is common creating a steep sided river valley. Interlocking spurs form as the river has to take a winding course due to the highland areas that jut out.

Waterfalls

The rock a river flows over is not uniform and waterfall and result after a river has flowed over hard rock and meets a band of soft rock. The velocity increases as the water nears the edge of the fall, because of a decrease in friction (the water is no longer in contact with the river bed).

The soft rock is worn away and over time, the hard rock is undercut and may collapse. Plunge pools are common at the base of waterfalls.

Rapids

If the gradient of the river increases quickly or flows over gently dipping areas of harder rock rapids will result.

Flood Plains

At its most basic level this is land more likely to experience flooding. If a river floods, silt is deposited on the land increasing its fertility, and the height of the flood plain increases. The flood plain can be made wider by the lateral erosion of meanders. .

Levees

If a river floods, material is deposited on the banks, the material that is dropped first is the coarsest, and creates a natural embankment. When the amount of water in the river is low deposition takes place, the bed of the river rises and flooding is again likely.

Braiding

At some times of the year, due to snowmelt, rivers may carry a very highload in comparison to their velocity. The channel can become full of sediment.

Deltas

These are made of fine sediment and occur when a river has a decrease in energy as it enters a lake or the sea.

Pools, riffles, and meanders

These features occur due to both erosion and deposition. Pools are areas of deeper water and riffles are areas of shallower water. A pool has more erosion than a riffle, and is caused by turbulence.

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The graded river and base level 

Changes in base level

Base level: is the lowest point to which erosion by running water can occur. In the case of rivers the theoretical limit is the sea, although there are exceptions where a local base level may exist, such as with the great Lakes of the USA. Changes in base level result from:

  1. Climatic change:Glaciations and changes in rainfall.
  2. Tectonic change:Where land is uplifted after plate movement or volcanic activity. Changes can bepositivesea level rises in relation to the land, ornegativesea level fall in relation to the land.


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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.

Effects of floods

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

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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