Top Banner

of 16

River Basin Processes

May 30, 2018

Download

Documents

3alliumcourt
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
  • 8/14/2019 River Basin Processes

    1/16

    River Basins

  • 8/14/2019 River Basin Processes

    2/16

    The Hydrological (Water) Cycle97% of the worlds water iscontained in the oceans.

    2% of the worlds water is stored

    as ice and snow.That leaves just 1% is either freshwater on land or water vapour

    As the amount of fresh water islimited, it has to be recycled over andover again. This is known as thehydrological cycle.

    As no water is added or lost from

    the hydrological cycle it is called aclosed system.

  • 8/14/2019 River Basin Processes

    3/16

  • 8/14/2019 River Basin Processes

    4/16

    The Drainage Basin System

    The drainage basin forms part of the hydrologicalcycle, but unlike the hydrological cycle it is anopen system. This is because and open system has:

    INPUTS where water enters the system throughprecipitation (rain and snow) OUTPUTS where water is lost to the system either by

    rivers carrying it to the sea or throughevapotranspiration. (Evapotranspiration is the loss ofmoisture directly from rivers or lakes evaporation, or

    from vegetation transpiration.

    Definition:A drainage basin is the land area or catchmentarea drained by a single river and its tributaries. An

    imaginary line called the watershed separates onedrainage basin from another. The watershed usuallyfollows a ridge of high land; any water falling on the otherside flows into an ad acent draina e basin.

  • 8/14/2019 River Basin Processes

    5/16

    The Drainage Basin System

    Within the system are storesand transfers(flows)

    STORES are places where water is held, eg: in lakes, on the surface, in soil

    and in rocks underground. TRANSFERS are processes by which water flows or moves through the

    system, eg: infiltration, surface runoff, throughflow, percolation.

  • 8/14/2019 River Basin Processes

    6/16

    Diagram of a typical drainage system.

    Source River Basin

    Confluence

    Tributary

    WatershedRiver

    Channel Mouthof river Sea or Lake

  • 8/14/2019 River Basin Processes

    7/16

    From its source, a river flows downhill. This is because of the Earth's gravity. Small trickles of

    water join up to form a stream. The amount of water in the stream increases steadily as morestreams, called tributaries, join it. Eventually the stream becomes big enough to be a river.

    A drainage basin is all the land that supplies ariver and its tributaries with water. If you couldlook down on a river from above, you would seethat it branches. This is called a drainagepattern. The shape of the pattern depends onrocks, soil, climate and the changes made to theriver. Radial drainage happens when streams

    flow down from a central high point, such as amountain top. Other rivers, such as the Amazon,form a pattern like the branches of a tree. This

    is called dendritic drainage.

    Rivers and their tributaries can form many

    other types of drainage patterns. Paralledrainagehappens when streams flow in valleysthat are parallel to one another. This mightbe.because movements in the Earth, millionsof years ago, made the rocks 'fold' intoparallel lines. Trellis drainage is commonwhere massive layers of sedimentary rockhave 'slipped'. Streams flow in channels thatare parallel to each other, and tributaries join

    the streams at right angles.

  • 8/14/2019 River Basin Processes

    8/16

    The force of a river depends on 2 things - the energy and the volume of the river.Energy = Speed + volumeThe steeper it is the faster it flows.Speed can be reduced by friction and turbulence, so a lot depends on the natureof the river bed. This is related to two things:

    the material it is made ofthe cross sectional shape of the river bed.

    The most efficient shape is where the depth is equal to half the width W = 2D

    http://library.thinkquest.org/28022/erosion/index.html
  • 8/14/2019 River Basin Processes

    9/16

    A wide shallow channelmeans little erosion, butoften plenty of deposition

    because of increasedfriction which slows downthe flow.

    A narrow, deep channelincreases the speed ofthe water and so theamount of erosionincreases anddeposition decreases.

  • 8/14/2019 River Basin Processes

    10/16

    Corrasion, otherwise known as abrasion, isthe grinding of rock fragments carried by

    the river against the bed and banks of the

    river. This action both widens and deepens

    the channel. The power of the grinding is

    especially powerful in floods when large

    fragments of rock are carried along theriver bed eroding the bed and the banks.

    Attrition is the knocking of rock

    fragments in the water against one

    another. The fragments are

    broken into smaller pieces and

    become smoother along the

    process.

    Solution, also known as corrosion, is the

    process by which river water reacts

    chemically with soluble minerals in therocks and dissolves them.

    For example, when a river stream flows

    over an area of limestone (calcium

    carbonate), it erodes the limestone by

    reacting chemically with it and dissolves it.

