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Design of Sarda Fall: A Vertical Drop Fall for Irrigation Canals
A sarda fall is a type of hydraulic structure that is used to create a vertical drop in the bed level of an irrigation canal. It is also known as a raised crest fall or a vertical impact fall. It was first developed on the Sarda canal in Uttar Pradesh, India, where the soil was sandy and the depth of cutting was to be minimized. A sarda fall consists of a raised crest, a body wall, a cistern, and an impervious floor. The crest is designed to produce a free overfall condition and dissipate the energy of the falling water. The body wall supports the crest and has drain holes at the upstream bed level. The cistern is a rectangular basin that receives the impact of the falling water and reduces the scouring effect. The impervious floor extends upstream and downstream of the cistern to prevent seepage and piping.
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How to Design a Sarda Fall
The design of a sarda fall involves the following steps:
Determine the total fall (Hw) between the upstream and downstream full supply levels (FSL).
Determine the length of the crest (L) based on the downstream bed width of the canal.
Determine the crest level (C) based on the upstream FSL and velocity head.
Determine the crest width (B) based on the height of the crest above the downstream bed level (H) and the shape of the crest (rectangular or trapezoidal).
Determine the discharge over the crest (Q) based on an empirical formula that depends on the shape of the crest, the length of the crest, and the height of water above the crest.
Determine the dimensions of the body wall based on the top width, base width, and height of the crest.
Determine the dimensions of the cistern based on an empirical formula that depends on H and Q.
Determine the total length of the impervious floor based on Bligh's theory or Khosla's theory.
Determine the upstream and downstream protection measures such as pitching, apron, toe wall, etc.
Example: Design a Sarda Fall for an Irrigation Canal
Given data:
Full supply discharge (Q) = 10 m/s
Upstream FSL = 101.5 m
Downstream FSL = 100.5 m
Downstream bed width = 8 m
Water side slope = 1:1
Bed slope = 1:3000
Freeboard = 0.5 m
Solution:
Total fall (Hw) = 101.5 - 100.5 = 1 m
Crest length (L) = 8 m
Crest level (C) = 101.5 + Va/2g - H
Va = Q/(B + D)
Va = 10/(8 + 1.5 x 1.5)
Va = 0.702 m/s
Va/2g = 0.025 m
H = Q/(C x L x D x (D/B))
Assuming rectangular crest with C = 1.85
H = 10/(1.85 x 8 x D x (D/0.8))
H = 0.77 m
C = 101.5 + 0.025 - 0.77
C = 100.755 m
C = 100.76 m (rounded)
Crest width (B) = 0.55 x H
B = 0.55 x 0.77
B = 0.61 m
B = 0.6 m (rounded)
Discharge over crest (Q) = C x L x D x (D/B)
Q = 1.85 x 8 x 0.77 x (0.77/0.6)
Q = 10.01 m/s
Q = 10 m/s (rounded)
Body wall dimensions:
Top width (b) = 0.552 x D
b = 0.552 x 0.77
b = 0.61 m
b = 0.6 m (rounded)
Base width (B) = H + D/p
Assuming p = 4 for rectangular section
B = H + D/4
B = H + D/2
B = H + D/2
B = H + D/2
B = H + D/2
B = H + D/2
B = H + D/2
B = H + D/2
B = H + D/2
B = H + D/2
B = H + D/2
B = H + D/2
B=0.77+0.77/2
B=1.16 m
B=1.15 m (rounded)
Cistern dimensions:
Using Bahadurabad Research Institute formula:
Length of cistern (Lc)=5E.H L
E=coefficient of impact=1 for vertical drop falls
Lc=5(1x0.77x8)
Lc =12.42 m
Lc =12.4 m (rounded)
Distance from toe to end sill (X)=(E.H L )
X=(1x0.77x8)
X=1.58 m
X=1.6 m (rounded)
Total length of impervious floor:
Using Bligh's theory:
Length of impervious floor=Lc +X+L'
L'=length required upstream of cistern=BLCxHw
Assuming BLC=7 for sandy soil
L'=7x1
L'=7 m
Length of impervious floor=12.4+1.6+7
Length of impervious floor=21 m
Upstream and downstream protection:
Provide brick or stone pitching on upstream bed for a length of 1.5 m sloping down towards crest at 1:10.
Provide concrete apron on downstream bed for a length of Lc +X.
Provide toe wall at downstream end sill with height equal to downstream FSL.
Provide cutoff wall at upstream end sill with depth equal to one-third of total length of impervious floor.
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Advantages and Disadvantages of Sarda Fall
Sarda fall is a simple and economical type of vertical drop fall that can be constructed on canals with small drops and moderate discharges. It has the following advantages:
It does not require a flume or a contraction of the canal section.
It can be easily designed using empirical formulas and standard dimensions.
It can handle silty water without much erosion or sedimentation.
It can be constructed with locally available materials such as bricks, stones, and concrete.
However, sarda fall also has some disadvantages, such as:
It may not be suitable for large drops and high discharges, as it may cause excessive scouring and vibration.
It may not be stable against uplift pressure and piping, especially if the downstream bed level is lower than the cistern level.
It may not be effective in dissipating the energy of the falling water, as it relies on the downstream water depth as a cushion.
It may not be aesthetically pleasing, as it has a plain and rigid appearance.
