River Profiles

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  • Created by: Hannah
  • Created on: 05-04-13 17:32

The Hydrological Cycle

The hydrological cycle is a closed system.

The drainage basin system is an open system.

Both have transfers, stores, inputs of water but in the hydrological system, no gains or losses are from outside the system.

In the drainage basin system, heavy rainfall, drought and human activity such as deforestation, can easily upset the balance of its natural state.

There are four main processes that operate within the hydrological cycle.

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The Hydrological Cycle

Interception

  • This is where plants prevent some rainfall from directly reaching the ground
  • It may later reach the ground via stem flow (flowing down the stem) or through-fall (dripping)
  • Secondary interception occurs at ground level where water hits undergrowth

Evapotranspiration

  • Some water on vegetation returns to the atmosphere via evapouration and transpiration

Potential evapotranspiration is the amount of water that can be lost via evapotranspiration. It is potentially high in deserts, but the amount that can take place is limited due to the minimal moisture available. Actual evapotranspiration is what actually occurs. In the UK, there is more water available for evapotranspiration than takes place.

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The Hydrological Cycle

Infiltration

This is where water slowly soaks into the soil from the ground. The maximum rate at which this can occur is the infiltration capacity. It is dependent on the amount of water already present in soil structure and vegetation.

Precipitation

This is the most important input into the system. It includes rain, snow, hail and fog.

Percolation is when the water is the soil does not remain there but moves down slowly into the lower layers of soil and rock. It creates groundwater storage found in rocks and this may later be moved sideways through the rock via groundwater flow.

 

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The Hydrological Cycle

Water flows through the hydrological cycle in various ways:

Throughflow- Where water moves downwards through layers of soil

Channel flow- Downhill movement of water in rivers

Groundwater flow- Lateral movement of water from the water table

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Drainage Basins

Drainage basin system (http://www.s-cool.co.uk/a-level/assets/learn_its/alevel/geography/river-profiles/drainage-basins/2007-10-17_161200.gif)

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Drainage Basins

The boundaries of a drainage basin are called the watershed and are usually mmarked by areas of higher land.

Land is drained by rivers in a variety of ways that are exhibited as drainage patterns:Drainage patterns (http://www.s-cool.co.uk/a-level/assets/learn_its/alevel/geography/river-profiles/drainage-basins/2007-10-17_161245.gif)

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Drainage Basins

Drainage Density

This relates to the number of streams in a particular drainage basin and can be measured by dividing total length of all streams in a basin by its are. The higher the drainage density, the more quickly water drains to a river.

High density drainage basins- Impermeable land surface, steep slopes, limited vegetation cover, limited rainfall, gentle slopes, large channel frequency (tributaries).

Low density drainage basins- Permeable rock (eg. chalk), a lot of vegetation cover, limited rainfall, gentle slopes, lower channel frequency

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Storm Hydrographs

Storm hydrographs are graphs that show how a drainage basin responds to a period of rainfall. They are useful in planning for flood situations and times of drought as they show the discharge that originated as precipitation.

They have a variety of characteristics which influence how quickly or slowly river discharge increases after a storm.

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Storm Hydrographs

(http://2.bp.blogspot.com/_tsYrUmiiPyw/TFHAZalp_yI/AAAAAAAAAEo/yODcl9JCtHo/s1600/Storm+Hydro.jpg)

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Influences on Hydrographs and Drainage Basin

Size of Basin, Shape and Relief

  • Size- The smaller the basin, the less time it takes for water to drain to the river, resulting in a shorter lag time.
  • Shape- The shape of the basin that lends itself to most rapid drainage is circular. In a long, narrow basin, water takes longer to reach the river.
  • Relief- The steeper the basin, the more quickly it drains.

Forms of Precipitation

  • Heavy Storms- In heavy storms, rainfall is often far in excess of the infiltration capacity of the soil leading to much overland flow, and rapid rises in river levels.
  • Lengthy Rainfall- This leads to the ground being saturated and overland flow.
  • Snowfall- Until snow melts, potential discharge for a river is held in storage. Rapid melting can lead to flooding.
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Influences on Hydrographs and Drainage Basin

Temperature

  • High rates of evapotranspiration reduce amounts of discharge, and low temperatures can store water in the form of ice and snow.

Land Use

  • Vegetation- Important in reducing discharge as it intercepts precipitation and adds to rates of evapotranspiration. Roots of plants take up water reducing throughflow. Interception is less in winter in the UK due to the shedding of leaves from deciduous trees. Flooding is more likely to occur in deforested areas.

