Category: Irrigation Engineering

# Find out the Discharge of a Siphon Spillway – Problem & Solution

## Find out the Discharge of a Siphon Spillway

Mathematical Problem:

Find out the discharge of a siphon spillway from following data:

• Number of siphon units = 4.
• Area at throat in m2 = 3.
• Full reservoir level = 150 m.
• R.L of centre of outlet = 128.
• Tailwater level on D/S side during rains = 130.
• Tailwater level during winter = 125.
• Discharge coefficient = 0.60.

Solution:

Case 1

In rainy days outlet remains submerged and hence discharge depends upon the tailwater level.

Working head = R.L of reservoir – R.L. or T.W.L. = 150 – 130 = 20 m.

$Q = CA \sqrt{2gh}$

Or, $Q = 0.60\times 3 \sqrt{2g\times 20}$

Or, Q = 35.75 cumecs.

So, the discharge of four units = 4 × 35.75 = 143 cumecs.

Case II

In winter T.W.L falls down and spillways discharge free in the air.

Available head = Reservoir level – R.L of center of outlet = 150 – 128 = 22 m.

$Q = CA \sqrt{2gh}$

Or, $Q = 0.60\times 3 \sqrt{2 \times 9.81 \times 22}$

Or, Q = 37.5 cumecs.

Discharge of four units = 4 × 37.5 = 152 cumecs.

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Standard Irrigation Water

How to Calculate Annual Runoff – Problem & Solution

How to Calculate Discharge of the Water if the Depth and Crop Period are Given

# Determine the Discharge of the Channel for which it is to be Designed

## Determine the Discharge of the Channel for which it is to be Designed

Mathematical Problem

A channel is to be designed for irrigating 6000 hectares of Kharif crop and 5500 hectares of Rabi crop. The water requirement for the Kharif crop is 70 cm and the water requirement for Rabi is 35 cm. The Kor period for Kharif is 2 weeks and for Rabi is 3 weeks and 4 days. Determine the discharge of the channel for which it is to be designed.

Solution:

Using the relation

∇ = (8.64 × B)/ D

Discharge for Kharif crop

Here,

• ∇ = 70 cm = 0.70 m
• B = 2 weeks = 14 days

Duty (D) = (8.64 × 21)/ 0.70 = 259.2 hectares/cumec.

Area to be irrigated = 6000 hectares.

Required discharge of channel for Kharif crop = (6000/259.2) = 23.15 cumec.

Discharge for Rabi crop

Here,

• ∇ = 35 cm = 0.35 m.
• B = 3 weeks and 4 day = (21 + 4) = 25 days.

Duty (D) = (8.64 × 25)/ 0.35 = 617.14 hectares/cumec.

Area to be irrigated = 5500 hectares.

Required discharge of channel for Kharif crop = (5500/617.14) = 8.91 cumec.

Therefore, the channel is to be designed for the maximum discharge of 23.15 cumec, because this discharge capacity of the channel will be able to supply water to both seasons.

Read More:

Relation Between Base, Delta, and Duty

Factors Affecting the Duty of Water

How to Calculate Discharge of The Water If The Depth & Crop Period are Given

# How to Calculate Discharge of The Water If The Depth & Crop Period are Given

## How to Calculate Discharge of The Water If The Depth & Crop Period are Given

Mathematical Problem

A Waterhouse has a culturable commanded area of 2200 hectares. The intensity of irrigation for crop A is 50% and for B is 35%, both the crops being Rabi crops. Crop A and B have a crop period of 25 days and 20 days respectively. Calculate the discharge of the water if the depth for crops A and B is 12 cm and 18 cm respectively.

Solution:

Culturable land for irrigation = 2200 hectares.

Area of irrigation for Crop A = 2200 × 50% = (2200 × 50)/100 = 1100 hectares.

Area of irrigation for Crop B = 2200 × 35% = (2200 × 35)/100 = 770 hectares.

Using the relation.

D = (8.64 × B)/ ∇

Discharge for crop A

Here,

• ∇ = 12 cm = 0.12 m.
• B = 25 days

Duty (D) = (8.64 × 25)/0.12 = 1800 hectares/cumec.

Required discharge for crop A = (1100/1800) = 0.61 cumec.

Discharge for crop B

Here,

• ∇ = 18 cm = 0.18 m
• B = 20 days

Duty (D) = (8.64 × 20)/0.18 = 960 hectares/cumec.

Required discharge for crop B = ( 770/960) = 0.80 cumec.

S0, the total discharge of water course = (0.61 + 0.80) = 1.41 cumec.

Read More:

Relation Between Base, Delta, and Duty

Factors Affecting the Duty of Water

Determine the Discharge of the Channel for which it is to be Designed

# How to Calculate Annual Runoff? Problem & Solution

## How to Calculate Annual Runoff?

Mathematical Problem:

For a catchment in M.P., India, the mean monthly rainfall and temperatures are given. Calculate the annual runoff by Khosla’s formula.

Solution:

In Khosla’s formula,

Rm = Pm – Lm

Where,

• Rm = Monthly runoff
• Pm = Monthly rainfall in cm
• Lm = (0.48 × T) Or Pm whichever is less.
• T = Monthly temperature in degrees.

1. For January month

Lm = (0.48 × 12) = 5.76

So, 5.76 is greater than the corresponding Pm = 4.

So, the value of Lm will be taken as 4.

Now,

Rm = (Pm – Lm) = (4 – 4) = 0

2. For July month

Lm = (0.48 × 31) = 14.88

So, 14.88 is less than the corresponding value of Pm = 32

So, we will take the value of Lm = 14.9

Now,

Rm = (Pm – Lm) = (32 – 14.88) = 17.1

I think you can able to solve the rest of the months by using the above method.

