Learn the Design Principles of Sarda Type Fall and Compare it with Other Types of Canal Falls (PDF)
Design of Sarda Type Fall PDF Download
A canal fall is a solid masonry structure that is constructed on a canal when the natural ground slope is steeper than the designed channel bed slope. It helps to reduce the excess energy of water and prevent erosion and scouring of the canal bed and banks. A canal fall also helps to control the water level and flow in the canal.
Design Of Sarda Type Fall Pdf Download
There are different types of canal falls, such as ogee fall, rapid fall, stepped fall, trapezoidal notch fall, well type fall, etc. Each type has its own advantages and disadvantages depending on the site conditions, discharge capacity, economy, and maintenance requirements. One of the types of canal falls that is widely used in India is the Sarda type fall.
A Sarda type fall is a simple vertical drop fall that consists of a single vertical drop that allows the upstream water to fall with sudden impact on downstream. The downstream acts like a cushion for the upstream water and dissipates extra energy. This type of fall was first tried in Sarda Canal in Uttar Pradesh, India, and therefore it is also called Sarda Fall.
A Sarda type fall has some advantages over other types of falls. It is easy to construct and maintain, as it does not require any complicated curves or slopes. It is also economical and suitable for low discharges and small drops. It can be constructed on weak soils, as it minimizes the depth of cutting. However, a Sarda type fall also has some disadvantages. It may cause excessive turbulence and splashing of water, which may damage the downstream structures. It may also create backwater effect and reduce the effective head available for irrigation.
Design Principles of Sarda Type Fall
The design of a Sarda type fall involves determining the crest level and dimensions, calculating the discharge over the crest, designing the body wall and cistern dimensions, and checking the stability and imperviousness of the floor.
How to determine the crest level and dimensions
The crest level and dimensions of a Sarda type fall depend on the discharge capacity, bed width, depth of flow, and drop height of the canal. The following formulas can be used to determine them:
For canal discharge 15 cumec or more: Crest length = Bed width
For distributaries and minors: Crest length = Bed width + Depth of flow
For rectangular body wall: Top width = 0.552 d; Base width = H + d/p
For trapezoidal body wall: Top width = 0.522 (H + d); Base width = determined by upstream batter 1:3 and downstream batter 1:8
Crest level = Upstream bed level - H
Here H is depth of water above the crest in metres (including velocity of approach), d is height of crest above downstream bed level in metres, p is specific weight of water in kN/m, and L is length of crest in metres.
How to calculate the discharge over the crest
The discharge over the crest of a Sarda type fall can be calculated by using the following formulas:
For free flow condition: Q = CLH H/b
For submerged flow condition (above 33% submergence): Q = CdL(H - h2)
Here Q is discharge in cumec, C is coefficient of discharge (2 for trapezoidal crest and 1.85 for rectangular crest), Cd is coefficient of discharge (0.65), H is drop in water surface in metres, and h2 is depth of downstream water level over top of crest in metres.
How to design the body wall and cistern dimensions
The body wall of a Sarda type fall can be either rectangular or trapezoidal in section, depending on the discharge capacity of the canal. The edges of the body wall are rounded with a radius of 0.3 m. The stability of the body wall should be checked by usual procedure when the drops exceed 1.5 m. Drain holes may be provided at the upstream bed level to dry out the canal during closures for maintenance.
The cistern dimensions of a Sarda type fall can be fixed by using the Bahadurabad Research Institute formula, which is:
Cistern length = 5EHL
Cistern width = (EHL)
Here E is coefficient of impact (0.5 for vertical impact and 0.7 for inclined impact), and HL is head loss in metres.
How to check the stability and imperviousness of the floor
The stability and imperviousness of the floor of a Sarda type fall can be checked by using Bligh's theory or Khosla's theory, depending on the size of the structure. Bligh's theory is based on the assumption that the seepage flow under the floor follows a straight line, while Khosla's theory is based on the assumption that the seepage flow under the floor follows a curved line.
According to Bligh's theory, the total length of impervious floor should be equal to Bligh's creep coefficient multiplied by the total head causing seepage. According to Khosla's theory, the total length of impervious floor should be equal to Khosla's creep coefficient multiplied by the total head causing seepage.
The creep coefficients are empirical values that depend on various factors, such as soil type, permeability, uplift pressure, exit gradient, etc. They can be obtained from standard tables or charts.