    Hydraulic action is the breaking

    down of rocks and dragging them

    away from the bed and banks bythe force of the running water

    itself. When water from a fast

    moving stream enters cracks in a

    rock, the force breaks up the

    force into pieces.

  • 8/14/2019 River Basin Processes

    11/16

    Some rivers have greater volume than others. There are four factorsthat influence volume.

    Vegetation

    The presence of vegetation affects the rate at which

    water is intercepted and is able to infiltrate the ground.

    If an area is covered with vegetation, the vegetation will

    intercept the rain and allows some of it to infiltrate into

    the ground. As a result, there will be less water as surface

    runoff into the river and so the volume of water into the

    river wil l be lowered. On the other hand, in an area with

    no vegetation, there will be no interception and little

    infiltration into the ground. This will result in more water

    as surface runoff into the river and so the volume of the

    water is higher than that of a vegetated area.

    Permeability of rocks

    The permeability of rocks is determined by the size of the por

    between the rock particles. If the rock has small pores, water cann

    easily infiltrate into the rock and this means that the rock i

    impermeable. On the other hand, if the rock has large pores, water c

    easily infiltrate and thus the rock is permeable. When water flow

    through an area of impermeable rock, little water infiltrates th

    ground, as a result there is high surface runoff and leads to a hig

    volume of flow of water.

    Drainage basin size

    A larger drainage basin will have a greater volume ofwater. This is because there is a greater amount of

    tributaries bringing water to the river resulting in a

    higher surface runoff.

    Drainage Basin refers to the area drained by a river and

    its tributaries.

    Climate

    The amount of precipitation and temperature affects the volume oriver. During dry season, a river channel may be almost emp

    therefore the volume is low. During the wet season, the volume will b

    high and water may overflow the channel and flood the surroundin

    land.

    Temperature also plays a part in influencing the volume of rivers. Whe

    the temperature is high, there is more evaporation and transpiratio

    which reduces the amount of water in the river.

  • 8/14/2019 River Basin Processes

    12/16

    The three factors that influence the velocity of a river are the river

    gradient,channel roughness and channel shape.Gradient

    A river flowing down a

    steep slope or

    gradient has higher

    velocity than one

    which flows down a

    gentler gradient. For

    example, the speed

    of flow in a river that

    plunges down a steepslope in the form of a

    waterfall is much

    higher than the speed

    of flow in a river that

    winds down a gentler

    slope.

    Channel Roughness

    In figure A, the channel

    is smooth while that in

    figure B is rough or

    uneven with boulders on

    the river bed as well as

    rocks that protrude out

    from the bank. A river

    that flows through such a

    river has to overcomesuch obstacles and

    therefore there will be

    more friction and the

    velocity of the river is

    reduced.

    Channel shape

    The shape of the

    channel or its cross

    section affects the

    wetted perimeter. The

    wetted perimeter

    refers to the extent to

    which water is in

    contact with its channel.

    The greater the wettedperimeter, the greater

    the friction between

    the water and the banks

    and the bed of the

    channel, and the slower

    the flow of river.

  • 8/14/2019 River Basin Processes

    13/16

    Diag A. Diag B.

    In figure A, the channel is smooth while that in figure B is rough

    or uneven with boulders on the river bed as well as rocks thatprotrude out from the bank. A river that flows through such a riverhas to overcome such obstacles and therefore there will be morefriction and the velocity of the river is reduced.

  • 8/14/2019 River Basin Processes

    14/16

    Wetted perimeter is calculating byadding the length and the breadth of

    the channel in contact with water.

  • 8/14/2019 River Basin Processes

    15/16

    A river uses its energy to carry or transport eroded

    materials such as mud, sand, boulders and dissolvedmaterials. These materials are called its load. Rivertransport their load by four processes. Theseprocesses are:

    Saltation | Traction | Solution | Suspension |

    http://library.thinkquest.org/28022/transport/saltation.html
  • 8/14/2019 River Basin Processes

    16/16

    A river uses its energy to carry or transport eroded materials such as mud, sand,boulders and dissolved materials. These materials are called its load. River

    transport their load by four processes. These processes are traction, saltation,suspension and solution.

    Larger particles like pebbles and boulders roll and slide along the river bed.

    Dissolved materials containing minerals like calcium and sodium are carried in thewater. Trying to look out for these type of load in a river? Too bad, they cannot be seenby a naked eye.

    Smaller particles such as clay, silt and fine sand are carried along without contact withthe river bed. Materials carried in suspension usually forms the greatest part of the

    total river bed.

    http://library.thinkquest.org/28022/transport/suspension.htmlhttp://library.thinkquest.org/28022/transport/solution.htmlhttp://library.thinkquest.org/28022/transport/traction.htmlhttp://library.thinkquest.org/28022/transport/saltation.html