Common types of cross-sections for irrigation canals are trapezoid, parabolic, triangular, circular, and irregular (natural). Square / rectangular cross sections are used normally for concrete ducts.Out of these trapezoidal cross section is widely used due to following reasons :-
1)Heavy rains - The trapezoid has its width gradually increasing towards the top. In case of heavy rains , since the water level increases ,the trapezoid geometry ensures that the channel doesn’t overflow as the water is accomodated in the larger areas at the top of the trapezoid.
2) Velocity - The ratio of flow area to the contact area with the wall is very high. This decreases the effect of viscous forces on the flow velocity of water,thus making it move faster.
3) Safety - If someone or something falls into the canal by mistake, it is easier to climb a wall with a gradual gradient like the trapezoid unlike a vertical wall.
4.)Discharge-The best thing about a trapezoidal channel is that it has maximum discharge i.e., maximum volume of water flows through it within 1 second, when compared to rectangular, circular and triangular channels.
5.)This trapezoidal section offers least frictional resistance.
6.)It is easy to maintain such as dredging and tile work in trapezoidal canal can be done by even unskilled labour and less equipments.
Excellent presentation :-) :-)
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