Comparison of Sarda Type Fall with Other Types of Falls
A Sarda type fall is different from other types of falls in terms of its shape, design, performance, and suitability. The following table compares some of the common types of falls with Sarda type fall:
Type of Fall Shape Design Performance Suitability ------------------------------------------------------- Ogee Fall Combination of convex and concave curves Complex curves and slopes Smooth transition and low impact Steep slope and high discharge Rapid Fall Long sloping glacis Simple slope and curtain walls Moderate turbulence and impact Plane and long slope Stepped Fall Vertical steps at gradual intervals Simple steps and drops High turbulence and impact High upstream level Trapezoidal Notch Fall High crested wall with trapezoidal notches Simple notches and wall Low turbulence and impact Low discharge and drop Well Type Fall Inlet well with pipe at bottom Complex wells and pipes No turbulence and impact High discharge and drop Sarda Type Fall Single vertical drop Simple drop and wall High turbulence and impact Weak soil and low discharge The criteria for choosing the suitable type of fall for a canal depend on various factors, such as site conditions, discharge capacity, economy, maintenance, etc. Some general guidelines are:
Ogee fall is suitable for steep slope and high discharge canals where smooth transition and low impact are desired.
Rapid fall is suitable for plane and long slope canals where moderate turbulence and impact are acceptable.
Conclusion
In this article, we have discussed the design of Sarda type fall, which is a simple vertical drop fall that is widely used in India. We have explained the design principles of Sarda type fall, such as how to determine the crest level and dimensions, how to calculate the discharge over the crest, how to design the body wall and cistern dimensions, and how to check the stability and imperviousness of the floor. We have also compared Sarda type fall with other types of falls, such as ogee fall, rapid fall, stepped fall, trapezoidal notch fall, and well type fall. We have highlighted the advantages and disadvantages of Sarda type fall and the criteria for choosing the suitable type of fall for a canal.
We hope that this article has been informative and useful for you. If you are interested in learning more about canal falls and their design, you can download a PDF file of this article by clicking on the link below. You can also share your feedback and comments with us by leaving a reply at the end of this page.
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Frequently Asked Questions
What is the difference between free flow and submerged flow conditions in Sarda type fall?
What are the advantages and disadvantages of Sarda type fall?
What are the factors that affect the creep coefficient in Bligh's theory or Khosla's theory?
How to design a Sarda type fall for a canal with a discharge of 10 cumec and a drop of 1.5 m?
What are some examples of canals where Sarda type fall is used?
Answers
Free flow condition occurs when the downstream water level is below the crest level of the fall. Submerged flow condition occurs when the downstream water level is above the crest level of the fall. The discharge formula for free flow condition is Q = CLH H/b, while the discharge formula for submerged flow condition is Q = CdL(H - h2).
The advantages of Sarda type fall are: it is easy to construct and maintain, it is economical and suitable for low discharges and small drops, it can be constructed on weak soils, as it minimizes the depth of cutting. The disadvantages of Sarda type fall are: it may cause excessive turbulence and splashing of water, which may damage the downstream structures, it may create backwater effect and reduce the effective head available for irrigation.
The factors that affect the creep coefficient in Bligh's theory or Khosla's theory are: soil type, permeability, uplift pressure, exit gradient, etc. The creep coefficient is an empirical value that represents the length of seepage flow under the floor per unit head causing seepage.
To design a Sarda type fall for a canal with a discharge of 10 cumec and a drop of 1.5 m, we can use the following steps:
Assume a rectangular body wall with top width b = 0.552 d = 0.552 1.5 = 0.85 m and base width B = H + d/p = 1.5 + 1.5/9.81 = 2.03 m.
Assume a free flow condition with C = 1.85 and solve for H using Q = CLH H/b. We get H = 0.67 m.
Determine the crest level as R.L. of crest = U/S bed level - H = 100 - 0.67 = 99.33 m.
Determine the cistern length as LC = 5EHL = 50.5x1.5 = 4.33 m and cistern width as X = (EHL) = (0.5x1.5) = 0.47 m.
Check the stability and imperviousness of the floor using Bligh's theory or Khosla's theory and adjust the dimensions if necessary.
Some examples of canals where Sarda type fall is used are: Sarda Canal in Uttar Pradesh, India, Lower Ganga Canal in Uttar Pradesh, India, and Upper Bari Doab Canal in Punjab, India.
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