Fluvial Processes – How does a river erode transport

Document Sample
Fluvial Processes – How does a river erode transport Powered By Docstoc
					Becky Dearne


Fluvial Processes – How does a river erode, transport and
              why does it deposit sediment?


Fluvial erosion –

Fluvial erosion is the removal of rock and other mineral particles from the
channel bad and banks by stream flow.

There are various types of erosion;

Process:                                 Description:
Abrasion                                 Abrasion is the grinding effect of a
                                         channel caused by sediments in the
                                         process of being transported. The
                                         material being transported
                                         undermines river banks and valley
                                         slopes. However, the sediment itself
                                         is eroded by colliding with other
                                         pieces of material, this is known as
                                         attrition.
Hydraulic action                         Hydraulic action is the erosive effect
                                         of flowing water without the
                                         assistance of rock particles. The sheer
                                         force of the flow causes erosion to
                                         occur. This type of erosion is most
                                         effective in areas of the channel
                                         which consist of incoherent materials,
                                         such as; sand and gravel.
Capitation                               Cavitations occur when tiny bubbles
                                         of air implode in fissures and cracks
                                         in the channel banks. The tiny shock
                                         waves that result in the weakening of
                                         river banks and result in the collapse
                                         of the landforms.
Corrosion                                Corrosion is the chemical action of
                                         stream water, which dissolves
                                         carbonate rocks such as chalk and
                                         limestone.


A river has three main stages, sediment supply zone, sediment transport zone
and the sediment storage zone. Within these three stages various erosion
occurs within each stage.
Becky Dearne


Within the sediment supply zone there are four types of erosion occurring;
hydraulic action, corrosion, corrosion and attrition. Within this stage the
erosion is mainly directed vertically and head ward, this is due to the river not
having a lot of spare energy as it is using 90% of its energy to overcome
obstacles such as; large rocks and boulders. This erosion leads to landforms
such as; rapids, small water falls and Steep River bed to form.

Within the sediment transport zone, it is mostly corrosion and attrition which
occurs due to the sediment being transported and colliding with both the
channel bed and with each other. However, there is little hydraulic power and
corrosion still taking place within this stage. Due to the river having fewer
obstacles to over come it has more spare energy to use to erode allowing it to
erode more laterally rather than vertically, widening the river channel. This
erosion leads to the development of landforms such as; rapids, small
meanders, small floodplains, pools and riffles.

In the sediment storage zone less to no erosion occurs, besides some lateral
erosion on the outside bends of meanders. This leads to the development of
larger meanders and floodplains.

Fluvial Transport – Solid and soluble particles eroded from the channel,
together with materials input by mass movements and weathering from valley
slopes, are transported down the stream (from the upper stage to the lower
stage). The material transported by a river is known as the rivers LOAD. The
load is divided into three fractions;

Type of load:              Sediment                         Transport process:
                           Characteristics:
Bedload                    Coarse caliber particles         Intermitting sliding,
                           such as; boulders, cobbles       rolling and hopping
                           and gravel.                      along the stream bad at
                                                            high velocity – for large
                                                            particles, lift and
                                                            eddying are very
                                                            important.
Suspended or wash load.    Fine-caliber particles of silt   Silts and clays are
                           and clay, and medium size        entrained at high flow
                           sand particles.                  velocities and
                                                            transported along
                                                            distances in suspension
                                                            – sand sized particles
                                                            move at lower velocities,
                                                            bouncing along the
                                                            stream bed and lifted
                                                            into the current and are
                                                            transported.
Solution load.             Dissolved minerals from          Minerals in solution –
                           the weathering and               transport occurs
Becky Dearne


                           erosion of carbonate rocks   continually and is
                           that crop out in the         independent of velocity
                           channel and in the           and discharge.
                           catchment.
   Material may be transported by a river in four main ways: solution,
   suspension, saltation and traction (see diagram).




    The type of transport taking place depends on 1. The size of the sediment
   and 2. The amount of energy that is available to undertake the transport.
   In the upper course of the river there is more traction and saltation going
   on due to the large size of the bedload, as a river enters its middle and
   lower course there is a lot of finer material eroded from further upstream
   which will be carried in suspension.

   River Deposition –

   Deposition is where material carried by the river is dropped. This will occur
   when there is no longer sufficient energy to transport material. Deposition
   of material may result in the formation of distinctive features such as slip
   off slopes (on the inner bends of meanders); levees (raised banks) and of
   course the floodplain itself. Remember - it is the largest material that will
   be dropped first as it requires the most energy to be transported. Eroded
   from further upstream which will be carried in suspension.

   Here is a website which has an animation which shows the sediment
   deposition as a river enters a lake -

   http://www.classzone.com/books/earth_science/terc/content/visualizations/
   es0604/es0604page01.cfm?chapter_no=visualization.