What are Canal Falls and their significance and typesArticles > What are Canal Falls and their significance and types
Whenever the natural ground slope is steep than the designated bed slope of the channel, the difference is adjusted by constructing vertical “falls” or “drops” in the canal beds at a suitable intervals. Such a drop in a natural canal bed will not be stable and, therefore, in order to retain this drop, a masonry structure is constructed. Such a structure is called Canal Fall or Canal Drop.
A canal is not constructed according to the slope of land, because if a canal is constructed parallel to the slope of land, the velocity of flowing water will increase up to extraordinary level.
Why is a canal fall constructed?
- A canal fall is constructed across the canal to lower down the bed level to maintain the designed slope when there is a change of ground level to maintain the designed slope when there is change of ground level.
- Canal fall is constructed to reduce the slope of canal bed. If the slope of canal bed is not reduced, the velocity of flowing water will increase to an extra-ordinary level.
- If at a sloppy ground, the level of canal bed is not lowered down, the construction of canal, at a level higher than that of ground, becomes impossible.
- Construction of Canal fall can be used of generation of energy.
- Most of the canal falls are made in slope to lower the speed of falling water at a canal fall. If the canal fall is constructed vertical, the freely falling water will damage the bottom of the canal.
There are many types of canal falls, among these following are the most important.
(1). Ogee canal fall:
The purpose of this type of fall is to provide smooth transition to the flowing water from upstream to the downstream. This type of fall is designed by giving a concave curve to the bed of canal at the fall region. The water at this type of fall does not fall freely. The curved bottom of the canal makes the water move smoothly with out colliding with any part of the bottom-bed of the canal. There is not any sudden up and down in the bottom of the canal, thus, giving the water a smooth move from the upstream to the down stream.
(2). Rapid fall or rapid:
Rapid fall consists a long sloping glacis. It is constructed if the available natural ground surface is plane and long. For this, a bed of rubble masonry is provided and it is finished with cement mortar of 1:3 ratio. To maintain the slope of bed curtain walls are provided at both upstream and downstream.
(3). Stepped canal fall:
The canal falls is either constructed in the form of a smooth slope or in the form of steps of stairs. The purpose of the steps is to prevent the transiting water from directly collision with the bottom of the canal at down stream. The ground slope is adjusted in stepped slope with in the bed of canal. Another significance of the stepped fall is to lower down the velocity of flowing water from the upstream to the down stream.
(4). Trapezoidal notch fall:
The purpose of trapezoidal notches is to disperse the water flowing downstream. A trapezoid means a flat shaped block having four equal sides, while notches means a deep narrow passage. Four sided square of rectangular or square shaped blocks of concrete are constructed in the way of water escorted from upstream at equal intervals while allowing the passage of water through the notches between the trapezoids.
(5). Vertical drop type fall:
When there is a 90 degree vertical slope in the canal bed, it is known as Rapid Fall or Rapid. The purpose of vertical fall is also the same as that of the other types of falls. However, the demerit of such type of fall is, the freely falling water collides with the bed of canal. Thus, the chances of erosion of the bed of the canal are more likely than in any other type of fall.
(6). Glacis type fall:
This is the modern type of construction, in which a raised crest is constructed across the canal and a gentle straight inclined surface is provided from raised crest to the downstream.
(7). Meter or non-meter fall:
A meter falls is a type of fall, in which the discharges water can be measured. And the non-meter fall is a fall in which the discharged water can not be measures. Generally a Glacis type of fall is suitable as a meter whereas vertical drop is not suitable due to formation of partial vacuum under the nape.
(8). Flume and full-width fall:
A fall may either construct the full channel width or it may distract. The falls of contracted width know as flamed falls. For a flamed fall, the smooth upstream transition should provide to avoid turbulence and to maintain an accurate depth discharge relationship.
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