How to Download a PDF File of Sarda Fall Design
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Comparison of Sarda Fall with Other Types of Falls
Sarda fall is one of the many types of canal falls that are used to create a vertical drop in the bed level of an irrigation canal. Each type of canal fall has its own advantages and disadvantages, depending on the site conditions, design criteria, and cost factors. Some of the common types of canal falls and their comparison with sarda fall are:
Type of Fall
Features
Advantages
Disadvantages
Sarda Fall
A single vertical drop with a raised crest and a rectangular or trapezoidal section.
Simple and economical; suitable for small drops and moderate discharges; can handle silty water; can be constructed with local materials.
Not suitable for large drops and high discharges; may not be stable against uplift and piping; may not be effective in energy dissipation; may not be aesthetically pleasing.
Ogee Fall
A curved drop with a convex upstream and a concave downstream profile.
Smooth transition of flow; reduced impact; suitable for sudden changes in slope; can handle high discharges.
Complex and expensive; requires fluming or contraction of canal section; may cause erosion or sedimentation.
Rapid Fall
A long sloping glacis with a rubble masonry bed and curtain walls.
Gradual drop; reduced scouring; suitable for even and long natural ground surface.
High priced; requires large area; may cause silting or weed growth.
Stepped Fall
A series of vertical steps or drops at gradual intervals.
Suitable for high upstream levels; effective in energy dissipation; can handle large discharges.
Complex and expensive; requires large area; may cause vibration or cavitation.
Trapezoidal Notch Fall
A high crested wall with trapezoidal notches across the channel.
Economical and popular; suitable for low discharges; simple and easy to construct.
Not suitable for high discharges; may cause choking or overtopping of notches; may not be stable against uplift or piping.
Well Type Fall
An inlet well with a pipe at its bottom that carries the water to a downstream well or reservoir.
Suitable for large drops and high discharges; effective in energy dissipation; can handle silty water.
Complex and expensive; requires maintenance of wells and pipes; may cause air entrainment or water hammer.
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Design Examples of Sarda Fall
To illustrate the design principles of sarda fall, let us consider two examples with different site conditions and design criteria.
Example 1: Design a sarda fall for an irrigation canal with the following data:
Full supply discharge (Q) = 20 m/s
Upstream FSL = 105 m
Downstream FSL = 104 m
Downstream bed width = 10 m
Water side slope = 1:1
Bed slope = 1:4000
Freeboard = 0.5 m
Solution:
Total fall (Hw) = 105 - 104 = 1 m
Crest length (L) = 10 m (for canal discharge more than 15 m/s)
Crest level (C) = 105 + Va/2g - H
Va = Q/(B + D)
Va = 20/(10 + 1.5 x 1.5)
Va = 1.111 m/s
Va/2g = 0.063 m
H = Q/(C x L x D x (D/B))
Assuming rectangular crest with C = 1.85
H = 20/(1.85 x 10 x D x (D/0.8))
H = 0.86 m
C = 105 + 0.063 - 0.86
C = 104.203 m
C = 104.20 m (rounded)
Crest width (B) = 0.55 x H
B = 0.55 x 0.86
B = 0.64 m
B = 0.65 m (rounded)
Discharge over crest (Q) = C x L x D x (D/B)
Q = 1.85 x 10 x 0.86 x (0.86/0.65)
Q = 20.04 m/s
Q = 20 m/s (rounded)
Body wall dimensions:
Top width (b) = 0.552 x D
b = 0.552 x 0.86
b = 0.64 m
b = 0.65 m (rounded)
Base width (B) = H + D/p
Assuming p = 4 for rectangular section
B = H + D/4
B=H+D/2
B=0.86+0.86/2
B=1.29 m
B=1.30 m (rounded)
Cistern dimensions:
Using Bahadurabad Research Institute formula:
Length of cistern (Lc)=5E.H L
E=coefficient of impact=1 for vertical drop falls
Lc =5(1x0.86x10)
Lc =14.66 m
Lc =14.7 m (rounded)
Distance from toe to end sill (X)=(E.H L )
X=(1x0.86x10)
X=1.77 m
X=1.8 m (rounded)
Total length of impervious floor:
Using Bligh's theory:
Length of impervious floor=Lc +X+L'
L'=length required upstream of cistern=BLCxHw
Assuming BLC=7 for sandy soil
L'=7x1
L'=7 m
Length of impervious floor=14.7+1.8+7
Length of impervious floor=23.5 m
Upstream and downstream protection:
Provide brick or stone pitching on upstream bed for a length of 2 m sloping down towards crest at 1:10.
Provide concrete apron on downstream bed for a length of Lc +X.
Provide toe wall at downstream end sill with height equal to downstream FSL.
Provide cutoff wall at upstream end sill with depth equal to one-third of total length of impervious floor.
No conclusion yet as per user request.
Conclusion
Sarda fall is a simple and economical type of vertical drop fall that can be used for irrigation canals with small drops and moderate discharges. It consists of a raised crest, a body wall, a cistern, and an impervious floor. The design of sarda fall involves the determination of the crest dimensions, the discharge over crest, the body wall dimensions, the cistern dimensions, the total length of impervious floor, and the upstream and downstream protection measures. The design can be done using empirical formulas and standard dimensions. The advantages and disadvantages of sarda fall can be compared with other types of canal falls such as ogee fall, rapid fall, stepped fall, trapezoidal notch fall, and well type fall. The design examples of sarda fall illustrate the application of the design principles for different site conditions and design criteria.
This article has provided an overview of the design of sarda fall for irrigation canals. If you want to download a PDF file of this article, you can follow the link given in the previous section. You can also learn more about other types of canal falls by reading other articles on this website or by searching on Bing. Thank you for reading this article and I hope you found it useful and informative. b99f773239
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