Erosion and deposition process

EROSION consists of three components, which are Corrasion, Hydraulic Action and Solution.
Any surplus energy is used by a river to carry out erosion. Erosion is the picking up and removal of material. There are three main processes by which a river will erode its channel.

Solution: water contains dissolved carbon dioxide from the air and this may react with limestone and chalk, causing it to dissolve.

Corrasion: this is where particles of rock carried by the river grind away at the bank and bed. It

is the most significant type of erosion in most rivers.

Hydraulic action: this is the sheer power of the water as it crashes onto the bed or against the banks. It is particularly significant at waterfalls and raids or during times of flood. Air bubbles may burst in areas of great turbulence sending our shockwaves which may increase erosion. This is cavitation.

DECOMPOSITION occurs throughout the course of a river wherever the speed of flow drops such that particles can no longer be carried. This could occur on the inside bend of a meander on the bed of the ricer or close to its banks where friction is at its greatest or where the ricer enters the sea or a lake and its flow is checked.

In addition, erosion mainly happened at upper course and middle course of a river, where holds the largest energy, and also the fastest speed. Decomposition mainly happened at the lower course of a river, where has the lowest speed or energy, even though its volume may be the largest.

Done by Fangwei

Waterfalls and plunge pools

A WATERFALL is a vertical step in the river. It is usually found along the upper course of the river where velocity is usually greater. Waterfalls usually form when there is a band of more resistant rock cutting across less resistant rock, as seen in the picture below (:



As the river flows, the more resistant rock gets eroded slower than the less resistant rock. Thus, a step is formed. Over time, as water goes down the vertical step its kinetic energy hits the bottom, causing a deep pool under the waterfall. This is called the PLUNGE POOL.

As the plunge pool gets larger and larger, the overhang of the waterfall (the more resistant rock, in the picture) will eventually drop into the plungepool due to a lack of support. Overtime, the waterfall receeds. Apparently, the niagara falls retreats about three feet a year! :O

The biggest waterfalls in the world attract many tourists. These include niagara falls, victoria falls etc. Here's a pic of the picturesque Niagara Falls (:




Grace.

Urbanisation

As familiar as this question may seem to us, only a few selected ones actually know the true answer to this puzzling question - how does urbanisation affect the hydrograph? First of all, in order to answer this question with precision, this has to be broken down into 2 parts, the water balance equation as well as the hydrograph.

Water Balance Equation: Ppt = Q + E + Change in S
Legend: Ppt = percipitation
E = evapotranspiration
Q = surface runoff
(delta S) = change in soil moisture

During urbanisation, there is a decrease in Evapotranspiration (E) due to the fact that trees are being cut down for the sake of development. As a result, this results in a decrease in soil moisture as well, as when trees are cut down, this reduces interception capabilities through throughfall and stemflow. As such normally, we would expect an increase in surface runoff (Q) by rainfall soil erosion. However, this is not the case as in urbanised areas, there is little soil exposed as soil is mainly covered up by concrete which is impermeable. As such, this reduces the infiltration capacity of soil greatly. As a direct result, as surface runoff = total ppt - rate of infiltration, there will be an increase in Q.



Now that we know that there is an increase in surface runoff, this would also lead us into discovering that there is basically a higher peak now in the hydrograph as well as a steeper rising limb as those 2 are dependant on amount of surface runoff generated as surface runoff is the fastest to enter the river. There will too be a lesser lag time as seen in graph above as basically, in urbanised areas, to protect against flooding as a result of high amounts of surface runoff, there are advanced drainage systems which will too speed up in the generating of surface runoff, decreasing lag time. As such, we now know more about effects of urbanisation on hydrographs. Oh yea. Great..

And ya, kudos to MOE for the graph above.

Joshua
JH40615