Sedimentation – Types, Effects, and Measurement

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SEDIMENT:

  • Products of hydric erosion which are set in motion are called Sediments.

SEDIMENTATION:

  • “It is removal, and deposition of eroded material in channels, dams, ponds reservoirs, etc”.
  • Erosion and sedimentation are interrelated in complex ways and each aggravates the other. Rain produces erosion à Erosion produces sediments and sedimentation in channels which in turn raises flood stages and damages property.
  • Erosion affects the stability of soil, and usefulness of land water for man purposes like drinking, agriculture, industrial, etc. WHILE sedimentation affects the channels, ponds, reservoirs, etc.



SEDIMENT YIELD:

  • It is the weight of detached properties of soil from rocks in ton or tones which have been eroded from a unit area in unit time.

Denudation:

– “It is the rate of lowering of the land surface usu expressed as uniform thickness of depths (inch or cm) in a month or year.”

TYPES OF SEDIMENTS:

Trash Load:

The amount of sediments floating on the surface is called Trash load.

Suspended Sediments:

It is the particular matter (organic or inorganic) suspended in and carried by moving water. It affects the turbidity (dirtiness and transparency) or water.

Dissolved Load:

It refers to all organic and inorganic material carried in solution by moving water. It affects the conductivity of water.

Bed Load or Contact Load:

It refers to coarser materials, usu minerals, moved along the bottom of the streams. Due to force of velocity and buoyancy, gravels, stones, and boulders move along the bottom of the stream by rapping (saltation _ direct pressure of wind causes soil particles to collide with other particles, as a result, particles are pushed along the surface of the ground and suddenly leap, vertically as in detrusion.), rolling and sliding.

Total Stream load:

It includes all the organic and inorganic material carried part a sampling station in the form of dissolved suspended, as bedload material.

Sediment Balance:

Sediment deposited = Sediment incoming – (sediment outgoing + sediment settled).



EFFECTS OF SEDIMENTATION:

  • The lifespan of our reservoirs, dams, ponds, channels, is decreasing.
  • It affects the irrigation system
  • Floods occur in plains which greatly damages the properties
  • Damages agricultural as well as forest crops, esp young crops
  • Affects the turbidity of the water
  • Affects the conductivity of water
  • Affects the fertility of the soil
  • Decreases the activities of hydro-electric reservoirs
  • Channels bank erosion increases due to sedimentation
  • It causes great economic loss of the country

MEASUREMENT OF SEDIMENTATION/ HYDRIC EROSION:

            Following methods are generally used:

By Surveying:

  • Sediment discharge can be measured from deposits in reservoirs by surveying with reference to permanent pillars
  • Elevation of sediments deposited is taken by direct surveying
  • In submerged areas by ‘sediment feeler’ or ‘sounding equipment’

Cross section of Gullies:

  • Sediments from gullies and other sources can be measured by a detailed topographic survey of gullies or channels at particular points.
  • Deposition and removal of sediments measured by surveying and multiplying with the average length, total sediments is calculated in (tons/ ha)

Measuring sediment in water samples:

  • A sample of the runoff water from the stream is taken
  • Suspended load is estimated by filtering this sample and is represented in parts per million (ppm)

Sediment samples:

  • An equipment to collect the representative sample of streamflow runoff for measurement of the amount of sediment and hence to compute the sediment in the total stream flow or runoff

Metal stacks:

  • There are buried at different places. The portion of the stack, which has exposed, is the soil which has been eroded.
  • It measures sheet erosion or denudation only.

Exposure of tree roots:

  • Painting collar of the tree at surface or erosion pavement.
  • The depth up to which the roots exposed in the erosion
  • The height up to which a heavy martial is laying over the surface shows the height of material which has been eroded away.
  • It also measures sheet erosion and denudation only

Runoff plots:

  • Small runoff plots are laid out and the soil eroded from these plots measured by analyzing the sediment content in runoff from these plots.
  • This measures all the eroded material from a single uniform slope and does not account for the material which will be deposited on the way towards the measuring station.

Soil profile description:

  • Studying soil profile at different places, the eroded material deposited at a site during a certain period can be estimated.

Old tree stumps and rock:

  • 1st: mark the collar of about 10 to 12 trees in the watershed area.
  • 2nd: After an interval, say 5 yrs, check the mark of the collar.
  • If there is an average difference of say 7cm of all the trees forms the initial mark position in the watershed area, then, the total amount of sediments can be calculated as:

As, 7cm = 0.07m

Multiply it with the area of w-shed (say 10,000 m2)

Then, 0.07 m × 10, 000 m2

= 700 m3 has been eroded

By Construction of Flumes:

  • (Most accurate method of measuring erosion.)
  • Sediments are collected in a circular cemented tank with a pipe having 30 holes of equal dimensions
  • All holes are closed except 1 hole which is piped in another hole of the same dimension. And similarly the second one in the third one. And finally in a tank.

Measurement of Sedimentation and Erosion - Forestrypeida

  • Now calculate volume (Volume = Area × height) from the last tank and multiply it with ‘3’. Finally, calculate the total volume
  • Take water samples -> calculate ppm by sensitive weight balance -> then filter it and place it in the oven
  • For convenience, take 100 cc water -> filter it -> sediments remain on filter paper (measure the initial weight) -> again in oven (measure the final weight) and finally calculate the amount of ‘sediments’.

For correction and improvements please use the comments section below.




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