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River landscapes

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Key idea Specification content
The shape of river valleys changes as rivers flow downstream.

 

The long profile and changing cross profile of a river and its valley.

Fluvial processes: • erosion – hydraulic action, abrasion, attrition, solution, vertical and lateral erosion • transportation – traction, saltation, suspension and solution • deposition – why rivers deposit sediment.

Distinctive fluvial landforms result from different physical processes.

 

Characteristics and formation of landforms resulting from erosion – interlocking spurs, waterfalls and gorges.

Characteristics and formation of landforms resulting from erosion and deposition – meanders and ox-bow lakes. Characteristics and formation of landforms resulting from deposition – levées, flood plains and estuaries.

An example of a river valley in the UK to identify its major landforms of erosion and deposition.

Different management strategies can be used to protect river landscapes from the effects of flooding. How physical and human factors affect the flood risk – precipitation, geology, relief and land use.

The use of hydrographs to show the relationship between precipitation and discharge.

The costs and benefits of the following management strategies: • hard engineering – dams and reservoirs, straightening, embankments, flood relief channels • soft engineering – flood warnings and preparation, flood plain zoning, planting trees and river restoration.

An example of a flood management scheme in the UK to show: • why the scheme was required • the management strategy • the social, economic and environmental issues.

River processes

 

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Objective – To understand river processes – What work does a river do?

A river never sleeps. It works non-stop, day and night, cutting and shaping and smoothing the land.

There are three main process that influence rivers, they are:

  1. Erosion
  2. Transportation 
  3. Deposition

To understand the above processes we need to firstly look at the long profile of a river.

The long profile of a river

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River energy – energy is required for rivers to erode and transport material. Loss of energy causes rivers to deposit material.

River processes

Erosion

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A river erode by four processes:

  1. Abrasion – sediment carried along rub and abrade the channels sides and bed; pebbles swirled in cavities in the bed drill downwards – just like the potholes in the picture above.
  2. Attrition – transported rocks collide, break up and become smaller and more rounded downstream.
  3. Hydraulic action – the immense weight and force of the flowing water removes material from the bed and channel.
  4. Corrosion – water reacts with minerals (chemicals) in the rocks which are carried away in solution.

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The faster it flows, and the more water it has, the faster the river erodes.

Transportation

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The material carried by a river is called its load. There are four ways a river can transport the material:

  1. Traction – (bedload) rolled and pushed along river bed. These rocks become smaller and more rounded downstream (attrition).
  2. Saltation – large sand grains bounced along stream bed.
  3. Suspended load – sand, silt and clay carried in suspension (increases downstream).
  4. Solution – minerals dissolved in water.

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The faster the river flows the more water it has, the larger the load the river can carry.

Deposition

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Deposition occurs when a river lacks sufficient energy to transport the load it is carrying so it deposits it (drops it off). The deposited material is called sediment. Deposition happens when:

  1. a decrease in gradient (refer back to the long profile of the river)
  2. the river flows into a lake or sea – often forming deltas.
  3. the river flows more slowly on the inside of a bend (meander) and deposits material (picture above).

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In the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster:

  • As the river erodes laterally, to the right side then the left side, it forms large bends, and then horseshoe-like loops called meanders.

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  • The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
  • The force of the water erodes and undercuts the river bank on the outside of the bend where water flow has most energy due to decreased friction.
  • On the inside of the bend, where the river flow is slower, material is deposited, as there is more friction.
  • Over time the horseshoe become tighter, until the ends become very close together. As the river breaks through, eg during a flood when the river has a higher discharge and more energy, and the ends join, the loop is cut-off from the main channel. The cut-off loop is called an oxbow lake.

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Lower course

In the lower course, the river has a high volume and a large discharge. The river channel is now deep and wide and the landscape around it is flat. However, as a river reaches the end of its journey, energy levels are low and deposition takes place.

Floodplains

The river now has a wide floodplain. A floodplain is the area around a river that is covered in times of flood. A floodplain is a very fertile area due to the rich alluvium deposited by floodwaters. This makes floodplains a good place for agriculture. A build up of alluvium on the banks of a river can create levees, which raise the river bank.

Levee

In times of flood a river may overflow its banks and spread over the flood plain. As it does so it loses energy and deposits its material across the flood plain. As it takes more energy to carry larger particles, these are deposited first and therefore build up along the banks of the river to form a natural embankment which are called levees. The levee will become higher every time the river floods.

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Deltas

Deltas are found at the mouth of large rivers – for example, the Mississippi. A delta is formed when the river deposits its material faster than the sea can remove it.

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River Case Study

Flood Hydrographs

a01c003c2445f4946103ae49c84306d08a9c1babThe peak rainfall is the time of highest rainfall. The peak discharge (the time when the river reaches its highest flow) is later because it takes time for the water to find its way to the river (lag time) . The normal (base) flow of the river starts to rise (rising limb) when run-off, ground and soil water reaches the river. Rock type, vegetation, slope and situation (ie is this an urban river?) affect the steepness of this limb. The falling limb shows that water is still reaching the river but in decreasing amounts. The run-off/discharge of the river is measured in cumecs – this stands for cubic metres per second. Precipitation is measured in mm – this stands for millimetres.

River management – hard and soft engineering

Hard engineering – Hard engineering uses heavy machinery to build artificial structures which work against nature to reduce the risk of flooding.

Types of hard engineering are: dams & reservoirs, channel straightening, embankments and flood relief.

Soft engineering – soft options are more ecologically sensitive. They involve adapting to the river.

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Restoration of the river Lea

Types of soft engineering include: flood plain zoning, planting trees, river restoration and flood warnings

Flooding – Boscastle 2004

Objective – understand the physical and human causes of flooding.

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Boscastle is a village popular with tourists on the North Cornish coast. The river Valency, with a catchment of 18km², flows through the village and  joins with the river Jordan in the centre of Boscastle.

Causes (physical and human):

Physical:

  1. Weather + climate– A depression formed in the Atlantic shortly before the flood with the remains of Hurricane Alex which slowed down on the land making the rainfall more immense. The storm was localised (this meant it stayed in one place and surrounding areas received a mere 3mm of water) because there was a trough situated right on top on Boscastle.
  2. Winds– A convergence of the prevailing South-Westerly winds and the path of the depression cause a vertical uplift of air. This lead to the creation of cumulonimbus clouds and more rainfall.
  3. Topography/relief– Boscastle is at the bottom of a steep hill so like a funnel it attracts more overland flow.

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Human:

  1. Land use – The upper part of the village has been developing (urbanisation!) Also, hedges have been removed to make fields bigger.
  2. Lack of any flood control system – in the form of either raised banks around the river channel or emergency drainage ditches to catch overflowed water.
  3. The sewer & drainage systems – Boscastle had old sewage systems which had a small capacity. This encouraged surface runoff.

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