Soil

  • A control on the rate of infiltration, amount of soil moisture storage and rate of throughflow. Larger pore spaces (eg. sand) allow for greater water storage and limit the risk of flooding.
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Influences on Hydrographs and Drainage Basin

Geology

  • Permeable Rocks (allows water through)- Can be porous (eg. chalk) that store water within them or they can be pervious (eg. limestone) where water flows along bedding plains.
  • Impermeable Rocks (doesn't allow water through)- Encourage greater amounts of surface run-off and a more rapid increase in discharge than permeable rocks.

Drainage Density

  • The higher the density, the greater the risk of flooding.

Tides and Storms

  • High spring tides prevent water from entering the sea and increase the risk of flooding.
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Influences on Hydrographs and Drainage Basin

Urbanisation

  • Interception increased and overland flow reduced by trees
  • Farmland: Arable and pasture for dairy cattle
  • Large floodplain allows waters to drain
  • River channel can hold a flow of 15,000 cubic metres/second

OR

  • Many trees removed for building and fuel
  • Few trees left
  • Inceased overflow due to less obstruction
  • Town built: concrete, drainage, gutters and sewers all lead to rivers
  • Modified river narrower and deeper
  • Fields ploughed and seeded
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The Water Balance

The water balance is the balance between inputs and outputs.

Precipitation = Run-off + Evapotranspiration +/- Change in Storage

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The Water Balance

Soil Moisture

Surplus: If precipitation exceeds evapotranspiration and the excess is not being used by plants.

Deficiency: If evapotranspitation exceeds precipitation.

Recharge: If replacement of water is lost during drier periods

Field Capacity

This is the maximum amount of water that soil can hold. A water surplus can result in wet soils, high river levels and run-off, whereas a deficit leads to dry soil, falling river levels and possibly drought.

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The Water Balance

Water Deficit

Evapotranspiration is in excess of precipitation and any previously available moisture has been used, in soil moisture utilisation.

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Regimes

Variations in a River Flow

The regime of a river is expected to have a seasonal pattern of discharge during the year due to factors such as climate, local geology and human interaction. Equatorial rivers have regular regimes but in the UK where seasons exist, one or two peaks may be recognisable.

Simple Regimes

These show times of high water levels followed by lower levels. They exist as a result of a glacier melt, snowmelt or seasonal rainfalls such as monsoons.

Complex Regimes

If a river has more than one period of high and/or low water levels, a more complex regime results. It's more common on large rivers that flow through a variety of relief and receive their water supply from large tributaries.

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Types of Flow

A river has two main functions. One to transport water and one to transport sediment. The type of flow depends on factors such as gradient, valume of water, channel shope adn friction.

Laminar Flow: This rarely occurs, water flows smoothly in a straight channel. It is most common in the lower parts of a river.

Turbulent Flow: This is far more common, it occurs where the shape of the rivers channel is varied with pools, meanders and rapids. A great deal of turbulence results in sediment being disturbed. The greater the velocity the larger the quantity and size of particles that can be transported

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Channels

Channel Roughness

The velocity of a river is at its greatest near the end of the river. At the start of a river, large angular boulders create a rough channel shape and therefore, a large amount of its bed friction. This creates more resistance to flow than a river with smooth clays and silt forming its banks.

Channel Shape

THe efficiency of a rivers channel is measured by finding its hydraulic radius. It is the ratia between the length of wetted perimeter and the cross section of a river channel.

Wetted Perimeter: the entire length of the riverbed and sides in contact with the water.

 

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Channels

 

Hydralic Radius = Cross sectional area (km) / Wetted Perimeter

 

Channel Slope

The steeper the gradient, the greater the velocity, because of the influence of gravity.

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The Long Profile

(http://thebritishgeographer.weebly.com/uploads/1/1/8/1/11812015/5852640_orig.gif?336)

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The Long Profile

Upper Course

Verticle erosion, waterfalls, potholes, rapids, gorges. Overland flow is found in depressions making lakes. Evdentually, the channel gets deeper and waterfalls become rapids.

Middle Course

Lateral erosion, transportation, floodplain, meanders, truncated spurs, river cliffs. The gradient of the river is increasingly even and smooth and the flood plain begins to develop.

Lower Course

Transpotation, deposition, large channel, deltas, levees, bluffs, meanders. The flood plain increases in size as meanders migrate downstream.

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