So, the total annual runoff = ( 17.1 + 15.1 + 2.6) = 34.8 in cm.

Read More:

Types of Rainfall

Types of Irrigation

Methods of Improving Duty of Water

# Benching – Excavation or Cutting

## Benching

The art of cutting side slopes in steps is called benching. When the embankment is constructed on hillside slopes, it is necessary to bench the surface of the hill slope to add stability to the slope.

These benches should have a gentle fall towards the hillsides. Thus benching reduces the probability of sides of slopes.

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Borrow Pits

Spoil Bank

# What is Spoil Bank? Earthwork

## Spoil Bank

The bank constructed from surplus excavated earth on the side cutting parallel to the road alignment or main canal bank is known as a spoil bank.

In other words, when the excavated earth is not completely required for the forming of the canal bank or the road embankment. In such a case, the extra earth is deposited in the form of small banks which are known as spoil banks.

The height of the spoil bank should be nearly 1.5 m. They should be constructed at a distance of 3 m from the top edge of the cutting.

The spoil bank is not continuous along the main bank, adequate spaces are left between the two adjacent spoil banks for proper drainage.

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Procedure For Doing Earthwork in Embankment

# 11 Factors to be Considered While Fixing Canal Alignment

## Factors to be Considered While Fixing Canal Alignment

The following factors should be considered while fixing the canal alignment:

1. The canal should be straight, as a result, the length of the canal will be minimum, which helps to reduce the loss of water due to percolation and evaporation.

2. It should be spotted that the canal does not pass through a village or town but by the side of it.

3. Cross-drainage work should be avoided since such work is very costly.

4. Deep cuttings and high filling should be avoided.

5. The alignment should avoid fissure and brackish formations. Fissured formations cause loss of water through fissures, and brackish formations render water useless.

6. To get off excessive percolation losses, the alignment of the canal should avoid passing through sandy tracks.

7. The alignment should avoid rock formation. Required a lot of labor for the construction of canals in the rocky pathways.

8. Idle length of the canal should be minimum.

9. Suitable foundations and construction materials should be available for the works like falls, cross regulators, head regulators, etc.

10. Unnecessary curves in the canals should be avoided, if the curves are necessary, they should be as large a radius as possible.

11. The cutting and filling should be such that( i.e. cutting should be equal to filling as possible), it will be most economical.

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Advantages and Disadvantages of Canal Lining

Methods of River Bank Protection

Standard of Irrigation Water

# 10 Advantages and Disadvantages of Canal Lining

Canal lining has some advantages and some disadvantages which are as follows:

## Advantages of Canal Lining

The following are the advantages of canal lining:

### 1. Seepage Control

Canal lining helps to prevent water loss due to seepage. Heavy seepage losses in canals would require a large reservoir and a bigger dam to be built. Therefore, preventing seepage by lining would decrease their impounding capacity, thus reducing the construction costs of these works.

### 2. Prevention of Waterlogging

In simple words, the rising of the water table near the ground surface is known as waterlogging. The water table in the surrounding field of the canal rises near the ground surface level because of the continuous seepage through the unlined canal.

As a result, it brings up harmful salts near the ground surface, which makes the land unsuitable for cultivation. Thus, the lining of the canal prevents waterlogging, and it helps to increase food production.

### 3. Increase in Commanded Area

A lined canal reduces the evaporation loss as well as seepage loss. Which saves more water for irrigation. Additional areas may be brought under irrigation from the water saved by the lining.

### 4. Prevent Weed Growth

Unlined canal sides and beds are very favorable for the growth of weeds. It reduces the velocity of the water flow through the canal. Hence, by providing lining, the growth of weeds can be prevented.

### 5. Others

• High velocity prevents silting of the canal.
• The lining makes the bank more stable.

## Disadvantages of Canal Lining

The following are the disadvantages of canal lining:

1. It requires a very heavy initial expenditure.

2. When the lining gets old, cracks usually develop. It is very hard to check leakage through cracks.

3. The portion of the lining joints always creates problems.

4. The outlet shifting process is very difficult.

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Methods of River Bank Protection

# Soil-Cement Lining || Civilnoteppt

## Soil-Cement Lining

When the lining is done by using a mixture of soil and cement is known as soil-cement lining. The quantity of the used cement in the soil-cement lining is 5 to 10 % of the weight of dry soil.

The mixture, in a proportion of 5 to 10% cement and 90 to 95 % is uniformly mixed dry, and then water is added, and again thoroughly mixed so as to bring the soil to its optimum moisture content. The mixture is placed at the site, and it is made properly compact. For at least seven days it is kept covered with wet sand for curing.

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4 Methods of River Bank Protection

# Failure of Tube Well || Reasons – Corrosion & Incrustation

## Failure of Tube Well

A tube well may fail for the following reasons:

• Corrosion
• Incrustation

### Corrosion of the Well Pipes

The Corrosion of the well pipes is caused by the presence of acid, chloride and sulphate in the groundwater. The following measures can be taken to reduce the corrosion of the well pipes.

1.  Always thick pipes should be used.
2.  Always galvanized or other anti-corrosion coated pipes should be used.
3.  Periodic cleaning of the well pipe should be performed with sulphuric acid.

### Incrustation

The groundwater also holds calcium bicarbonate, magnesium, sulphate, etc. For a long duration, these components are deposited inside the tube wells pipes, this reduces the diameters of the pipe. This is known as incrustation.
The incrustation can be detained as follows:

1. In order to determine the presence of alkali salts, the water of the tube well is tested in the laboratory. Salts responsible for incrustation may be removed by titration.

Titration is performed by forcing sufficient doses of acid into the well. The water in the tube well is then washed by pumping.

2. The tube well should not be left unused for a long period.

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