Design Of Sarda Fall.pdf
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How to Design a Sarda Fall for Canal Irrigation
A Sarda fall is a type of vertical drop fall that is used to dissipate the excess energy of water flowing in a canal. It consists of a raised crest with a rectangular or trapezoidal shape, a downstream expansion, and a cistern with a sloping glacis. Sarda falls are commonly constructed on sandy soils where the depth of cutting is to be minimized.
In this article, we will explain the design principles and steps for a Sarda fall, based on the information available in the PDF document \"Design Of Sarda Fall (Vertical Canal Section)\"[^1^] [^2^]. We will also provide some examples and diagrams to illustrate the process.
Design Principles for Sarda Fall
The design of a Sarda fall is based on the following hydraulic parameters:
Total fall (Hw): The difference between the upstream and downstream full supply levels (FSL) of the canal.
Crest length (L): The length of the crest parallel to the canal axis. It is usually equal to the downstream bed width of the canal.
Crest width (B): The width of the crest perpendicular to the canal axis. It depends on the shape of the crest (rectangular or trapezoidal) and the head above the crest (H).
Head above the crest (H): The difference between the upstream total energy level (TEL) and the crest level. It is determined by trial and error using the discharge equation.
Discharge (Q): The flow rate of water in the canal. It is given by Q = C x L x H x (H/B), where C is a constant depending on the shape of the crest.
Velocity of approach (Va): The average velocity of water in the upstream channel. It is given by Va = Q/A, where A is the cross-sectional area of the channel.
Velocity head (ha): The kinetic energy per unit weight of water due to its velocity. It is given by ha = Va/2g, where g is the acceleration due to gravity.
Effective head (He): The sum of the head above the crest and the velocity head. It represents the total energy available for dissipation at the fall.
Total energy level (TEL): The sum of the FSL and the velocity head. It represents the total energy of water in the upstream channel.
Crest level: The elevation of the top surface of the crest. It is obtained by subtracting H from TEL.
Height of crest above upstream bed level (hc): The difference between the crest level and the upstream bed level.
Hydraulic jump: A phenomenon that occurs when water flows from a high-velocity state to a low-velocity state, resulting in a sudden rise in water level and a loss of energy.
Hydraulic head (Hh): The difference between the crest level and the downstream bed level. It represents the maximum height that water can rise after passing through the fall.
Total length (LT): The length of the floor required to prevent undermining of the foundation due to seepage. It is obtained by multiplying Hh by Bligh's coefficient (c), which depends on the soil permeability.
Length of cistern (Lc): The length of
the floor where a hydraulic jump occurs. It is obtained by multiplying 5 by
the square root of Hw x He.
Depth of cistern (x): The depth of water at
the end of cistern. It is obtained by multiplying 0.25 by Hw.
Cistern level: The elevation