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Rivers

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Hydrological cycle

Objective – understand the workings of the hydrological cycle / water cycle.

The hydrological cycle is also known as the water cycle.

Hydro cycle399d2c5bf2051be03926ff82df8fe6d491776051

Seas and oceans contain 97 per cent of the world’s water, and ice holds 2 per cent. That leaves just 1 per cent of the world’s water as fresh water on land or in the air.

River processes

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 – there are three processes erosion, transportation and deposition.

Erosion

Erosion

Transportation

Transport b769ef8874c0494f292d9fe793bc4c89f0707b84

Deposition

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.

River Landforms

Objective – understand that a river will pass through three stages on its course from source to mouth and within each of these stages different landforms are found.

landforms 7562912_orig

Upper course

As the river moves through the upper course it cuts downwards. The gradient here is steep and the river channel is narrow. Vertical erosion in this highland part of the river helps to create steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges.

Waterfalls

waterfall ba40f0b45c5418bca547288fc5fd8cbac6ab380d

Middle course

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.

meander1.5H_meander_formation

  • 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.

ox-bow_lake formation

Upper 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.

leveesAbove information BBC Bitesize

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.

delta 4998046_orig

River Severn from Source to mouth – whilst you watch the video below take notes of the landforms.

Reading – using the worksheet below build up a comprehensive case study of the River Tees.

RIVER LANDFORMS: THE RIVER TEES (GeoActive worksheet)

YouTube video is about the features on the River Tees

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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Typography of Boscastle – steep sided v-shape valley. Human causes to the flooding are deforestation and building too close to the channel with no flood defences.

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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Objective – understand that there are social, environmental and economic impacts on the communities where floods occur.

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IMPACTS (social, environmental and economic):
Social:
  1. 58 proporties flooded and 4 were swept away by the flood
  2. Fortunately no-one died but there was one injury – a broken thumb!
  3. 32 cars were swept away and never to be seen again
  4. A range of infrastructure was badly damaged
  5. Sewer pipes were blocked and washed away so raw sewage contaminated flood water which caused a severe health risk.
  6. Four footbridges along the Valency were washed away.
  7. Long term disruptions and a major re-building project needed to take place.
  8. Long-term stress and anxiety to people traumatised by the incident.
Economic:
  1. Damage to buildings and services was £2 million.
  2. Rescue operation including helicopters, police, fire service was costly.
  3. Loss of tourism a major source of income to the village of Boscastle.
Environmental:
  1. Damage was caused to local wildlife habitats.
  2. There was coastal pollution as debris and fuel from cars flowed out into the sea.

Flood hydrograph for Boscastle

Objective – understand how to interpret the data from a flood hydrograph.

Boscastle-living-storm-hydrograph-Prae

Click on the link for a clear understanding of flood hydrographs.

MANAGEMENT
  1. The environmental agency made a decision to lay a large relief culvert that would carry excess rainwater. It is twice the size of the old one.
  2. The River Valency is being widened and lowered from the lower bridge to the car park, so that it has a bigger capacity and can carry more water.
  3. The height of the car park is being raised using stone removed from from the river bed. Also barriers are being made for the car park so that if it floods it takes much longer for the cars to be swept away.
  4. There are plans to demolish the lower bridge near the harbour and replace it with a higher bridge further downstream. This will give the river more capacity and help to reduce flooding.

Boscastle flood prevention scheme 

Indicative Landscape Plan.qxd

 

A comparative study between Boscastle UK (HIC) and Bangladesh (LIC)

Boscastle, UK
Dhaka, Bangladesh
16th August 2004

Village built at the confluence of the River Jordan and the River Valency

Video

Severe floods in 1998 and 2004

Confluence of three rivers: Meghna, Ganges, Brahmaputra

Video

Causes Natural
  • Torrential rain: over 500mm rain fell in 4 hours, remnants of Hurricane Alex. Weather system remained stationary over area due to converging winds.
  • Small catchment area (about 23km2)
  • Impermeable upland area (Bodmin Moor)
  • Steep-sided valleys

Extremely wet summer, by August ground was saturated.

  • Confluence of three large rivers
  • Snow melt from Himalayas
  • 70% land area <1m above sea level
  • Cyclones create storm surges
  • Increased surface runoff and more soil erosion means silt blocks river channels reducing carrying capacity of river
  • 80% floodplain and delta makes Bangladesh very susceptible to flooding

Monsoon rainfall

Human
  • Building along river
  • Construction of small bridges across river (trapped material behind them creating small dams)
  • River narrowed through Boscastle reducing capacity

Deforestation in valleys

  • Deforestation in Himalayas
  • Ganges diverted for irrigation increasing silt deposition
  • Rapid unplanned urbanisation in Dhaka
Effects Social
  • Destroyed homes
  • Stress and anxiety for local people

Cars belonging to more than 1,000 people washed away

  • Hundreds killed
  • Millions made homeless

Contaminated water led to disease

Economic
  • 25 business properties destroyed
  • Bridges and roads damaged
  • 4 footbridges washed away
  • Visitor centre destroyed
  • Area relies on tourism for 90% income, most of which earned in summer months. Floods dramatically affected earnings.
  • Insurance companies paid out an estimated £20 million to repair damaged property
  • Over 900 bridges destroyed
  • 15,000km road damaged
  • Crops lost leading to food shortages
  • Financial losses for businesses and shops
  • Communications damaged making co-ordinating rescue efforts difficult
Environmental
  • Gardens and pavements destroyed by weight of floodwater
  • Agricultural land flooded and contaminated by polluted water
Responses Short-term Major incident declared at 5pm. RAF search and rescue were alerted to rescue trapped people. No-one died mainly due to the rapid response of the emergency services.
  • Farmers provided with free seeds
  • Foreign aid (including £21million from UK)
  • Water purification tablets
  • Food aid
Long-term To reduce the risk of future floods, a £4.6 million flood defence scheme was completed in 2008.

  • Low bridges removed
  • River widened through Boscastle
  • New flood defence wall built in village centre
  • River bed lowered by an average of 0.75m
  • Trees near to river removed
  • Landowners encouraged to maintain vegetation cover on valley sides
Dhaka Integrated Flood Protection Project

  • Cluster villages (raised by 2m) and raised homesteads (individual homes raised by 2m on earth banks)
  • Flood shelter: about 2ha of raised land where people can bring livestock. Each shelter has space for over 100 families and includes a toilet
  • Radios to issue warnings
  • Embankments to hold back rising water
  • Sluice gates (able to close channel when water rises)
  • Slope protection (to reduce erosion of embankments)

Revision – use the worksheet below to help with your revision.

Year 9 Rivers – revision

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